Protection of Steel Components Against Marine Corrosion by Thermally Sprayed Anodic Coatings

Abstract

Abstract It is well known that thermally sprayed aluminum and aluminum alloys can be used to protect low-alloyed steel against marine corrosion in offshore applications. The efficiency and service life of this protection can be, however, severely limited by the amount and distribution of defects, which are usually present in coating microstructures. In thermal spraying, microstructures and properties are strongly influenced by the type of spray system used for the production of coatings. To investigate the influence of defects like pores, oxides and cracks on the corrosion performance, coatings were processed by conventional thermal spray techniques, such as Flame Spraying (FS) and Arc Spraying (AS). In addition, the more recently introduced High Velocity Combustion Wire (HVCW) spraying technique was used, which, due to higher particle velocities, results in lower porosity and finer coating microstructures as compared to conventional processes. The influence of spray conditions and related microstructures on the performance in corrosion tests was investigated for protective coatings of Al99.5, AlMg5 and Al - 30 wt. % W2C. The resistance against corrosion was analyzed by different electrochemical methods, such as corrosion potential monitoring, polarization resistance and potentiodynamic anodic polarization measurements. Additionally, the microstructures of the coatings were examined before and after the electrochemical tests. The results from these tests are correlated and attributed to the different microstructures obtained by the various spray techniques and different compositions of the feedstock material.