Analysis on surface film formed on high-strength carbon steels in acidic phosphate solution and its relationship with localized corrosion in a 3.5% NaCl solution

The nature of a passive film formed on high-strength carbon steel and degradation of its passivity in acidic phosphate solutions were characterized by X-ray photoelectron spectroscopy and electrochemical polarization measurements. This study reveals that the steels show typical passivation and localized corrosion behaviors. The passivating film is composed mainly of FePO4·2H2O at a lower potential of 0.4 VSCE, but the major component of the film is changed to γ-Fe2O3 at a higher potential of 1.1 VSCE. At more than 1.5 VSCE, a number of shallow pores and/or pits were observed primarily at coarse-sized second phase particles. The four-point bending test suggested that the pre-existed pits/pores act as stress intensifiers under the subsequently applied tensile loading condition, resulting in higher anodic dissolution rates and lower resistance to stress corrosion cracking (SCC). Keywords: Carbon steels, Corrosion, Surface film, Phosphate solution