Anodic stress corrosion cracking

Normal anodic stress corrosion cracking is caused by a combination of mechanical tensile stress and loeal eleetrolyte dissolution processes when certain conditions are met. First, the corrosive medium must have a specific effect on the respective alloy, and in addition, the alloy in contact with the electrolyte in this material/corrosive medium system must be prone to stress corrosion cracking. The tensile stress must also be suffleiently high. Susceptible systems, for example, are stainless austenitic steels in chloride-eontaining solution or unalloyed and low-alloy steels in nitrate solutions. In contrast, unalloyed and low-alloy steels are not susceptible to stress eorrosion eraeking in ehloride solutions. 

A more recent theory on the mechanism of anodic stress corrosion cracking, based partly on tests, combines the electrochemical process of local metal dissolution with hydrogen embitterment at the base of the crack caused by the atomic hydrogen forming during corrosion, which may be of major significance to crack propagation. 

Transgranular stress corrosion cracking also occurs in solutions of CO-CO2-H2O containing oxygen. Recent results also indicate that unalloyed and low-alloy steels may be sensitive to anodic stress corrosion cracking in aqueous CO2 solutions. The considerable hydrogen permeation has been observed under critical conditions t > 40°C, > 1 Mpa, o > 0.5a therefore, a certain amount... 

Treseder, R. S., Influence of Yield Strength on Anodic Stress Corrosion Cracking Resistance of Weldable Carbon and Low Alloy Steels with Yield Strengths Below 100 ksi, WRC Bulletin, Vol. 243, November 1978, pp. 27-33. 

This is one of the most unpieasant forms of corrosion, since it occurs suddeniy and can quickiy iead to faiiure of the components. The so-caiied anodic stress corrosion cracking is caused hy the interaction of mechanical tensile stresses of sufficient height, and locally acting anodic dissolution processes. Generally, this type of corrosion originates from cracks and damage in the protective passive top layer of the material. 

R. C. Newman and M. Saito, Anodic stress-corrosion cracking Slip-dissolution and film-induced cleavage, in Ref 12, p. 3. 

The effect of strain rate on SCC velocity by slip-dissolution for a system showing a f" current decay on a fresh surface, for various values of the decay time constant. (From Newman, R.C. and Saito, M., Anodic stress-corrosion cracking SKp-dissolution and film-induced cleavage, in Corrosion-Deformation Interactions (T. Magnin and J.-M. Gras, eds.), Les Editions de Physique, Les Ulis, France, 1993, p. 3.)... 

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. 

In addition to the alloy compositions being of importance with regard to susceptibility to stress-corrosion cracking, the resistance of the alloy can be altered by microstructural factors. Hanninen has reviewed the available literature quite thoroughly and has concluded that a fine grain size is likely to be beneficial. Strain imposed prior to use tends to be deleterious because deformed material usually acts anodic with respect to unstrained material and because the introduction of plastic deformation may also... 

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