Effect of environmental variables and main alloying elements on the repassivation potential of Ni–Cr–Mo–(W) alloys 59 and 686

Chloride-induced crevice corrosion (E_R,CREV) of alloys UNS N06059 and UNS N06686 was studied at different temperatures in 0.1, 1 and 10 M chloride solutions. Crevice corrosion occurred several degrees below the reported critical crevice temperatures obtained through standard immersion tests. The repassivation potential of the tested alloys as a function of temperature and chloride concentration was given by E_R,CREV = (A + BT)log[Cl−] + CT + D for a range of environmental conditions. When temperature and chloride concentration increased, E_R,CREV showed a lesser dependence on the environmental variables. The repassivation potential of Ni–Cr–Mo–(W) alloys was described by a new proposed equation in terms of [Cl−], T, Cr, Mo and W, alloys in wt%. The dependence of E_R,CREV with the weight % of main alloying elements was 5–6 mV/%Cr, 17–18 mV/%Mo and ∼9 mV/%W, at 85 °C in chloride solutions. An optimal main alloying elements relationship was noted that maximizes the E_R,CREV value. The optimal alloy ratio would be 1:3.3:1.65 for wt%Cr, wt%Mo and wt%W, the same factors as in the PRE equation. The optimal alloying ratio would be independent of the alloy composition since it is not a function of the content of main elements.