Crevice Corrosion of Alloy 625 in Natural Seawater

An experimental study was conducted to determine the influence of temperature on crevice corrosion initiation for Alloy 625 (UNS N06625) in natural seawater. These tests showed that there was a critical potential-temperature-time relationship needed to initiate crevice corrosion. The potential necessary to cause crevice corrosion on Alloy 625 decreased (became less noble) when the temperature was increased from ambient to 40°C. The crevice initiation potential decreased from 300 mV for ambient temperature seawater to 100 mV for 40°C seawater. Crevice initiation potentials were essentially unchanged between 40°C and 65°C, while the time required to initiate crevice corrosion decreased as temperature increased. In a second aspect of this work, natural seawater exposure studies were conducted to determine if there is a mechanistic connection between ennoblement (the gradual elevation of corrosion potential that occurs during long-term continuous immersion in natural seawater) and crevice corrosion initiation. It was found that ennoblement produced corrosion potentials that exceed the crevice corrosion initiation potential in ambient temperature natural seawater. At 65°C, the open-circuit potential did not exceed the crevice initiation potential. However, temperature transients from ambient to elevated temperature created temporary conditions where the corrosion potential was substantially higher than the crevice initiation potential if ennoblement had previously occurred at ambient temperatures.