Corrosion does not stand for a single phenomenon but is a generalized term to cover a destructive
attack on a metal as a result of either a chemical or electrochemical reaction between the metal and
various elements present in the environment. For instance, iron is converted into various oxides or
hydroxides when reacting with the oxygen present in air/water, when in contact with a more noble
metal such as tin or when exposed to certain bacteria.
The international standard defines corrosion
more specifically as a “physicochemical interaction between a metal and its environment which
results in changes of the properties of the metal and which may often lead to impairment of the
function of the metal, the environment, or the technical system of which these form a part.” According to other authors, corrosion derives from “the natural tendency of materials to return to their most thermodynamically stable state.”
Table 3 below identifies four broad categories of
refinery elements that can contribute to corrosion risk.
Corrosion of a metal occurs either by the action of specific substances or by the conjoint action of
specific substances and mechanical stresses. Depending upon environmental conditions, corrosion
can occur in various forms such as stress corrosion, pitting corrosion, embrittlement and cracking.
The particular type of corrosion occurring in a specific component can often be difficult to classify.
For example, several forms of corrosion (e.g., galvanic corrosion, pitting corrosion, hydrogen
embrittlement, stress sulphide corrosion cracking) are characterized by the type of mechanical force
to which the metal component is exposed. It is not within the scope of this work to address in depth
either corrosion electrochemistry or the identification of different forms of corrosion. The basics of
corrosion mechanisms are described as a basis for understanding the conditions that make corrosion
risks highly relevant for refinery operations and more specifically to provide some insight into the
underlying causes of the corrosion events leading to the accidents analysed in this report. Also,
corrosion of certain metals (e.g. aluminium) enhances their corrosion resistance, but in this work
corrosion is assumed to be solely an undesirable phenomenon.
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