Implications of hydrogen uptake and transport for corrosion fatigue crack growth
The effect of long term pre-exposure on the corrosion fatigue crack growth rates of low alloy steels has been studied in order to provide guidance on the reliability of short term testing. Tests were conducted in aerated 3.5% NaCl at a potential of -1100 mV(SCE) using a sinusoidal loading wave with a stress ratio of 0.25 and a frequency of 0.167 Hz. In order to calculate the pre-exposure periods for the corrosion fatigue tests, the effective diffusivity of hydrogen in low alloy steels was measured using the electrochemical permeation technique. The diffusivity in BS4360 SOD steel in 3.5% NaCl at a potential of -llOOmV(SCE) was 9.7 x 10 " m V compared to 2.0 x 10 m V in 3.5% Ni-Cr-Mo-V steel. Pre-exposure of AISI 4340 for 21 weeks enhanced the corrosion fatigue crack growth I rates by up to a factor of 7 relative to data from conventional 4 week tests. Pre-exposure of 3.5% Ni-Cr-Mo-V steel for 55 weeks increased the crack growth rates by up to a factor of 4 relative to results from a conventional 4 week test. There was some indication of an effect of pre-exposure on the crack growth rates of BS4360 50D at values of AK below 13 MPam^ but the scatter in the data prevented a definitive conclusion. For certain environmental test conditions, conventional corrosion fatigue tests may seriously underestimate crack growth rates in service. The reliability of the data depends critically on the exposure time. In conditions for which hydrogen uptake is greater at the external surface than at the crack tip, the specimen should be pre-exposed prior to testing to allow a steady-state concentration of hydrogen to develop through the specimen.