Corrosion fatigue and electrochemical reactions in steels

For corrosion fatigue crack growth in steels in aqueous environments, the fundamental question is as follows: "Is the crack growth response controlled by the rate of electrochemical reactions at the crack tip?" If so, thefollowing more specific question needs to be addressed: "Is the anodic or the cathodic process in control for a specific set of conditions?" To address these questions, critical experiments involving coordinated measurements of crack growth rates and of rates of electrochemical reactions were carried out. The crack growth experiments were designed to probe the effects of reaction time and rate through changes in cyclic-load frequency and temperature. These experiments were conducted in an acetate buffered solution (pH = 4.2) under constant crack driving force (AK) and freely corroding conditions. The electrochemical experiments were designed to provide confirmatory data on the kinetics and extent of reactions as a function of temperature, and to determine the mechanisms and the rate-controlling processes for the reactions. The crack growth experiments, along with other crack growth data, provided strong support for an affirmative answer. The proposed supporting electrochemical experiment, however, was judged not to be sufficiently critical to identify the rate-controlling process and the mechanisms of reactions, because of the presence of extraneous charges introduced during specimen preparation. The overall approach and direction are reviewed and are deemed to be correct. New and modified experiments, designed to circumvent the indicated shortcoming and to broaden the understanding of corrosion fatigue crack growth in aqueous environments, are considered.