Magnesium alloy nickel-plated layer interface oxidation prevention low damage metallographic sample preparation method

Abstract

This invention relates to the field of metallographic sample preparation technology, and discloses a method for metallographic sample preparation with low-damage oxidation prevention at the interface of nickel-plated magnesium alloy layers. The method includes: using a nickel-plated magnesium alloy sample as a working electrode in contact with a non-aqueous electrolyte containing a magnesium corrosion inhibitor, a nickel corrosion inhibitor, and a conductive salt; periodically collecting response current density and open-circuit potential during initial electrochemical mechanical polishing, and establishing an interface exposure criterion based on continuous sampling data before interface exposure; determining interface exposure based on the current response current density and the rate of negative shift of the open-circuit potential during continued polishing; switching to dual-potential pulse control after interface exposure, sequentially applying a magnesium protection potential and a nickel passivation potential, and determining the polishing endpoint based on the change in interface response current density at the end of the same phase. This invention reduces interfacial galvanic corrosion and excessive polishing damage through time-division detection and dual-potential pulse control, solving the problem of interface artifacts affecting accurate observation.

Description

Technical Field

[0001] This invention relates to the field of metallographic sample preparation technology, and more specifically, to a metallographic sample preparation method for preventing oxidation and minimizing damage at the interface of nickel-plated magnesium alloy layers. Background Technology

[0002] Magnesium alloys possess characteristics such as low density, high specific strength, good thermal conductivity, and excellent electromagnetic shielding properties, making them widely used in aerospace, automotive parts, electronic product housings, and lightweight structural components. However, magnesium alloys have high chemical reactivity and are prone to oxidation and corrosion in humid, saline, or acidic / alkaline environments, which limits their surface stability and service life. To improve the corrosion resistance, wear resistance, and decorative properties of magnesium alloy surfaces, a nickel plating is typically applied.

[0003] The interfacial bonding state, coating continuity, interfacial porosity, and microcrack conditions of nickel-plated magnesium alloys are important bases for evaluating coating quality and service reliability. Metallographic sample preparation and microscopic observation are common methods for analyzing the cross-sectional morphology and interfacial state of nickel-plated magnesium alloys. In order to obtain clear, flat cross-sectional specimens that can reflect the true interfacial structure, it is usually necessary to sequentially cut, mount, grind, polish, and perform necessary surface treatments on the nickel-plated magnesium alloy specimens.

[0004] Magnesium alloy substrates have low hardness and are chemically reactive, making them prone to plastic dragging, surface oxidation, localized peeling, and edge chamfering during traditional mechanical polishing. Nickel plating differs significantly from magnesium alloy substrates in hardness, plastic deformation capacity, and chemical stability, and nickel plating is also susceptible to problems such as coating edge curling, interface chamfering, coating peeling, or distortion of cross-sectional morphology during polishing. These issues affect the assessment of interfacial bonding, coating thickness, and defect distribution.

[0005] In existing technologies, electrochemical polishing or electrochemical mechanical polishing is also used to prepare magnesium alloys or their surface coatings to reduce deformation damage caused by pure mechanical polishing. These methods typically improve the surface smoothness of the sample and reduce some mechanical scratches and deformation layers by combining applied potential, electrolyte, and mechanical polishing.

[0006] For dissimilar metal interfaces such as magnesium alloys with nickel plating, a significant electrochemical potential difference exists between the magnesium alloy substrate and the nickel plating. When the polishing process reaches the interface region, both the magnesium alloy substrate and the nickel plating may be simultaneously exposed to the electrolyte, easily leading to localized galvanic interactions between them. This can accelerate oxidation or corrosion of the magnesium alloy substrate near the interface. Furthermore, existing electrochemical polishing or electrochemical mechanical polishing methods are mostly based on a single applied potential or a single material surface state, making it difficult to simultaneously adapt to the different electrochemical responses of the magnesium alloy substrate and the nickel plating when dissimilar metal interfaces are exposed.

[0007] These phenomena can create unrealistic gaps, holes, corrosion pits, or crack-like morphologies at the cross-sectional interface, thus interfering with the judgment of the bonding state of the coating interface. Therefore, how to reduce galvanic corrosion and interface damage caused by the simultaneous exposure of the magnesium alloy substrate and the nickel coating at the interface during the metallographic sample preparation of magnesium alloy nickel plating, and obtain metallographic samples that can reflect the true interface morphology, is a technical problem that needs to be solved in this field. Summary of the Invention

[0008] In view of this, the present invention proposes a metallographic sample preparation method for preventing oxidation and minimizing damage at the interface of nickel-plated magnesium alloys. This method aims to solve the problem that in the existing electrochemical mechanical polishing sample preparation process for nickel-plated magnesium alloys, when the magnesium alloy substrate and the nickel plating are simultaneously exposed to the electrolyte in the interface area, galvanic corrosion is prone to occur, resulting in non-real gaps, holes, corrosion pits or crack-like morphologies at the interface, which affects the accurate judgment of the bonding state of the nickel plating interface.

[0009] In one aspect, this invention proposes a metallographic sample preparation method for preventing oxidation and minimizing damage at the interface of nickel-plated magnesium alloy layers, comprising: A magnesium alloy nickel-plated sample was installed as a working electrode in an electrochemical mechanical polishing device, and the magnesium alloy nickel-plated sample was brought into contact with a non-aqueous electrolyte containing magnesium corrosion inhibitor, nickel corrosion inhibitor and conductive salt, wherein the response current density was positive in the anodic direction and negative in the cathodic direction. The magnesium alloy nickel-plated sample was subjected to initial electrochemical mechanical polishing, and the response current density and open circuit potential of the working electrode were collected at intervals during the initial electrochemical mechanical polishing process. The applied potential was kept constant when the response current density was collected, and the applied power supply was disconnected when the open circuit potential was collected. A baseline data window is determined based on continuous sampling data before interface exposure, and an interface exposure criterion is established based on the changes in response current density and open circuit potential within the baseline data window. During the continued polishing process, the interface between the magnesium alloy substrate and the nickel plating is determined to be exposed based on the comparison results of the current response current density and the negative shift rate of the adjacent open circuit potential with the interface exposure criterion. After the interface is exposed, the potential control mode is switched to dual-potential pulse control. Within the same pulse cycle, a magnesium protection potential and a nickel passivation potential are applied sequentially. The magnesium protection potential is a negative cathode potential, and the nickel passivation potential is a positive anode potential. The duration of the magnesium protection potential is longer than the duration of the nickel passivation potential. Electrochemical mechanical polishing continues under the control of the dual-potential pulse, and the polishing endpoint is determined based on the change in interface response current density at the end of the same phase within several consecutive pulse cycles. When the polishing endpoint is reached, electrochemical mechanical polishing is stopped, and a metallographic sample of the magnesium alloy nickel plating layer interface is obtained.

[0010] Furthermore, during the initial electrochemical mechanical polishing process, the response current density and open-circuit potential of the working electrode are collected at intervals, including: Electrochemical mechanical polishing is performed continuously for several detection cycles. Each detection cycle includes a polishing control sub-period and an open-circuit detection sub-period. During the polishing control sub-period, an external power supply is turned on and a nickel plating polishing potential is applied to the working electrode. The response current density at the end of the polishing control sub-period is collected. During the open-circuit detection sub-period, the external power supply is turned off and the working electrode is connected to a high-resistance potential acquisition channel. The open-circuit potential at the end of the open-circuit detection sub-period is collected. The response current density and open-circuit potential collected in the same detection cycle are used as a set of sampling data.

[0011] Furthermore, the duration of the open-circuit detection sub-period is determined based on the open-circuit potential sampling stability, including: After disconnecting the external power supply, the open circuit potential is continuously collected. When the absolute value of the difference between adjacent open circuit potential sampling values ​​is less than or equal to the potential noise amplitude of the high resistance potential acquisition channel, the open circuit detection sub-period ends and the next grinding and polishing control sub-period begins.

[0012] Furthermore, when determining the baseline data window based on continuous sampling data prior to interface exposure, the following steps are included: Several sliding sampling windows are formed with the same number of consecutive detection cycles, and each sliding sampling window includes at least three detection cycles; the range of response current density and the unidirectional drift of open circuit potential in each sliding sampling window are calculated respectively; the sliding sampling window with the smallest range of response current density and the smallest unidirectional drift of open circuit potential is selected as the reference data window; the average value of response current density in the reference data window is calculated, and the difference between the maximum and minimum values ​​of response current density in the reference data window is taken as the response current density fluctuation.

[0013] Furthermore, when establishing the interface exposure criterion based on the response current density change and open-circuit potential change within the reference data window, the following are included: The average response current density is added to the fluctuation of the response current density to obtain the current criterion; the reference open circuit potential change rate is calculated based on the open circuit potential difference and detection interval of adjacent open circuit detection sub-periods within the reference data window; the maximum absolute value of the reference open circuit potential change rate is used as the potential criterion; the current criterion and the potential criterion are used together as the interface exposure criterion.

[0014] Further, when determining whether the interface between the magnesium alloy substrate and the nickel plating is exposed based on the comparison result of the current response current density and the negative shift rate of the adjacent open-circuit potential with the interface exposure criterion, the following steps are included: The response current density at the end of the current polishing control sub-period is compared with the current criterion; the open circuit potential of the current open circuit detection sub-period is subtracted from the open circuit potential of the previous open circuit detection sub-period, and the difference is divided by the detection interval to obtain the real-time open circuit potential change rate; when the response current density is greater than the current criterion, and the real-time open circuit potential change rate is negative and its absolute value is greater than the potential criterion, the interface is determined to be exposed.

[0015] Furthermore, the determination of the magnesium protection potential and the nickel passivation potential includes: Before switching to dual-potential pulse control, the bare magnesium alloy samples and nickel-plated samples from the same batch were placed in the non-aqueous electrolyte for polarization scanning. The polarization scanning used the same reference system as the electrochemical mechanical polishing device. The potential corresponding to the stable change of the cathode current in the cathodic polarization curve of the bare magnesium alloy sample was selected as the magnesium protection potential. The potential corresponding to the passivation plateau of the anodic current in the anodic polarization curve of the nickel-plated sample was selected as the nickel passivation potential.

[0016] Furthermore, when selecting the potential corresponding to the stable change range of the cathode current from the cathodic polarization curve of the exposed magnesium alloy sample as the magnesium protection potential, the following steps are taken: The cathode polarization curve is divided into several continuous potential segments based on the potential step size of the polarization scan. The ratio of the difference in cathode current density to the potential difference between adjacent sampling points in each potential segment is calculated, along with the average absolute value, standard deviation, and range of cathode current density within that potential segment. When at least two consecutive potential segments satisfy the condition that the difference in the average absolute value of the ratio between the segment and the previous segment is less than or equal to the standard deviation of the ratio within that segment, and the range of cathode current density within that segment is less than or equal to the range of cathode current density within the previous segment, these two consecutive potential segments are defined as a stable cathode current variation segment. The potential corresponding to the end with the smallest absolute potential value within the stable variation segment is taken as the magnesium protection potential.

[0017] Furthermore, when the magnesium protection potential and the nickel passivation potential are applied sequentially within the same pulse cycle, the following is included: First, a magnesium protection potential is applied to the working electrode to form a magnesium protection phase; then, a nickel passivation potential is applied to the working electrode to form a nickel passivation phase; the response current density changes over time within the magnesium protection phase and the nickel passivation phase are collected respectively; the negative response current density within the magnesium protection phase is integrated over time to obtain the charge per unit area of ​​the cathode; the positive response current density within the nickel passivation phase is integrated over time to obtain the charge per unit area of ​​the anode; the durations of the magnesium protection phase and the nickel passivation phase are adjusted according to the charge per unit area of ​​the cathode and the charge per unit area of ​​the anode, while maintaining the duration of the magnesium protection phase as greater than the duration of the nickel passivation phase.

[0018] Further, when adjusting the duration of the magnesium protection phase and the nickel passivation phase based on the charge per unit area of ​​the cathode and the charge per unit area of ​​the anode, and determining the polishing endpoint based on the change in interface response current density at the end of the same phase within several consecutive pulse cycles, the process includes: When the charge per unit area of ​​the anode is greater than the absolute value of the charge per unit area of ​​the cathode, the duration of the magnesium protection phase in the next pulse cycle is increased by a control step, and the duration of the nickel passivation phase in the next pulse cycle is decreased by the same control step; the control step is the minimum adjustable time unit of the pulse power supply or an integer multiple of the minimum adjustable time unit; when the duration of the nickel passivation phase decreases to the passivation response time, the shortening of the duration of the nickel passivation phase is stopped; the passivation response time is the time corresponding to the anode response current density of the same batch of nickel-plated samples entering the passivation platform at the nickel passivation potential; The interface response current density is collected at the end of the nickel passivation phase of each pulse cycle, and adjacent statistical windows are formed with the same number of consecutive pulse cycles. Each statistical window includes at least three pulse cycles. The average value of the interface response current density in the previous statistical window and the next statistical window are calculated respectively, and the absolute value of the difference between the two average values ​​is taken as the average change. The difference between the maximum and minimum values ​​of the interface response current density in the next statistical window is calculated as the fluctuation within the window. When the average change is less than or equal to the fluctuation within the window, the grinding and polishing endpoint judgment result is generated.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: By periodically collecting the response current density and open-circuit potential of the working electrode during the initial electrochemical mechanical polishing process, the judgment of the interface exposure state is combined with the current response under the applied potential condition and the potential change under the open-circuit condition, which can reduce the error caused by judging interface exposure based solely on a single electrochemical signal; by determining the reference data window based on the continuous sampling data before interface exposure, and establishing the interface exposure criterion based on the reference data window, the comparison benchmark used for interface exposure judgment comes from the actual response data of the same sample under the same electrolyte and polishing conditions, reducing the influence of different samples, different coating states, or different electrolyte conditions on the judgment results; by adjusting the potential control method after determining interface exposure... The process switches to dual-potential pulse control, applying a magnesium protection potential and a nickel passivation potential sequentially within the same pulse cycle. This ensures that the magnesium alloy substrate and the nickel plating are in their respective potential-controlled phases during the polishing process after interface exposure, thereby reducing interfacial galvanic effects when both are simultaneously exposed to a non-aqueous electrolyte. By ensuring the duration of the magnesium protection potential is longer than that of the nickel passivation potential, oxidation and corrosion of the magnesium alloy substrate are preferentially suppressed during the interface exposure phase. Furthermore, by determining the polishing endpoint based on the change in interfacial response current density at the end of the same phase within several consecutive pulse cycles, the interface response changes can be compared under the same potential state, reducing errors caused by the inability to directly compare current data at different potential phases and avoiding damage to the interface between the magnesium alloy substrate and the nickel plating caused by excessive polishing. Therefore, this invention can reduce interfacial galvanic corrosion and polishing damage during the electrochemical mechanical polishing of magnesium alloy nickel plating samples, improving the authenticity and stability of the metallographic sample interface morphology. Attached Figure Description

[0020] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings: Figure 1 The flowchart illustrates a metallographic sample preparation method for preventing oxidation and minimizing damage at the interface of a nickel-plated magnesium alloy layer, as provided in this embodiment of the invention. Detailed Implementation

[0021] Embodiments of the present invention will now be described in more detail with reference to the accompanying drawings. While the accompanying drawings show exemplary embodiments of the present disclosure, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the disclosure to those skilled in the art. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0022] See Figure 1 As shown, this application proposes a metallographic sample preparation method for preventing oxidation and minimizing damage at the interface of nickel-plated magnesium alloy layers, including: S1: The magnesium alloy nickel-plated sample is installed as the working electrode in the electrochemical mechanical polishing device, and the magnesium alloy nickel-plated sample is brought into contact with a non-aqueous electrolyte containing magnesium corrosion inhibitor, nickel corrosion inhibitor and conductive salt, wherein the response current density is positive in the anodic direction and negative in the cathodic direction. S2: Initial electrochemical mechanical polishing was performed on the nickel-plated magnesium alloy sample, and the response current density and open circuit potential of the working electrode were collected at intervals during the initial electrochemical mechanical polishing process. The applied potential was kept constant when the response current density was collected, and the applied power supply was disconnected when the open circuit potential was collected. S3: Determine the baseline data window based on the continuous sampling data before the interface is exposed, and establish the interface exposure criterion based on the change in response current density and open circuit potential within the baseline data window; S4: During the continued polishing process, based on the comparison results of the current response current density and the negative shift rate of the adjacent open circuit potential with the interface exposure criterion, determine whether the interface between the magnesium alloy substrate and the nickel plating is exposed. S5: After the interface is exposed, the potential control mode is switched to dual potential pulse control. Within the same pulse cycle, the magnesium protection potential and the nickel passivation potential are applied sequentially. The magnesium protection potential is a negative cathode potential, and the nickel passivation potential is a positive anode potential. The duration of the magnesium protection potential is longer than the duration of the nickel passivation potential. S6: Continue electrochemical mechanical polishing under dual-potential pulse control, and determine the polishing endpoint based on the change in interface response current density at the end of the same phase within several consecutive pulse cycles; when the polishing endpoint is reached, stop electrochemical mechanical polishing to obtain a metallographic sample of the magnesium alloy nickel plating layer interface.

[0023] Specifically, before connecting the magnesium alloy nickel-plated sample to the electrochemical mechanical polishing device, the sample can be cut at a low speed using a diamond precision cutting disc with a thickness of no more than 0.3 mm. The cutting speed is, for example, 50 r / min-150 r / min. The cutting cooling medium is anhydrous ethanol containing 0.05%-0.2% stearic acid corrosion inhibitor to reduce cutting heat and moisture introduction. Immediately after cutting, the sample is ultrasonically cleaned with anhydrous ethanol for 20s-60s and then dried with high-purity argon or nitrogen. When installing a nickel-plated magnesium alloy sample, it can be connected to an electrochemical workstation via a conductive clamp or a back wire, making the end face where the nickel plating layer is located the sample surface to be prepared. The area other than the sample surface to be prepared is encapsulated with insulating resin resistant to non-aqueous electrolytes. If necessary, an inert protective layer of 50nm-100nm is formed in the non-detection edge area to reduce edge chipping and chamfering, but the protective layer does not cover the interface area between the magnesium alloy substrate and the nickel plating layer to be tested. The effective exposed area of ​​the working electrode is the area of ​​the sample surface to be prepared that is in contact with the non-aqueous electrolyte and participates in the electrochemical response. The response current density is obtained by dividing the collected working electrode current by the effective exposed area, and it is uniformly stipulated that the anode direction is positive and the cathode direction is negative. Non-aqueous electrolytes can use anhydrous ethanol, ethylene glycol, or a mixture of both as solvents, with the addition of magnesium corrosion inhibitors, nickel corrosion inhibitors, and conductive salts. For example, the magnesium corrosion inhibitor can be hexadecyltrimethylammonium bromide at a concentration of 0.1%-0.3%; the nickel corrosion inhibitor can be benzotriazole at a concentration of 0.05%-0.1%; and the conductive salt can be lithium perchlorate at a concentration of 0.3%-0.8%. The above concentrations are not arbitrary fixed thresholds, but can be determined by immersing exposed magnesium alloy samples and nickel-plated samples from the same batch in the candidate electrolyte, observing their conductivity, and performing polarization scanning. The selection criteria are that no significant oxide film thickening appears on the magnesium alloy surface, no sudden increase in pitting current appears in the nickel plating, and the electrolyte conductivity meets the potential switching response requirements. The initial electrochemical mechanical polishing (EMF) stage primarily targets material removal from the nickel plating surface. The applied potential can be the nickel plating polishing potential, which can be obtained by anodic polarization scanning of nickel plating samples from the same batch under the same non-aqueous electrolyte and reference system. Specifically, the potential is selected when the anodic current enters the stable polishing zone and before a sharp increase in pitting current occurs; for example, it can be 0.1V-0.3V relative to the non-aqueous Ag / Ag+ reference electrode. Mechanical parameters can be determined based on the plating thickness and the depth of pre-grinding scratches. For example, the polishing disc speed can be 50-200 r / min, the polishing pressure 0.05MPa-0.2MPa, and the polishing medium can be anhydrous ethanol-based diamond suspension or anhydrous polishing solution containing corrosion inhibitors, ensuring that the mechanical and electrochemical removal amounts remain within a controllable range. The mechanical polishing parameters for the initial EMF stage can be determined based on the nickel plating thickness, the depth of pre-grinding scratches, and the electrochemical removal rate.Specifically, a short-term trial polishing can be performed using nickel-plated magnesium alloy samples from the same batch. The thickness of the nickel plating removed per unit time is recorded to obtain the mechanical polishing rate. Simultaneously, under the same non-aqueous electrolyte and the same nickel plating polishing potential, the electrochemical removal amount is estimated based on the integral of the response current density over time. Then, the polishing pressure and polishing disc speed are adjusted to ensure that the total removal amount within a single detection cycle is less than the detection resolution allowed by the change in nickel plating thickness. For example, when the nickel plating thickness is 10 μm and the pre-grinding scratch depth is approximately 1 μm, the polishing pressure can be set to 0.1 MPa and the polishing disc speed to 100 r / min. The material removal rate measured through short-term trial polishing is approximately 0.5 μm / min. If the response current density changes smoothly under these conditions and no obvious dragging scratches appear on the cross-section, then this pressure and speed are used as the initial polishing parameters for formal sample preparation. When collecting response current density and open-circuit potential at intervals, it is not required that the current and potential change freely at the same electrochemical state. Instead, sampling is performed in a time-division manner within a detection cycle: during the polishing control sub-period, the external power supply is turned on and the polishing potential of the nickel plating is kept constant, and the response current density is collected at the end of this sub-period; then, during the open-circuit detection sub-period, the external power supply is turned off, and the working electrode is connected to the high-resistivity potential acquisition channel to collect the open-circuit potential; the detection cycle can be determined according to the response speed of the acquisition system and the polishing rate, for example, 0.5s-5s, and the open-circuit detection sub-period is preferably shorter than the polishing control sub-period. The potential noise amplitude of the high-resistivity potential acquisition channel can be measured under the condition that the sample is connected to a non-aqueous electrolyte but not polished or subjected to an external potential. For example, at least 30 open-circuit potential sampling points are continuously collected, the absolute value of the difference between adjacent sampling points is calculated, and the maximum value or root mean square value is taken as the potential noise amplitude. During the actual open-circuit detection sub-period, when the absolute value of the difference between adjacent open-circuit potential sampling values ​​is less than or equal to the potential noise amplitude, the current open-circuit detection ends and the next polishing control sub-period begins, in order to reduce the duration of the open-circuit state. The reference data window is a stable data segment selected from the continuous sampling data before the interface is exposed. A sliding sampling window can be formed with at least three consecutive detection cycles, and several sliding sampling windows can be formed in chronological order. For each sliding sampling window, the difference between the maximum and minimum values ​​of the response current density is calculated as the response current density range of that window, and the cumulative change of the open-circuit potential in the same direction is calculated as the unidirectional drift of the open-circuit potential. The sliding sampling window with the smallest response current density range and the smallest unidirectional drift of the open-circuit potential is selected as the reference data window. When establishing the interface exposure criterion, the average value of each response current density in the reference data window can be calculated first, and then the average value can be added to the difference between the maximum and minimum response current densities in the window to obtain the current criterion. At the same time, based on the open circuit potential difference and detection interval of adjacent open circuit detection sub-time periods in the reference data window, several reference open circuit potential change rates can be obtained respectively, and the maximum absolute value of these reference open circuit potential change rates can be taken as the potential criterion.During the continued polishing process, the response current density at the end of the current polishing control sub-period is compared with the current criterion, and the current open circuit potential is subtracted from the previous open circuit potential and divided by the time interval between two adjacent open circuit detections to obtain the real-time open circuit potential change rate. When the response current density is greater than the current criterion, and the real-time open circuit potential change rate is negative and its absolute value is greater than the potential criterion, it indicates that the current electrochemical response has deviated from the stable state of the nickel plating layer's isolated exposure stage, and the interface between the magnesium alloy substrate and the nickel plating layer is determined to be exposed. After the interface is exposed, the potential control method is switched from initial constant potential polishing to dual potential pulse control. The magnesium protection potential and nickel passivation potential can be predetermined before switching by polarization scanning of samples from the same batch. That is, the exposed magnesium alloy samples from the same batch are placed in the same non-aqueous electrolyte for cathodic polarization scanning. The potential at which the cathode current enters the stable change range and no violent gas evolution current appears is taken as the magnesium protection potential. For example, it can be selected in the range of -1.3V to -1.8V by scanning curve. The nickel plating samples from the same batch are placed in the same non-aqueous electrolyte for anodic polarization scanning. The potential corresponding to the anode response current density entering the passivation platform is taken as the nickel passivation potential. For example, it can be selected in the range of 0.1V to 0.5V by scanning curve. Dual-potential pulse control divides a pulse cycle into a magnesium protection phase and a nickel passivation phase. The magnesium protection potential is applied first, followed by the nickel passivation potential. The pulse cycle can be determined based on the pulse power supply's response capability and the rate of change of the interface current, for example, 0.1ms-5ms, with the duration of the magnesium protection phase being longer than that of the nickel passivation phase. During the pulse, the data on the change of response current density over time in both phases are recorded. The negative response current density in the magnesium protection phase is integrated over time to obtain the cathode charge per unit area, and the positive response current density in the nickel passivation phase is integrated over time to obtain the anode charge per unit area. When the anode charge per unit area is greater than the absolute value of the cathode charge per unit area, the magnesium protection phase is extended in the next pulse cycle by the minimum adjustable time unit of the pulse power supply or an integer multiple thereof, while the nickel passivation phase is shortened accordingly. However, the nickel passivation phase must not be shorter than the passivation response time. The passivation response time can be determined by the step response test of the same batch of nickel-plated samples at the nickel passivation potential, i.e., recording the time required for the anode response current density to rise from its initial state to the passivation plateau.To avoid comparing current data at different potential states when determining the polishing endpoint, the interface response current density can be collected at the same phase end, for example, all at the end of the nickel passivation phase. A statistical window is formed with at least three consecutive pulse cycles, and adjacent statistical windows are formed. The average value of the interface response current density in the previous and subsequent statistical windows is calculated, and the absolute value of the difference between the two average values ​​is taken as the average change. At the same time, the difference between the maximum and minimum values ​​of the interface response current density in the subsequent statistical window is calculated as the fluctuation within the window. When the average change is less than or equal to the fluctuation within the window, it is determined that the interface response change caused by continued polishing has fallen into the electrochemical fluctuation range under the same phase, and the polishing endpoint has been reached. After stopping electrochemical mechanical polishing, the relative movement between the polishing disc and the sample should be stopped first, then the external power supply should be disconnected, the sample should be taken out of the non-aqueous electrolyte, rinsed immediately with anhydrous ethanol, and dried with high-purity argon or nitrogen. When scanning electron microscopy observation is required, an ultrathin carbon film of 10nm-20nm can be formed on the cross section of the sample to be prepared or it can be directly vacuum preserved to reduce secondary oxidation of the magnesium alloy substrate during the transfer and observation process.

[0024] In some embodiments of this application, when periodically acquiring the response current density and open-circuit potential of the working electrode during the initial electrochemical mechanical polishing process, the following is included: Electrochemical mechanical polishing was performed continuously for several detection cycles. Each detection cycle included a polishing control sub-period and an open-circuit detection sub-period. During the polishing control sub-period, the external power supply was turned on and the polishing potential of the nickel plating was applied to the working electrode. The response current density at the end of the polishing control sub-period was collected. During the open-circuit detection sub-period, the external power supply was turned off and the working electrode was connected to the high-resistance potential acquisition channel. The open-circuit potential at the end of the open-circuit detection sub-period was collected. The response current density and open-circuit potential collected in the same detection cycle were used as a set of sampling data.

[0025] Specifically, the detection cycle refers to a set of timed control cycles that are repeatedly executed during the initial electrochemical mechanical polishing process. It includes the grinding and polishing control sub-period and the open-circuit detection sub-period, which do not overlap in time. The number of detection cycles can be determined according to the nickel plating thickness, the expected grinding and polishing rate, and the response time of the electrochemical sampling system. For example, when the nickel plating thickness is 5μm-50μm, each detection cycle can be 0.5s-5s, the grinding and polishing control sub-period can be 0.45s-4.8s, and the open-circuit detection sub-period can be 0.05s-0.2s. It can also be dynamically terminated by the open-circuit potential sampling stabilization condition. During the polishing control sub-period, the external potential output terminal of the electrochemical workstation or pulse power supply is connected to the working electrode to apply a nickel plating polishing potential to the magnesium alloy nickel-plated sample. The nickel plating polishing potential is not an arbitrary set value; it can be obtained by anodic polarization scanning of the same batch of nickel-plated samples under the same non-aqueous electrolyte and the same reference system. Specifically, the potential at which the nickel plating enters the stable dissolution or stable polishing current region and no pitting current surge occurs is selected from the anodic polarization curve as the nickel plating polishing potential. For example, it can be 0.1V-0.3V relative to the non-aqueous Ag / Ag+ reference electrode. The response current is collected at the end of the polishing control sub-period because the external potential has completed capacitor charging and initial transient decay at this moment, and the collected value is closer to the stable electrochemical response of the polished surface. The collected working electrode current is then divided by the effective exposed area of ​​the sample surface to be prepared to obtain the response current density corresponding to this detection cycle. When entering the open-circuit detection sub-period, the external power supply is disconnected via a relay, electronic switch, or the open-circuit test mode of the electrochemical workstation, so that the working electrode no longer receives external polarization current, and a high-resistance potential acquisition channel is connected between the working electrode and the reference electrode; the high-resistance potential acquisition channel can use an input impedance of not less than 10Ω. 9 The potential acquisition module is Ω to reduce disturbances to the open-circuit potential during the acquisition process. The open-circuit potential acquired at the end of the open-circuit detection sub-period refers to the potential of the working electrode relative to the reference electrode under conditions of no applied current. This potential differs from the constantly applied external potential during the polishing control sub-period and varies with the type of exposed material and the interface state. After each detection cycle, the response current density at the end of the polishing control sub-period and the open-circuit potential at the end of the open-circuit detection sub-period within that cycle are bound together in chronological order as a set of sampling data, for example, denoted as the k-th set of sampling data, which includes the response current density and open-circuit potential of the k-th detection cycle. Subsequent calculations, such as establishing a reference data window, calculating the negative shift rate of the open-circuit potential, and determining interface exposure, are all based on multiple consecutive sets of sampling data. This time-division acquisition method allows the response current density to be obtained under constant applied potential conditions and the open-circuit potential to be obtained under conditions of disconnected external power supply, avoiding the requirement for simultaneous free changes in current and potential under the same electrochemical state.

[0026] In some embodiments of this application, the duration of the open-circuit detection sub-period is determined based on the open-circuit potential sampling stability, including: After disconnecting the external power supply, the open circuit potential is continuously collected. When the absolute value of the difference between adjacent open circuit potential sampling values ​​is less than or equal to the potential noise amplitude of the high resistance potential acquisition channel, the open circuit detection sub-period ends and the next grinding and polishing control sub-period begins.

[0027] Specifically, the open-circuit detection sub-period is used to acquire the open-circuit potential of the working electrode under conditions without external potential interference. Therefore, its duration should not be a fixed long waiting period, but rather dynamically determined based on whether the open-circuit potential sampling has reached a stable state, in order to reduce the time that the magnesium alloy substrate and nickel plating are exposed together in the open-circuit state. The potential noise amplitude of the high-resistivity potential acquisition channel can be calibrated before formal sample preparation. For example, the reference electrode and the working electrode can be connected to the same non-aqueous electrolyte system. Under conditions of no polishing, no external potential applied, and consistent temperature, the open-circuit potential can be continuously acquired at the same sampling frequency as the formal detection. The absolute value of the difference between two adjacent sampling values ​​in the continuous sampling data is taken to obtain several adjacent difference values. The maximum value, the average value plus three times the standard deviation, or the root mean square value of the adjacent difference can be taken as the potential noise amplitude. When using the average value plus three times the standard deviation, the misjudgment caused by random noise points can be reduced; when using the maximum value, the judgment condition is more conservative. During the formal grinding and polishing process, once the open-circuit detection sub-period begins, the control module disconnects the external power supply and immediately starts continuously acquiring open-circuit potential. If the absolute value of the difference between adjacent open-circuit potential sampling values ​​is still greater than the potential noise amplitude, it indicates that the working electrode potential is still undergoing significant changes. In this case, the open-circuit detection continues, and the next open-circuit potential sampling value is acquired. If the absolute value of the difference between adjacent open-circuit potential sampling values ​​is less than or equal to the potential noise amplitude, it indicates that the current open-circuit potential change has fallen within the potential fluctuation range of the acquisition channel itself. At this time, the current open-circuit potential sampling value is taken as the open-circuit potential for this detection cycle, and the open-circuit detection sub-period ends immediately. The external power supply is then reconnected to enter the next grinding and polishing control sub-period. For example, when the potential noise amplitude of the high-resistivity potential acquisition channel in a non-aqueous electrolyte is calibrated to 0.8mV and the sampling frequency is 1kHz, if the absolute values ​​of the differences between adjacent open-circuit potential samples obtained after disconnecting the external power supply are 3.5mV, 1.6mV, and 0.7mV respectively, then the open-circuit detection sub-period ends when 0.7mV is obtained, and the corresponding open-circuit potential is taken as the open-circuit potential for that detection cycle. This dynamic termination method based on sampling stability avoids both excessively short open-circuit detection sub-periods leading to insufficient stabilization of the open-circuit potential and excessively long open-circuit detection sub-periods causing the interface area to remain unprotected for too long.

[0028] In some embodiments of this application, determining the reference data window based on continuous sampling data before the interface is exposed includes: Several sliding sampling windows are formed with the same number of consecutive detection cycles, and each sliding sampling window includes at least three detection cycles; the range of response current density and the unidirectional drift of open circuit potential in each sliding sampling window are calculated respectively; the sliding sampling window with the smallest range of response current density and the smallest unidirectional drift of open circuit potential is selected as the reference data window; the average value of response current density in the reference data window is calculated, and the difference between the maximum and minimum values ​​of response current density in the reference data window is taken as the response current density fluctuation.

[0029] Specifically, the continuous sampling data before interface exposure refers to multiple sets of sampling data obtained continuously according to the detection cycle after the initial electrochemical mechanical polishing begins but before the interface between the magnesium alloy substrate and the nickel plating is determined to be exposed. Each set of sampling data includes a response current density and an open-circuit potential. Since the electrochemical response of the working electrode is relatively stable when the nickel plating is exposed alone, while grinding and polishing disturbances, polishing fluid flow, and acquisition noise can cause fluctuations, a single sampling point is not directly used as a benchmark. Instead, a sliding sampling window is used for statistical analysis. The sliding sampling window consists of the same number of continuous detection cycles, with each sliding sampling window including at least three detection cycles. For example, three, five, or eight continuous detection cycles can be used as a window, and several sliding sampling windows are formed by sliding sequentially along the time interval according to the step size of one detection cycle. When the sampling frequency is high or the grinding and polishing rate is slow, the number of detection cycles included in the window can be appropriately increased to reduce the impact of occasional current spikes on the benchmark data. For each sliding sampling window, the range of the response current density is the difference between the maximum and minimum values ​​of the response current density within that window. The unidirectional drift of the open-circuit potential can be calculated as the cumulative value of the changes in the same direction among adjacent open-circuit potential changes within that window. For example, first calculate the difference between the open-circuit potential of the later detection cycle and the open-circuit potential of the previous detection cycle, and then accumulate the differences that are both positive or both negative to obtain the amount of continuous shift of the open-circuit potential in a single direction within that window. If the range of the response current density within a certain window is small, it indicates that the nickel plating response under the applied potential inside and outside that window is relatively stable; if the unidirectional drift of the open-circuit potential is small, it indicates that the surface potential under the open-circuit state within that window has not undergone continuous shift. When selecting the reference data window, one can first screen one or more sliding sampling windows with the smallest range of the response current density, and then select the window with the smallest unidirectional drift of the open-circuit potential as the reference data window; alternatively, the range of the response current density and the unidirectional drift of the open-circuit potential can be normalized and summed, and the sliding sampling window with the smallest sum can be selected as the reference data window. After determining the baseline data window, the arithmetic mean of the response current density for each detection cycle within the window is calculated to obtain the average response current density. Simultaneously, the difference between the maximum and minimum response current density within the window is taken to obtain the response current density fluctuation. For example, if a baseline data window contains 5 detection cycles with corresponding response current densities of 2.1 μA / cm², 2.3 μA / cm², 2.2 μA / cm², 2.4 μA / cm², and 2.2 μA / cm², then the average response current density is 2.24 μA / cm², and the response current density fluctuation is 0.3 μA / cm². The baseline data window obtained in this way reflects the stable electrochemical response during the individual exposure stage of the nickel plating. The current and potential criteria in subsequent interface exposure judgments are calculated based on the data within this window.

[0030] In some embodiments of this application, when establishing an interface exposure criterion based on changes in response current density and open-circuit potential within a reference data window, the following steps are included: The average response current density is added to the response current density fluctuation to obtain the current criterion; the reference open circuit potential change rate is calculated based on the open circuit potential difference and detection interval of adjacent open circuit detection sub-periods within the reference data window; the maximum absolute value of the reference open circuit potential change rate is used as the potential criterion; the current criterion and the potential criterion are used together as the interface exposure criterion.

[0031] Specifically, the interface exposure criterion comprises two parts: a current criterion and a potential criterion. The current criterion is derived from the normal fluctuation range of the response current density within the reference data window, while the potential criterion is derived from the normal fluctuation range of the open-circuit potential change rate within the reference data window. When establishing the current criterion, the response current density of each detection cycle within the reference data window is first arithmetically averaged to obtain the average response current density. Then, the difference between the maximum and minimum response current density values ​​within the window is calculated to obtain the response current density fluctuation. Finally, the average response current density and the response current density fluctuation are added together to obtain the current criterion. This current criterion is equivalent to using the stable response current density of the nickel plating layer in a single exposure stage as a basis, and then superimposing the judgment boundary formed by the fluctuation amplitude of that stage. Therefore, it is not an artificially fixed current threshold. For example, if the average response current density within the reference data window is 2.24 μA / cm², and the fluctuation of the response current density is 0.30 μA / cm², then the current criterion is 2.54 μA / cm². During subsequent polishing, if the response current density collected at the end of the current polishing control sub-period is greater than 2.54 μA / cm², then the interface exposure judgment condition on the current side is met. When establishing the potential criterion, the open-circuit potential at the end of each open-circuit detection sub-period within the reference data window is extracted according to the sampling time sequence. The difference between two adjacent open-circuit potentials is calculated, and then this difference is divided by the detection interval between two adjacent open-circuit detection sub-periods to obtain the corresponding reference open-circuit potential change rate. The detection interval can be the time difference between two adjacent open-circuit potential sampling times. When the detection period is constant, it is equal to the duration of one detection period; when the detection period changes dynamically, the actual time difference is used. The absolute values ​​of the open-circuit potential change rates obtained within the reference data window are taken, and the maximum value is used as the potential criterion. This potential criterion reflects the maximum allowable rate range of natural drift of the open-circuit potential during the individual exposure stage of the nickel plating. For example, if the absolute values ​​of adjacent open-circuit potential change rates within the reference data window are 0.6 mV / s, 0.8 mV / s, 0.5 mV / s, and 0.7 mV / s, respectively, then the potential criterion is 0.8 mV / s. Using both the current criterion and the potential criterion as the interface exposure criterion means that when subsequently determining whether the interface is exposed, not only must the response current density exceed the current fluctuation boundary of the individual exposure stage of the nickel plating, but the open-circuit potential must also show a negative shift exceeding the reference drift range. Only when both conditions are met simultaneously is the interface exposed, thus reducing misjudgment of a single signal caused by polishing slurry disturbance, transient poor contact, or mechanical polishing vibration.

[0032] In some embodiments of this application, when determining whether the interface between the magnesium alloy substrate and the nickel plating is exposed based on a comparison of the current response current density and the negative shift rate of the adjacent open-circuit potential with the interface exposure criterion, the following steps are included: The response current density at the end of the current polishing control sub-period is compared with the current criterion; the open circuit potential of the current open circuit detection sub-period is subtracted from the open circuit potential of the previous open circuit detection sub-period, and the difference is divided by the detection interval to obtain the real-time open circuit potential change rate; when the response current density is greater than the current criterion, and the real-time open circuit potential change rate is negative and its absolute value is greater than the potential criterion, the interface is determined to be exposed.

[0033] Specifically, during the continued polishing process, the control module continuously acquires the response current density and open-circuit potential according to the detection cycle, and uses the response current density collected at the end of the polishing control sub-segment within the current detection cycle as the current response current density. Since the response current density is positive in the anodic direction, when the magnesium alloy substrate is not yet exposed, the working electrode mainly exhibits a stable response of the nickel plating layer at the polishing potential of the nickel plating layer. As the polishing process gradually approaches the interface between the magnesium alloy substrate and the nickel plating layer, the magnesium alloy substrate locally begins to participate in the electrochemical reaction, and the current response current density will increase relative to the current criterion obtained from the reference data window. Therefore, the current response current density is compared with the current criterion. If the current response current density is not greater than the current criterion, it is considered that the current current change is still within the normal fluctuation range of the nickel plating layer's isolated exposure stage, and the interface exposure judgment is not triggered. If the current response current density is greater than the current criterion, the direction and rate of change of the open-circuit potential are further determined. In the specific calculation, the open-circuit potential collected at the end of the current open-circuit detection sub-period is subtracted from the open-circuit potential collected at the end of the previous open-circuit detection sub-period to obtain the difference between adjacent open-circuit potentials. Then, the difference between adjacent open-circuit potentials is divided by the detection interval between the two open-circuit potential collection times to obtain the real-time open-circuit potential change rate. When the detection cycle is fixed, the detection interval can be the duration of one detection cycle. When the open-circuit detection sub-period ends dynamically based on sampling stability, the detection interval is the actual time difference between the current open-circuit potential sampling time and the previous open-circuit potential sampling time. If the real-time open-circuit potential change rate is positive or its absolute value is not greater than the potential criterion, it indicates that the open-circuit potential has not undergone a negative shift exceeding the reference drift range, and interface exposure is not determined. If the real-time open-circuit potential change rate is negative, and its absolute value is greater than the potential criterion, it indicates that the current open-circuit potential has undergone a negative shift exceeding the normal drift range of the nickel plating's individual exposure stage relative to the previous detection cycle. In this case, combined with the condition that the current response current density is greater than the current criterion, it is determined that the interface between the magnesium alloy substrate and the nickel plating has been exposed. For example, if the current criterion is 2.54 μA / cm², the potential criterion is 0.8 mV / s, the current response current density is 3.10 μA / cm², the current open-circuit potential is -0.42 V, the previous open-circuit potential was -0.40 V, and the detection interval between the two open-circuit potential acquisition times is 10 s, then the real-time open-circuit potential change rate is -2 mV / s, and its absolute value is greater than 0.8 mV / s. Therefore, the two conditions of current increase and potential negative shift are met, and the interface is determined to be exposed. If the current response current density increases but the real-time open-circuit potential change rate does not show a negative value, or if the open-circuit potential changes negatively but its absolute value is not greater than the potential criterion, then the initial electrochemical mechanical polishing and periodic sampling will continue, and the dual-potential pulse control will not be switched.

[0034] In some embodiments of this application, the determination of the magnesium protection potential and the nickel passivation potential includes: Before switching to dual-potential pulse control, the bare magnesium alloy samples and nickel-plated samples from the same batch were placed in a non-aqueous electrolyte for polarization scanning. The polarization scanning used the same reference system as the electrochemical mechanical polishing device. The potential corresponding to the cathode current entering the stable change section was selected from the cathodic polarization curve of the bare magnesium alloy samples as the magnesium protection potential. The potential corresponding to the anode current entering the passivation plateau was selected from the anodic polarization curve of the nickel-plated samples as the nickel passivation potential.

[0035] Specifically, the magnesium protection potential and nickel passivation potential are not arbitrarily set after interface exposure, but are obtained in advance through polarization scanning of samples from the same batch before or at the initial stage of sample preparation, and stored in the control module for direct retrieval after interface exposure is determined. Exposed magnesium alloy samples from the same batch refer to samples with the same magnesium alloy grade and heat treatment state as the nickel-plated magnesium alloy samples to be prepared, but without a nickel plating layer or with the magnesium alloy substrate exposed after the nickel plating layer has been removed. Nickel-plated samples from the same batch refer to samples that have formed a nickel plating layer using the same nickel plating process as the samples to be prepared. During polarization scanning, the exposed magnesium alloy samples and nickel-plated samples are used as working electrodes, placed in a non-aqueous electrolyte with the same composition as the formal electrochemical mechanical polishing, and using the same reference system and counter electrode system as the electrochemical mechanical polishing device, such as a non-aqueous Ag / Ag+ reference electrode and a platinum sheet counter electrode. The reason for using the same reference system is that the potential values ​​under different reference electrodes cannot be directly equated. If the reference system is different during scanning and sample preparation, potential conversion is required, which can easily introduce control deviations. In some implementations, to reduce the impact of non-aqueous reference electrode drift on the calculation of magnesium protection potential, nickel passivation potential, and the rate of negative shift of open-circuit potential, the non-aqueous Ag / Ag+ reference electrode can be calibrated using a ferrocene / ferroceneon standard redox system before sample preparation. Specifically, a ferrocene standard is dissolved in a solvent system compatible with the non-aqueous electrolyte, and the same supporting electrolyte is added. The redox peak potential of the ferrocene / ferroceneon is measured using cyclic voltammetry. The half-wave potential is calculated based on the oxidation and reduction peak potentials, and then compared with the calibration reference value. When the deviation exceeds the allowable range, the subsequently recorded potential values ​​are corrected, or the reference electrode is replaced and recalibrated. For example, the potential measurement deviation after reference electrode calibration can be controlled within ±5mV to ensure that the calculation of magnesium protection potential, nickel passivation potential, and the rate of negative shift of open-circuit potential uses a consistent potential reference. Before polarization scanning, the sample can be allowed to stand in a non-aqueous electrolyte until the open-circuit potential change tends to stabilize. Then, a potential scan is performed at a low scan rate, such as 0.5 mV / s to 2 mV / s. During the scan, the working electrode potential and response current density are recorded simultaneously. For exposed magnesium alloy samples, scanning is performed along the cathode direction to obtain the cathode polarization curve. As the potential shifts negatively, anodic dissolution on the magnesium alloy surface is suppressed, and the cathode response current gradually increases. When the change in cathode current with the continued negative shift of potential decreases, and no sudden current surge caused by severe gas evolution occurs, this segment is called the stable change segment of cathode current, and the corresponding potential in this stable change segment is selected as the magnesium protection potential.In practical selection, the end with the smaller absolute value of the potential within the stable change range can be prioritized to avoid unnecessary cathode side reactions caused by an excessively negative protection potential. For example, in a non-aqueous electrolyte and a non-aqueous Ag / Ag+ reference system, the stable change range of the cathode for an exposed magnesium alloy sample appears between -1.35V and -1.55V. Therefore, a potential of -1.35V or close to this end can be selected as the magnesium protection potential. For nickel-plated samples, a scan is performed along the anodic direction to obtain the anodic polarization curve. When the anodic response current density no longer increases significantly with increasing potential within a certain potential range, or the increase is significantly smaller than that of the previous activation and dissolution stage, this potential range is called the passivation plateau. The corresponding potential from this passivation plateau is selected as the nickel passivation potential. In practical selection, the transition zone at the beginning of the passivation plateau and the pitting or over-passivation zone at the high potential end can be avoided. The potential corresponding to the middle of the passivation plateau should be prioritized. For example, in a non-aqueous electrolyte and a non-aqueous Ag / Ag+ reference system, if the passivation plateau of the nickel-plated sample appears between 0.20V and 0.40V, then 0.30V can be selected as the nickel passivation potential. Through this method, the magnesium protection potential originates from the cathodic polarization response of the same batch of magnesium alloy substrate in the same electrolyte, and the nickel passivation potential originates from the anodic polarization response of the same batch of nickel plating in the same electrolyte. Both are matched with the material state of the sample, the electrolyte composition, and the reference system, providing callable potential parameters for subsequent dual-potential pulse control.

[0036] In some embodiments of this application, when selecting the potential corresponding to the stable change segment of the cathode current from the cathodic polarization curve of the exposed magnesium alloy sample as the magnesium protection potential, the following steps are taken: The cathode polarization curve is divided into several continuous potential segments based on the potential step size of the polarization scan. The ratio of the difference in cathode current density to the potential difference between adjacent sampling points in each potential segment is calculated, along with the average absolute value, standard deviation, and range of cathode current density within that segment. When at least two consecutive potential segments satisfy the condition that the difference in the average absolute value of the ratio between the current segment and the previous segment is less than or equal to the standard deviation of the ratio within that segment, and the range of cathode current density within that segment is less than or equal to the range of cathode current density within the previous segment, these two consecutive potential segments are defined as the stable change segment of the cathode current. The potential corresponding to the end with the smallest absolute potential value within the stable change segment is taken as the magnesium protection potential.

[0037] Specifically, after performing cathodic polarization scanning on exposed magnesium alloy samples from the same batch, the resulting cathodic polarization curves can be segmented according to the potential step size used in the polarization scan. For example, when the potential step size of the polarization scan is 1mV, every 10 or 20 consecutive sampling points can be divided into a potential segment, with adjacent potential segments arranged sequentially according to the scanning direction. Alternatively, several consecutive potential segments can be divided according to a fixed potential width of 10mV-20mV. For each potential segment, the difference in cathodic current density between every two adjacent sampling points within that segment is first calculated, and then divided by the potential difference between the two sampling points to obtain several ratios reflecting the degree of local current change. Since the cathodic current density is a negative value as defined in this application, the absolute value is used for subsequent comparisons to avoid the current direction affecting the judgment of the change amplitude. Subsequently, the arithmetic mean of the absolute values ​​of multiple ratios within the same potential segment is calculated to obtain the average absolute value of the ratios within that potential segment. The standard deviation of these ratios is then calculated to determine the degree of local fluctuation in the curve within that potential segment. Simultaneously, the absolute magnitude of the difference between the maximum and minimum cathode current densities within that potential segment is taken as the range of cathode current densities within that segment. Following a gradually negatively shifting scanning sequence, each current potential segment is compared with its predecessor: if the difference between the average absolute values ​​of the ratios between the current and previous potential segments is less than or equal to the standard deviation of the ratios within the current potential segment, it indicates that the difference in the slope of current change between adjacent potential segments has fallen within the fluctuation range of the current potential segment itself; furthermore, if the range of cathode current densities within the current potential segment is less than or equal to the range of cathode current densities within the previous potential segment, it indicates that continued negative potential shift has not led to an increase in cathode current fluctuations within that potential segment. When both of the aforementioned conditions are met within at least two consecutive potential segments, this continuous potential segment is defined as the stable change segment of the cathode current. If multiple continuous potential segments satisfying the conditions appear along the scanning direction, the first continuous potential segment to appear can be taken as the stable change segment of the cathode current to reduce side reactions caused by excessively negative potentials. After determining the stable change segment, the potential corresponding to the end with the smallest absolute potential value within the stable change segment is taken as the magnesium protection potential. That is, under the premise of being able to enter the stable change segment of the cathode current, the potential with a smaller degree of negative polarization is selected. For example, after a cathode polarization curve is divided into potential segments of 10mV, if the average change of the absolute value of the ratio is not greater than the corresponding standard deviation and the range of cathode current density does not continue to increase within the two continuous potential segments of -1.36V to -1.38V and -1.38V to -1.40V, then -1.36V to -1.40V can be defined as the stable change segment of the cathode current, and the end with the smallest absolute potential value, i.e., -1.36V, can be taken as the magnesium protection potential.

[0038] In some embodiments of this application, when the magnesium protection potential and the nickel passivation potential are applied sequentially within the same pulse cycle, the following are included: First, a magnesium protection potential is applied to the working electrode to form a magnesium protection phase; then, a nickel passivation potential is applied to the working electrode to form a nickel passivation phase; the response current density changes over time in the magnesium protection phase and the nickel passivation phase are collected respectively; the negative response current density in the magnesium protection phase is integrated over time to obtain the charge per unit area of ​​the cathode; the positive response current density in the nickel passivation phase is integrated over time to obtain the charge per unit area of ​​the anode; the duration of the magnesium protection phase and the nickel passivation phase are adjusted according to the charge per unit area of ​​the cathode and the charge per unit area of ​​the anode, and the duration of the magnesium protection phase is kept greater than the duration of the nickel passivation phase.

[0039] Specifically, after determining that the interface between the magnesium alloy substrate and the nickel plating is exposed, the control module stops the output of the nickel plating polishing potential during the initial electrochemical mechanical polishing stage and calls upon the pre-determined magnesium protection potential and nickel passivation potential, applying them alternately to the working electrode in a pulse manner. One pulse cycle includes a magnesium protection phase and a nickel passivation phase executed sequentially; the magnesium protection phase refers to the time period during which the magnesium protection potential is applied to the working electrode, during which the working electrode is in a negative cathodic polarization state, and the collected response current density is usually negative; the nickel passivation phase refers to the time period during which the nickel passivation potential is applied to the working electrode, during which the working electrode is in a positive anodic polarization state, and the collected response current density is usually positive. The initial length of the pulse period can be determined based on the pulse power supply response time and the electrochemical response speed of the interface region, for example, it can be 0.1ms-5ms. The initial duty cycle can be determined based on the polarization response of the bare magnesium alloy samples and nickel-plated samples of the same batch. For example, the initial duration of the magnesium protection phase can be set to 60%-80% of one pulse period, and the initial duration of the nickel passivation phase can be set to 20%-40% of one pulse period, so that the duration of the magnesium protection phase is longer than the duration of the nickel passivation phase. During pulse operation, the current acquisition module records the data of the response current density changing with time in the magnesium protection phase and the nickel passivation phase, respectively. The sampling frequency should be higher than the pulse switching frequency so that there are at least multiple effective sampling points in each phase. For example, when the pulse period is 1ms, a current sampling frequency of not less than 50kHz can be used to obtain the current change curve in each phase. When performing time integration on the negative response current density in the magnesium protection phase, the negative response current density of each sampling point in the phase can be multiplied by the time interval between adjacent sampling points and then summed to obtain the charge per unit area of ​​the cathode. Since this charge is negative in the direction of current, its absolute value can be used for subsequent comparisons. When integrating the positive response current density within the nickel passivation phase over time, the positive response current density at each sampling point within that phase can be multiplied by the time interval between adjacent sampling points and then summed to obtain the charge per unit area of ​​the anode. For example, if the negative response current density collected within a magnesium protection phase, after time integration, yields -0.80 mC / cm², then the absolute value of the charge per unit area of ​​the cathode is 0.80 mC / cm². If, within the same pulse cycle, the charge per unit area of ​​the anode in the nickel passivation phase, after time integration, is 0.60 mC / cm², then the intensity of the cathode process is greater than the intensity of the anode process in that cycle, and the current phase duration can be maintained unchanged. If the absolute value of the charge per unit area of ​​the anode is greater than that of the absolute value of the charge per unit area of ​​the cathode, it indicates that the proportion of the positive anode response is too large in that cycle. In the next cycle, the duration of the magnesium protection phase can be increased while the duration of the nickel passivation phase can be decreased. If the absolute value of the charge per unit area of ​​the cathode is significantly greater than that of the charge per unit area of ​​the anode, the current phase ratio can be maintained, or the duration of the nickel passivation phase can be appropriately restored without falling below the passivation response time.By using the charge per unit area as the adjustment basis, the magnitude and duration of the current in each phase can be incorporated into the control, rather than adjusting the pulse parameters only based on the instantaneous current value. Throughout the dual-potential pulse control process, the control module always ensures that the duration of the magnesium protection phase is greater than the duration of the nickel passivation phase, and maintains the nickel passivation phase for a minimum duration sufficient to complete the passivation response of the nickel surface.

[0040] In some embodiments of this application, when adjusting the duration of the magnesium protection phase and the nickel passivation phase based on the charge per unit area of ​​the cathode and the charge per unit area of ​​the anode, and determining the polishing endpoint based on the change in interface response current density at the end of the same phase within several consecutive pulse cycles, the process includes: When the absolute value of the charge per unit area of ​​the anode is greater than that of the charge per unit area of ​​the cathode, the duration of the magnesium protection phase in the next pulse cycle is increased by one control step, and the duration of the nickel passivation phase in the next pulse cycle is decreased by the same control step. The control step is the minimum adjustable time unit of the pulse power supply or an integer multiple of the minimum adjustable time unit. When the duration of the nickel passivation phase decreases to the passivation response time, the shortening of the duration of the nickel passivation phase is stopped. The passivation response time is the time corresponding to the anode response current density of the same batch of nickel-plated samples entering the passivation platform at the nickel passivation potential. The interface response current density is collected at the end of the nickel passivation phase of each pulse cycle, and adjacent statistical windows are formed with the same number of consecutive pulse cycles. Each statistical window includes at least three pulse cycles. The average value of the interface response current density in the previous statistical window and the next statistical window are calculated respectively, and the absolute value of the difference between the two average values ​​is taken as the average change. The difference between the maximum and minimum values ​​of the interface response current density in the next statistical window is calculated as the fluctuation in the window. When the average change is less than or equal to the fluctuation in the window, the grinding and polishing endpoint judgment result is generated.

[0041] Specifically, during the dual-potential pulse control process, the control module uses the cathode charge per unit area and the anode charge per unit area calculated within each pulse cycle as the basis for adjusting the phase duration. The cathode charge per unit area is derived from the integral of the negative response current density over time within the magnesium protection phase, while the anode charge per unit area is derived from the integral of the positive response current density over time within the nickel passivation phase. Since both have been normalized according to the effective exposed area of ​​the working electrode, they can reflect the relative intensity of the cathodic protection process and the anode passivation process in the same interface area within one pulse cycle. When the absolute value of the charge per unit area of ​​the anode is greater than that of the charge per unit area of ​​the cathode within a certain pulse cycle, it indicates that the proportion of the positive anode response is too high within that cycle. In the next pulse cycle, the duration of the magnesium protection phase is increased by one control step, and the duration of the nickel passivation phase is decreased by the same control step, so that the total duration of the pulse cycle remains unchanged. The control step is not an arbitrary adjustment amount, but is determined by the time resolution capability of the pulse power supply. It can be the minimum adjustable time unit of the pulse power supply, or an integer multiple of the minimum adjustable time unit. For example, when the minimum adjustable time unit of the pulse power supply is 1μs, the control step can be 1μs, 2μs, or 5μs. The actual value can be determined based on whether the change in response current density in adjacent pulse cycles shows oscillation. If the interface response current density frequently alternates between rising and falling in adjacent cycles after adjustment, the control step should be reduced. The duration of the nickel passivation phase cannot be shortened indefinitely; its lower limit is determined by the passivation response time. The passivation response time can be obtained before sample preparation through a potential step test of nickel-plated samples from the same batch. Specifically, nickel-plated samples from the same batch are placed in the same non-aqueous electrolyte and the same reference system. The potential is stepped from the open circuit potential or the nickel plating polishing potential to the nickel passivation potential, and the change of the anodic response current density over time is continuously recorded. When the anodic response current density changes from the initial transient change to the passivation platform, and the current density difference between several consecutive sampling points does not exceed the current noise amplitude of the test channel, the time taken to reach this state is taken as the passivation response time. For example, if the anodic response current density of a nickel-plated sample from the same batch enters the passivation platform after 0.12 ms at the nickel passivation potential, then the duration of the nickel passivation phase in the dual-potential pulse control will no longer decrease to below 0.12 ms. When the duration of the nickel passivation phase has decreased to the passivation response time, even if the absolute value of the charge per unit area on the anode is still greater than that on the cathode, the nickel passivation phase should not be shortened further. The current phase ratio can be maintained, or interface disturbances can be reduced by decreasing the mechanical polishing feed intensity. The polishing endpoint should be determined at the same phase end to avoid direct comparison of response current densities under different potential states. For example, the interface response current density can be collected at the end of the nickel passivation phase in each pulse cycle. The potential state, sampling time, and current attenuation degree corresponding to this position are consistent, making the obtained data comparable.The control module forms statistical windows with the same number of continuous pulse cycles. Each statistical window includes at least three pulse cycles; for example, 5, 10, or 20 consecutive pulse cycles can be used as a statistical window. The previous and subsequent statistical windows are considered adjacent statistical windows. The average value of the interface response current density within each statistical window is calculated, and the absolute value of the difference between the two is taken as the average change. Simultaneously, the difference between the maximum and minimum values ​​of the interface response current density within the subsequent statistical window is calculated as the fluctuation within the window. If the average change is greater than the fluctuation within the window, it indicates that there is still an overall change between the two statistical windows that exceeds the fluctuation range of the current window itself, and dual-potential pulse electrochemical mechanical polishing continues. If the average change is less than or equal to the fluctuation within the window, it indicates that the change in interface response current density caused by continued polishing is no longer significantly higher than the natural fluctuation under the same phase, and a polishing endpoint judgment result is generated. For example, if the average value of the interface response current density at the end of the nickel passivation phase in the first statistical window is 1.80 μA / cm², and the average value in the second statistical window is 1.72 μA / cm², then the average change is 0.08 μA / cm². If the maximum value of the interface response current density in the second statistical window is 1.76 μA / cm² and the minimum value is 1.67 μA / cm², then the fluctuation within the window is 0.09 μA / cm². Since the average change is less than the fluctuation within the window, a polishing endpoint determination result is generated. After generating the polishing endpoint determination result, the control module stops the relative polishing motion between the polishing disc and the sample, and stops the dual-potential pulse output. Then, the magnesium alloy nickel-plated sample is removed from the non-aqueous electrolyte, rinsed with anhydrous ethanol, and dried with high-purity argon or nitrogen, thus obtaining the metallographic sample of the magnesium alloy nickel-plated layer interface. In some implementations, for multiple samples from the same batch, with the same magnesium alloy grade, the same nickel plating process, and the same coating thickness range, a representative sample can be selected for polarization scanning and pulse response testing to obtain the initial parameters for the magnesium protection potential, nickel passivation potential, nickel coating polishing potential, passivation response time, and dual-potential pulse control corresponding to that batch, and to establish a sample preparation parameter database for the same batch. When preparing subsequent samples from the same batch, the potential parameters and initial pulse parameters in the parameter database can be directly called, eliminating the need to repeat a complete polarization scan for each sample; however, the reference data window is still determined based on the real-time sampling data of the current sample during the initial electrochemical mechanical polishing process to reflect the differences in the surface state, clamping state, and effective exposed area of ​​the current sample. This method improves the sample preparation efficiency of the same batch while maintaining the adaptability of the interface exposure criterion to the actual electrochemical response of individual samples.

[0042] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope defined by the present invention.

Claims

1. A method for preparing a metallographic sample of a magnesium alloy nickel-plated layer interface with anti-oxidation and low damage, characterized in that, include: A magnesium alloy nickel-plated sample was installed as a working electrode in an electrochemical mechanical polishing device, and the magnesium alloy nickel-plated sample was brought into contact with a non-aqueous electrolyte containing magnesium corrosion inhibitor, nickel corrosion inhibitor and conductive salt, wherein the response current density was positive in the anodic direction and negative in the cathodic direction. The magnesium alloy nickel-plated sample was subjected to initial electrochemical mechanical polishing, and the response current density and open circuit potential of the working electrode were collected at intervals during the initial electrochemical mechanical polishing process. The applied potential was kept constant when the response current density was collected, and the applied power supply was disconnected when the open circuit potential was collected. A baseline data window is determined based on continuous sampling data before interface exposure, and an interface exposure criterion is established based on the changes in response current density and open circuit potential within the baseline data window. During the continued polishing process, the interface between the magnesium alloy substrate and the nickel plating is determined to be exposed based on the comparison results of the current response current density and the negative shift rate of the adjacent open circuit potential with the interface exposure criterion. After the interface is exposed, the potential control mode is switched to dual-potential pulse control. Within the same pulse cycle, a magnesium protection potential and a nickel passivation potential are applied sequentially. The magnesium protection potential is a negative cathode potential, and the nickel passivation potential is a positive anode potential. The duration of the magnesium protection potential is longer than the duration of the nickel passivation potential. Electrochemical mechanical polishing continues under the control of the dual-potential pulse, and the polishing endpoint is determined based on the change in interface response current density at the end of the same phase within several consecutive pulse cycles. When the polishing endpoint is reached, electrochemical mechanical polishing is stopped, and a metallographic sample of the magnesium alloy nickel plating layer interface is obtained.

2. The method for preparing a magnesium alloy nickel-plated layer interface anti-oxidation and low-damage metallographic sample according to claim 1, characterized in that, During the initial electrochemical mechanical polishing process, the response current density and open-circuit potential of the working electrode were collected at intervals, including: Electrochemical mechanical polishing is performed continuously for several detection cycles. Each detection cycle includes a polishing control sub-period and an open-circuit detection sub-period. During the polishing control sub-period, an external power supply is turned on and a nickel plating polishing potential is applied to the working electrode. The response current density at the end of the polishing control sub-period is collected. During the open-circuit detection sub-period, the external power supply is turned off and the working electrode is connected to a high-resistance potential acquisition channel. The open-circuit potential at the end of the open-circuit detection sub-period is collected. The response current density and open-circuit potential collected in the same detection cycle are used as a set of sampling data.

3. The method for preparing a magnesium alloy nickel-plated layer interface anti-oxidation and low-damage metallographic sample according to claim 2, characterized in that, The duration of the open-circuit detection sub-period is determined based on the open-circuit potential sampling stability, including: After disconnecting the external power supply, the open circuit potential is continuously collected. When the absolute value of the difference between adjacent open circuit potential sampling values ​​is less than or equal to the potential noise amplitude of the high resistance potential acquisition channel, the open circuit detection sub-period ends and the next grinding and polishing control sub-period begins.

4. The method for preparing a metallographic sample of a magnesium alloy nickel-plated layer interface with anti-oxidation and low damage according to claim 3, characterized in that, When determining the baseline data window based on continuous sampling data prior to interface exposure, the following are included: Several sliding sampling windows are formed with the same number of consecutive detection cycles, and each sliding sampling window includes at least three detection cycles; the range of response current density and the unidirectional drift of open circuit potential in each sliding sampling window are calculated respectively; the sliding sampling window with the smallest range of response current density and the smallest unidirectional drift of open circuit potential is selected as the reference data window; the average value of response current density in the reference data window is calculated, and the difference between the maximum and minimum values ​​of response current density in the reference data window is taken as the response current density fluctuation.

5. The method for preparing a magnesium alloy nickel-plated layer interface anti-oxidation and low-damage metallographic sample according to claim 4, characterized in that, When establishing interface exposure criteria based on the changes in response current density and open-circuit potential within the reference data window, the following are included: The average response current density is added to the fluctuation of the response current density to obtain the current criterion; the reference open circuit potential change rate is calculated based on the open circuit potential difference and detection interval of adjacent open circuit detection sub-periods within the reference data window; the maximum absolute value of the reference open circuit potential change rate is used as the potential criterion; the current criterion and the potential criterion are used together as the interface exposure criterion.

6. The method for preparing a magnesium alloy nickel-plated layer interface anti-oxidation and low-damage metallographic sample according to claim 5, characterized in that, When determining whether the interface between the magnesium alloy substrate and the nickel plating is exposed based on the comparison between the current response current density and the negative shift rate of the adjacent open-circuit potential with the interface exposure criterion, the following steps are taken: The response current density at the end of the current polishing control sub-period is compared with the current criterion; the open circuit potential of the current open circuit detection sub-period is subtracted from the open circuit potential of the previous open circuit detection sub-period, and the difference is divided by the detection interval to obtain the real-time open circuit potential change rate; when the response current density is greater than the current criterion, and the real-time open circuit potential change rate is negative and its absolute value is greater than the potential criterion, the interface is determined to be exposed.

7. The metallographic sample preparation method for preventing oxidation and minimizing damage at the interface of nickel-plated magnesium alloy as described in claim 6, characterized in that, The determination of the magnesium protection potential and the nickel passivation potential includes: Before switching to dual-potential pulse control, the bare magnesium alloy samples and nickel-plated samples from the same batch were placed in the non-aqueous electrolyte for polarization scanning. The polarization scanning used the same reference system as the electrochemical mechanical polishing device. The potential corresponding to the stable change of the cathode current in the cathodic polarization curve of the bare magnesium alloy sample was selected as the magnesium protection potential. The potential corresponding to the passivation plateau of the anodic current in the anodic polarization curve of the nickel-plated sample was selected as the nickel passivation potential.

8. The metallographic sample preparation method for preventing oxidation and minimizing damage at the interface of nickel-plated magnesium alloy as described in claim 7, characterized in that, When selecting the potential corresponding to the stable change range of the cathode current from the cathodic polarization curve of the exposed magnesium alloy sample as the magnesium protection potential, the following steps are taken: The cathode polarization curve is divided into several continuous potential segments based on the potential step size of the polarization scan. The ratio of the difference in cathode current density to the potential difference between adjacent sampling points in each potential segment is calculated, along with the average absolute value, standard deviation, and range of cathode current density within that potential segment. When at least two consecutive potential segments satisfy the condition that the difference in the average absolute value of the ratio between the segment and the previous segment is less than or equal to the standard deviation of the ratio within that segment, and the range of cathode current density within that segment is less than or equal to the range of cathode current density within the previous segment, these two consecutive potential segments are defined as a stable cathode current variation segment. The potential corresponding to the end with the smallest absolute potential value within the stable variation segment is taken as the magnesium protection potential.

9. The metallographic sample preparation method for preventing oxidation and minimizing damage at the interface of nickel-plated magnesium alloy as described in claim 8, characterized in that, When magnesium protection potential and nickel passivation potential are applied sequentially within the same pulse cycle, the following applies: First, a magnesium protection potential is applied to the working electrode to form a magnesium protection phase; then, a nickel passivation potential is applied to the working electrode to form a nickel passivation phase; the response current density changes over time within the magnesium protection phase and the nickel passivation phase are collected respectively; the negative response current density within the magnesium protection phase is integrated over time to obtain the charge per unit area of ​​the cathode; the positive response current density within the nickel passivation phase is integrated over time to obtain the charge per unit area of ​​the anode; the durations of the magnesium protection phase and the nickel passivation phase are adjusted according to the charge per unit area of ​​the cathode and the charge per unit area of ​​the anode, while maintaining the duration of the magnesium protection phase as greater than the duration of the nickel passivation phase.

10. The metallographic sample preparation method for preventing oxidation and minimizing damage at the interface of nickel-plated magnesium alloy as described in claim 9, characterized in that, When adjusting the duration of the magnesium protection phase and the nickel passivation phase based on the charge per unit area of ​​the cathode and the charge per unit area of ​​the anode, and determining the polishing endpoint based on the change in interface response current density at the end of the same phase within several consecutive pulse cycles, the process includes: When the charge per unit area of ​​the anode is greater than the absolute value of the charge per unit area of ​​the cathode, the duration of the magnesium protection phase in the next pulse cycle is increased by a control step, and the duration of the nickel passivation phase in the next pulse cycle is decreased by the same control step; the control step is the minimum adjustable time unit of the pulse power supply or an integer multiple of the minimum adjustable time unit; when the duration of the nickel passivation phase decreases to the passivation response time, the shortening of the duration of the nickel passivation phase is stopped; the passivation response time is the time corresponding to the anode response current density of the same batch of nickel-plated samples entering the passivation platform at the nickel passivation potential; The interface response current density is collected at the end of the nickel passivation phase of each pulse cycle, and adjacent statistical windows are formed with the same number of consecutive pulse cycles. Each statistical window includes at least three pulse cycles. The average value of the interface response current density in the previous statistical window and the next statistical window are calculated respectively, and the absolute value of the difference between the two average values ​​is taken as the average change. The difference between the maximum and minimum values ​​of the interface response current density in the next statistical window is calculated as the fluctuation within the window. When the average change is less than or equal to the fluctuation within the window, the grinding and polishing endpoint judgment result is generated.

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