Causes and prevention methods of stainless steel pitting corrosion

Views: 195     Author: China CHANGWEN Cookware      Publish Time: November 14, 2023      Origin:https://www.cookwarecwpee.com

We all know that the excellent corrosion resistance of stainless steel is due to the formation of an invisible oxide film on the surface of the steel, so it is passive. However, this does not mean that stainless steel is immune to corrosion.
Factors causing pitting corrosion of stainless steel
Pitting corrosion is most likely due to the presence of chloride ions combined with the depolarizing agent. Pitting corrosion of passive metals such as stainless steel is often caused by the local destruction of the passivation film by certain corrosive anions. Protecting a passivated state with high corrosion resistance usually requires an oxidizing environment, but this is also the condition for pitting corrosion to occur. The medium that causes pitting corrosion is the presence of heavy metal ions such as Fe3+, Cu2+, Hg2+ or chloride solutions containing Na+, Ca2+ alkali and alkaline earth metal ions containing H2O2, O2, etc. in Cl-, Br-, I-, ClO4- solutions.
The pitting corrosion rate increases with increasing temperature. For example, in a solution with a concentration of 4% to 10% sodium chloride, the maximum weight loss caused by pitting corrosion is reached at 90°C; for more dilute solutions, the maximum value occurs at a higher temperature.
Methods to prevent pitting corrosion of stainless steel
⒈Avoid the concentration of halogen ions;
⒉ Ensure the uniformity of the oxygen or oxidizing solution, stir the solution and avoid small areas where the liquid is not flowing;
⒊ Either increase the concentration of oxygen, or remove oxygen;
⒋Increase PH value. Clearly alkaline chloride solutions cause less or no pitting corrosion than neutral or acidic chlorides (hydroxide ions act as corrosion inhibitors);
⒌Work at the lowest possible temperature;
⒍Add passivating agent to corrosive media. Low concentrations of nitrate or chromate are effective in many media (inhibiting ions from preferentially adsorbing on metal surfaces, thus preventing chloride ions from adsorbing and causing corrosion);
⒎Adopt cathodic anti-corrosion. There is evidence that stainless steel cathodically protected with mild steel, aluminum or zinc will not cause pitting corrosion in seawater.
Austenitic stainless steel containing 2% to 4% molybdenum has good pitting corrosion resistance. The use of molybdenum-containing austenitic stainless steels can significantly reduce pitting or general corrosion from corrosive media such as sodium chloride solutions, seawater, sulfurous acid, sulfuric acid, phosphoric acid and formic acid.

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