Influence of twin wire arc spraying and machine hammer peening on the corrosion fatigue of ZnAl4 coatings on S355 J2C + C substrate

Abstract

ZnAl-based coatings are used as corrosion protection for offshore application, e.g., offshore wind turbines. These applications are subjected to superimposed mechanical and corrosive stresses, e.g., wind, tide, seawater and mist. Thermally sprayed coatings processed by twin wire arc spray process and the mechanical post-treatment process machine hammer peening (MHP) were investigated. The coating consisting of ZnAl4 was deposited on a S355 J2C + C substrate. In addition to two conditions produced with different process parameters during MHP post-treatment, uncoated and as-sprayed conditions were evaluated as reference. The aim was to determine the effect of the coating and its properties, such as hardness, porosity and roughness, on the corrosion and corrosion fatigue behavior. In this study, potentiodynamic polarization tests and instrumented corrosion fatigue tests using a self-designed corrosion cell were performed in 3.5% NaCl solution. Mechanical and electrochemical measurement techniques in combination with fractographic and microstructural analyses were used to characterize corrosion fatigue damage mechanisms.MHP resulted in a uniform coating thickness with lower roughness and porosity in comparison to the as-sprayed coatings, which were further reduced by adjusting the process parameters of MHP. The electrochemical potential of the ZnAl coating provides passive corrosion protection for the substrate. The coating system improved corrosion fatigue resistance at high cycle fatigue regime. While as-sprayed and MHP post-treated specimens run out at 280 MPa, uncoated specimens run out at 240 MPa. The open circuit potential (OCP) curves plotted during the corrosion fatigue tests can be correlated with the damage mechanisms. Crack initiation and propagation in the coating and crack propagation in the substrate can be identified from the OCP curves.