Hydrogen-Related Stress Corrosion Cracking

A description of the trapping process may be based on the chemical potentials of the hydrogen absorbed in the metal. In the general case, the chemical potential of interstitial or diffusible hydrogen may be described by the equation 

The chemical potential of trapped hydrogen may similarly be expressed by 

Under equilibrium conditions, the chemical potential of diffusible hydrogen equals the chemical potential of trapped hydrogen, i.e.. 

The electrochemical hydrogen permeation technique has been used in efforts to establish threshold hydrogen concentrations in steel below which no cracking occurs. The threshold concentration depends largely on the type of failure under investigation, the chemical and physical properties of the steel, and the magnitude of applied and residual stress. 

Under steady-state conditions, the concentration of interstitially absorbed hydrogen can be established at any depth of the membrane (dotted line in Fig. 25). By subsequent metallographic examination of the mem- 

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The method can be used for studies on hydrogen diffusion and trapping in metals, which, for example, are relevant within the field of hydrogen-related stress corrosion cracking. Critical hydrogen concentrations for various types of cracking can be assessed. 

V. Nielsen, Evaluation of Hydrogen-Related Stress Corrosion Cracking in High-Pressure Pipelines, Technical report, Department of Manufacturing Engineering, Corrosion and Surface Technology, The Technical University ofDenmark, 1996. 

Much earlier in this book (Section 7.10), the point was made that the path and rds for hydrogen evolution divide themselves into those (e.g., H30+-> Hads+H20 2Hads- H2) in which 9H is 1 and those (e.g., H30++ e H20 + H H30+ + Hads + e —> H20 + H2) in which 6 —> 1. The distinction between these mechanisms is important for Fe because of the effect of the value of the coverage with 9 on embrittlement and the related stress corrosion cracking (Section 12.6.5). This is more likely if 9 is larger than when it is small because a large 9 increases the driving force for the permeation of H into the metal. 

Flis J. (1991). Stress corrosion cracking of structural steels in nitrate solutions. In Corrosion of Metals and Hydrogen-Related Phenomena. Materials Science Monograph, Vol. 59 (ed. J Flis). Amsterdam Elsevier, pp. 57-94. 

Crack propagation models are then presented for both short and long cracks. Anodic dissolution and hydrogen effects at the crack tip are analyzed. Finally, the possible relation between stress corrosion cracking and CF is shown for crack initiation and crack propagation processes. 

The chapter builds on our critical reviews on Mg corrosion [1-4] and Mg SCC [5,6]. SCC [5-8] involves (1) a stress, (2) a susceptible alloy and (3) an environment. SCC is related to hydrogen embrittlement (HE). HE is SCC that is caused by hydrogen (H), which can be gaseous, can come from corrosion, or can be internal from prior processing. HE is often postulated as the SCC mechanism. SCC can be extremely dangerous. Under safe loading conditions, SCC causes slow crack growth. Fast fracture occurs when the crack reaches a critical size. SCC, for any alloy + environment combination, can be characterised by [7,8] the threshold stress, ctscc> threshold stress intensity factor, iscc> the stress corrosion crack velocity. 

Several tests are not related to any particular part of the corrosion process, but involve only a specific test specimen that responds to corrosion by complete failure. These tests are used in the measurement of certain forms of corrosion involving factors such as stress. Examples are corrosion fatigue, stress corrosion cracking, and hydrogen embrittlement In designing such corrosion tests, the variety of test specimens parallels the number of apphcations. 

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