Contribution of anodic dissolution and hydrogen embrittlement to corrosion-fatigue crack growth

Abstract

1. The corrosion fatigue crack growth rate depends not only on the stress intensity factor but also on the electrochemical conditions at the crack tip that can be characterized by the pH values of the environment and the electrode potential of the metal. 2. Any value of the corrosion fatigue crack growth rate in steels (under variable electrochemical conditions at the crack tip) corresponds to a certain fixed pH value and a fixed value of the electrode potential at the crack tip; moreover, any section of the crack growth rate diagrams corresponds to certain fixed ranges of the indicated parameters. 3. If the stress intensity factor is fixed, then the crack growth rate varies as a function of the pH value of the environment and the electrode potential of the metal at the crack tip. 4. We proposed a method for the evaluation of the contributions of the mechanisms of local anodic dissolution and hydrogen embrittlement to the increment of the crack growth rate in aqueous environments. The method is based on the analysis of the pH value of the environment and the electrode potential of the metal characterizing the intensity of the processes of local anodic dissolution and hydrogen depolarization at the crack tip as well as on the application of the results of fatigue crack growth tests in air and environments taking into account the electrochemical conditions at the crack tip. 5. As an example, the increments of the fatigue crack growth rate in environments caused by local anodic dissolution and hydrogen embrittlement were determined for two different metal-environment systems (40Kh steel in water and 20Kh13 steel in a 3% NaCl solution). The maps of action of these mechanisms were constructed depending on the stress intensity factor and the loading frequency. Hydrogen embrittlement was proved to be the predominant mechanism of changes in the crack growth rate for 40Kh steel in water. At the same time, local anodic dissolution was predominant for 20Kh13 steel in a 3% NaCl solution. 6. The proposed method for the quantitative evaluation of local anodic dissolution and hydrogen embrittlement is fairly universal and allows one not only to determine the predominant mechanism but also to study the properties of both known and newly created types of corrosion-resistant steels.