Role of Hydrogen in Metal Oxidation-Implication to Irradiation Enhanced Corrosion of Ni-Based Alloys and Stainless Steels in High Temperature Water

Hydrogen plays various roles in metals or at metal-environment interfaces. Well known effects on metals are hydrogen embrittlement, hydrogen enhanced local plasticity, hydrogen enhanced strain-induced vacancy, hydrogen accelerated oxidation, hydrogen-induced creep, and their synergy. In this study, the potential roles of hydrogen in materials degradation are demonstrated and studied by two different tests. One is the high temperature oxidation of Ni-based alloy in various environments with hydrogen penetration, and the other is the effects of neutron flux/fluence on the oxidation kinetics and SCC of 316L and 316LN stainless steels, regarding a possible role of transmuted H from N. The results emphasize that the hydrogen either permeated into metals from surrounding environments, such as high temperature water or gaseous hydrogen, or generated in metals by nuclei transmutation, such as hydrogen transmuted from N atoms in metals, which can promote metal oxidation through multiple mechanisms. Apparently, the oxidation/corrosion phenomenon is a synergy of sub-mechanisms. For instance, dissolved hydrogen (DH) is usually believed to slow down the corrosion process for lowering the open circuit potential (OCP). However, H also facilitates the transport of the cations in oxide, thereby accelerating the corrosion process. In this bi-mechanism system, two different, contradictory mechanisms work and exist simultaneously. Therefore, whether the metallic materials are benefited or degraded by the H during its oxidation process depends on which sub-mechanism is dominant. Namely, hydrogen can play the role an oxidant in the metal and metal/oxide interface to pre-oxidize metal elements, such as Cr, Ni, and Fe, and possibly promote inward oxygen diffusion and the oxidation rate at the interface. Moreover, hydrogen may play a role as a reductant in oxides where existing oxides can be reduced. Then, the protective capability of oxides will be decreased to result in corrosion acceleration at the metal-oxide interface. These phenomena were observed in Ni-based alloy and possibly austenitic stainless steel containing N such as 316LN SS. This work demonstrates a part of the role of hydrogen on oxidation, and more extensive and systematic work is needed to delineate the role of hydrogen on oxidation with and without irradiation.