Stress-corrosion cracking mechanisms appearance

There appear to be some inconsistencies in the assignment of the growth mechanism of the oxide film in melts for aluminum and tantalum " owing to an apparent negative temperature coefficient of the Tafel slopes. Also, the apparently improved stress corrosion cracking (s.c.c.) resistance for... 

Part (b) of the Figure 11.48 shows the superposition of the behaviors of mechanical fatigue and stress corrosion cracking. In general, the influence of the latter shows up mostly at low strain rates (d /dy = 10 - 10 s ). In the graph that represents Aa/AN as a function of log K, a step appears at the point where the stress intensity factor that corresponds to the maximum value of the applied cyclic stress, reaches the value iscc. the threshold stress intensity for stress corrosion cracking. 

Only certain specific environments appear to produce stress corrosion of copper alloys, notably ammonia or ammonium compounds or related compounds such as amines. Mercury or solutions of mercury salts (which cause deposition of mercury) or other molten metals will also cause cracking, but the mechanism is undoubtedly differentCracks produced by mercury are always intercrystalline, but ammonia may produce cracks that are transcrystalline or intercrystalline, or a mixture of both, according to circumstances. As an illustration of this, Edmundsfound that mercury would not produce cracking in a stressed single crystal of brass, but ammonia did. 

FYom the multitude of intricate corrosion processes in the presence of mechanical action (friction, erosion, vibration, cavitation, fretting and so on) it is justified to touch upon corrosion types joined under a single failure mode induced by mechanical stresses. These are the stresses that govern the corrosion wear rate of metals during friction. Such processes are usually called corrosion stress-induced cracking in the case that the mechanical action is effective only in one definite direction, or otherwise termed corrosion fatigue in the case that compressive and tensile stresses alternate within cycles. In spite of the differences between the appearance of these corrosion types, they have much in common, e.g. fundamental mechanisms, the causes, and they overlap to a certain degree [19]. 

To this point, it has been assumed that failure occurs when K =T (or G=R) but, in studies of fracture, it is sometimes found that crack growth can occur at lower values of or G. Thus, kinetic effects must be included in any general formalism. There are several mechanisms that can give rise to sub-critical crack growth, but most attention has been directed to stress corrosion. This behavior has been extensively studied in silicate glasses but it can also occur in many polycrystalline ceramics. Figure 8.72 shows a typical response of ceramics to stress corrosion, with crack velocity v plotted as a function of K (or G). At low values of K, there often appears to be a threshold value of the stress intensity factor below which... 

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