Corrosion fracture processes

Many corrosion processes, e.g., stress corrosion cracking and other corrosion-fracture processes, cannot be described completely by a chemical reaction. These processes are complex interactions among chemistry, physical properties of the metal and mechanical stress. Therefore, the corrosion tendency of the metal-environment interaction cannot be evaluated using Equation 1. At the present time, empirical criteria (which are beyond the scope of this chapter) are used. 

In more recent work embrittlement in water vapour-saturated air and in various aqueous solutions has been systematically examined together with the influence of strain rate, alloy composition and loading mode, all in conjunction with various metallographic techniques. The general conclusion is that stress-corrosion crack propagation in aluminium alloys under open circuit conditions is mainly caused by hydrogen embrittlement, but that there is a component of the fracture process that is caused by dissolution. The relative importance of these two processes may well vary between alloys of different composition or even between specimens of an alloy that have been heat treated differently. 

It should be noted that costs due to stress corrosion were included in both the corrosion and fracture studies. There is thus some double counting of the costs of this process. The magnitude of this is unknown since costs were not accounted for on the basis of types of corrosion and fracture process, but is not expected to affect the results substantially. 

In our concluding remarks we can emphasize that depending on the nature of interactions between the components that constitute the medium and the solid, as well as on a combination of external conditions, one may observe the effects of various types and intensity. These include the facilitation of plastic flow of solids, or, alternatively, brittle fracture due to the action of lowered stresses mechanochemical phenomena in the zone of contact mechanically activated corrosion (the stress corrosion) the processes that are close to the spontaneous dispersion (the so-called quasi-spontaneous dispersion), and the true spontaneous dispersion, leading to the formation of thermodynamically stable lyophilic system. A great variety of types of interactions that exist between the stressed solids and the medium in contact with it requires careful and thorough examination of conditions under which... 

Under increasing load or other external actions (imposed deformations, temperature) in an element built of composite material, various forms of local failure appear cracks in the matrix, fractures of the fibres, failures of bonds between phases, plastic deformations, etc. All these effects develop in the fracture processes under the influence of increasing external actions and also with ageing, corrosion, shrinkage, fatigue and so on. To study the failure process, not only individual phenomena should be considered separately but also their interactions. In many cases, the homogenization and superposition of effects are not valid or should be used with caution within well-defined limits. 

The type of corrosive environment responsible for brittle fracture of suspension composite insulators was established. A series of FTIR experiments was performed to identify chemical functionalities formed during the degradation process of composite insulators affected by brittle fracture. It was shown that the brittle fracture process was caused by the formation of nitric acid either outside or inside an insulator leading to stress corrosion cracking of the glass/polymer composite rod material. Nitrate was detected on the composite fracture surfaces inside a 115 kV suspension composite insulator which failed in service by brittle fracture. 17 refs. 

Fracture Mechanics Methods These have proved very usebd for defining the minimum stress intensity K[scc. t which stress corrosion cracking of high-strength, low-ductihty alloys occurs. They have so far been less successful when apphed to high-ductility alloys, which are extensively used in the chemicm-process industries. 

Mechanism The mechanism of cracking has not been established. Even the corrosion reaction that is responsible for the initial attack has not been determined. Early work led to the suggestion that chlorine gas was generated and could cause fracture by a cyclic process requiring the formation and decomposition of TiCl2 ... 

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