Stress Corrosion Cracking

Stress Corrosion Cracking TriangleStress corrosion cracking (SCC) is a progressive fracture mechanism in metals that is a result of the simultaneous interaction of a corrodent and a sustained tensile stress. Structural failure due to SCC is often sudden and unpredictable, occurring after as little as a few hours of exposure, or after months or even years of satisfactory service. Metal components frequently experience SCC in the absence of any other obvious kinds of corrosive attack. Virtually all alloy systems are susceptible to SCC by a specific corrodent under a specific set of conditions.

The tensile stresses necessary for SCC are “static”, and they may be residual and/or applied (see chart below).

Residual
Applied
    • Welding, Shearing, Punching, Cutting, Bending, Crimping, Riveting, Machining (Lathe-Mill-Drill) Heat Treating, EDM, Laser/Wire Cutting
    • Grinding
    • Quenching, Thermal Cycling, Thermal Expansion, Vibration, Rotation, Bolting, Pressure
    • Dead Load

Progressive cracking due to “cyclic” stresses in a corrosive environment is termed “corrosion-fatigue”. The boundary between SCC and corrosion-fatigue is sometimes vague. However, because the environments that cause SCC and corrosion-fatigue are not the same, the two are treated as separate and distinct metal fracture mechanisms. Compressive residual stresses, such as those induced in the surface layers of a structure by controlled shot peening, could prevent or delay SCC and corrosion-fatigue.

Intergranular Corrosion

It was discovered at Atomics International that intergranular corrosion can be prevented in austenitic stainless steels by shot peening prior to exposure to sensitizing temperatures. For this purpose, the surfaces must be severely cold worked by the shot peening to break up surface grains and grain boundaries. When exposed to sensitizing temperatures, carbides will precipitate on the multitude of nucleation sites (i.e., slip planes, dislocations) formed within grains rather than preferentially along continuous grain boundaries to support intergranular attack in a corrosive medium.

Intergranular Corrosion
LEFT – Peened ~~ RIGHT – Unpeened

CWST has published a technical report of applications which is available upon request.

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