Cases in which carbon steel fixtures or similar are used to handle stainless steel, thereby intensifying corrosion conditions in certain areas, should be avoided

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It should be avoided to use carbon steel fixtures, etc., for handling stainless steel, as this can lead to intensified corrosion conditions in certain areas. Example 1: At a certain plant, the heating coil was covered with Type 304 stainless steel (with a coating thickness of 0.2–0.3 mm). It was used after being stored at the construction site for over 3 years. During installation, it was found that nearly 20% of the heating coils were broken; cracks appeared in the heating coils in many places. There are brownish-red rust spots on various parts of the coil surface, with cracks starting from these rust spots. The cause of corrosion is the contamination of the stainless steel coil surface with iron particles during processing, and these iron particles rust under humid conditions. Corrosion products are hygroscopic, thereby causing corrosion beneath the deposits. In the marine atmosphere at the construction site, the adsorption and concentration of chloride ions are accelerated, leading to stress corrosion cracking. Example 2: A 304 stainless steel pipeline in a factory leaked during pressure testing. Inspection revealed numerous longitudinal scratches on the surface of the stainless steel tube, with rust forming on those scratches ； Leakage starts at the rust spots. Upon removing the rust, cracks became visible. It turns out that carbon steel fixtures were used during the processing of this stainless steel pipe, resulting in wear and scratches on its surface as well as the embedding of iron particles. In a humid atmosphere, the scratched area rusts, leading to stress corrosion cracking and resulting in cracks in the pipeline. Analysis: These two cases illustrate that in the processing and manufacturing of stainless steel equipment, it is necessary to prevent surface damage and contamination, especially iron particle contamination. This is because the corrosion resistance of stainless steel comes from its ability to easily become passivated, forming a protective surface film. Many factors can cause the breakdown of the passivation film; for example, active ions in the environment (with chloride ions being the most common) can lead to localized damage of the passivation film, thereby triggering local corrosion such as pitting, crevice corrosion, and stress corrosion cracking. Measures: Keeping the stainless steel surface uniformly clean is beneficial for enhancing the stability of the passive film and preventing localized corrosion. Surface dirt, corrosion products, rust, and dust will lead to intensified corrosion conditions in localized areas, thereby reducing their corrosion resistance. Using wire brushes, carbon steel fixtures, etc. to treat stainless steel causes surface contamination with iron particles, which is particularly harmful and should be avoided.