Stress corrosion cracking material parameters

Intergranular stress-corrosion cracking (IG-SCC) can occur in some sensitized materials when placed under tensile stress. Thus DL-EPR has been used to study the effects of aging time on the susceptibility of Alloy 600 to IGSCC, as shown in Fig. 41 (39). This work also shows the need to modify the experimental parameters of the test to achieve optimal correlation for alloys other than Type 304SS, in this case lowering the KCNS concentration and the temperature while raising the peak potential and the scan rate. 

In metal alloys the combination of stress and environment can also lead to premature failures, indicated as Stress Corrosion Cracking, SCC [1]. The influence of the environment on SCC is generally of a chemical nature a chemical reaction occurs between the metal and the environment. Most of the research published on the ESC of polymers focuses on ESC in which the environment influences the material only physically [2-8]. In such cases the mechanism of ESC is studied and models are established for ESC prediction [9]. These models for physical ESC are based predominantly on the solubility parameters of the considered polymer/environment combination. In other words, ESC is mainly a consequence of polymer softening, i.e. it is a reduction of the interaction between the polymer chains that lowers the yield stress. 

Figure 4.28 Interactions between the parameters associated with cold work and their effect on the conjoint material and stress conditions for stress corrosion crack propagation [98].

It has long been recognized that, based on plant and laboratory observations, there are conjoint material, stress, and environment requirements that have to be met to sustain stress corrosion cracking in stainless steels (and other materials) in BWR systems (Figure 18.15a). The reason why the slip-oxidation model was adopted as the "working hypothesis" was that the basic controlling parameters at the crack tip in that model (creation of a localized environment, the periodic rupture of the protective oxide, and the oxidation kinetics on the bared crack tip surface) could be correlafed with these empirical observations (Figure 18.15b). 

The development of life prediction capabilities for LWR components subject to stress corrosion cracking are crucial to the success of proactive management of these problems and economic and safe operation of LWRs. Various approaches have been taken, including those based on (a) past-plant experience, (b) correlations based on the analysis of the effect of key stress, environment and material parameters on the cracking kinetics and, finally, (c) those that draw on an understanding of the mechanism of cracking. 

In equation (5.103), V is the crack velocity, Ki the stress intensity coefficient, and B and n are constants dependent on the material and its environment. The constant B is sometimes written as V to signify that it represents the critical velocity of crack growth at failure. The exponent n is an important parameter known as the stress-corrosion susceptibility coefficient of the ceramic. 

Some of the most obvious examples of problems with gas and materials are frequently found in refining or petrochemical applications. One is the presence of hydrogen sulfide. Austenitic stainless steel, normally a premium material, cannot be used if chlorides are present due to intergranular corrosion and subsequent cracking problems. The material choice is influenced by hardness limitations as well as operating stresses that may limit certain perfonnance parameters. 

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