Guidelines on Material Selection

Materials should be selected based on functional suitability and abihty to maintain function safely at a reasonable cost for an economical period of time. The particular material selected should be be accurately determined. 

The whole material complex should be considered, as an integrated entity, rather than each material separately. The more highly resistant materials should be chosen for the critical components and where relatively high fabrication costs are involved. It may be necessary to compromise and sacrifice some mechanically advantageous properties to satisfy corrosion requirements, and vice versa. 

Where the corrosion rate is either very low or very high, the choice of materials is simple where it is moderately low, a thorough analysis of all aspects is required. 

In dry environments and carefully controlled fluids, many materials can be used - and these often may be left unprotected. Under atmospheric conditions, even polluted atmospheres, such metals as stainless steels and aluminum alloys do not need protection. Also, copper and lead have long lives. In a more severe wet environment, for example in marine conditions, it is generally more economical to use relatively cheap structural materials (mild steel) and apply additional protection, rather than use more expensive options. For the severest corrosive conditions it is preferable in most cases to use materials resistant to the corrosive, than to use cheaper material with expensive protection. 

Materials more expensive than absolutely necessary should not be chosen unless it is economical in the long run and necessary for safety of personnel or product, or for other important reasons. Using fully corrosion-resistant materials is not always the correct choice, a balance between the initial cost and the cost of subsequent maintenance should be found over the full estimated life of the designed utility. 

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