Hydrogen-enhanced fatigue crack growth of Alloy 600

Abstract

The fatigue crack growth (FCG) behavior of mill annealed Alloy 600 in 0.1 M NaCl solution, 25 °C, was studied by a fracture mechanics test method. The rates of FCG were affected by the applied cathodic potential, load frequency f, and load ratio R (R = Pmax/Pmax). At an applied cathodic potential of −1300 mVSCE, the FCG rates increased with decreasing load frequency and increasing load ratio. In comparison with the FCG rate in air, the FCG rate of Alloy 600 was enhanced at −1300 mVSCE in 0.1 M NaCl solution, 25 °C. The enhancement increased with decreasing load frequency, and the trend was more pronounced as the load ratio increased. However, the enhancement showed saturation behavior for lower load frequencies at R = 0.2 and 0.5, and a reverse trend was even found at R = 0.5. The fractographs of Alloy 600 fatigued at cathodic potential showed a brittle fracture mode. The enhancement of the FCG rates of Alloy 600 in 0.1 M NaCl solution, 25 °C, at −1300 mVSCE was probably due to hydrogen-assisted cracking. The critical load frequency fcr, which could induce a significant FCG rate enhancement, was identified, and the value of fcr was found to increase with increasing load ratio. At R = 0.5, the peak load frequency (about 0.03 Hz) that could result in the largest FCG rate was found. A possible mechanism for the effect of hydrogen-assisted cracking on the FCG rate enhancement is discussed in this paper.