Abstract
The term ‘corrosion’ is used to cover the many and varied effects of an environment upon a solid object. The environment may include solid, liquid and gaseous phases, vapours, electromagnetic radiation (visible light plus infra-red and ultra-violet rays), mechanical vibrations and energy, heat and electrical effects, or a combination of two or more characteristics which may promote or restrict its action upon the object subject to such influence.
An appreciation of the mechanisms which produce corrosive effects enables preventive action to be taken in the design stage. This in particular involves the correct selection of materials and finishes, but incorporates also a knowledge of the particular environment to which the part will be exposed. When there is a clear understanding of the possible sources of corrosion, then means of protection may be specified or developed.
The correlation of laboratory testing procedures to actual outdoor exposure is a difficult and time-consuming exercise which involves the selection of an open-air or part-covered site which has such constancy of conditions that it may be used all the year round without consideration of season, providing that the exposure time is not less than approximately three months.
In temperate zones the times of sunrise and sunset deviate more than four hours throughout the year and similarly the temperature, the water vapour content of the atmosphere, and the cloud cover may vary enormously. This leads to the conclusion that the location of a suitable test site must be on or near the equator, it must have a maritime exposure, and also have characteristic features of uniform temperature, consistent sunshine, high humidity, and copious rainfall.
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