Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It is a progressive fracture mechanism in susceptible metals that is a result of the 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 months to years of service. Metal components frequently experience SCC in the absence of any other obvious kinds of corrosive attack. Many alloys 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). It is important to note that all 3 sides of the SCC triangle must be present for SCC to occur. Shot peening eliminates the tensile stress portion of the triangle which prevents SCC from occurring.
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.
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 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.
CWST has published a technical report of applications which is available upon request.