Corrosion, corrosion manifestation, corrosion damage, and corrosion protection

Steel pipelines are used for the transportation of gases. water, mineral oil, long-distance heating water and chemical products as well as for the hydraulic transportation of solid materials. In most cases the pipe material is unalloyed or low alloy steel. There is practically no difference in the chemical properties of these materials. High alloy steels - so-called stainless steels -used for special applications only, have totally different chemical properties varying considerably from type to type. The weldability of line pipe steels allows the construction of almost homo-geneous and mechanically durable pipeline systems.

In this presentation mainly the corrosion behavior of unalloyed and low alloy steels is covered. Minor differences in the chemical compositions of these may be ignored: so for simplification, in the following chapters this group of steels will just be called (steel). Corrosion is understood to be reactions of the material in question with chemical constituents of its environment. The changes resulting from these reactions are manifestations of corrosion. id the case of steel in water and humid soil the corrosion manifestation Is always the transformation of iron into corrosion products, mostly solid, called rust. The question, whether or not this results In damage, is answered exclusively by the extent of the process in relation to the required performance of the construction element.

Incase the construction element does no longer perform its task or if it may stop functioning within its projected service life, then there is damage, In the case of a pipeline the word (damage) means the pipe wall Is perforated or does no longer fulfill the service requirements. Generally corrosion damage may be taken to have occurred, if the wall thickness falls unduly short of the specified minimum. In this way of differentiating the various corrosion related concepts from one another, as put down in DIN 50 900, the fact is taken into account that all the metals in common use generally corrode more or less rapidly without this causing damage in every case.

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