The Role of Alloying Elements on the Crevice Corrosion Behavior of Ni-Cr-Mo Alloys

The roles of the alloying elements Mo, Cr, and W in resisting crevice corrosion of UNS N06022, UNS N06625, and UNS N10362 have been studied under galvanostatic conditions in 5 mol/L NaCl at 150°C. Corrosion damage patterns were investigated using surface analytical techniques such as scanning electron microscopy and optical imaging, and the corrosion products characterized by energy dispersive x-ray spectroscopy. While the Cr content of the alloy is critical in controlling initiation of crevice corrosion, the rate of activation (passive-to-active transition) is influenced by both the Cr and the Mo (and W) contents. The alloy’s Mo content also determines the distribution of corrosion damage within the crevice. In alloys with high Mo content, corrosion propagates laterally across the surface, while in alloys with low Mo content, it penetrates into the alloy. This can be attributed to the accumulation of molybdates (and tungstates), which stifle alloy dissolution at active sites. Thus, as the Mo content of the alloy increases in the order N06625 (9 wt% Mo) < N06022 (13 wt% Mo [3 wt% W]) < N10362 (22 wt% Mo), the depth of corrosion penetration decreases. In addition, once crevice corrosion initiates and the crevice acidifies, metal oxidation can also couple to proton reduction inside the crevice. The role of internal proton reduction in driving the crevice corrosion of these Ni alloys was found to be quite significant; greater than 50% of the corrosion damage is caused by proton reduction inside the crevice.