Evaluation of environmental effects on fatigue crack growth behaviour of a high strength steel in a saline solution with cathodic protection

Crack growth rates of small fatigue cracks in a high strength steel tested in 3.5% NaCl solution with cathodic protection are analysed in the Paris regime through the comparison with the corresponding results obtained in air or in high vacuum. Environmental effects in the saline solution are due to hydrogen produced by cathodic polarisation, which causes intergranular and transgranular brittle fracture surfaces. By comparison to fatigue crack growth rates obtained in air, it could be concluded that hydrogen effects are negligible at low ΔK and then increase with ΔK. But in fact, when compared to results obtained in a non-active media such as high vacuum, hydrogen effects in the saline solution are very high at low ΔK and decrease slightly when ΔK increases. In air, adsorption of the different gaseous species as well as hydrogen effects due to water vapour dissociation strongly enhance crack growth rate compared to vacuum especially for low ΔK. Consequently, a comparison with fatigue crack growth results obtained in air does not allow to quantify properly environmental effects due to another active environment such as the saline solution with cathodic protection considered in the present work.