A perspective on environmentally-induced cracking

The premature failure of engineering alloys in service is often associated with exposure to unintended environmental chemistry. High strength carbon steels and stainless steel will fail prematurely if exposed under tensile stress to absorbed atomic hydrogen. This is known as hydrogen embrittlement. Similarly, when aluminum alloys are exposed to liquid mercury, they are likely to crack prematurely if tensile stresses are present This is an example of liquid metal embrittlement. Silicate glass cracks in the presence of water, a phenomenon known as static fatigue of glass. Plastics fail prematurely in the presence of organic solvents. Service environments that contain soluble chlorides may lead to premature transgranular cracking of austenitic stainless steels. Interestingly, the same environments will not cause such failure in ferritic stainless steels. Likewise, caustic solutions are known to promote the premature failure of carbon steels and austenitic stainless steels. All of the above phenomena are described as environmentally-induced embrittlement or environmentally-induced cracking, EIC. These phenomena have been known for a very long time. Materials engineers are fully able to make materials selection decisions for the construction of engineering systems that see service in virtually any environment. What is also not known with certainty at this stage is the mechanism by which these examples of environmentally-induced embrittlement occur. There are multiple schools of thought regarding the mechanisms of each phenomenon mentioned above. As such, it is not uncommon to find such phenomena described broadly as stress corrosion cracking even though it is clear that corrosion is not a general prerequisite to such phenomena. It should be noted that in many cases of EIC, the alloy is virtually corrosion-free over most of its surface, including the fracture surface. As such, chemical or electrochemical dissolution has minimal if any effect on the fracture process. There has been an enormous amount of effort directed toward identifying the mechanism or mechanisms of environmentally-induced cracking, but there is wide disagreement and debate on this subject. Most involve in some fashion either the adsorption of specific embrittling species and subsequent lowering of the surface energy for fracture or localized anodic electrochemical processes such as dissolution or film formation. Our goal in this presentation is to assess the current state of knowledge of EIC in terms of the above phenomena, to identify what is known in a mechanistic sense and what remains to be understood in terms of the path forward toward a more complete mechanistic understanding

Evolution of Microstructure and Texture during Warm Rolling Of a Duplex Steel

The effect of warm rolling on the evolution of microstructure and texture in a duplex stainless steel (DSS) was investigated. For this purpose, a DSS steel was warm rolled up to 90 pct reduction in thickness at 498 K, 698 K, and 898 K (225 °C, 425 °C, and 625 °C). The microstructure with an alternate arrangement of deformed ferrite and austenite bands was observed after warm rolling; however, the microstructure after 90 pct warm rolling at 498 K and 898 K (225 °C and 625 °C) was more lamellar and uniform as compared to the rather fragmented and inhomogeneous structure observed after 90 pct warm rolling at 698 K (425 °C). The texture of ferrite in warm-rolled DSS was characterized by the presence of the RD (〈011〉//RD) and ND (〈111〉//ND) fibers. However, the texture of ferrite in DSS warm rolled at 698 K (425 °C) was distinctly different having much higher fraction of the RD-fiber components than that of the ND-fiber components. The texture and microstructural differences in ferrite in DSS warm rolled at different temperatures could be explained by the interaction of carbon atoms with dislocations. In contrast, the austenite in DSS warm rolled at different temperatures consistently showed pure metal- or copper-type deformation texture which was attributed to the increase in stacking fault energy at the warm-rolling temperatures. It was concluded that the evolution of microstructure and texture of the two constituent phases in DSS was greatly affected by the temperature of warm rolling, but not significantly by the presence of the other phas

Deterministic analysis of processes at corroding metal surfaces and the study of electrochemical noise in these systems

The control of metal dissolution processes is important in fields ranging from battery technology to corrosion. Nevertheless, metal dissolution remains a poorly understood class of electrochemical reactions. Although the reaction sequence for dissolution of many metals have been elucidated, kinetic descriptions are based on area averaged values of parameters that describe the relationship between the rate of reaction and the activation energy. Theoretical and experimental research on metal dissolution has focused traditionally on macroscopic processes and features. However, in the last few years there has been considerable interest in examining these processes on a microscopic scale. This proposal describes a research program to continue our work on theoretical modelling of metal dissolution. Over the next few years, results from theoretical modelling of dissolution, in conjunction with new experimental techniques such as in situ scanning tunneling microscopy, are expected to provide a new insight into the processes controlling metal dissolution and corrosion