On interpretation of the mechanism of stress-corrosion crack growth and relevant terminology

Abstract

Currently, there is an established concept that there exist two alternative mechanisms of growth of stresscorrosion cracks, namely, local anodic dissolution and hydrogen embrittlement of material in the vicinity of a crack tip [1-3]. Although there is no denying the correctness of this approach in general, it should be noted that the terminology used (local anodic dissolution and hydrogen embrittlement) is inadequate for processes that take place in a crack and, thus, it is not completely valid. Local anodic dissolution refers to the case where a crack grows due to the localization of corrosion immediately at its tip, which is possible when the corrosion rate is significantly higher in a small vicinity of the crack tip than on its sides. In this case, active corrosion occurs, for all practical purposes, right at the crack tip, and corrosion of the walls is small and usually neglected. Theoretical electrochemistry regards dissolution of metals as the following two processes: dissolution caused by an external current (anodic dissolution) and dissolution caused by interaction with the components of the medium, i.e., the corrosion itself [4]. Thus, anodic dissolution of a metal is understood as its oxidation under anodic polarization by an external current. Hence, corrosion inside a crack bears no relation to anodic dissolution. There is one more significant difference between anodic dissolution and corrosion that should be mentioned, namely, anodic dissolution is a nonspontaneous process sustained by external energy (electric current), whereas corrosion is a spontaneous process, during which energy is released in the form of corrosion current. Consequently, it is totally incorrect to apply the term "local anodic dissolution" to local corrosion in a crack. We think that the pure corrosion mechanism of crack growth should be treated as a mechanical--electrochemical mechanism because it is the mechanical effect that initiates galvanic cells in a crack, which give rise to local electrochemical corrosion of the crack tip. It is also incorrect to call the alternative mechanism according to which a crack goes deeper into metal weakened by hydrogenation, "hydrogen embrittlement of material in a vicinity of a crack". This term gives a simplistic idea of abrupt crack growth. We consider the so-called hydrogen embrittlement mechanism in greater detail. During this process, a crack propagates abruptly [1, 3] and a highly corrosion-active freshly formed surface emerges in the jump area. The corrosion of the freshly formed surface acting together with the crack walls as a galvanic cell is exactly the main cause of local purely corrosion deepening of a crack as well as hydrogen regeneration and charging of the metal with it, i.e., embrittling hydrogenation. Hydrogen charging of metal gives rise to cells of differential hydrogenation in a crack [5], which accelerates corrosion of the tip. Purely corrosion deepening of a crack can be quite significant and commensurable with purely mechanical growth per jump. Thus, at this stage, a crack exhibits not only mechanical but also corrosion growth, and the mechanism of its growth is of a mechanical-hydrogen-electrochemical nature. Thus, in view of the argument above, it is more appropriate to call the mechanism of local anodic dissolution and the mechanism of hydrogen embrittlement "the mechanical-electrochemical mechanism" and "the mechanicalhydrogen-electrochemical mechanism."