Stainless steel pitting, causes and prevention methods

1、 The concept of pitting corrosion in stainless steel

As mentioned earlier, the corrosion resistance of stainless steel is due to the formation of an invisible oxide film on the surface of the steel, making it appear dull. The formation of this passivation film is due to the reaction of steel with oxygen when exposed to the atmosphere, or the result of contact with other oxygen-containing environments. If the passivation film is damaged, stainless steel will continue to corrode. In many cases, the passivation film is only damaged locally on the metal surface, and the effect of corrosion is to form small holes or pits, resulting in irregular distribution of small pit like corrosion on the material surface.

2、 Factors Causing Pitting Corrosion of Stainless Steel

The occurrence of pitting corrosion is likely due to the presence of chloride ions that combine with the depolarizer. The pitting corrosion of passive metals such as stainless steel is often caused by the local damage of certain corrosive anions to the passivation film. Protecting the passive state with high corrosion resistance usually requires an oxidizing environment, but this is also a condition for pitting corrosion to occur. The medium that produces 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 such as H2O2 and O2 in CI -, Br -, l -, and C104- solutions.

The pitting rate increases with increasing temperature. For example, in a solution with a concentration of 4% to 10% sodium chloride, the weight loss caused by pitting corrosion at 90 ℃ is significant; For more dilute solutions, it occurs at higher temperatures.

3、 Methods for preventing pitting corrosion of stainless steel

1. Avoid concentration of halogen ions.

2. Ensure the uniformity of oxygen or oxidizing solutions, stir the solution and avoid it being static.

3. Increase the concentration of oxygen or remove oxygen.

4. Increase the pH value. Compared to neutral or acidic chlorides, significantly alkaline chloride solutions cause less or no pitting corrosion (hydrogen and oxygen ions act as corrosion inhibitors).

5. Work at the lowest possible temperature.

6. Add a passivating agent to a corrosive medium. Low concentrations of nitrate or chromate are effective in many media (inhibiting ions from preferentially adsorbing on the metal surface, thus preventing chloride ions from adsorbing and causing corrosion).

7. Adopting cathodic corrosion protection. There is evidence to suggest that stainless steel with cathodic protection combined with low-carbon steel, aluminum, or zinc will not cause pitting corrosion in seawater.

Austenitic stainless steel with a molybdenum content of 2% to 4% has good resistance to pitting corrosion. The use of molybdenum containing austenitic stainless steel can significantly reduce pitting or general corrosion, with corrosive media such as sodium chloride solution, seawater, sulfite, sulfuric acid, phosphoric acid, and formic acid.