Abstract
Offshore installations, e.g., offshore wind turbines and pipelines, are exposed to various mechanical loads due to wind or waves and corrosive loads such as seawater or mist. ZnAl-based thermal sprayed coatings, often in conjunction with organic coatings, provide sufficient corrosion protection and are well established for applications in marine environments. In this study, machine hammer peening (MHP) is applied after twin wire arc spraying to improve corrosion fatigue behavior through increased hardness, reduced porosity, and roughness compared to as-sprayed coatings. Mn-alloyed structural steel S355 J2 + C with and without ZnAl4 coating as well as with MHP post-treated ZnAl4 coating were cyclically loaded in 3.5% NaCl solution. MHP leads to a uniform coating thickness with lower porosity and roughness. ZnAl4 coating and MHP post-treatment improved corrosion fatigue behavior in the high cycle fatigue regime with an increase of the stress amplitude, applied to reach a number of cycles 1.2 × 106, up to 115% compared to sandblasted specimens. Corrosive attack of the substrate steel was successfully avoided by using the coating systems. Stress- and microstructure-dependent corrosion fatigue damage mechanisms were evaluated by mechanical and electrochemical measurement techniques.
Highlights
Offshore wind turbines, pipelines, or bridges are examples of applications in the marine environment
Corrosive protection of metallic coatings is based on various principles
The mean values of the arithmetical mean roughness Ra and mean roughness depth Rz have been reduced by more than 50% for machine hammer peening (MHP) post-treated (III) specimen compared to the ZnAl-coated (II) specimen
Summary
Pipelines, or bridges are examples of applications in the marine environment. A combination of structural steel and corrosive metallic protective coatings has proved to be the best solution. Corrosive protection of metallic coatings is based on various principles. Barrier coatings separate the substrate steel from the corrosive environment. Coatings must have sufficient adhesion to the substrate, adequate strength to resist first damage, and be suitably ductile to resist cracking [2]. Cathodic protection coatings work as a sacrificial anode. Substrate material and coating form a galvanic couple with the metallic coating having a more negative electrode potential. If the coating is damaged and the substrate steel comes into contact with the corrosive medium, the coating will preferentially degrade
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