Active Path Dissolution
Definition - What does Active Path Dissolution mean?
Active path dissolution is an important mechanism of stress corrosion cracking (SCC) failure. It is localized preferential corrosion (dissolution) at the crack tip, along a susceptible path, with the bulk of the material remaining in a more passive state. According to this mechanism, cracks initiate at the surface sites of localized concentration of tensile strength (trenches, pits).
Active paths generally occur in locations where the corrosion-resistant alloying elements are segregated due to manufacturing processes.
Active path dissolution may happen without stress, leading to intergranular corrosion that is uniformly distributed over the material.
Corrosionpedia explains Active Path Dissolution
The active path dissolution process involves accelerated corrosion along a path of higher-than-normal corrosion susceptibility, with the bulk of the material typically being passive. The most common active path is the grain boundary, where segregation of impure elements can make it marginally more difficult for passivation to occur.
For example, when an austenitic stainless steel has been sensitized by precipitation of chromium carbide along the grain boundary, the local chromium concentration at the grain boundary is reduced, and this region is slightly less easily passivated. Consequently, a form of crevice corrosion can occur, whereby the grain boundary corrodes, with the specimen surface and the crack walls remaining passive.
In the case of applied or residual stress, the stress helps to open up the cracks, thereby allowing easier transport of corrosion products away from the crack tip and allowing the crack tip to corrode faster. Active path corrosion processes are inherently limited by the rate of corrosion of the metal at the crack tip, which limits the maximum crack growth rate to around 10-2 mm/s, and crack growth rates are often much lower, down to around 10-8 mm/s (about 1 mm in 3 years) or less.The passivation rate is an important factor in this mechanism.
It should be noted that the rate of metal dissolution can be several orders of magnitude higher when an alloy is in its active state, compared to its passive condition, as, for example, indicated by potentiodynamic polarization curves.
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