Environmentally induced corrosion

The first recognized case of environmentally induced corrosion was that of seasonal cracking of brass in the rainy season. It was a classical example of SCC of brass in ammoniacal environment. Soon upon being realized as a major mechanism, it became the subject of intense research and a brief review of literature would show that thousands of papers have been written on the subject. The most intriguing is the mechanism of stress corrosion cracking and stress corrosion crack propagation. A full discussion of the mechanism is beyond the scope of this chapter. An online search would reveal thousands of articles and update references. Four basic categories of mechanisms have been proposed ... 

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. 

General description. In incomplete fusion, complete melting and fusion between the base metal and the weld metal or between individual weld beads does not occur (Fig. 15.8). Incomplete fusion that produces crevices or notches at surfaces can combine with environmental factors to induce corrosion fatigue (Chap. 10), stress-corrosion cracking (Chap. 9), or crevice corrosion (Chap. 2). See Fig. 15.9. 

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 ... 

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. 

In summary, the DuraCrete service-life design is based on equations modelling the deterioration processes of the structure and its resistance against the environmental actions. Mathematical models of the deterioration processes are formulated in physical/chemical terms, including the effect of time. These models are the basis for the probabilistic method of service-hfe design. In the following sections, the DuraCrete models for carbonation and chloride-induced corrosion will be illustrated. 

The total cost of material fracture is about 4% of gross domestic product in the United States and Europe (88,89). Fracture modes included in the cost estimates were stress-induced failures (tension, compression, flexure, and shear), overload, deformation, and time-dependent modes, such as fatigue, creep, SCC, and embrittlement. The environmentally assisted corrosion problem is very much involved in the maintenance of the safety and reliability of potentially dangerous engineering systems, such as nuclear power plants, fossil fuel power plants, oil and gas pipelines, oil production platforms, aircraft and aerospace technologies, chemical plants, and so on. Losses because of environmentally assisted cracking (EAC) of materials amount to many billions of dollars annually and is on the increase globally (87). 

In addition, upon exposure to atmospheric and environmental factors, even an SRB-induced corrosion may produce corrosion products that are in colors other than black (such as reddish brown, which could be hematite-ferric hydroxide). Figure 4.34 shows a case of SRB-induced MIC where orange tubercles were the main corrosion products available. 

Corrosion science and engineering is a complex and broad subject that is not well defined and is still evolving as the subject itself expands beyond the traditional one, the destructive oxidation of metals, to the subject of this report, environmentally induced degradation of a material that involves a chemical reaction. The newer subject matter encompasses a wide spectrum of environments and all classes of materials, not just metals, and it intentionally excludes degradation due to nonchemical processes such as creep, fatigue, and tribology. 

Erosion is one of several wear modes involved in tribocorrosion. Solid particle erosion is a process by which discrete small solid particles, with inertia, strike the surface of a material, causing damage or material loss to its surface. This is often accompanied by corrosion due to the environment. A major environmental factor with significant influence on erosion-corrosion rates is that of flow velocity, but this should be set in the context of the overall flow field as other parameters such as wall shear stress, wall surface roughness, turbulent flow intensity and mass transport coefficient (this determines the rate of movement of reactant species to reaction sites and thus can relate to corrosion wall wastage rates). For example, a single value of flow velocity, referred to as the critical velocity, is often quoted to represent a transition from flow-induced corrosion to enhanced mechanical-corrosion interactive erosion-corrosion processes. It is also used to indicate the resistance of the passive and protective films to mechanical breakdown [5]. 

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