Crevice Corrosion Effect on Alloy 625 in Untreated Seawater Environment

Abstract

The main objective of this paper is to present the impact of crevice corrosion not only on ordinary metals but also on highly Corrosion Resistance Alloys. The paper highlights the failure of Inconel 625 in sea water service at an elevated temperature, in addition to the recommendation for alternative metals for such service conditions. In order to examine the influence of temperature on crevice corrosion specifically for Alloy 625 (UNS N06625) in highly chlorinated environment such as untreated seawater, an intensive study was conducted with consideration of service corrosively, material composition, operating conditions, etc. The main driver of this study was the sever crevice corrosion pitting founded at PTFE, Polytetrafluoroethylene, coated Inconel gasket seating area. The failure occurred at the inlet gas coolers containing hydrocarbons in the shell side, and sea water on the tube side for one of the new offshore facilities. The incident resulted in a leakage of the sea water at flange joints for both ends of the channel and shell side. Although Inconel 625 is the world class Corrosion Resistant Alloy (CRA) which offers superior protection from corrosion and eliminates the need for expensive maintenance and repairs, it repeatedly failed to serve as expected. The outcome of the investigation showed a direct relationship between the potential of crevice corrosion and service temperature in the system where Alloy 625 is being used. The study results showed that crevice corrosion can aggressively attack the surfaces of Alloy 625 when the service temperature exceeds 40°C. As an action and a way forward to resolve this problem, a higher Molybdenum (Mo) containing alloy shall be utilized for flange face repairs and for the gaskets, accordingly Alloy 686 "Hastalloy" is the recommended alloy for this application. Unlike Inconel 625 CCT, Hastalloy is known for its excellent resistance to crevice corrosion and its Critical Crevice Temperature up to 85°C.