Stress corrosion cracking crack morphology

The changes in the anode composition and morphology are also responsible for the erratic response of the current density vs. potential difference tendency and the decrease in the electrocatalytic activity. Thermo-corrosion and thermal stress failure, stress corrosion cracking during gas evolution, and overcritical local pressure conditions can cause extensive pitting and large ohmic voltage drops. 

Corrosion-fatigue fractures, just as those due to stress corrosion cracking, can be either intergranular or transgranular and in general they exhibit a morphology typical for brittle fractures. On the other hand, the two phenomena differ in a number of ways. 

Swann, P. R., "Morphological Aspects of Stress Corrosion Failure," in Theory of Stress Corrosion Cracking in Alloys, R. M. Latanison, Ed., NATO, Brussels, Belgium, 1971, pp. 113-126. 

Asaro, R. J. and Tiller, W. A. (1972), Interface morphology development during stress corrosion cracking Part I. Via surface diffusion. Metallurgical Transactions 3, 1789-1796. 

Stress corrosion cracking and corrosion fatigue may have common features and there is a continuum of morphology and mechanisms between the two phenomena, CF becoming similar to SCC as the loading frequency is decreased. In the same way, the distinction between HE and SCC or CF is... 

Classification by the morphology of the attack The attack may be homogeneous but the attack may also be rather localized (pitting corrosion, stress corrosion cracking, intergranular corrosion, etc.), or the material may remain virtually intact but the interface to a coating may be destroyed (cathodic delamination). 

Fig. 14.40 Morphology of stress corrosion cracking, a Trans-intergranular with branching b intergranular with branching. SEM analysis c brittle fracture d intergranular [73]...

Susceptibility to aqueous cracking occurs to different degrees. Some alloys will break in moist air in the pre-cracked conditions, others require immersion in distilled water, while others require immersion in water containing appreciable amounts of dissolved halide. Different heat treatments may produce these different levels of susceptibility in one alloy. The Ti-8AI-1 Mo-1 V alloy, for example, will fail in laboratory air in the step-cooled condition, but requires immersion in distilled water in the mill-annealed condition and in 0.6 m KCl in the duplex annealed condition. Heat treatment of titanium alloys produces a variety of phase structures, morphology and composition, and the effects upon stress-corrosion susceptibility are complex. Generally, processes increasing the yield stress low A, c and A iscc> while... 

Figure 1.19 shows typical secondary cracks on the specimen gage length. Only half of the fractured specimen is shown since the other half exhibited similar cracking morphology. These cracks are typically developed in ductile materials susceptible to SCC. This type of specimen fracture is attributable to SCC, which is corroborated by the drop-off in ductility and the formation of secondary cracks. Therefore, the combination of an applied stress and applied potential in a corrosive environment degrade the material mechanical properties. 

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