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 center dot 2H(2)O at a lower potential of 0.4 V-SCE, but the major component of the film is changed to gamma-Fe2O3 at a higher potential of 1.1 V-SCE. At more than 1.5 V-SCE, 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). (C) 2018 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier Editora Ltda.