Corrosion factors of 304 stainless steel cold-rolled plates for welded pipes
The corrosion resistance of stainless steel is mainly determined by the protective passivation film. However, due to uneven defects, inclusions and solutes in steel types, when the medium contains certain active anions (such as CI~ ions), these anions are first It is adsorbed on the metal surface at certain points, thereby destroying the passivation film on the surface of the stainless steel. Once this layer of passivation is damaged and lacks the ability to self-oxidize, corrosion will occur on the stainless steel surface. This is because the base metal is easily exposed in the fast-sinking areas of the metal surface, making it in an activated state, and the passivation film is in a passive state. This forms an active-passive corrosion battery, so the corrosion develops deeper and the metal surface It is quickly corroded into small holes, which is called pitting corrosion.
2. CI~ ions can cause pitting corrosion
Stainless steel can withstand strong nitric acid, but cannot withstand weaker hydrochloric acid and dilute sulfuric acid. C ions in hydrochloric acid and sulfur in sulfuric acid are both accelerators for stainless steel corrosion. Especially after C ions are adsorbed by the metal surface, they destroy the density of the stainless steel surface protective film, which can significantly accelerate corrosion and cause the stainless steel surface to lose its protective effect. Causes pitting corrosion on the surface of stainless steel. It can be seen from the energy spectrum curve of the sample that the rusted areas of the sample contain C ions, that is, the C ions in the medium cause corrosion on the surface of the steel plate. 4.3 Effect of chemical composition on stainless steel passivation film The corrosion resistance of stainless steel is mainly caused by the production of a chromium-rich oxide film with a thickness of more than 10A. The most effective way to increase the corrosion resistance of stainless steel is to increase the content of elements chromium, molybdenum and nickel under certain conditions. Chromium increases the pitting corrosion resistance of steel. Increasing the chromium content in stainless steel can increase the chromium content of the oxide film and increase the chemical stability of the oxide film. The increase in nickel content can avoid the reduction of the oxide film due to the enrichment of nickel under the film, thereby improving the stability of the film. The increase in the content is particularly beneficial to improving the acid resistance of stainless steel. The oxidation ability of aluminum is between that of chromium and shen.
3. The influence of stainless steel surface finish on corrosion resistance
The surface finish of stainless steel has a certain influence on the corrosion resistance, and the improvement of the smoothness can improve the corrosion resistance. When the surface of stainless steel is rough, it is easy for scale to accumulate on the surface and pitting corrosion is likely to occur.
4. Measures and suggestions for inhibiting stainless steel night surface corrosion
Since the medium contains CI~ ions, CI~ ions cause slight rust on the surface of the 304 cold-rolled plate. Rust is pitting corrosion caused by CI~ ions. In order to inhibit stainless steel surface corrosion, it is recommended to take the following measures:
1) Improve the chemical composition of 304 stainless steel, increase the chromium and nickel content under certain conditions, increase the corrosion resistance of stainless steel and improve the passivation film structure.
2) Improve the surface finish of 304 stainless steel cold-rolled plate. The improvement of smoothness can improve the corrosion resistance.
3) In order to prevent the stainless steel surface from being stained by scale or rust and causing pitting corrosion, the user must ensure that the stainless steel surface is clean and dry, and provide a good environment to avoid corrosion on the steel plate surface.