The efficiency of sodium salts of sulfate, nitrate, and molybdate as crevice corrosion inhibitors was evaluated for nickel-based alloys of varying chemical composition. A set of commercial high-temperature and corrosion-resistant alloys, with Pitting Resistance Equivalent (PRE) values ranging from 16 to 88, was tested. The crevice corrosion repassivation potential (E R,CREV ) was determined in NaCl solutions of 0.01, 0.1, and 1 mol/L, at various inhibitor-to-chloride molar ratios (R) and temperatures. The critical ratio (R CRIT ), above which crevice corrosion was fully inhibited, was used to compare inhibitor efficiency. R CRIT values depended on alloy composition, chloride concentration, and temperature. Efficiency generally decreased with increasing temperature and chloride content, though some cases showed little sensitivity. R CRIT varied nonlinearly with PRE in an inhibitor-dependent manner. For nitrate and sulfate, R CRIT decreased with PRE at low values, plateaued, and rose again at high PRE. Molybdate efficiency increased steadily with PRE. Nitrate was the most efficient inhibitor overall, except for the highest-PRE alloy (HYBRID-BC1, PRE = 88), for which molybdate outperformed the others. Sulfate was the least efficient. The observed trends are consistent with proposed inhibition mechanisms: competitive electromigration and suppression of crevice corrosion stabilization for sulfate, pH increase via nitrate reduction, and molybdate reduction followed by precipitation of molybdic acid or polymeric species. The increased efficiency of molybdate at high PRE was attributed to enhanced reduction and precipitation kinetics under the more acidic conditions typical of crevice solutions in highly alloyed materials. • Inhibitor efficiency varies with alloy composition, chloride, and temperature • Crevice corrosion inhibition trends differ by alloy type and inhibitor species • Molybdate is most effective for highly alloyed Ni-based materials (high PRE) • Inhibitor efficiency generally ranks: sulfate < molybdate < nitrate
González et al. (Sun,) studied this question.