Residual stress measurements in wire-arc sprayed ZnAl15 coatings

Corrosion protection constitutes a major challenge for the cost-efficient design of offshore structures. In this area, thermal spray ZnAl15 has been proven to provide an excellent combination of cost-efficiency and longevity. In recent studies by the authors, crack formation was observed in oxidized areas of such coatings.

Wire-arc sprayed coatings usually exhibit a certain amount of tensile residual stresses. With regard to crack formation and propagation, such stresses could result in a premature coating failure. Determining the stress state in such coatings was not reliable using whole drilling and measurements based on the substrate curvature due to intrinsic restrictions of these methods. Thus, residual stress measurements were conducted on wire-arc sprayed ZnAl15-coatings using the sin²ψ method. This method is based on X-Ray diffraction. The results indicate slightly lower stress levels at a higher stand-off distance, which correlates with the assumed lower particle temperatures at higher stand-off distances. Furthermore, a strong dependence of the residual stress loading severity on the ductility was identified. Based on the results, a hypothesis for the root cause of the crack formation has been derived and is currently validated by ongoing long-term corrosion tests.