Abstract
Structural elements of offshore facilities, e.g., offshore wind turbines, are subject to static and dynamic mechanical and environmental loads, for example, from wind, waves, and corrosive media. Protective coatings such as thermal sprayed ZnAl coatings are often used for protection, mainly against corrosive stresses. The Machine Hammer Peening (MHP) process is an innovative and promising technique for the post-treatment of ZnAl coating systems that helps reducing roughness and porosity and inducing compressive residual stresses. This should lead to an enhancement of the corrosion fatigue behavior. In this paper, the effect of a thermally assisted MHP process was investigated. The softening of the coating materials will have a direct effect on the densification, residual porosity and the distribution of cracks. The investigation results showed the influence of thermally assisted MHP on the surface properties, porosity, residual stresses, and hardness of the post-treated coatings. The best densification of the coating, i.e., the lowest porosity and roughness and the highest compressive residual stresses, were achieved at a process temperature of 300 °C. A further increase in temperature on the other hand caused a higher porosity and, in some cases, locally restricted melting of the coating and consequently poorer coating properties.
Highlights
Structural elements used for offshore facilities, e.g., wind turbines and oil rigs, are exposed to various loads such as mechanical stresses and corrosive environments
The process forces applied to the workpiece surface and coating system were measured by machine hammer peening (MHP) trials conducted on uncoated specimens of S235 JR structural steel (1.0037)
Due to the dynamic behavior of the force measuremeasured before the MHP experiments
Summary
Structural elements used for offshore facilities, e.g., wind turbines and oil rigs, are exposed to various loads such as mechanical stresses and corrosive environments. Prevéy and Cammett analyzed the influence of the mechanical post-treatment on the corrosion fatigue of aluminum EN AW-7075 by means of low plasticity burnishing [26] In their studies, the introduction of compressive residual stresses led to a shift of the fatigue origin from the surface to the sub-surface area and a significant increase of the fatigue strength of both the uncorroded, machined surface and the corroded state. Mechanical compacting has been applied to influence the microstructure and surface of coatings or functional surfaces to reduce the roughness and porosity, induce compressive residual stresses and increase the surface hardness of ZnAl-based corrosion protection coatings [2,27]. A thermally assisted MHP-process (TaMHP) is presented and analyzed by means of experimental investigation of the influence on a TWAS sprayed ZnAl4-coating. TaMHP treatment on the residual stresses, surface hardness, and structural composition of the coating is investigated
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