Puncture corrosion and crevice corrosion

Puncture corrosion and crevice corrosion are commonly encountered localized corrosion. After the point corrosion and crevice corrosion environments are determined, the alloy’s resistance to such localized corrosion depends on the composition of the alloy. Chromium and molybdenum play the most significant role, usually with surplus. The pitting resistance equivalent number ranks the corrosion resistance of the alloy. The calculation of PREN for iron-nickel-based and nickel-based corrosion-resistant alloys is different from that of stainless steel. Its formula is:
PREN = w( Cr) + 1. 5w( Mo + W + Nb) + 30w(N)
The greater the PREN value, the better the pitting and crevice corrosion resistance of the alloy.
Corrosion tests in the laboratory and in the actual environment are also effective methods for evaluating the alloy’s resistance to such corrosion. At present, the critical pitting temperature (CPT) and critical crevice corrosion temperature in acid chlorides and oxidizing acid chlorides are widely used in the evaluation. Corrosion tests in seawater and in pulp production process media and in the actual environment of flue gas desulphurization are more resistant to pitting and crevice corrosion. Regardless of the test method, the absolute value of the result may vary, but the order of corrosion resistance of the alloy does not change.