Stress corrosion cracking cases

Wei, R. P., KJier, K., Simmons, G. W., and Chou, Y. T., Fracture Mechanics and Surface Chemistry Investigations of Envi-ronmental-Assisted Crack Growth, A.R. Troiano Festschrift on Hydrogen Embrittlement and Stress Corrosion Cracking, Case Western Reserve University, 2-3 June 1980. 

Although nickel alloys on the whole offer a better resistance to stress corrosion cracking over the steels, their application in high temperature chloride or alkaline environment and hydrogen sulfide environment may put them to the risk of stress corrosion cracking. Cases of SCC have been observed in high temperature pressurized water in steam generating turbines. Incoloy 800... 

Corrosion fatigue, therefore, is a special case of stress-corrosion cracking and fatigue failure. Figure 4-451 shows an example of pipe failures due to corrosion fatigue. Corrosion fatigue can be prevented or reduced by ... 

The corrosive environments which cause SCC in any material are fairly specific, and the more common combinations are listed in Table 53.2. In the case of chloride stress corrosion cracking of the 530 series austenitic stainless steels it is generally considered that the risk is... 

In practice, by far the most common case of stress corrosion is that occurring when austenitic stainless steels are simultaneously exposed to tensile stresses and hot, aqueous, aerated, chloride-containing environments. In this case the major variable is alloy composition and structure virtually all austenitic stainless steels are more or less susceptible to stress-corrosion cracking in these environments, while ferritic and ferritic/austenitic stainless steels are highly resistant or immune. 

As with alloys of other metals, nickel alloys may suffer stress-corrosion cracking in certain corrosive environments, although the number of alloy environment combinations in which nickel alloys have been reported to undergo cracking is relatively small. In addition, intergranular attack due to grain boundary precipitates may be intensified by tensile stress in the metal in certain environments and develop into cracking. Table 4.28 lists the major circumstances in which stress corrosion or stress-assisted corrosion of nickel and its alloys have been recorded in service and also shows the preventive and remedial measures that have been adopted, usually with success, in each case. 

Fig. 8.2S Long-time constant-load tests demonstrating a distinct stress-corrosion cracking threshold stress in the case of a straight l3Cr martensitic SS as opposed to a nickel-bearing SS...

Stress relief is of little practical value as a means of preventing stress-corrosion cracking in austenitic steels, as cracking occurs at quite low stress levels even in fully softened material and it is difflcult to ensure that stresses are reduced to a safe level in a real structure. The technique can however be useful in small items but, even in this case, phase changes which reduce stress-corrosion resistance or have other deleterious effects can occur at the stress relieving temperature. 

In common with many of the alloy-environment systems described so far, if the alloy is not susceptible to stress-corrosion cracking under constant stress or stress intensity, then little or no effect of environment on fatigue crack growth is observed. In these cases, frequency, R ratio and potential within the passive or cathodically protected ranges for titanium have no effect on growth rates. 

Duquette, D. J., Fundamentals of corrosion fatigue behaviour of metals and alloys , in Proc. Conf. Hydrogen Embrittlement and Stress Corrosion Cracking, Cleveland, Ohio, 1-3 June 1980, Case Western Reserve University Department of Metallurgy and Materials Science, pp. 249-70 (1980)... 

Localised corrosion The various forms of localised corrosion are a greater source of concern to the plant designer (and operator) since it is usually difficult to predict an accurate rate of penetration, difficult to monitor, and consequently can be (especially in the case of stress-corrosion cracking) catastrophically rapid and dangerous. 

New alloys with improved corrosion-resistance characteristics are continually being marketed, and are aimed at solving a particular problem, e.g. improved stress-corrosion cracking resistance in the case of stainless steels improved pitting resistance or less susceptibility to welding difficulties. 

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