An impressed current intelligent cathodic protection system for stainless steel water tanks

Abstract

This invention discloses an impressed current intelligent cathodic protection system for stainless steel water tanks, belonging to the field of metal corrosion protection technology. The system includes a potentiostat, an auxiliary anode, a reference electrode, an intelligent ion concentration monitoring unit, and a control module. This invention first utilizes a 0.5 m... 3 An impressed current cathodic protection experiment was conducted on a stainless steel water tank experimental platform to obtain the potential distribution and metal ion dissolution patterns under different seasons and protection parameters. The potentiostat adopts a portable integrated structure, including a chassis, front panel, rear panel, and main control module. The front panel integrates indicator lights, output display area, information display area, and terminal blocks, with a dual-channel potential acquisition terminal block on the left and a power output terminal block on the right. The rear panel has a fan opening and a power interface. The main control module is built into the chassis and electrically connected to each component, realizing the integrated display, control, acquisition, and output functions. This invention has the advantages of good protection effect, high degree of intelligence, and portability and ease of use.

Description

Technical Field

[0001] This invention relates to the field of metal corrosion protection technology, and specifically to an impressed current cathodic protection system method for 304 stainless steel water storage containers. Background Technology

[0002] 304 austenitic stainless steel, due to its excellent mechanical and processing properties, is widely used in water storage equipment such as building water supply tanks and inner tanks of water-storage water heaters. However, in tap water environments containing chloride ions and sufficient dissolved oxygen, 304 stainless steel is highly susceptible to pitting and crevice corrosion, especially in the heat-affected zone of the weld and the bottom deposition zone, often leading to early perforation and leakage of equipment. Impressed current cathodic protection is an effective electrochemical means of controlling metal corrosion. According to the cathodic protection criterion, effective protection is achieved when the potential of the protected metal shifts negatively by 100 mV relative to its self-corrosion potential. However, when traditional ICCP technology is directly applied to thin-walled stainless steel water tanks, the following technical challenges exist: First, there is a significant gradient in the potential distribution inside the water tank. If the reference electrode is placed in a strong electric field region near the auxiliary anode, the detected potential will be biased negatively towards the actual potential in the far field, causing the potentiostat to reduce current prematurely, resulting in insufficient protection in the far-end region due to a potential shift of less than 100 mV. Secondly, when the protection potential is too negative, the cathode reaction causes the interface pH to rise, and Cr2O3 in the passivation film undergoes alkaline dissolution to form soluble CrO4. 2- or Cr(OH)4 - The entry of Cr ions into the solution leads to an abnormally high concentration of Cr ions. Simultaneously, anodic dissolution of Fe is inhibited, and the amount of Fe dissolved remains at a low level. At this point, the Fe / Cr dissolution ratio decreases significantly. Therefore, monitoring the potential alone cannot identify the overprotection state, while synergistic monitoring of the Fe / Cr dual ion concentration can sensitively reflect the occurrence of overprotection.

[0003] Therefore, this application is submitted. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide an impressed current cathodic protection system and method for stainless steel water tanks based on numerical simulation optimization and dual ion monitoring.

[0005] (1) Overprotection early warning mechanism based on dual ion monitoring

[0006] This invention uses Fe / Cr dual-ion synergistic monitoring as an early warning indicator for stainless steel overprotection. The mechanism is as follows: under normal protection conditions, anodic dissolution of Fe is effectively suppressed, and the dissolution of Cr is low due to the stability of the passivation film, with the Fe / Cr dissolution ratio typically greater than 5.0.

[0007] Under overprotected conditions, interfacial alkalization leads to selective alkali dissolution of Cr in the passivation film, resulting in a rapid increase in Cr ion concentration. Fe dissolution is still suppressed, and the Fe / Cr dissolution ratio drops significantly to below 3.0.

[0008] (2) Seasonal potential regulation strategy

[0009] This invention sets differentiated protection parameters based on water temperature conditions in different seasons:

[0010] Winter low temperature (<25℃): The electrochemical reaction rate decreases, and a protection potential of -0.70 V to -0.75 V is used, with a Cr ion concentration change rate threshold of 0.2 μg / (L·d).

[0011] High temperatures in summer (≥25℃): The electrochemical reaction rate increases, and the risk of overprotection increases. Use a protection potential of -0.65V to -0.70V and a Cr ion concentration change rate threshold of 0.5 μg / (L·d) to strengthen the monitoring of Cr ion concentration.

[0012] (3) Control methods

[0013] The self-corrosion potential was measured, and a negative shift of 100 mV was used as the initial protection potential setpoint. The potentiostat adjusted the output current based on feedback from the reference electrode to form a closed-loop dynamic equilibrium. The Fe and Cr ion concentrations were measured periodically, and the Fe / Cr dissolution ratio was calculated.

[0014] If the rate of change in Cr ion concentration exceeds the seasonal threshold, or the Fe / Cr ratio falls below the lower limit threshold, over-protection is detected, and the protection potential is automatically lowered by 10-20 mV. This monitoring-detection-adjustment cycle is repeated until the indicators return to normal.

[0015] (4) Potentiostat

[0016] The potentiostat described in this invention adopts a portable, integrated structure, highly integrating display, control, data acquisition, and output functions into a compact chassis. The overall dimensions are only 140 mm × 140 mm × 275 mm, and the weight is 2.5 kg. Specifically, the potentiostat includes a chassis housing, a front panel, a rear panel, and a main control module. The front panel integrates indicator lights, an output display area, an information display area, and a set of terminals. The left side has dual-channel potential acquisition terminals, and the right side has power output terminals, achieving a partitioned layout for data acquisition and output. The rear panel has a fan opening and a 220 V power interface. The main control module is built into the chassis housing and electrically connected to each component. This structure overcomes the technical shortcomings of traditional potentiostats, such as large size, dispersed functions, and chaotic field wiring, and features portability, ease of use, and high functional integration.

[0017] (5) Beneficial effects

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] Scientifically optimized electrode arrangement: Multiple spatial arrangement schemes can optimize the protection current, number of anodes, size and position, achieving 100% protection coverage.

[0020] Sensitive and reliable over-protection early warning: Establish Fe / Cr dual-ion synergistic monitoring index to realize self-sensing of over-protection and adaptive adjustment of protection potential.

[0021] All-season adaptive adjustment: Differentiated protection parameters are set for the water temperature difference between winter and summer to achieve safe and efficient operation in all climates.

[0022] Sufficient durability verification: 20-year long-term simulation shows that the protective current flows continuously and effectively, and the system has good durability and reliability. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the external current cathodic protection system for stainless steel water tanks according to the present invention.

[0024] Figure 2 This is a schematic diagram of the front structure of the potentiostat of the stainless steel water tank impressed current cathodic protection system of the present invention.

[0025] Figure 3 This is a schematic diagram of the back structure of the potentiostat of the stainless steel water tank impressed current cathodic protection system of the present invention.

[0026] Figure reference numerals: 1. Potentiostat; 2. Auxiliary anode; 3. Reference electrode; 4. Positive terminal connection; 5. 304 stainless steel; 6. Positive terminal connection for potential acquisition; 7. Negative terminal connection for potential acquisition; 8. Negative terminal connection; 9. Power signal; 10. 4G signal; 11. Overload signal; 12. Alarm signal; 13. Output display area; 14. Information display area; 15. Dual-channel potential acquisition terminal 1; 16. Dual-channel potential acquisition terminal 2; 17. Power output terminal; 18. Network cable interface; 19. Power interface; 20. Power switch; 21. Fan opening. Detailed Implementation

[0027] [Example 1: Monitoring Operations at Low Temperatures in Winter]

[0028] The water temperature in winter was 5±2℃, and the initial protection potential of the system was set to a negative shift of 100mV from the reference potential under natural conditions. The experimental medium was a 3.5wt% NaCl solution, and the experimental model was a 1×1×0.5 m 304 stainless steel water tank. As shown in Table 1, after 93 days of operation, historical data curves showed that the Fe ion concentration in the protection group remained between 4.9-14.0 μg / L, and the Cr ion concentration remained between 0.3-1.9 μg / L, with a protection efficiency of over 85%.

[0029] Table 1. ICP test results of 304 stainless steel water tanks in winter (with and without ICCP)

[0030]

[0031] [Example 2: Summer High Temperature Over-protection Early Warning and Adaptive Adjustment]

[0032] The summer water temperature was 30±2℃, and the initial protection potential was set to a negative shift of 100 mV from the reference potential under natural conditions. The experimental medium was a 3.5wt% NaCl solution, and the experimental model was a 1×1×0.5 m 304 stainless steel water tank. As shown in Table 2, on day 21, the Cr ion concentration was detected to increase from 0.8 μg / L to 5.3 μg / L, with a change rate of 0.63 μg / (L·d) > the summer threshold of 0.5 μg / (L·d); the Fe / Cr dissolution ratio decreased from 10.2 to 2.8 < the lower limit threshold of 5.0. The protection potential was then lowered, and the operation log was recorded. On day 28, the Cr ion concentration dropped back to 1.5 μg / L, the Fe / Cr ratio recovered to 6.5, and the indicators returned to normal.

[0033] Table 2. ICP test results of 304 stainless steel water tanks in summer (with and without ICCP)

[0034]

[0035] The above description is merely a preferred embodiment of the present invention and does not limit the scope of the patent. Any equivalent structural modifications made based on the inventive concept of the present invention and the description and drawings, or direct / indirect applications in other related technical fields, are included within the scope of patent protection of the present invention.

Claims

1. An impressed current cathodic protection system for a stainless steel water tank, characterized in that, include: A potentiostat (1) is used to output an adjustable DC protection current; an auxiliary anode (2) is electrically connected to the positive electrode of the potentiostat and installed on the inner wall of the water tank; a reference electrode (3) is electrically connected to the feedback input terminal of the potentiostat and is used to collect the protection potential of the inner wall of the water tank in real time; a control module (4) is connected to the control terminal of the potentiostat and is used to adjust the protection potential setting value; a smart management and monitoring subsystem (5) includes a device access module, a data acquisition module, a data storage module, a real-time display module, a threshold judgment module, and an alarm notification module; wherein, the spatial arrangement scheme of the auxiliary anode (2) and the reference electrode (3) can be selected from multiple schemes, and the smart management and monitoring subsystem (7) uses a negative shift of 100 mV relative to the self-corrosion potential of the protected structure as an effective protection criterion, and automatically adjusts the protection potential setting value or issues an alarm prompt according to the changing trend of Fe and Cr ion concentrations.

2. The stainless steel water tank impressed current cathodic protection system according to claim 1, characterized in that: The auxiliary anode (2) is arranged in a single column and is vertically installed at the center of the bottom of the water tank, 50-100 mm from the bottom surface; the reference electrode (3) is arranged near the auxiliary anode (2) to achieve rapid feedback adjustment; the auxiliary anode (2) is a rod-shaped titanium-based mixed metal oxide anode with a diameter of 150 mm.

3. The stainless steel water tank impressed current cathodic protection system according to claim 1, characterized in that: This includes protection current optimization. By simulating the potential distribution under different protection currents, with the optimization goal of achieving 100% protection coverage and no over-protection, the optimal protection current was determined to be 0.25 A.

4. The stainless steel water tank impressed current cathodic protection system according to claim 1, characterized in that: Comparing the two schemes of reference electrode placement near and far from the anode, the scheme of placement near the anode has a more stable current density and more uniform potential distribution in most areas of the water tank, and the current attenuation in the edge area is effectively alleviated. Auxiliary anode position optimization: Comparing the schemes of placing the anode in the middle area of ​​the water tank and placing it near the wall, the current density in the edge area of ​​the middle placement scheme is approximately 1.89 μA / m², which is higher than the 1.38 μA / m² of the scheme near the wall, resulting in better protection uniformity. Optimization of the number of auxiliary anodes: Comparing the single-anode and dual-anode schemes, the single-anode scheme has a more reasonable potential distribution, while the dual-anode scheme has an excessively large total output current leading to over-protection; the single-anode scheme is more advantageous.

5. The stainless steel water tank impressed current cathodic protection system according to claim 1, characterized in that: The auxiliary anode (2) is selected from one of the following materials: a mixed metal oxide coated titanium anode, a platinum-niobium anode, a high-silicon cast iron anode, or a graphite anode. The mixed metal oxide coated titanium anode has an IrO2-Ta2O5 or RuO2-IrO2 metal oxide coating on its surface, with a recommended maximum operating current density of 50-100 A / m². The platinum-niobium anode has a platinum layer structure electroplated or coated on a niobium substrate, with a platinum layer thickness of 1-10 μm, and a recommended maximum operating current density of 100-200 A / m². The high-silicon cast iron anode has the following composition by mass percentage: silicon 14.5%, chromium 4.5%, with the balance being iron, and a recommended maximum operating current density of 5-8 A / m². Preferably, the auxiliary anode is a mixed metal oxide coated titanium anode. The reasons for choosing this anode are: its consumption rate is extremely low, and its theoretical lifespan can reach more than 20 years, meeting the long-life design requirements of stainless steel water tanks; its chlorine evolution overpotential is high, and it has few side reactions in chloride ion-containing environments, making it less likely to produce harmful chlorine gas; its dimensional stability is good, and the coating is firmly bonded to the titanium substrate, making it less prone to peeling off; its current density has a wide applicable range and can be flexibly adjusted according to protection needs.

6. The stainless steel water tank impressed current cathodic protection system according to claim 1, characterized in that: The reference electrode (3) is selected from one of the following materials: silver / silver chloride electrode (Ag / AgCl), high-purity zinc electrode, saturated calomel electrode (SCE), or copper / copper sulfate electrode (CSE). Preferably, the reference electrode is a silver / silver chloride electrode (Ag / AgCl). The reason for selecting this electrode is that it has a stable potential in the chlorinated tap water medium in the stainless steel water tank, and the change in chloride ion concentration has little effect on its equilibrium potential; the electrode potential drift is small, and the long-term stability is good; the structure is compact and easy to install inside the water tank; it is environmentally friendly and does not contain toxic substances such as mercury.

7. The stainless steel water tank impressed current cathodic protection system according to claim 1, characterized in that: The potentiostat (1) includes: an indicator light group (9-12), an output display area (13), an information display area (14), and a terminal block group (15-17) on the front panel; the indicator light group includes a power status indicator light (9) and an alarm signal indicator light (12); the output display area (13) is used to display the output voltage, output current, output power, and real-time temperature; the information display area (14) is used to display the set potential value and the acquired potential value; the terminal block group includes a dual-channel potential acquisition terminal block (15-16) located on the left side of the front panel, and a power output terminal block (17) located on the right side of the front panel.

8. The stainless steel water tank impressed current cathodic protection system according to claim 1, characterized in that: The potentiostat (1) includes: a rear panel, which is located on the rear side of the chassis housing, and the rear panel is provided with a fan opening (21) and a power interface (19), the power interface being used to connect to a 220 V AC power supply; and a main control module, which is located inside the chassis housing and is electrically connected to the indicator light group, output display area, information display area, terminal block group and power interface respectively.

9. A control method for an impressed current cathodic protection system for a stainless steel water tank, applied to the system described in any one of claims 1 to 8, characterized in that, Includes the following steps: S1. Measure the self-corrosion potential Ecorr of the protected 304 stainless steel under the actual water quality conditions in the water tank, and use Ecorr-100mV as the initial protection potential setting value Vset. S2. The potentiostat (1) adjusts the output current according to the measured potential Vreal fed back by the reference electrode (3) to stabilize Vreal near Vset; S3. Every preset sampling period T, the ion concentration monitoring unit (6) collects a water sample from the water tank, measures the Fe ion concentration and Cr ion concentration therein, and calculates the Fe / Cr dissolution ratio; S4. Calculate the change rate ΔCr / Δt of the Cr ion concentration and the change rate ΔR / Δt of the Fe / Cr dissolution ratio within the current sampling period; S5. When any of the following conditions is satisfied simultaneously, it is determined that there is a risk of overprotection, and the control module (4) lowers the protection potential set value by one step ΔV from Vset: Condition 1: ΔCr / Δt > K1, where K1 is the threshold value of the change rate of the Cr ion concentration; Condition 2: R < Rmin, where Rmin is the lower limit threshold value of the Fe / Cr dissolution ratio; S6. Repeat steps S3 to S5 until the change rate of the Cr ion concentration drops below K and the Fe / Cr dissolution ratio recovers above Rmin. According to the adaptive control method described in claim 1, wherein: the threshold value K1 of the change rate of the Cr ion concentration, the value of K1 is: in the summer working condition, K1 = 0.5 μg / (L·d), and in the winter working condition, K1 = 0.2 μg / (L·d).

10. The adaptive control method according to claim 1, characterized in that: The value of the lower limit threshold Rmin of the Fe / Cr dissolution ratio is 3.0 - 5.0, and the potential down - adjustment step ΔV is 10 - 20 mV.

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