How stress influences Stress corrosion cracking
The role of stress is generally considered or the stress intensity factor in case of precracked samples as an effect with the passage of time in a given system. There is a threshold stress value below which corrosion does not occur even for long test duration.
The threshold stress is not just a function of alloy chemistry and structure even also counts the environmental conditions such as solution composition, capability and temperature, hence it is not a specific characteristic of a material similar to yield or tensile strength. Additionally while the threshold stress is referred as the stress below which cracking does not occur, it is not important and it is better stated as the stress above which total failure occurs as or some systems cracks have been found to occur below the threshold however stopped to distribute after some growth. It is hard to explain why cracks must stop to propagate on any stress based argument, as stress content or intensification would be expected to increase with crack growth under constant load.
A probable explanation is that it is not stress per se however the strain rate it produces that is the controlling factor and the cracks stop to flow when the crack tip strain rate decreases below some critical value associated to the rate of film growth. This explanation is consistent with the effect of the relative times at which the stress and the environment conditions for cracking are obtained, creep at stable load before the settlement of environmental conditions delaying or preventing cracking in lab tests. Certainly it is also consistent with the demonstration of the effect of applied strain rate upon cracking for a great range of combinations of metal and environment, for some of which it has been seen that adequately slow rates of strains can cause ductile failure without loss of tensile strength of the alloy irrespective of its exposure to a strong cracking condition.
There is another significant result of strain rate in causing crack growth and it relates to cyclic as opposed to static, loading. Hysteresis effects are well known to attain cyclic loading and the cyclic loading offers microplastic deformation. It is understood from such effects that load cycling may cuss stress corrosion cracking at much lower stress values as compare to those required with static loading.
The conditions that will cause corrosion fatigue, using ferritic steels is more beneficial. For stress corrosion cracking to occur, the conditions should have characteristics that assist in the retention of crack geometry by filming of crack sides however with large amplitude stress mechanical crack sharpening occurs so that there are less harsh needs for the properties of the environment.
Prevention of Stress Corrosion Cracking
As occurrence of SCC needs a sensitive alloy to be subjected to a specific environment at stress values above the limiting value, by using effective SCC resistant alloy materials such as Inconel 718 wire, Hastelloy C276 and Incoloy 825, the SCC can be controlled.