Corrosion protection and fatigue performance of HFMI-treated welded joints in offshore wind turbine structures

Abstract This study investigates the fatigue performance of HFMI-treated welded joints in offshore wind turbine monopile foundations. Fatigue tests were carried out on longitudinal butt welds (LB) and T-butt welds (TB) made from S355J2 + N, S355ML, and S500ML in both as-welded (AW) and HFMI-treated conditions. In addition, single-sided transverse stiffeners (TS) were tested after blast cleaning to simulate conventional surface preparation prior to coating. The results show that HFMI significantly improves the fatigue strength of TB welds, shifting the failure location away from the critical weld toe when all weld toes, including intermediate layers, are treated. In contrast, LB welds exhibited a high fatigue resistance already in AW condition, indicating limited benefit of HFMI treatment under axial loading. Blast cleaning, whether applied before or after HFMI, was found to be compatible with organic coatings and thermal sprayed zinc and did not reduce fatigue performance; a slight positive effect was even observed. These findings support the selective use of HFMI treatment for critical details in offshore monopiles and confirm its compatibility with standard corrosion protection systems.