Crevice corrosion repassivation of Ni-Cr-Mo alloys by cooling

Abstract

Ni-Cr-Mo alloys offer outstanding corrosion resistance in hot chloride solutions. Crevice corrosion is the expected form of localized corrosion as pitting corrosion requires very harsh conditions to occur in Ni-Cr-Mo. Crevice corrosion may proceed only if specific conditions are met (e.g., temperature and potential should be above the critical values for each alloy). The classical approach for assessing the occurrence of localized corrosion in service conditions is by laboratory testing at a constant temperature. Thus, the outcome of a set of tests at different fixed temperatures is used for predicting the alloys’ susceptibility to corrosion, even though the temperature of the industrial process may change during operation. There are many applications of Ni-Cr-Mo alloys in processes with changing operating temperatures, such as heating followed by cooling. It is unclear what would happen when localized corrosion initiates at a higher temperature and then the temperature drops. There is an extensive database of critical potentials for Ni-Cr-Mo alloys in a wide range of environmental conditions which may be used to predict repassivation by cooling. The objective of this work was to verify if the literature data can be used with confidence in situations of fluctuating temperature. Repassivation of crevice corrosion by cooling was studied on selected Ni-Cr-Mo alloys by a five-step technique, in 0.1 mol/L to 10 mol/L chloride solutions, at cooling rates from 0.333 K/h to 33.3 K/h. Stable and relatively low current densities of crevice corrosion propagation were obtained in the potentiostatic step before cooling. Therefore, the current density drop in the final step was entirely due to the cooling process. In most cases, repassivation kinetics of formed crevices were not affected by the chloride concentration. Cooling rates of 3.33 K/h and 33.3 K/h led to similar repassivation temperatures while a cooling rate of 0.333 K/h led to higher repassivation temperatures than 3.33 K/h and 33.3 K/h. For the lowest cooling rate, the tested alloys repassivated from 61°C to 66°C, with the only exception of alloy UNS N06625 which repassivated at 45°C. Crevice corrosion which initiated at a higher temperature can proceed at lower temperatures than those predicted by isothermal data if cooling occurs at 3.33 K/h or 33.3 K/h, for most of the tested alloys. However, for cooling of 0.333 K/h, almost all the tested alloys are expected to repassivate at a temperature higher than that predicted by isothermal data. Comparison of the alloys behavior on repassivation by cooling at similar crevice corrosion propagation rates indicated that alloy UNS N10362 outperformed alloys UNS N06022, UNS N06059, UNS N07022, and UNS N06686, which, in turn, outperformed alloy UNS N06625.