Environment-sensitive cracking related

Some films are termed "passive," for stainless steels or aluminum alloys, for instance. These films can play an important role in environment-sensitive crack initiation and fracture. Under thermodynamic equilibrium conditions, the film stability may be inferred from E =/(pH) diagrams, where E is the electrical potential related to the chemical free energy G by G = -nEF, and F is Faraday s number. At equilibrium, one can define the electrode potential (related to AG) and the current density I (I e here AG is the activation energy of dissolution). 

From studies of service behavior and from extensive laboratory investigations, the well-established terms stress-corrosion cracking (SCC) and corrosion fatigue have been shown to relate to a continuum of failure modes classified as environment-sensitive fracture. In many environments, the addition of stress, with associated strains, introduces a variable that can result in brittle failure in the sense of very limited plastic flow in otherwise ductile materials such as the stainless steels. Environment-sensitive fractures propagate at an advancing crack tip at which, simultaneously, the local stresses can influence the corrosion processes, and the corrosion can influence the crack-opening processes. Since these processes proceed by kinetic mechanisms, they are time and stress dependent with the result that the crack propagation rate can become very sensitive to the stress application rates. Conventional SCC usually has been associated with static stress, but this is seldom realized... 

It may be felt that the initiation of a stress-corrosion test involves no more than bringing the environment into contact with the specimen in which a stress is generated, but the order in which these steps are carried out may influence the results obtained, as may certain other actions at the start of the test. Thus, in outdoor exposure tests the time of the year at which the test is initiated can have a marked effect upon the time to failure as can the orientation of the specimen, i.e. according to whether the tension surface in bend specimens is horizontal upwards or downwards or at some other angle. But even in laboratory tests, the time at which the stress is applied in relation to the time at which the specimen is exposed to the environment may influence results. Figure 8.100 shows the effects of exposure for 3 h at the applied stress before the solution was introduced to the cell, upon the failure of a magnesium alloy immersed in a chromate-chloride solution. Clearly such prior creep extends the lifetime of specimens and raises the threshold stress very considerably and since other metals are known to be strain-rate sensitive in their cracking response, it is likely that the type of result apparent in Fig. 8.100 is more widely applicable. 

Polymer product quality control The molecular architecture of a polymer is very sensitive to reaction environment. The actual customer specifications are often represented by nonmolecu-lar parameters (e.g., tensile strength, impact strength, color, crack resistance, thermal stability, etc.) that must be somehow related to fundamental polymer properties such as molecular weight distribution, composition, composition distribution, branching, crosslinking, etc. Many of these properties are influenced by more than one reaction or process variable and hence, one needs to understand complex and nonlinear relations between reaction variables and fundamental polymer properties. The lack of online... 

Representative environments for which SCC has been reported in carbon steels are included in Table 7.7. The sensitivity of these steels to changes in composition and environment are illustrated by the effects of potential in Fig. 7.78 to 7.80 and by the slow strain-rate data of Fig. 7.82 and 7.83. These data support the conclusion that environment cracking is related to the susceptibility of the passive films to crack under stress, to the subsequent crack growth due to anodic dissolution and/or hydrogen embrittlement during the period of exposure of the alloy substrate, and to rates of repassivation of the exposed areas. Actual crack-front growth mechanisms are discussed in some detail in a later section. 

The sensitivity of concrete structures to sulphate attack is strongly related to the exposure conditions. Structures in an environment of high sulphate content in the air or in w ater, for example sewage tunnels, are particularly vulnerable. After sulphate ions penetrate the pore system of cement paste, complex reactions start with C3 A leading principally to two kinds of processes gypsum corrosion and sulphoaluminate corrosion (Mindess etal. 2003). The products of sulphate reactions with cement expand and can cause cracking and destruction. The permeability of the material s structure and the quality of cement decide upon the rate of these processes. Special Portland cements as well as high alumina cements may be used for elements exposed to sulphates (cf. Section 4.1.1). 

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