Corrosion behavior of (Fe,Ni)–Gd intermetallic compounds in FeNi-based neutron-absorbing steels

Abstract

In this paper, microstructures of Fe–12Ni–1.4Mn–(0–4 wt%)Gd steels were quantitatively analyzed, and corrosion behavior of the alloys in solutions containing H3BO3 and Cl– was investigated through potentiodynamic polarization tests and corrosion morphology observation. The addition of Gd to the Fe–12Ni–1.4Mn alloy formed secondary phases, mainly (Fe,Ni)17Gd2 and a small amount of (Ni,Fe)4Gd. As the Gd content increased, the total amount of the secondary phase linearly increased. In a NaCl solution, the pitting corrosion resistance of the alloys was reduced by the addition of Gd. In a HCl solution, both uniform and pitting corrosion occurred, and the resistance to both types of corrosion decreased as the Gd content increased. In the two solutions, the corrosion susceptibility increased in the order of matrix < (Fe,Ni)17Gd2 < (Ni,Fe)4Gd. The high susceptibility to corrosion of the Gd-rich phase is due to the fact that Gd is electrochemically active compared to the other constituent elements of the matrix and also has a low ability to form passive films. Also, in a H3BO3 solution, both uniform and pitting corrosion were observed in the alloys. Although Gd is electrochemically active compared to Fe–Ni–Mn matrix, uniform corrosion resistance was improved by addition of Gd and the Gd-rich phase behaved as a noble phase only in the H3BO3 solution. This is considered to be because H3BO3 selectively adsorbs to the Gd-rich phase and protects it from dissolution. However, because the protective ability of the H3BO3 adsorbed layer was weaker than that of the passive film of the matrix, pitting occurred at the Gd-rich phase.