Pitting initiation and propagation of hypoeutectoid iron-based alloy with inclusions of martensite in chloride-containing nitrite solutions

Abstract

The pitting initiation and the pitting propagation of a hypoeutectoid iron-based alloy with inclusions of martensite in nitrite solutions containing chloride ions were studied by various electrochemical techniques. It was shown, by scanning reference electrode technique (SRET), that pitting occurred only on the martensite phase rather than hypoeutectoid areas. Anodic polarization behaviors of martensite electrode in nitrite solution with and without chloride ions are similar to those of hypoeutectoid electrode: both of martensite and hypoeutectoid electrodes were passive and the passive films could be broken by chloride ion. However, the open circuit potential of martensite electrode is more negative than that of hypoeutectoid electrode, and the passive current density of martensite electrode in the solution containing chloride ions was larger than that of hypoeutectod electrode. It was shown, by Mott-Schottky analysis, that the passive film of martensite electrode in the solution containing chloride ions had a higher donor concentration than that of hypoeutectoid electrode. There was a galvanic interaction between martensite and hypoeutectoid electrodes and it was this interaction that induced and accelerated the pitting corrosion on martensite area. Current and potential fluctuations, which reflected pitting initiation, were observed when martensite and hypoeutectoid electrodes were coupled together.