Intergranular corrosion sites

The orientation of the cracks reveals that cyclic bending stresses or cyclic axial stresses were active. The intensification of these stresses at pits and intergranular corrosion sites produced the cracks observed. 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

Localised Corrosion (or localised attack) accelerated corrosion at certain sites only of a metal surface, usually induced by spatial separation of the anodic and cathodic sites. Examples include pitting corrosion, stress-corrosion cracking and intergranular corrosion. 

In aluminum alloys exposed to aqueous chloride solutions, corrosion fatigue cracks frequently originate at sites of pitting or intergranular corrosion. Initial crack propagation... 

The sites for the oxidation reactions are called anodes, and the sites for the reduction reactions are called cathodes. Anodes and cathodes can be spatially separated at fixed locations associated with heterogeneities on the electrode surface. Alternatively, the locations of the anodic and cathodic reactions can fluctuate randomly across the sample surface. The former case results in a localized form of corrosion, such as pitting, crevice corrosion, intergranular corrosion, or galvanic corrosion, and the latter case results in nominally uniform corrosion. 

Another danger is the formation of Cr-carbides at grain boundaries. They cause the depletion of the adjacent metal in Cr and thus reduce locally the protection of the alloy. As a consequence, pits may form at these sites at more negative potentials. They may merge together and cause intergranular corrosion. 

Pits that reach a critical depth can act as crack initiation sites if they lead to a higher local stress intensity. The crack initiation time in this case corresponds to the incubation time of pits of a critical size. Alternatively, precipitation reactions at the grain boundaries can render an alloy sensitive to intergranular corrosion. The preferentially corroded grain boundary then serves as initiation site of a crack. Inclusions, preexisting microcracks, or other structural defects are also likely crack initiation sites. The crack initiation time, in this case, is defined as the time required for a crack to reach a detectable size. Crack initiation may also be the result of hydrogen formed by a corrosion reaction that may cause embrittlement of the metal or of successive ruptures of a passive film or tarnish layer, but these mechanisms are more important for the propagation than the initiation of cracks. Because of the multitude of possible crack initiation mechanisms, and because of the statistical nature of the phenomenon, it is not possible to predict the crack initiation time from first principles. 

Intergranular corrosion is preferential dissolution either at grain boundaries or within immediately adjacent regions. It is explained electrochemically by the concept that selective dissolution occurs at weak sites, e.g. segregation and/or precipitation of alloy elements along the boundaries. It is said that most heat-treated aluminum alloys have some susceptibility to intergranular corrosion if poor heat-treatment is applied. 

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