Response to Dr. A. Turnbull's discussion on “Development of fast algorithms for estimating stress corrosion crack growth rate” [Corrosion Science 41 (12) (1999) 2267]

摘要

In his previous paper [1] and in his recent letter, to which we are responding, Dr. Turnbull clearly indicates that his approach in analyzing the potential drop external to the crack works in the following manner. By assuming some value for the elec- trode potential at crack mouth, E_mouth, he calculates the potential at crack tip, E_tip, and the current density flowing from the crack at the crack mouth, I_mouth, (by solving the transport equations inside the crack) and subsequently obtains the corrosion potential, E_corr, (by solving the Laplace's equation outside the crack). In doing so, the relationship between E_tip and E_corr is obtained. Dr. Turnbull offers this approach as an alternative to convergence on I_mouth, which we described a decade ago in devel- oping the original coupled environment fracture model (CEFM) [2]. We cannot agree with Turnbull's approach, because the corrosion potential, E_corr, is fully de- termined by the properties of the external environment (including the substrate) at a point that is sufficiently remote from the crack that it cannot be influenced by the potential at the crack mouth. In other words, the corrosion potential is not a crack variable and hence cannot be used as the variable in an iterative convergence cycle.