Fatigue Crack Growth Behavior of a Mn-Ni-Cr Steel in 3.5 % NaCl Medium and Its Modeling

Abstract

Understanding the fatigue crack growth behavior of marine steel at low frequencies in a corrosive environment under cathodic protection is essential for the design and prognosis of offshore structures. Experimental results demonstrate that the corrosion fatigue crack growth rate increases with a decrease in frequency, and this behavior can be mitigated by reducing the corrosion rates with the application of a cathodic potential. Knowledge of an optimum cathodic protection potential for corrosion fatigue crack growth without entering the domain of hydrogen-assisted cracking potentials is vital. To that end, the corrosion processes within the enclave of a stationary crack and a pulsating fatigue crack under different crack mouth potentials are elucidated through modeling of mass transport of electrolytic species and electrode reactions. Effect of crack tip strain enhanced electrochemical reaction rates on electrochemical parameters such as pH, potential, and corrosion current density at the crack tip is evaluated. The corrosion current density at the crack tip is reduced significantly when the applied crack mouth potential is changed from −550 to −1,050 mV SCE for both stationary and pulsating cracks. In the case of strain-enhanced corrosion processes, the corrosion current density is increased significantly when compared with the equivalent model without considering the straining effects. However, the corrosion current density is the same for the applied crack mouth potential below −900 mV SCE. The crack solution becomes alkaline as the crack tip pH increases with a decrease in cathodic potential. This result agrees with the experimental observation of the minimum corrosion fatigue crack growth rate of a steel in sodium chloride solution at around −900 and −950 mV SCE.