Appearance Fabricability

Figure 3. Materials selection criteria.

• Availability. Availability is a less obvious feature of the material selection process. Many steel producers will require a minimum order of, say, 3 tons for a nonstandard material. Clearly, the equipment manufacturer is not going to buy this large quantity when the job that is to be done may only require the use of one ton of material.

• Strength. Strength is a factor that is taken into account at the design stage but, as with all the others, cannot be considered in isolation. For example, many of the new stronger stainless steels, although more expensive on a ton-for-ton basis than conventional stainless steels, are less expensive when considered on a strength/cost ratio.

• Fabricability. There is little point in considering materials that are either unweldable (or unfabricable) or can be welded only under conditions more akin to a surgical operating theater than a general engineering fabrication shop.

• Appearance. Appearance may or may not be an important requirement. Equipment located outside must be resistant to environmental weathering and therefore may require the application of protective sheathing, which could double the basic material cost.

• Maintenance. Is the equipment to be essentially maintenance-free or is some maintenance, such as periodic repainting, tolerable? How long will the equipment operate without the need for major servicing?

When all these interrelated criteria have been considered, the long list of possible starters will have been reduced to maybe one or two. Also, somewhere through the selection process some of those materials initially rejected because, for example, of their high cost, may have to be reconsidered because of other factors.

TYPES OF CORROSION Defining Corrosion

Before embarking on a discussion of the various forms of corrosion, it is worthwhile considering exactly what corrosion is. There are several definitions of corrosion. For example, Fontant (7) defines it as extractive metallurgy in reverse using the diagram, shown in Figure 4, to illustrate the point.

A more general and descriptive definition is "it is the deterioration or destruction of a material through interaction with its environment." This covers all materials of construction including rubber and plastics as well as metal. However, the primary object of this article is to deal with corrosion of metals, in particular stainless steels, and how this corrosion can be classified.

There are two basic forms of corrosion—wet corrosion and dry corrosion. Dry corrosion is concerned with the oxidation of metals at high temperature and clearly outside the scope of this text. Wet corrosion occurs in aqueous solutions or in the presence of electrolytes and is an electro-

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