Mechanisms of Hydrogen Embrittlement

Despite the major technical importance of hydrogen embrittlement, and the wealth of research work on the subject, the mechanism (or perhaps mechanisms) of hydrogen embrittlement remains uncertain. Much of the book edited by Oriani, Hirth and Smialowski is concerned with mechanistic aspects of hydrogen embrittlement, and the reader is referred in particular to the summary by Thomson and Lin . 

As shown in Figure 6.49a, the cracks grow by slip dissolution due to diffusion of active water molecules, halide ions, etc., to the crack tip, followed by a rupture of the protective oxide film by strain concentration, fretting contact between the crack faces. This is followed by dissolution of the fresh exposed surface and growth of the oxide on the bare surface. For the alternative mechanism of hydrogen embrittlement in aqueous media, the critical steps involve diffusion of water molecules or hydrogen ions to the crack tip reduction to hydrogen atoms at the crack tip surface diffusion of adsorbed atoms to preferential surface locations absorption and diffusion to critical locations in the... 

Figure 6.70 Possible mechanisms of hydrogen embrittlement (a) chemical adsorption of hydrogen (b) absorption of atomic hydrogen (c) decohesion of atoms (d) possible brittle hydride particle at the tip1...

Hil] Hill, M.L., Johnson, E.W., Hydrogen in Cold Worked Iron-Carbon Alloys and the Mechanism of Hydrogen Embrittlement , Trans. Metall. Soc. AIME, 215(4), 717-725 (1959) (Experimental, Morphology, Transport Phenomena, 33)... 

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