Selecting Materials Of Construction

Designing equipment is a multidiscipline exercise involving mechanical engineers, materials-corrosion engineers, stressing experts, draftsmen, etc. The corrosion engineer has an important role in this team effort, namely, to ensure the materials specified will offer a corrosion resistance that is just adequate for all the environmental conditions likely to be encountered during normal operation of the equipment. A piece of equipment that prematurely fails by corrosion is as badly designed as one in which the materials have been overspecified. Unfortunately, all too often the functional requirements for a piece of equipment are analyzed in a somewhat arbitrary manner and all too often, the basic cost of the material tends to outweigh other equally important considerations (6).

Figure 3 shows the primary criteria that must be considered in the initial selection process.

• Corrosion Resistance. For any processing operation, there will be a range of materials that will offer a corrosion resistance that is adequate (or more than adequate) for a particular job. When considering corrosion resistance, the operational environment is the obvious one, but the other point must be whether the material will also offer corrosion resistance to the chemicals used for cleaning and sanitizing.

• Cost. Many of the materials originally considered will be eliminated on the grounds of their high cost. For example, there is no point in considering a highnickel alloy when a standard 300 series stainless steel at a lower cost will be perfectly satisfactory.

Corrosion U)St resistance i Availability \ ^ /

Maintenance —Material -*— Strength

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