Method for testing corrosion rate of rare earth magnesium alloy under different stress states and application thereof

Abstract

The application discloses a testing method for corrosion rates of rare earth magnesium alloys under different stress states and application thereof, and mainly comprises the following steps: polishing and polishing the sample after wire electrical discharge machining; cleaning, drying and weighing the polished sample; setting corrosion parameters of the corrosion experiment under different stress states; cleaning the sample surface after corrosion for a certain time to remove surface corrosion products; weighing the mass of the sample after corrosion test; and calculating the corrosion mass loss per unit area of the test sample. The application introduces different stress states in the traditional immersion corrosion experiment, obtains a novel corrosion experiment testing technology coupling a stress field and a corrosion environment, and is low in equipment cost and operation threshold, high in experiment efficiency, and high in reliability of the obtained testing data.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of corrosion detection, and particularly relates to a testing method for corrosion rates of rare earth magnesium alloys under different stress states and application thereof. BACKGROUND

[0002] Rare earth magnesium alloy is a kind of metal material with excellent properties. Due to its biocompatibility and biodegradability, the rare earth magnesium alloy can be widely applied to the preparation of biomedical materials such as bone endosseous implants, cardiac stents and the like. However, its poor corrosion resistance becomes an obstacle for practical application, and the service environment and working condition of the biological endosseous implant material bring a series of challenges to the biodegradable rare earth magnesium alloy endosseous implant material. Therefore, the evaluation of the corrosion rates of the rare earth magnesium alloy material under different stress states is of great significance to the safe service of the rare earth magnesium alloy device. However, the current research has no clear understanding of the change of the alloy corrosion rate under different stress states. The lack of simulation of the alloy corrosion rate under the actual service state is extremely unfavorable for the development and optimization of new biodegradable rare earth magnesium alloy biological endosseous implant materials.

[0003] At present, the research on the service failure behavior of magnesium alloy under the combined action of stress field and corrosion environment mainly focuses on stress corrosion and stress corrosion cracking, however, the research on the stress field accelerating the corrosion rate has no systematic evaluation. Many in-vitro environmental corrosion test methods cannot reflect the corrosion behavior under the actual service environment, if the corrosion rate is faster under the in-vivo service environment, the rare earth magnesium alloy biological endosseous implant material will fail prematurely before bone healing, and the uncontrollable degradation behavior will occur. Therefore, the evaluation of the corrosion rate of the rare earth magnesium alloy under the simulated service stress environment has important engineering significance for understanding the corrosion mechanism of the rare earth magnesium alloy, optimizing the corrosion resistance, and optimizing the processing technology and application scene. SUMMARY

[0004] To solve the above problems, the purpose of the present application is to provide a testing method for the corrosion rates of rare earth magnesium alloys under different stress states and application thereof. The present application introduces different stress states in the traditional immersion corrosion experiment to obtain a new corrosion experiment test technology coupling stress field and corrosion environment.

[0005] The testing technical scheme adopted by the present application is as follows:

[0006] A testing method for the corrosion rates of rare earth magnesium alloys under different stress states, comprising the following steps:

[0007] S1, a plurality of rare earth magnesium alloy test samples of the same material and size are subjected to corrosion experiments under different stress states and for different times;

[0008] S2, after each of the rare earth magnesium alloy test samples in S1 reaches the preset time, the rare earth magnesium alloy test sample is cleaned to remove the surface corrosion product;

[0009] S3, the rare earth magnesium alloy test sample from which the surface corrosion product is removed is cleaned, dried, and weighed;

[0010] S4, the corrosion mass loss of the rare earth magnesium alloy test sample per unit area under different stress states is calculated to obtain the corrosion rate information of the rare earth magnesium alloy test sample.

[0011] Preferably, before S1, the process further comprises pretreatment of the rare earth magnesium alloy test sample:

[0012] The rare earth magnesium alloy is subjected to wire cut electrical discharge machining to obtain rare earth magnesium alloy test samples of uniform size. The surface of the rare earth magnesium alloy test sample is polished smooth to remove the oxide skin on the surface of the rare earth magnesium alloy test sample, thereby obtaining a rare earth magnesium alloy test sample with a mirror surface and no scratches. The rare earth magnesium alloy test sample is then polished to remove scratches on the surface of the rare earth magnesium alloy test sample, and then the rare earth magnesium alloy test sample is cleaned and dried. The pretreatment is completed.

[0013] The pretreated rare earth magnesium alloy test sample is used for S1.

[0014] Preferably, in the pretreatment process, S2, and S3, the rare earth magnesium alloy test sample is first washed with deionized water and then with anhydrous ethanol and is dried to obtain a dry rare earth magnesium alloy test sample.

[0015] Preferably, when the corrosion experiment is performed, the stress states include a state of zero stress, a constant stress state, and an alternating stress state, and each rare earth magnesium alloy test sample is subjected to one of the stress states.

[0016] In the constant stress state and the alternating stress state, the rare earth magnesium alloy test sample is subjected to a three-point bending test. When the alternating stress state is applied, the rare earth magnesium alloy test sample is subjected to a vibration of a preset frequency at the midpoint.

[0017] Preferably, in the constant stress state and the alternating stress state, the applied pressure is 1-20 N.

[0018] When the alternating stress state is applied, the rare earth magnesium alloy test sample is subjected to a vibration of a preset frequency of 0.1-5 Hz at the midpoint.

[0019] Preferably, in S1, when the corrosion experiment is performed, the corrosion solution is a NaCl solution with a mass fraction of 0.9%, and the temperature is 36.5-37.5℃.

[0020] In S2, when removing the surface corrosion product, the cleaned rare earth magnesium alloy test sample is immersed in a chromic acid solution to remove the surface corrosion product.

[0021] Preferably, the concentration of the solute in the chromic acid solution is 200 g / L.

[0022] Preferably, the calculation formula of the corrosion mass loss per unit area of the rare earth magnesium alloy test sample under different stress states is as follows:

[0023]

[0024] Wherein, S is the surface area of the rare earth magnesium alloy test sample before corrosion; W Loss is the mass loss per unit area of the rare earth magnesium alloy test sample, unit: mg·cm -2 ; m1-m2 is the mass difference of the rare earth magnesium alloy test sample before and after the corrosion experiment.

[0025] Preferably, in S4, according to the corrosion mass loss per unit area of the rare earth magnesium alloy test sample under different stress states, a curve of the corrosion per unit area changing with the corrosion time under different stress states is drawn, and the corrosion rate information of the rare earth magnesium alloy test sample is obtained.

[0026] The application of the rare earth magnesium alloy corrosion rate testing method under different stress states as described above is used for:

[0027] Optimizing the corrosion resistance of the rare earth magnesium alloy;

[0028] Simulating the corrosion behavior of the rare earth magnesium alloy under the complex stress environment in the actual service process and evaluating the corrosion resistance of the rare earth magnesium alloy under different stress states.

[0029] The present application has the following beneficial effects:

[0030] In the present application, a plurality of rare earth magnesium alloy test samples of the same material and size are subjected to corrosion experiments under different stress states and for different times, and the present application first introduces different stress states in the traditional immersion corrosion experiment, so that the experiment is closer to the actual service environment of the rare earth magnesium alloy, and the corrosion rate of the rare earth magnesium alloy under the simulated service stress environment can be evaluated, which has important engineering significance for understanding the corrosion mechanism of the rare earth magnesium alloy, optimizing the corrosion resistance, and optimizing the processing technology and application scene. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 The structure and size of the rare earth magnesium alloy test sample in the embodiments of the present application are shown in the drawings.

[0032] Fig. 2(a) is a schematic diagram of a dynamic mechanical analyzer environmental clamp used in the embodiment of the present application, and Fig. 2(b) is a schematic diagram of the overall structure of a dynamic mechanical analyzer used in the embodiment of the present application;

[0033] Fig. 3(a) is a curve of the mass loss per unit area versus the corrosion time in the unstressed state in Example 1 of the present application.

[0034] Fig. 3(b) is a curve of the corrosion rate versus the corrosion time in the unstressed state in Example 1 of the present application.

[0035] Fig. 3(c) is a surface corrosion morphology after corrosion for 240 minutes in the unstressed state in Example 1 of the present application.

[0036] Fig. 3(d) is a cross-section corrosion morphology after corrosion for 240 minutes in the unstressed state in Example 1 of the present application.

[0037] Fig. 3(e) is a result of the hydrogen evolution corrosion experiment of the original alloy and the alloy solid-solution treated for 6 hours in Example 1 of the present application.

[0038] Fig. 3(f) is a surface morphology after corrosion of the original alloy in Example 1 of the present application.

[0039] Fig. 3(g) is a surface morphology after corrosion of the alloy solid-solution treated for 6 hours in Example 1 of the present application.

[0040] Fig. 4(a) is a curve of the mass loss per unit area versus the corrosion time in the constant stress state in Example 2 of the present application.

[0041] Fig. 4(b) is a curve of the corrosion rate versus the corrosion time in the constant stress state in Example 2 of the present application.

[0042] Fig. 4(c) is a surface corrosion morphology after corrosion for 240 minutes in the constant stress state in Example 2 of the present application.

[0043] Fig. 4(d) is a cross-section corrosion morphology after corrosion for 240 minutes in the constant stress state in Example 2 of the present application.

[0044] Fig. 4(e) is a tensile curve of the original rare earth magnesium alloy in Example 2 of the present application.

[0045] Fig. 4(f) is a tensile curve of the alloy solid-solution treated for 6 hours in Example 2 of the present application.

[0046] Fig. 5(a) is a curve of the mass loss per unit area versus the corrosion time in the alternating stress state in Example 3 of the present application.

[0047] Fig. 5(b) is a curve of the corrosion rate versus the corrosion time in the alternating stress state in Example 3 of the present application.

[0048] Figure 5(c) is a surface corrosion morphology after 240 minutes of corrosion under alternating stress state in Example 3 of the present application.

[0049] Figure 5(d) is a cross-section corrosion morphology after 240 minutes of corrosion under alternating stress state in Example 3 of the present application.

[0050] Figure 5(e) is a hydrogen evolution corrosion test result after 10 hours of solid solution in Example 3 of the present application.

[0051] Figure 5(f) is a surface corrosion morphology after 10 hours of solid solution in Example 3 of the present application. DETAILED DESCRIPTION

[0052] The present application will be further described in conjunction with the accompanying drawings and examples.

[0053] The process of the test method for the corrosion rate of the rare earth magnesium alloy under different stress states comprises the following steps:

[0054] Step 1. Use wire electrical discharge machining to cut the rare earth magnesium alloy plate into a size that can be accommodated by the dynamic mechanical thermal analyzer environmental clamp, such as Figure 1 As shown in the figure, the rare earth magnesium alloy test sample (hereinafter referred to as test sample) is a long strip sample.

[0055] Step 2. Polish the six surfaces of the test sample with 2000 grit sandpaper to remove the surface oxidation scale caused by wire electrical discharge machining and prevent the oxidation scale from affecting the corrosion behavior.

[0056] Step 3. Use a polishing machine to polish the test sample with the surface oxidation scale removed to remove surface scratches and prevent the existence of scratches from affecting the corrosion rate. Rinse the surface polished test sample with anhydrous ethanol and dry it.

[0057] Step 4. Weigh the polished, cleaned, and dried test sample, with a weighing accuracy of 0.1 mg, and record the mass obtained as m1.

[0058] Step 5. Build a corrosion rate test system under different stress states, as shown in Figures 2(a) and 2(b), and use the dynamic mechanical thermal analyzer environmental clamp as a simulated service environment for coupling different stress states and corrosion media.

[0059] Step 6. Add the corrosion medium to the dynamic mechanical thermal analyzer environmental clamp, and at the same time, immerse the test sample in the corrosion medium, and close the furnace cover of the environmental clamp.

[0060] Step 7. Set the stress state, temperature, and corrosion time of the dynamic mechanical thermal analyzer.

[0061] Step 8. After a certain time of corrosion, the furnace cover is opened to take out the corroded test sample, and the corroded test sample is cleaned. First, the surface corrosion product is rinsed with deionized water, then the corroded test sample is immersed in a chromium acid solution while being ultrasonically treated for 5 minutes to dissolve the surface corrosion product, and finally the test sample is taken out, rinsed with deionized water and anhydrous ethanol in sequence, and the surface residual anhydrous ethanol is blown dry with a hair dryer to obtain a dry corrosion test sample.

[0062] Step 9. The test sample after corrosion and cleaning and drying is weighed, with a weighing accuracy of 0.1 mg, and the obtained mass is recorded as m2.

[0063] Step 10. The mass loss per unit area of the rare earth magnesium alloy after corrosion for a certain time under different stress states is calculated, with a unit of mg·cm -2 .

[0064] The experimental conditions and processes of the embodiments of the present application are as follows:

[0065] As shown in Figure 1 , the metal test sample used in the following embodiments of the present application has a length of 30 mm, a width of 5 mm, and a thickness of 1.2 mm, and the main corrosion surface is two rectangular surfaces of 30 mm x 5 mm.

[0066] As shown in Figs. 2(a) and 2(b), the stress state corrosion rate test device used in the following embodiments of the present application mainly consists of a dynamic thermal mechanical analyzer and an environmental clamp. The fixed clamp provides the corrosion medium and the accommodation space for the test sample, and the movable clamp provides the three-point bending pressure. The three-point bending clamp is used to measure rigid, highly elastic materials such as metals, ceramics, highly filled thermosetting polymers, highly filled and highly crystalline thermoplastic polymers, and the like. When performing a stress-free state, a very small three-point bending contact pressure (1 x 10 -3 N) needs to be applied, at which time the movable clamp only slightly contacts the surface of the test sample, which can be considered as a zero stress state, avoiding the influence of non-experimental factors on the experimental data; when performing a constant stress state, a constant three-point bending pressure needs to be applied; and when performing a dynamic stress state, i.e. oscillation test, a static force needs to be applied.

[0067] The specific experimental operation steps are as follows:

[0068] Step 1. The rare earth magnesium alloy plate is processed into the required size by using wire electrical discharge machining, the surface oxide skin of the test sample is polished off using 2000 mesh sandpaper, and the polished sample is polished to the extent that no obvious scratches can be observed with the naked eye, obtaining a test sample with a mirror surface and no scratches; finally, the test sample is rinsed with deionized water and anhydrous ethanol in sequence and dried to obtain a dry test sample. The test sample after polishing, cleaning and drying is weighed, with a weighing accuracy of 0.1 mg, and the obtained mass is recorded as m1.

[0069] Step 2. Add the corrosion medium into the dynamic mechanical analyzer environmental clamp, and the specific parameters of the corrosion medium are 11 ml of 0.9%wt. NaCl solution; place the prepared dry test sample on the environmental three-point bending clamp, and the sample is placed in the middle of the clamp; close the furnace cover.

[0070] Step 3. Set the corrosion parameters. The temperature is 37℃, and the same corrosion time and different three-point bending stress states are set for corrosion tests under different stress states.

[0071] Step 4. After the corrosion is completed, the corrosion test sample is taken out, and the surface corrosion products are washed with deionized water and anhydrous ethanol in sequence, then immersed in a 200g / L chromium acid solution and ultrasonically oscillated for 5 minutes. The sample is taken out and washed with deionized water and anhydrous ethanol in sequence to remove the residual chromium acid solution on the surface, and the surface is dried with a hair dryer to obtain a dry corrosion test sample. The test sample after corrosion and cleaning and drying is weighed, and the weighing precision is 0.1mg. The mass obtained by weighing is denoted as m2.

[0072] Step 5. Calculate the mass loss of the rare earth magnesium alloy per unit area after corrosion under different stress states for a certain time, and the calculation formula is as follows:

[0073]

[0074] Wherein, S is the surface area of the test sample before corrosion, W Loss is the mass loss per unit area, and the unit is mg·cm^(-2).

[0075] Step 6. Repeat the above steps 1-5 to obtain the mass loss of the test sample after corrosion under different corrosion times and different stress states.

[0076] Step 7. According to the data obtained in step 6, the curve of the corrosion per unit area under different stress states with the corrosion time is drawn.

[0077] The test method for the corrosion rate of the rare earth magnesium alloy under different stress states in the embodiment utilizes a dynamic thermal mechanical analyzer environmental clamp to create a stress field and a corrosion medium coupling simulation of the real service environment of a biomedical bone implant, and sets three stress state conditions, namely, a stress-free state, a constant stress state and an alternating stress state, so as to compare the corrosion rates of the rare earth magnesium alloy under different stress states, and meanwhile, the three-point bending pressure is equal, the corrosion medium composition and volume and the concentration are the same, and the environmental temperature is the same, which makes the test result reliable. In addition, since the corrosion rate is finally calculated by the mass loss per unit area, if the same sample is used for the test at the same time interval, firstly, the protection effect of the corrosion product on the alloy sample matrix will be affected, and the error will be increased; secondly, the corrosion environment temperature will be affected by opening the dynamic thermal mechanical analyzer furnace cover many times during the corrosion process, and the test efficiency will be affected. Therefore, the test scheme of the embodiment is that the test samples observed at the same time are all new samples, that is, the sample corroded for 0 minutes, and the test scheme of the embodiment avoids the increase of the experimental error and improves the experimental efficiency, and has a certain universality.

[0078] The test method for the corrosion rate of the rare earth magnesium alloy under different stress states can be used for optimizing the corrosion resistance of the rare earth magnesium alloy, simulating the corrosion behavior of the rare earth magnesium alloy under the complex stress environment in the actual service process, and evaluating the corrosion resistance of the rare earth magnesium alloy under different stress states.

[0079] Embodiment 1

[0080] The corrosion conditions of the test sample in this embodiment are as follows: the corrosion medium temperature is 37℃, the corrosion medium is 11ml of a NaCl solution with a mass concentration of 0.9wt.%, and the three-point bending pressure is 1×10 -3 N, and the vibration frequency is 0Hz.

[0081] Under the above corrosion conditions, the extruded ZE62 magnesium alloy test sample is subjected to corrosion experiments for 40 minutes, 80 minutes, 120 minutes, 160 minutes, 200 minutes and 240 minutes respectively. The unit area mass loss change curve of the stress-free state is shown in Fig. 3(a), and the corrosion rate change curve of the stress-free state is shown in Fig. 3(b). It can be concluded that the corrosion rate gradually decreases and tends to a certain value with the prolongation of the corrosion time, indicating that the corrosion tends to be saturated and continues to corrode at a constant corrosion rate. The surface corrosion morphology after corrosion for 240 minutes in the stress-free state is shown in Fig. 3(c), and the cross-section corrosion morphology after corrosion for 240 minutes in the stress-free state is shown in Fig. 3(d), the surface corrosion morphology presents a typical immersion filamentous corrosion morphology, and the cross-section corrosion surface depth is shallow and relatively flat, which indicates that the corrosion type in the stress-free state is the same as the immersion corrosion.

[0082] Based on the experimental results and micro-morphology observation of Example 1, the corrosion resistance of the rare earth magnesium alloy as a bone biological implant material can be optimized as follows: surface treatment, heat treatment. For surface treatment, chemical conversion film is an effective method for corrosion protection treatment of magnesium alloy surface. By contacting the magnesium alloy substrate with a certain specific solution, a layer of well-adhered insoluble compound film is formed on the metal surface. This film can protect the substrate from water and other corrosive environments, and can improve the paint film adhesion of subsequent coating steps. A good performance passivation film can prevent harmful anions and oxidizing agents from flowing from the outside to the metal surface. The protective ability of the passivation film is closely related to the structure and composition of the film. For heat treatment, since the corrosion of the rare earth magnesium alloy is micro-electric corrosion between the second phase particles contained in the alloy and the alloy substrate, the content of the second phase particles in the alloy can be reduced by high temperature solid solution heat treatment, thereby reducing the corrosion rate. The rare earth magnesium alloy and the original alloy were subjected to hydrogen evolution corrosion comparison experiment by selecting 450℃ solid solution for 6 hours. The experimental results are shown in Figure 3(e). After 48 hours of corrosion, the corrosion rate of the magnesium alloy material subjected to high temperature 450℃ solid solution heat treatment was reduced by 57.81%. The alloy surface corrosion morphology before solid solution heat treatment is shown in Figure 3(f), and the alloy surface corrosion morphology after solid solution heat treatment is shown in Figure 3(g). Due to the reduction of the content of the second phase particles, the tendency of surface corrosion product cracking is greatly reduced. The alloy after 6 hours of solid solution heat treatment has a relatively complete corrosion product layer after corrosion, which effectively protects the alloy substrate and reduces the corrosion rate.

[0083] Example 2

[0084] The corrosion conditions of the test sample in this example are as follows: the corrosion medium temperature is 37℃, the corrosion medium is 11ml of NaCl solution with a mass concentration of 0.9wt.%, the three-point bending pressure is 8N, and the vibration frequency is 0Hz.

[0085] Under the above corrosion conditions, the extruded ZE62 magnesium alloy test sample was subjected to corrosion experiments for 40 minutes, 80 minutes, 120 minutes, 160 minutes, 200 minutes and 240 minutes, respectively. The obtained unit area mass loss versus corrosion time curve under constant stress state is shown in Fig. 4(a); the corrosion rate versus corrosion time curve under constant stress state is shown in Fig. 4(b). It can be concluded that the corrosion rate gradually decreases with the extension of corrosion time and tends to a certain value, indicating that the corrosion tends to saturation, and the corrosion continues at a constant corrosion rate, but the corrosion rate is sharply increased compared with the corrosion rate under stress-free state in Example 1. The surface corrosion morphology after corrosion for 240 minutes under constant stress state is shown in Fig. 4(c), and the cross-section corrosion morphology after corrosion for 240 minutes under constant stress state is shown in Fig. 4(d). The surface corrosion morphology presents extremely serious pitting morphology, and the spalling morphology appears when the corrosion product falls off, the cross-section corrosion surface depth is deep, and cracks extending from the surface to the inside and holes extending from the pitting initiation appear, which indicates that the reason for the sharp increase of the corrosion rate under constant stress state can be attributed to the fact that the surface cracks generated under constant stress state increase the contact area between the alloy sample matrix and the corrosion medium, thereby sharply increasing the corrosion rate.

[0086] Based on the experimental results and micro-morphology observation of Example 2, the following requirements are made for the corrosion resistance performance optimization of the rare earth magnesium alloy as a bone biological implant material: for surface treatment, since the properties of the surface film can determine the effect of corrosion control, the requirements for surface plating are that the surface film has a certain elasticity, deforms with the alloy at the same time when the alloy is bent, and the deformation difference between the two is less than 1%, so as to prevent large gaps from being generated between the film layer and the alloy matrix due to the deformation difference, thereby causing the film layer to lose the protection effect. For heat treatment, although high-temperature heat treatment can reduce the corrosion rate by reducing the content of the second phase particles, high-temperature solid solution will also cause the grain size to grow, which will affect the mechanical properties of the alloy. As a bone biological implant material, the alloy material needs to have mechanical properties similar to human bones when serving as a load-bearing part. The tensile curves of the original rare earth magnesium alloy and the rare earth magnesium alloy after 6 hours of high-temperature solid solution are shown in Figs. 4(e) and 4(f), respectively. After solid solution heat treatment, the yield limit, tensile strength and elongation of the alloy do not decrease significantly, which indicates that the rare earth magnesium alloy after 450℃ solid solution for 6 hours meets the service requirements in terms of corrosion resistance and mechanical properties.

[0087] Based on the experimental results and micro-morphology observation of Example 2, the following requirements are made for the service environment of the rare earth magnesium alloy as a bone biological implant material: in the static load-bearing part, the load value should be as low as possible to avoid bending, stretching and other deformations.

[0088] Example 3

[0089] The corrosion conditions of the test sample in this embodiment are as follows: the corrosion medium temperature is 37°C, the corrosion medium is 11 ml of a NaCl solution with a mass concentration of 0.9 wt.%, the three-point bending pressure is 8 N, and the vibration frequency is 1.0 Hz.

[0090] Under the above corrosion conditions, the test sample of the extruded ZE62 magnesium alloy was subjected to corrosion experiments for 40 minutes, 80 minutes, 120 minutes, 160 minutes, 200 minutes, and 240 minutes, respectively. The obtained curve of the mass loss per unit area of the test sample under alternating stress conditions versus the corrosion time is shown in FIG. 5(a), and the curve of the corrosion rate under alternating stress conditions versus the corrosion time is shown in FIG. 5(b). It can be concluded that the corrosion rate continuously increases with the extension of the corrosion time, indicating that the corrosion continuously accelerates at an uncontrollable corrosion rate, and the corrosion rate sharply increases compared with the corrosion rate under the stress-free condition in Example 1 and the corrosion rate under the constant stress condition in Example 2. The surface corrosion morphology after corrosion for 240 minutes under alternating stress conditions is shown in FIG. 5(c), and the cross-sectional corrosion morphology after corrosion for 240 minutes under alternating stress conditions is shown in FIG. 5(d). The surface corrosion morphology presents a very serious pitting morphology and a micro-crack morphology, accompanied by a peeling morphology caused by the falling of corrosion products. The cross-sectional corrosion surface is deep, and cracks extending from the surface to the interior and holes extending from the pitting to the interior are observed. At the same time, it can be observed that the alloy matrix also generates cracks extending to the interior, which indicates that the reason for the sharp increase in the corrosion rate under alternating stress conditions is that the surface cracks generated under alternating stress conditions increase the contact area between the alloy sample matrix and the corrosion medium, and the oscillating stress state causes the test sample to fatigue and generate more micro-cracks, which promotes the expansion of the micro-cracks, thereby sharply increasing the corrosion rate. The test sample corroded under this alternating stress condition increases the risk of fracture of the sample during service.

[0091] Based on the experimental results and micro-morphology observation of Example 3, the following requirements are made for the corrosion resistance of the rare earth magnesium alloy as a bone biological implant material: for surface treatment, when the surface film is locally damaged, the film can be quickly repaired, preventing the initiation and propagation of micro-cracks in the surface film due to mechanical fatigue. The rupture of the passivation film will cause serious corrosion. For heat treatment, although the mechanical properties of the alloy after high-temperature solid solution heat treatment do not decrease significantly, the rapid grain growth caused by excessive high-temperature holding time will reduce the grain boundary density, which will reduce the nucleation sites of corrosion products and make the product layer more easily damaged, promoting corrosion. The experimental data of hydrogen evolution corrosion of the original rare earth magnesium alloy, the rare earth magnesium alloy after high-temperature solid solution for 6 hours and the rare earth magnesium alloy after high-temperature solid solution for 10 hours for 48 hours are shown in Figure 5(e). After excessive high-temperature solid solution, the corrosion resistance of the alloy decreases. The micro-morphology of the surface of the rare earth magnesium alloy after corrosion after high-temperature solid solution for 10 hours is shown in Figure 5(f). The corrosion product layer on the surface has wide cracks, which loses the protective effect on the alloy substrate, and the corrosion rate does not decrease but increases.

[0092] Based on the experimental results and micro-morphology observation of Example 3, the following requirements are made for the service environment of the rare earth magnesium alloy as a bone biological implant material: in the dynamic load-bearing part, avoid vibration caused by movement. The vibration environment will cause fatigue cracks on the surface of the alloy, which will increase the corrosion rate. The fatigue cracks of the alloy in the alternating stress environment will rapidly expand, and the alloy will break during service.

[0093] The test device for the corrosion rate under different stress states described in the present application can be used to analyze the service time of different alloys in different corrosion media under different stress states by changing the corrosion medium type, concentration, environmental temperature, stress state stress size, vibration frequency, alloy type, and corrosion time. In addition, the surface and cross-sectional morphology after corrosion for a certain time under different stress states can be observed, which has a theoretical support for analyzing the corrosion mechanism of the difference in corrosion rate of the alloy under different stress states. This is conducive to further targeted optimization of the corrosion resistance of the alloy, and requirements can be made for the application conditions of the alloy during service. The device and test scheme have no excessive limitation on the corrosion conditions, the experiment is simple, efficient, and easy to popularize.

[0094] Specifically, the test technical scheme of the present application has the following advantages:

[0095] 1. The present application is based on the environmental clamp of the dynamic mechanical analyzer, which belongs to the expansion of the function of the dynamic mechanical analyzer. Therefore, the test means inherits the control accuracy of the dynamic mechanical analyzer for stress and frequency, and the test conclusion is more accurate and reliable.

[0096] 2. The experimental device designed by the present application has simple structure, is easy to build, and has low requirements on experimental environment and experimental technology.

[0097] 3. The present application has no strict restrictions on corrosion medium, corrosion time, corrosion temperature and alloy type, and can simulate corrosion conditions according to different actual service conditions to test corrosion rates under simulated actual service conditions.

[0098] 4. The test scheme of the present application has short duration and simple data processing.

[0099] 5. The test method for the corrosion rate of the rare earth magnesium alloy under different stress conditions as described above can be used to evaluate the corrosion resistance of the rare earth magnesium alloy under different stress states.

[0100] 6. The test method for the corrosion rate of the rare earth magnesium alloy under different stress conditions as described above can be used to require the service environment and state for the rare earth magnesium alloy as a bone biological endosseous implant.

Claims

1. A method for testing the corrosion rate of a rare earth magnesium alloy under different stress states, characterized by, The method comprises the following steps: S1, a plurality of rare earth magnesium alloy test samples of the same material and size are subjected to corrosion experiments under different stress states and for different time; S2, after each rare earth magnesium alloy test sample is subjected to the corrosion experiment for a preset time, the rare earth magnesium alloy test sample is cleaned to remove surface corrosion products; S3, the rare earth magnesium alloy test sample from which the surface corrosion products are removed is cleaned, dried and weighed; S4, the corrosion mass loss of the rare earth magnesium alloy test sample per unit area under different stress states is calculated to obtain the corrosion rate information of the rare earth magnesium alloy test sample; Before S1, the rare earth magnesium alloy test sample is pretreated: The rare earth magnesium alloy is subjected to wire cut electrical discharge machining to obtain rare earth magnesium alloy test samples of uniform size; the surface of the rare earth magnesium alloy test sample is polished to be smooth, and the oxide skin on the surface of the rare earth magnesium alloy test sample is removed to obtain a rare earth magnesium alloy test sample with a mirror surface and no scratches; then the rare earth magnesium alloy test sample is polished to remove scratches on the surface of the rare earth magnesium alloy test sample, and then the rare earth magnesium alloy test sample is cleaned and dried; The pretreatment is completed; The pretreated rare earth magnesium alloy test sample is used for S1; During the corrosion experiment, the stress states include a state of zero stress, a constant stress state and an alternating stress state, and each rare earth magnesium alloy test sample is subjected to one of the stress states; During the constant stress state and the alternating stress state, the rare earth magnesium alloy test sample is subjected to three-point bending test, and during the alternating stress state, the rare earth magnesium alloy test sample is subjected to vibration at a preset frequency at the midpoint; During the constant stress state and the alternating stress state, the applied pressure is 1-20 N; During the alternating stress state, the rare earth magnesium alloy test sample is subjected to vibration at a preset frequency of 0.1-5 Hz at the midpoint; During the corrosion experiment in S1, the corrosion solution is a NaCl solution with a mass fraction of 0.9%, and the temperature is 36.5-37.5℃; During the removal of the surface corrosion products in S2, the cleaned rare earth magnesium alloy test sample is immersed in a chromic acid solution to remove the surface corrosion products.

2. The method for testing the corrosion rate of a rare earth magnesium alloy under different stress states according to claim 1, characterized in that, In the pretreatment process, S2 and S3, the rare earth magnesium alloy test sample is washed with deionized water and anhydrous ethanol in sequence and is dried to obtain a dried rare earth magnesium alloy test sample.

3. The method for testing the corrosion rate of a rare earth magnesium alloy under different stress states according to claim 1, characterized in that, The concentration of the solute in the chromic acid solution is 200 g / L.

4. The method for testing the corrosion rate of a rare earth magnesium alloy under different stress states according to claim 1, characterized in that, The calculation formula of the corrosion mass loss of the rare earth magnesium alloy test sample per unit area under different stress states is as follows: Wherein, S is the surface area of the rare earth magnesium alloy test sample before corrosion; W Loss is the mass loss per unit area of the rare earth magnesium alloy test sample, and the unit is mg·cm -2 ; m1-m2 is the mass difference of the rare earth magnesium alloy test sample before and after the corrosion experiment.

5. The method for testing the corrosion rate of a rare earth magnesium alloy under different stress states according to claim 1, characterized in that, In S4, according to the corrosion mass loss of the rare earth magnesium alloy test sample per unit area under different stress states, a curve of the corrosion per unit area versus the corrosion time under different stress states is drawn to obtain the corrosion rate information of the rare earth magnesium alloy test sample.

6. Use of the rare-earth magnesium alloy according to any one of claims 1 to 5 for the testing of the corrosion rate under different stress states, characterized in that, The test method is used for: Optimization of the corrosion resistance of a rare earth magnesium alloy; Simulation of the corrosion behavior of the rare earth magnesium alloy under a complex stress environment in an actual service process and evaluation of the corrosion resistance of the rare earth magnesium alloy under different stress states.

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