Mode Definition

The "mode" of corrosion refers to the morphology and avenues by which materials are corroded or degraded as illustrated in Figure 1. The term, "submode," refers to variations of modes where the same mode in the same material in somewhat the same environment occurs but involves different dependencies on the principal variables. A good illustration of two submodes is the SCC that occurs in alkaline and acidic environments; both are of the SCC mode but depend differently on principle variables.

"Mode Definition" means to define the dependency of the possible occurrence of modes relevant to the LAi in terms of the principal variables, i.e. What is the rate of the mode of interest in the range of the environmental definition? Mode definition also means to define the domain, e.g. pH and potential, within which the mode is most likely. In addition, it is quite likely that more than one mode or submode can occur at a given LAi, e.g. SCC and IGC may occur simultaneously.

Defining the mode of corrosion also includes considering stages of both initiation and propagation. Often, these two stages depend differently upon principal variables.

Lifetime Prediction, Roger W. Staehle, Adjunct Professor, Department of Chemical Engineering and Materials Science, University of Minnesota, Staehle Consulting Co.