Role of metastable pitting in crevices on crevice corrosion stabilization in Alloys 625 and 22

The metastable pitting behavior inside crevices of Alloy 625 (UNS N06625) and Alloy 22 (UNS N06022) was examined to obtain insight into differences in crevice corrosion susceptibility between Alloys 625 and 22. Metastable corrosion event rates recorded as current-time transients were found to increase with increasing applied potential and temperature for both alloys. However. the increase was more significant for Alloy 625 as compared to Alloy 22, and the cumulative number of events was greater. A strong correlation was obtained between the increase in event rates and the decrease in crevice stabilization potential with temperature. The alloying content in Alloy 22, traced to increased Mo and W, was rationalized to decrease the metastable event rate and, hence. the cumulative number of events after a given time. However. metastable peak heights, values for peak integrated charge. and I/r ratios. as well as metastable peak shapes associated with individual events, were not strongly affected by alloy type in the narrow range of Mo contents explored here. Observed differences in resistance to crevice corrosion stabilization are rationalized to depend on differences in the cumulative number of metastable events occurring sufficiently close in space and time to contribute to the development of a critical crevice chemistry at specific depths in a crevice. The properties of individual events did not have a significant effect. Stable crevice corrosion eventually occurred at the sites where a row of metastable pits formed at a critical distance from the crevice mouth. This row of pit sites focused acidication, which contributed to local depassivation.