Influence of pH on Properties of Oxide Films Formed on Type 316L Stainless Steel, Alloy 600, and Alloy 690 in High-Temperature Aqueous Environments

Oxide films formed on Type 316L (UNS S31603) stainless steel (SS), Alloy 600 (UNS N06600), and Alloy 690 (UNS N06900) in high-temperature aqueous environments of pH 5, 8, and 10, which represent possible situations created in the secondary circuit of pressurized water nuclear reactors, were examined by scanning electron microscopy (SEM), capacitance measurements, and Auger electron spectroscopy (AES). The chemical composition of the films was strongly pH-dependent, with a marked enrichment in chromium in the films formed at pH 5. At pH 10, the films rather consisted of mixed iron-nickel oxides. The changes in chemical composition were accompanied by important changes in the electronic structure. Capacitance measurements using the Mott-Schottky approach showed that, depending on the pH of the corrosive medium, the films could be described as single layers of p-type or n-type semiconductivity. Moreover, heterojunctions (p-n) also were developed in the case of films formed at pH 8 on Type 316L SS and Alloy 690. Electronic band structure models, which describe the semiconducting character of the different films, are proposed.