Effect of Alloying Elements on Crevice Corrosion Inhibition of Nickel-Chromium-Molybdenum-Tungsten Alloys Under Aggressive Conditions: An Electrochemical Study

The effects of Cr, Mo, and W on the crevice corrosion of a number of commercial Ni-Cr-Mo(W) alloys in 1.0 mol/L sodium chloride (NaCl) were studied using the potentiodynamic-galvanostatic-potentiodynamic technique to measure film breakdown and repassivation potentials as well as protection temperatures. As expected, Cr is the key element determining resistance to crevice initiation, but a substantial Mo alloy content is required to achieve maximum film stability, especially at temperatures >60°C. Mo, not Cr, is the major element controlling crevice propagation and repassivation. If the protection temperature is accepted as the key indicator of overall alloy resistance, then the resistance increases in the order: Alloy 625 (UNS N06625) < C-4 (UNS N06455) < C-276 (UNS N10276) < C-22 (UNS N06022) ~ Alloy 59 (UNS N06059) ~ C-2000 (UNS N06200) < Alloy 686 (UNS N06686). More generally, this order could be written: high Cr-low Mo < low Cr-high Mo < high Cr-high Mo < high Cr-high (Mo+W). The individual influences of Mo and W appear to be inseparable and, while adding W improved the resistance, adding the equivalent amount of Mo could achieve the same improvement.