Effect of Heat Treatment on Pitting Corrosion of austenitic Cr-Ni-Mo steels in sodium chloride solution

The pitting corrosion resistance of Cr 17 Ni 12 Mo 2,5 type steel under potentiostatic polarization in a sodium chloride solution is adversely affected by previous annealing. Even though the reproducibility and accuracy of measurements are limited due to the statistical nature of pitting, the data obtained were systematically dependent on annealing temperature, time and surface roughness. The corrosion current, the number of pits or the mean area of pit opening and the corrosion rate within the pits were increased by previous annealing at 550 to 750 °C for 1–100 hrs. The highest corrosion rate estimated corresponded to heat treatments provoking severe sensitization to intergranular corrosion. The percentage area of corrosion pit openings and the estimated pit penetration rates were several times higher for as-machined than for polished surfaces. It can be assumed that pitting corrosion is little affected by the carbon content and that molybdenum depletion of grain-boundary zones is responsible for the reduced pitting resistance of annealed steels.