Fatigue crack growth tests on BM and HAZ specimens have been conducted in air and free-corrosion seawater environments. A new shape function solution has been derived using Finite Element simulations and is presented in this paper. For both BM and HAZ materials, the FCG rate is on average around 2 times higher in seawater compared to air. In both air and seawater environments the mean curves for BM and HAZ fall close to each other.