First, the concept of stainless steel pitting
As mentioned earlier, the excellent corrosion resistance of stainless steel is due to the formation of an invisible oxide film on the surface of the steel, rendering it passive. The passivation film is formed as a result of the reaction of the steel with oxygen when exposed to the atmosphere, or as a result of contact with other oxygenated environments. If the passivation film is destroyed, the stainless steel will continue to erode. In many cases, the passivation film is only destroyed in the local area of the metal surface. The effect of the corrosion is to form fine holes or pits, and irregular pitted corrosion occurs on the surface of the material.
Second, cause stainless steel pitting factors. duplex steel flange
Pitting corrosion is likely to be the presence of chloride ions combined with depolarization agents. The pitting of stainless steel and other passive metals often results from the local destruction of the passivation film by certain aggressive anions. Protecting passives with high corrosion resistance usually requires an oxidizing environment, but this is exactly the condition for pitting corrosion. The pitting-causing medium is a chloride solution of heavy metal ions such as Fe3+, Cu2+, Hg2+, or Na+, Ca2+ alkali, and alkaline earth metal ions containing H2O2, O2, etc. in the 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 at 90°C is large; for a more dilute solution, the superlarge value appears at a higher temperature. stainless steel pipe fittings
Third, prevent stainless steel pitting method
1. Avoid concentrated halogen ions.
2. Ensure the homogeneity of the oxygen or oxidizing solution, stir the solution and avoid the solution being static.
3, increase the oxygen concentration, or remove oxygen.
4, increase the PH value. Compared to neutral or acidic chlorides, the significantly alkaline chloride solution causes less pitting, or none at all (the hydroxide ions function as an anti-corrosion agent).
5. Work at the lowest possible temperature.stainless steel elbow
6. Add passivating agent in corrosive medium. Low concentrations of nitrates or chromates are effective in many media (the inhibiting ions preferentially adsorb on the metal surface, thus preventing the chloride ions from adsorbing and causing corrosion).
7, using cathode anti-corrosion. There is evidence that stainless steels cathodically protected with mild steel, aluminum or zinc do not cause pitting in seawater.
Molybdenum 2% to 4% austenitic stainless steel has good pitting resistance. The use of austenitic stainless steels containing molybdenum can significantly reduce pitting or general corrosion, corrosive media such as sodium chloride solution, seawater, sulfurous acid, sulfuric acid, phosphoric acid and formic acid.