Definition

Corrosion can be defined as the reaction of a material with its environment. The problem of corrosion arises in various environments ranging from urban and marine atmospheres to industrial chemical plant installations. It is a major factor governing the design and operation of plant and equipment as it reduces their useful life and can often result in unscheduled shutdowns or, in some cases, catastrophic failure. The control of corrosion presents a considerable challenge to engineers and, in spite of our best efforts, the annual costs of corrosion damage and corrosion related service failures run into many millions of pounds, estimated at about 4% of the GNP for an industrial country. However, there is scope to reduce this cost burden by making improvements in materials selection, methods of protection, design and in-service monitoring.

In aqueous environments, corrosion may occur as uniform (general) or non-uniform (local) attack. Uniform corrosion results in general wastage, is reasonably easy to inspect and to predict from weight loss experiments or electrochemical data. Local corrosion can take a number of various forms and is much less predictable. It can result in more serious damage to structures.
Passive Oxide Films

In order to understand both general and local forms of wet corrosion of metallic materials, the role of oxide films on their surfaces must be considered. All metals except gold will have a surface air-formed oxide film, the nature of the film depending on alloy composition and the conditions and temperature of its formation. Films which are strongly adherent and which do not contain imperfections are protective and can protect the underlying substrate against further dry oxidation or wet corrosive attack. Oxide films on the surfaces of metals are therefore seen to play a significant part in the mechanism of aqueous corrosion, as illustrated by the rusting of mild steel. Failures so caused are difficult to prevent due to the complex nature of the interaction of different corrosion mechanisms and residual and mechanical stresses in service. These mechanisms quite often give rise to non-uniform forms of corrosion that can result in severe local attack leading to failure.
Corrosion Mechanisms

Corrosion can proceed by several different mechanisms, including:

· Rusting of mild steel

· Differential aeration

· Pitting

· Galvanic attack

· Intergranular attack

· Leaching (selective corrosion)

· Corrosion and erosion

· Stress corrosion cracking (SCC)

· Corrosion fatigue

· Hydrogen damage
Design and Materials Selection

In short, corrosion damage takes many forms resulting from a variety of material/environment/stress interactions. In selecting materials to resist corrosion or taking corrosion protection measures, then total life cycle cost is probably the most significant factor. In this respect, the extra cost of inspection, maintenance and earlier replacement of cheaper, lower corrosion resistant materials have to be considered.

Materials have to be selected on the basis of their ability to resist specific corrosive environments and to withstand the levels of service stresses. The ease of fabricating the materials into the shapes required by the design is also an important consideration.

In the design process, potential corrosion problems may be prevented by avoiding:

· Shapes with crevices that might cause differential aeration

· Contact between incompatible materials which might give rise to galvanic attack

· Situations where small anodic sites are in contact with large cathodic areas