Relationships Between Corrosion Behavior of AISI 304 Stainless Steel in High-Temperature Pure Water and Its Oxide Film Structures

Abstract

Effects of dissolved oxygen (DO) concentration on corrosion rates of AISI 304 stainless steel (SS) in high-temperature pure water were examined at temperatures between 200 and 320 C using impedance analysis techniques. A significant reduction in the corrosion rate >260 C and independence of the corrosion rate on the DO concentration at 320 C were observed as unique phenomena. To understand them, the oxide film structures were examined by scanning electron microscopy (SEM), secondary ion mass spectroscopy (SIMS), and electrical capacity measurements. The oxide films could be divided into outer and inner layers. The outer layer consists of two sublayers, a porous precipitation sublayer and a cracked inner sublayer which is hypothesized to be formed by the breakdown of the inner layer. The outer layer was characterized by a decrease of Cr content towards the outer surface of the oxide film. The inner layer was defined as a residual oxide in which no decrease of Cr content occurred. The inner oxide film layer was insulating and effectively suppressed corrosion above 260 C. At 320 C, the thickness of the inner oxide film layer became independent of the DO concentration and showed a good correspondence with the corrosion rate behavior. It was hypothesized that the cracked sublayer was formed by the breakdown of the inner oxide film layer resulting from the induced internal stress as the inner layer became thicker.