The prediction of the maximum corrosion fatigue crack propagation rate in the low alloy steel-de-oxygenated water system at 2880C

There is increasing evidence that the environmental enhancement of fatigue crack growth rate in the A533B pressure vessel steel-de-oxygenated water system at 2880C is a function of the sulphur content in both the alloy and the environment. The objective of this investigation is to explain quantitatively this phenomenon and to predict the maximum theoretical increase in propagation rate due to this compositional effect at various stress amplitude, mean stress and loading frequency conditions. The film-rupture-slip-dissolution model of environmentally enhanced cracking was used as a working hypothesis. Measurements of oxidation rates on bared surfaces exposed to aqueous environments expected at the crack tip indicated that these rates were strongly dependent on the sulphur content in the electrolyte. Incorporating these oxidation rates into the film-rupture-slip-dissolution model of crack advancement indicated that such a model could explain quantitatively the observed corrosion fatigue rates over a wide range of frequency, ΔK and R values for both high and low sulphur content systems. The model predicted that the theoretical maximum environmental enhancement in the crack propagation rate may be in excess of the currently observed values, under certain material and loading condition combinations. (author)