The Passive Film on Alloy 22

This report describes oxide (passive film) formation on Alloy 22 surfaces when aged in air (25-750 C) and in solutions (90-110 C) over times ranging from days to 5 years. Most zero-valent metals (and their alloys) are thermodynamically unstable on the earth's surface and in its upper crust. Most will therefore convert to oxides when exposed to a surficial or underground environment. Despite the presence of thermodynamic driving forces, metals and their alloys may persist over lengthy timescales, even under normal atmospheric oxidizing conditions. One reason for this is that as metal is converted to metal oxide, the oxide forms a film on the surface that limits diffusion of chemical components between the environment and the metal. The formation of surface oxide is integral to understanding corrosion rates and processes for many of the more ''resistant'' metals and alloys. This report describes the correlation between oxide composition and oxide stability for Alloy 22 under a range of relevant repository environments. In the case in which the oxide itself is thermodynamically stable, the growth of the oxide film is a self-limiting process (i.e., as the film thickens, the diffusion across it slows, and the metal oxidizes at an ever-diminishing rate). In the case where the oxide is not thermodynamically stable, it dissolves at the oxide--solution interface as the metal oxidizes at the metal--oxide interface. The system achieves a steady state with a particular oxide thickness when the oxide dissolution and the metal oxidation rates are balanced. Once sufficient metal has transferred to solution, the solution may become saturated with respect to the oxide, which is then thermodynamically stable. The driving force for dissolution at the oxide--solution interface then ceases, and the first case is obtained. In the case of a complex alloy such as Alloy 22 (Haynes International 1997), the development and behavior of the oxide layer is complicated by the fact that different metal components (e.g., Ni, Cr, Mo, W) form distinct oxides, each of which may be stable under somewhat different environmental conditions. For one set of conditions, the oxide layer may be dominated by one or more of these metals, for another, by a different set. Furthermore, the oxide ''layer'' itself may consist of sub-layers of different composition. The purpose of this report is to characterize the oxide layer obtained from Alloy 22 over a range of environmental conditions and to demonstrate that the oxide shows passive behavior. Section 2 provides background information and theoretical predictions describing the role of pH and applied potential in oxide formation and stability. It includes a review of pertinent data on similar alloys. Section 3 presents data characterizing the oxide over a range applied potential and pH. Section 4 evaluates the oxide obtained from Alloy 22 samples aged for time periods extending from one month to over five years. Section 5 presents data showing that the oxide growth rate is logarithmic in time. Section 6 discusses the stability of the oxide as determined by short-term electrochemical tests. Section 7 describes the oxide scale that forms due to thermal processing (solution annealing and in air). Taken together, the various sections in this report present an understanding of the oxide layer obtained using a variety of methodologies, techniques, and testing conditions. An Appendix provides additional information regarding surface analysis techniques and electrochemical testing