Corrosion is the natural degradation of a material due to its dissolution, caused by reactions with the surrounding environment. Corrosion is nature seeking to recombine elements which have been reduced to an unnaturally pure form.
Corrosion of metals may be subdivided into: uniform corrosion, galvanic corrosion, pitting corrosion, crevice corrosion, intergranular corrosion, exfoliation corrosion, stress corrosion and filiform corrosion. In addition, stress-corrosion cracking and corrosion fatigue may occur as a result of a combination of applied loads and corrosion.
This is a common form of corrosion, where all areas of the metal corrode at a similar rate. Over a period of time, the exposed metal undergoes oxidation by aggressive ions. The corrosion then propagates at a rate determined by a combination of the corrosive environment and the alloy composition.
This type of corrosion occurs when two conducting materials of different chemical composition are joined and exposed to a conducting solution. If the corrosion potential of the two metals is significantly different, and they are in direct contact and immersed in an electrolyte, the more ‘noble’ metal will become the cathode and the more ‘active’ will become the anode. A measurable current may flow between the two. The corrosion rate of the anode will be increased and that of the cathode decreased. The increased corrosion of the anode is called ‘galvanic corrosion’.
Galvanic corrosion is very damaging because it concentrates on the less-noble metal at the metal-metal junction, where deep attack occurs. At the junction a large corrosion current can pass because the electrical resistance of the short path through the electrolyte is low. Common forms of metal joining, e.g. brazing, welding etc., provide junctions at which galvanic corrosion can develop.
Pitting is a form of localised corrosion which occurs when a film-protected metal is almost, but not completely, resistant to corrosion. Pitting may also occur in crevices, in which case it is called ‘crevice corrosion’.
Crevice corrosion is an intense form of localised corrosion ranging from small pits to extensive corrosion over the whole surface and occurs under the same conditions as pitting. It can occur within narrow crevices that may be formed by:
· the geometry of the structure, e.g. riveted plates, threaded joints etc.,
· contact of metal with non-metallic solids, e.g. plastics, rubber etc. and
· deposits of sand, dirt or permeable corrosion products on the metal surface.
Attack starts more easily in a narrow crevice than on an unshielded surface. Crevices, such as those found at flange joints or at threaded connections, are thus often the most critical sites for corrosion.
This type of corrosion consists of localised attack, which may occur if the area around the grain boundaries is less corrosion-resistant than the matrix in the medium in question.
Exfoliation corrosion is a specific type of selective attack that proceeds along multiple narrow paths, e.g. grain boundaries which run parallel to the surface of a metal. Generation of corrosion products forces the layers apart and causes the metal to swell; flakes may be pushed up and peel off.
This appears as a random non-branching white tunnel of corrosion product, either on the surface of non-protected metal or beneath surface coatings. It is a structurally insensitive form of corrosion which is often more detrimental to appearance than strength.
A material failure may be accelerated by the combined effect of corrosion and mechanical stress. The most common type is transgranular stress-corrosion cracking, SCC, which can occur unexpectedly – resulting in the failure of components. Often a material chosen for its corrosion resistance in a given environment is found to fail at a stress level well below its normal fracture stress. Rarely is there any obvious evidence of an impending failure, and it can occur in components which are externally unstressed.
A typically SCC attack takes the form of thin, branched cracks.
A material subjected to a cyclic load far below the ultimate tensile stress can fail, a process called fatigue. If the metal is simultaneously exposed to a corrosive environment, the failure can take place at even lower loads and after a shorter time. Contrary to a pure mechanical fatigue, there is no fatigue limit load in corrosion-assisted fatigue.
Although it is often possible to provide adequate protection for metallic parts which are stressed under static conditions, most surface films – including naturally protective oxides – can be more easily broken or disrupted under cyclic loading.
 In this context a noble refers to a metal which is distinguished from an active metal by its relatively low position in the electrochemical series and correspondingly low chemical reactivity
 The microstructures of rocks, metals, ceramics and other materials are made up of relatively rigid ‘grains’ separated by a network of ‘grain boundaries’.