An important issue to be considered in the structural integrity assessment of offshore structures, such as offshore wind turbines and Oil & Gas pipelines, is the characterisation of material degradation due to environmental damage and its subsequent effects on the remaining lifetime predictions. The main aim of this project is to investigate the importance of corrosion and corrosion-fatigue damage in the structural integrity assessment of offshore steel weldments (i.e. base metal, heat affected zone and weld metal) using experimental testing and numerical modelling techniques. S355 structural steel weldments, which are widely used in offshore industry, will be exposed to different corrosive environments for various durations and the corresponding changes in the mechanical response, fracture and fatigue crack growth behaviour of the material will be examined. Furthermore, a numerical model will be developed and validated to predict corrosion pitting, corrosion weight loss and corrosion-fatigue crack growth behaviour in S355 weldments in the seawater environment.