Effect of Low Concentrations of Hydrogen Sulfide in Seawater on Fatigue Crack Growth in a CMn Structural Steel

The fatigue crack growth behavior of a CMn steel and its weldments in seawater containing low concentrations of H₂S, in the range of 10 to 150 ppm, has been established. The tests were designed to simulate a hydrogen sulfide environment, which may exist under marine foulings on submerged structures, as a result of the anaerobic activity of sulfate-reducing bacteria. In the absence of H₂S, the increase in crack growth rate in seawater compared to that in air was due to a combination of anodic dissolution and hydrogen embrittlement at the crack tip. Under cathodic protection (CP) of −1.0 V (SCE), the anodic dissolution was suppressed and the da/dN vs ΔK curves showed plateau behavior in the intermediate ΔK range, with higher crack growth rates compared to free corrosion. Weldments, in general, showed faster crack growth rates compared to the base plate. Appropriate concentrations of hydrogen sulfide in seawater were achieved by bubbling H₂S/N₂ gas mixtures with specific volume fractions of H₂S in pure N₂, through seawater in a specimen testing chamber. Under a CP = −1.0 V (SCE), as little as 2 to 10 ppm H₂S in seawater caused a 2.5 times increase in crack growth rate in the intermediate ΔK range compared to that without H₂S. A 4.8 times increase in crack growth rate was recorded in the plateau region, compared to that without H₂S, when the H₂S concentration was increased to 100 to 150 ppm with a CP = −1.0 V (SCE). The crack growth enhancement resulting from CP and with increasing H₂S concentration was attributed to hydrogen assisted cracking. The plateau behavior during Stage II growth under CP and/or in the presence of H₂S was considered to be caused by the rate limiting diffusion of hydrogen atoms to the position of maximum triaxial stress in the plastic zone ahead of the crack tip. Hydrogen sulfide enhances the fatigue crack growth rate by firstly increasing the hydrogen fugacity at the crack surface and secondly by poisoning the hydrogen recombination reaction (H₂), thereby making more atomic hydrogen available for entry into the steel.