Dr. Dmitri Kopeliovich
Stress corrosion cracking (SCC) is a brittle fracture caused by the combined action of tensile stress and corrosive environment.
Cracks induced by stress corrosion may be intergranular, transgranular or mixed (combining intergranular and transgranular morphology).
Possible sources of internal tensile stresses:
In contrast to tensile stresses compressive stresses suppress stress corrosion cracking.
Stress corrosion cracks do not form if the stress is below a critical value, which is called threshold stress. Threshold stress for SCC is lower than tensile strength of the material.
Despite the fact that corrosion mechanism plays important role in stress corrosion cracking, very small amount of corrosion products form within the cracks.
Austenitic stainless steels, Aluminum alloys, Titanium alloys and brassess are susceptible to stress corrosion cracking in specific environments (acidic chloride solutions, sea water, sodium hydroxide solutions, ammonia solutions).
Stress corrosion cracking is associated with the process called anodic dissolution (slip dissolution, stress enhanced dissolution, active path corrosion).
According to this mechanism cracks initiate at the surface sites of localized concentration of tensile strength (trenches, pits).
A stress corrosion crack progresses along a specific path (active path), which is composed of grain boundaries or specific crystal planes within the grains.
The stress corrosion crack propagates by the repetitive process, cycle of which consists of the following stages:
Hydrogen assisted stress corrosion corrosion cracking is a separate type of SCC, which is principally different from the stress corrosion cracking undergoing by anodic dissolution.
In contrast to anodic dissolution mechanism Hydrogen assisted cracking is enhanced by cathodic reaction: H+ + e- = H occurring on the crack tip surface.
The atomic hydrogen dissolves in the metal where its ions interact with the dislocations of the crystal lattice causing decrease of the metal ductility (hydrogen embrittlement).
Hydrogen cracking effect is increased in harder materials and at higher temperatures.
Hydrogen assisted corrosion cracking may be prevented by selection of suitable materials and maintaining the environment solution at neutral or basic PH (PH>6). In contrast to anodic stress corrosion hydrogen cracking is enhanced by Cathodic protection.