Limitations of potentiostatic control in stress corrosion crack growth measurements

Abstract

The electrode potential distribution along a crack in a potentiostatically polarized specimen has been derived analytically by including polarization behavior and solution conductance considerations. The analysis has been applied to the stress corrosion cracks within low alloy steels in an 8M sodium hydroxide solution at 373 K and shows that the electrode potential at the tip falls to the normal equilibrium corrosion potential as the crack length increases. These results show that potentiostatic control at the tip of a stress corrosion crack is subject to large varying systematic errors. Consequently the validity of stress corrosion mechanisms based on potentiostatically controlled crack growth measurements which do not take into account such errors should be reexamined. List of Symbolsα the Tafel constant for the anodic dissolution reaction on a natural logarithm scale, V,β the Tafel constant for the cathodic reduction reaction on a natural logarithm scale, V,C the specific conductance of a solution, Cl'1 mδ the crack opening displacement, m,Ecthe free corrosion potential, V,Eappthe potentiostatically applied potential, V,Exthe potential at position x within a crack, VEyYoung's modulus of elasticity, MNm-22,icthe corrosion current density at the free corrosion potential, Am-2,iappthe net anodic current density supplied to polarize the specimen to a potential Eapp, Am-2,i1the cathodic current density at potential Eapp, Am-2,i2 the anodic current density at potential E, Am-2,ix the net anodic current density at potential Ex, Am-2,if the current flow per unit length along the crack at position x, Am-2,ρy the yield stress, MNm-2,K the stress intensity, MNm-3/2, w the width of the stress corrosion crack, m, and x the length of the stress corrosion crack, m.