Environmentally induced cracking

In some environments and under certain conditions, a microscopically brittle fracture of materials can occur at levels of mechanical stress that may be far below those required for general yielding or those that could lead to significant damage in the absence of an environment. This susceptibility also depends on the chemical composition and 

Intergranular and transgranular cracking often occur simultaneously in the same alloy. Such transitions in crack modes are observed in alloys with high amounts of nickel, iron chromium and brasses. In corrosion under tension, ruptures are fragile and are sometimes characterized by the presence of cleavages, notably in the case of hydrogen embrittlement.16 

Key factors of SCC. The stress applied on a metal is nominally static or slowly increasing tensile stress. The stresses can be applied externally, but residual stresses often cause SCC failures. Internal stresses in a metal can be due to cold work or a heat treatment. In fact, all manufacturing processes create some internal stresses. Stresses introduced by cold work arise from processes such as lamination, bending, machining, rectification, drawing, drift, and riveting. Stresses introduced by thermal treatments are due to the dilation and contraction of metal or indirectly by the modification of the microstructure of the material. Welded steels contain residual stresses near the yield point. Corrosion products have been shown to be another source of stress and can cause a wedging action. 

At intermediate stress intensity levels (stage 2) the crack propagation rate shows a plateau velocity Vpiateau that is virtually independent of the mechanical stress, but depends on the alloy/environment interface and the the rate-limiting environmental processes such as mass transport of the aggressive species to the crack tip. The plateau in a quenched and tempered low-alloy steel of 1700 MPa yield strength in deaerated water at 100°C 

Magnitude of the crack tip stress distribution (Stress intensity factor, (K), MPaVm or ksiVTn.) 

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Corrosion also occurs as a result of the conjoint action of physical processes and chemical or electrochemical reactions (1 3). The specific manifestation of corrosion is deterrnined by the physical processes involved. Environmentally induced cracking (EIC) is the failure of a metal in a corrosive environment and under a mechanical stress. The observed cracking and subsequent failure would not occur from either the mechanical stress or the corrosive environment alone. Specific chemical agents cause particular metals to undergo EIC, and mechanical failure occurs below the normal strength (5aeld stress) of the metal. Examples are the failure of brasses in ammonia environments and stainless steels in chloride or caustic environments. 

Concrete and fiberglass vaults are often used, although they can be subject to environmentally induced cracks. Soil and clay liners are not allowed. Flexible hner systems have been developed that may be a cost-effective and environmentally sound alternative. State-of-the-art liner technology has overcome many of the previous problems with seams, low mechanical strength, and chemical resistance. 

Environmentally-induced cracking has emerged as a significant problem in the following fluids ... 

Environmentally induced cracking consists of (i) stress corrosion cracking (ii) corrosion fatigue and (iii) hydrogen-induced cracking. The general features of these modes of failure are given below ... 

Mechanisms of Environmentally Induced Cracking (EIC). Environmentally assisted cracking spans a wide range from brittle fracture to electrochemical process and the underlying mechanisms are complex. 

The second part of the book consists of two chapters namely the forms of corrosion and practical solutions. The chapter, Forms of Corrosion consists of a discussion of corrosion reactions, corrosion media, active and active-passive corrosion behavior, the forms of corrosion, namely, general corrosion, localized corrosion, metallurgically influenced corrosion, microbiologically influenced corrosion, mechanically assisted corrosion and environmentally induced cracking, the types and modes of corrosion, the morphology of corroded materials along with some published literature on corrosion. 

Craig, B., Environmentally induced cracking, m Metals Handbook, 9th ed., Vol. 13, Corrosion, 169, Metals Park, OH ASM. 

Let us return to consider the properties of the critical stress intensity for environmentally induced crack growth ( Tiscc)- The fact that /sTiscc for sensitized TVpe 304SS apparently depends on the applied potential can be gleaned from the data shown in Fig. 42. Fur ermore, both experiment and theory have established that the crack growth rate for A bcc solution conductivity, temperature, ECP, flow rate, and possible ion type (e.g., sulfate versus nitrate). The most comprehensive database for the effect of conductivity on crack growth rate is probably that of Kassner et al. [82] at the Argonne National Laboratory. Their data... 

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