Stress-corrosion critical crack depth

Pits that reach a critical depth can act as crack initiation sites if they lead to a higher local stress intensity. The crack initiation time in this case corresponds to the incubation time of pits of a critical size. Alternatively, precipitation reactions at the grain boundaries can render an alloy sensitive to intergranular corrosion. The preferentially corroded grain boundary then serves as initiation site of a crack. Inclusions, preexisting microcracks, or other structural defects are also likely crack initiation sites. The crack initiation time, in this case, is defined as the time required for a crack to reach a detectable size. Crack initiation may also be the result of hydrogen formed by a corrosion reaction that may cause embrittlement of the metal or of successive ruptures of a passive film or tarnish layer, but these mechanisms are more important for the propagation than the initiation of cracks. Because of the multitude of possible crack initiation mechanisms, and because of the statistical nature of the phenomenon, it is not possible to predict the crack initiation time from first principles. 

Figure 9 Critical stress corrosion cracking-hardness-intergranular depth diagram [69] HE hydrogen embrittlement...

Stress increases corrosion attack and must be controlled through proper design and use of equipment, as well as by reduction of enviromnental corrosives. For example, corrosion pits concentrate stress and are prime initiation points for corrosion fatigue cracks, which are the major cause of drill pipe failure. It is easily understood that corrosion problems become more critical as well depth increases, because among other things, high temperature becomes one of the more critical problems faced in many deep drilling projects. 

Those alloy/environment systems that form relatively thick corrosion-product layers (e g., brass in ammonia environments) frequently exhibit preferential penetration of the corrosion products along grain boundaries. These are generally brittle products such that cracking will occur on reaching a critical depth in the presence of tensile stresses across the grain boundary. The environment again has access to the... 

Whether the application is fatigue-critical. If so, the depth of pitting corrosion might cause sufficient stress to initiate a fatigue crack. 

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