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.