Electrochemical Prediction and Control of Localized Corrosion and SCC of Stainless Steels

The conditions for prevention and control of localized corrosion (pitting, crevice, and SCC) of AISI⁽¹⁾ 304, 316 stainless steels (SSs), and superaustenitic 20Cr-25Ni-4.5Mo-1.5Cu SSs in chloride-containing waters have been defined using an experimental electrode potential E vs NaCl concentration diagrams. The existence has been established of a range of potentials, within which localized corrosion propagation is stopped by the deactivation mechanism, while the free surface of the SS remains passive; the technique driving the potential within the aforementioned range can be termed controlled cathodic protection. The effects of alloying elements Ni, Cu, Mo, and Cr on the behavior of localized corrosion have been exploited, especially with regard to protection conditions. The validity of the diagrams has been checked using long-term tests conducted under controlled conditions (in a laboratory loop and in natural seawater) on precorroded and non-precorroded specimens provided with crevice geometry. The transgranular stress corrosion cracking (TGSCC) in NaCl solution has been studied by using slow strain rate tests (SSRTs) performed either at the free corrosion potential or at an imposed potential and explained as the evolution of pitting corrosion under critical potential, temperature, NaCl concentration, pH, and strain rate. The controlled cathodic protection was shown to also prevent stress corrosion cracking (SCC).