Why can’t solution tests to screen different metal materials be conducted in the same container?

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Example: To remove chlorides from a strong acid mixture containing some residual chlorine and a large amount of salts, it is necessary to heat the solution in a container to 80–90°C. Due to the high corrosivity of the medium, glass-lined steel containers can only be used for 1–2 years, and it is even more difficult to choose materials for the heating coils. To this end, specimen pieces of several alloy materials were immersed in the production solution in the laboratory to conduct material selection tests. The selected materials include nickel, Inconel, Hastelloy, stainless steel, and titanium. The results show that only titanium is corrosion-resistant. However, the heating coil made of titanium suffered corrosion damage within two days, with a corrosion rate as high as 38 mm/year. The difference between the test results and the actual corrosion behavior of titanium is really that large! Comment: According to the description, the testing was carried out by placing test pieces of several materials in the same container. This approach is clearly incorrect. Because interactions may occur between different materials, leading to incorrect results. In this case, the Fe2+ ions generated by the corrosion of the iron-based alloy specimen located near the titanium specimen are oxidized to Fe3+ ions in this halide solution, and Fe3+ ions act as an extremely effective corrosion inhibitor for titanium. Therefore, a basic principle of corrosion testing is that only test pieces of one material should be placed in a test container, and shielding between the test pieces must be avoided to ensure full contact with the solution. Gaps should be avoided between the test piece and the container, as well as between the test piece and the support frame. No metal brackets are used. In short, it is necessary to avoid introducing corrosion factors that are difficult to estimate.