On the bi-modal long-term characteristic for metallic corrosion

The bi-modal characteristic of general or 'uniform' corrosion, or corrosion as measured by mass loss, as a function of exposure time now has been demonstrated extensively for various steels including low alloy, chromium and weathering steels, cast irons and, more recently also for aluminium alloys and for copper alloys. Also, it usually is independent of the environment. Further, it also occurs for the trend in maximum pitting as a function of time. This wide applicability demands explanation. Previously it was argued that the bi-modal character is the result of a change, at the metal-environment interface, from predominantly oxygen-rich corrosion conditions to predominantly locally anoxic corrosion conditions. It was argued also that pitting plays an important role in this process. Herein these concepts are extended from accepted theory for pitting under nominally oxic conditions that, within pits, become anoxic and causes the pitting process to become autocatalytic but eventually to lead to cessation of pit depth growth. There are two key outcomes. The first is the observation that pits grow episodically, that is, they grow in depth to a given limit, grow sideways and possibly amalgamate or coalesce, leaving plateaus of corroded surface in which new pitting can occur. The cycle time for this is faster under already anoxic conditions than earlier when it takes time for anoxic conditions to develop. The second outcome is that mass loss as a function of time is a function both of the pit depth and thus the pitting process and the increase in the number of pits with time. A considerable body of observational evidence is available to support these mechanisms. There also are well-developed theories that already explain the component mechanisms.