The results of corrosion weight loss and corrosion rate of coupon samples after salt spray test at different times are shown in Figure 1. It can be seen that with the extension of salt spray corrosion time, the corrosion of samples first increases and then decreases. In the preliminary stage of 1-4h, the corrosion characteristics of samples gradually change from pitting corrosion to full-scale corrosion, and the corrosion degree continues to deepen, There is a large exposed area on the surface, which can make the corrosion products grow rapidly. From the 8h sample, the corrosion products gradually cover the whole surface and become dense, covering the metal matrix tightly. The corrosion can only be carried out in the holes not covered by rust, and the rust will gradually block the holes, increasing the area of rust layer and making it more dense, The exposed area of the metal matrix is further reduced, and the corrosion rate is also decreasing.
Cut one end face of the impact abrasive wear sample after the salt spray test, and observe the typical rust on the end face surface with an optical microscope, so as to more clearly analyze the change of the volume and morphology of the rust on the sample surface with the salt spray time. The results are shown in Fig. 2. In Fig. 2a, there are many parallel scratches on the surface of the sample after 1h salt spray corrosion. These scratches are worn on the surface of the sample with sandpaper before the salt spray test. The metal matrix can be identified by scratches. The corrosion morphology is mainly pitting corrosion, which is a typical morphology at the beginning of corrosion, and the color of rust has not completely changed to red. In 2B, a large number of oxide balls and rust spots appeared in the sample after 2H salt spray corrosion, and the rust was loose and discrete. It can be seen from Fig. 2C that after 4H salt spray corrosion, the rust on the surface of the sample changes obviously, and the rust in some areas becomes dense and completely connected, while the rust in other areas is still loose and discrete point or block. As shown in Figure 2D, after 8h salt spray corrosion, the area of rust connected together further increases, and the color is closer to dark red. It is a typical color of iron oxide, and its shape is also more dense. The dense rust layer covers the metal matrix, resulting in the decline of the corrosion rate of the sample. When the salt spray corrosion time reaches 24h, the typical rust area reaches the maximum, the thickness also increases obviously, the color is dark red, and the metal matrix in the field of view is basically covered by the rust layer, resulting in the decline of the corrosion rate of the sample.
The surface rust of salt spray test samples at different times was observed by SEM, and the element distribution was determined by surface scanning. The results are shown in Fig. 3. In general, the SEM results are consistent with the optical microscope results. The sample of LH in 3a is a typical pitting pit, the amount of rust is small, and the distribution of oxygen element is basically unchanged. 3b and C it can be seen that when the salt spray time is 2H and 4h, the distribution of oxygen changes obviously, there are typical rust, and its edge is irregular sawtooth. 3D and e show that when the salt spray time reaches more than 8h, large dense rust layers begin to cover the sample, and the metal matrix with scratches has been invisible.
In order to better characterize the surface rust morphology of the salt spray test sample, the typical rust morphology of the sample at different salt spray times is shown in Fig. 4 with a larger multiple of SEM photos, and the typical morphology of the 1H salt spray sample is the concave pitting pit shown in FIG. 4A. As shown in Fig. 4b, at 2h, the sample surface began to appear convex corrosion products with a diameter of about 100pm. Fig. 4C shows that needle flocculent corrosion products begin to appear on the surface of 4H salt spray sample, which are typical products before and in the middle of corrosion. These products are loose and prone to cracks. It can be seen from Fig. 4D that the corrosion products on the surface of 8h salt spray sample change from needle floc to block and become more dense. When the salt spray time reaches 24h, large clusters of corrosion products can be observed in 4E. These large clusters further increase the thickness of the rust layer and cover the metal matrix more tightly.
The impact abrasive wear test is carried out on the samples corroded by salt spray, and the SEM photos of the worn surface are shown in Fig. 5. In Fig. 5a, the wear surface of 1H salt spray corrosion sample after impact abrasive wear test is similar to that of non corroded sample in the previous section, and its wear mechanism is still characterized by furrow, strain fatigue and embedded abrasive particles. There are a large number of corrosion products on the surface of 2H salt spray corrosion sample, which also has a great impact on the impact abrasive wear behavior of the material. These corrosion products will be destroyed in the early stage of impact abrasive wear, fly away from the friction area or be pressed into the matrix. The morphology of furrow and embedded corrosion at this time is pointed out in Fig. 5b, and the wear mechanism mentioned in the above section, The furrow on the wear surface of the material with good wear resistance is long and shallow, on the contrary, in the material with poor wear resistance, the furrow is short and deep. From the morphology of the furrow in Fig. 5B to e, it can be seen that with the extension of salt spray corrosion time, the furrow becomes shorter and deeper, and the wear weight loss of the corresponding sample is more. This is because the salt spray corrosion products damage the surface of the bain horse multiphase wear-resistant cast steel, The strength of the metal matrix is reduced, so that the metal matrix is easier to be pushed and cut.
The morphology of the wear sub surface layer formed by the salt spray corrosion sample after the impact abrasive wear test is shown in Fig. 6. By observing the sub surface layer of the impact abrasive wear test, the deformation and work hardening behavior of the experimental steel in the impact abrasive wear test can be analyzed. From figures 6a to C, it is not difficult to see that there is an obvious deformation area between the wear surface and the substrate on the wear sub surface of the sample corroded by salt spray for 0-2h, and its width is 18pm, 25pm and 30pm respectively. The deformation is caused by the deformation and hardening of the surface of the metal substrate due to the impact. The deformation area of the sample without salt spray corrosion is the smallest, because the wear surface of the sample is the cleanest, the strength of the metal matrix is the best, the absorption rate of the sample for impact energy is the highest, and the deformation can be fully completed in a small area. With the increase of salt spray corrosion time, the corrosion products on the wear surface of the sample increase and the strength of the metal matrix decreases, The impact energy will also be dispersed by corrosion products, so the deformation zone will gradually increase. From figures 6D to F, it can be seen that when the salt spray corrosion is more than 8h, the deformation area can not be obviously observed, and only a small amount of deformation is distributed on the edge of embedded corrosion. This is because with the further increase of corrosion products, the impact energy is dispersed and absorbed more obviously, and the metal matrix is not only difficult to be effectively strengthened, but also the strength is reduced by corrosion, At this time, the worn surface is often accompanied by catastrophic delamination and spalling failure.
Through the analysis of the wear behavior of the samples corroded by salt spray in the impact abrasive wear test, it can be found that there is a close relationship between corrosion failure and wear failure. They affect each other and lead to more serious weight loss of bainitic Martian multiphase cast steel.
Therefore, a formula is tried to estimate the wear weight loss of bainitic Martian multiphase wear-resistant cast steel under the synergistic action of salt spray corrosion and impact abrasive wear:
Where, f is the wear weight loss of the sample (g), t is the corrosion and wear time of the sample (H), and m, N and P are the correction factors. By calculating the salt spray corrosion and impact abrasive wear data of bainitic Martian multiphase wear-resistant cast steel, the value of positive factor is obtained, and it is obtained that FF = 0.0611n / + 0.077 / + 0.0722.