Experimental Determination of the Electrochemistry in Corrosion Fatigue Cracks in Structural Steel in Artificial Seawater

Abstract

An experimental method has been developed for measuring the electrode potential and solution pH at the tip of a growing fatigue crack in BS 4360 5OD steel in seawater. The technique involved insertion of separate 1- to 2-mm diameter glass and reference electrodes in holes drilled to intersect the growing crack. A compact tension specimen was used with the holes drilled in the transverse direction, i.e., perpendicular to the crack front, to minimize perturbation of the local stresses. Measurements of the crack tip pH and potential were made for a range of applied potentials, at varying ΔK (range of the stress intensity factor) and R-value (minimum load/maximum load). At cathodic protection potentials, the pH was alkaline but generalizations about the potential drop were not possible, although the potential in the crack was always in a region of “protection” for the conditions investigated. At the free corrosion and anodic potentials the pH was neutral to slightly alkaline. The potential drop was small at the free corrosion potential but increased markedly with anodic polarization. Reasonable agreement was found between experimental measurements and theoretical predictions of pH and potential.