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"Defining the environment" is the most important step in predicting performance. It is also the step that is given least attention and for which there is little understanding by the technical community.
An approach to defining environments is illustrated in the "dot diagram" of Figure 2. Each of the numbers in brackets, [], identifies an explicit action that needs to be considered in the step of environmental definition. The end point of the process in Figure 2 is an input to a Location for Analysis, LAi in Figure 4. (LAi is used to indicate that there may be, and likely are, several, "i," "Locations for Analysis"). Before elaborating on the definition of environment there are two items of background to consider:
Chronology: Often, a steady state operating environment is assumed to be the design-basis environment for design of components and selection of materials. However, corrosion-related failures are often related to environments which occur during shutdown, startup and layup environments. Sometimes the causative environment is one picked up during manufacturing or storage. Each of these stages needs to be assessed and accounted for.
Surface: Environments of specific concern to degradation are those on the surface. The chemical composition of the bulk environment, while important, is often transformed at a surface by local heat transfer, local electrochemical cells, deposits from insoluble materials in the bulk environment, or by the growth of reaction product films. Environments on surfaces, but inside occluded geometries such as crevices or microbial pustules, may vary greatly from bulk environments.
Lifetime Prediction, Roger W. Staehle, Adjunct Professor, Department of Chemical Engineering and Materials Science, University of Minnesota, Staehle Consulting Co.