Abstract: The electrochemical behavior of X20 steel after 230 000 hours service in power plant, and the correlation between microstructure degradation and electrochemical behavior were investigated by electrochemical analysis and micromorphology observation. The results show that the carbides distributed in prior austenite grain boundary and martensite lath boundary were severely coarsened after long-term service. The precipitation and growth of chromium-rich carbides at the boundaries may lead to the reduction of the resistance to pitting nucleation and repassivation of the steel in chloride environment. The microstructure observation show that the pits on the surface of X20 steel were easy to be initiated at the original austenite grain boundary. This was because the stable carbide precipitated near the grain boundary separated the chromium element from the grain boundary, and local austenite grain boundary and martensite lath boundary formed a localized chromium-depleted zone.