Behaviour of oxide films in boric acid – lithium hydroxide solutions

In this paper we try to relate Stress Corrosion Cracking (SCC) initiation in stainless steels (SS) to the performance and nature of the anodic oxide film that develops during exposure to 300°C water containing boric acid and lithium hydroxide in an autoclave. In-situ electrochemical impedance spectra (EIS) have been obtained while exposing the samples. EIS reveals the behaviour of the oxide layer and the electrochemical processes that take place at the material-oxide-solution interface. After these exposures the oxide films were analyzed by X-ray photoelectron spectroscopy (XPS) to obtain the composition of the oxide film. The results showed that the oxide layers consisted of a mixed oxihydrate (iron-nickel based), dominating at the subsurface region and in larger depth, mainly consisted of chromium-iron oxides. The behaviour of the oxide film is being modelled using an extension to the point defect (PDM) / mixed conduction (MCM) series of models describing interface chemistry and vacancy and electron transport in the film.