Crevice Corrosion Repassivation of Ni Cr

Ni-Cr-Mo alloys offer outstanding corrosion resistance in hot chloride solutions. Crevice corrosion is the expected form of localized corrosion since pitting corrosion requires very harsh conditions to occur in Ni-Cr-Mo. Crevice corrosion may proceed only if specific conditions are met: temperature and potential, for instance, 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.