Corrosion resistance of micro-arc oxidation composite coatings on S355 offshore steel

Composite coatings were prepared by laser cladding combined with micro-arc oxidation technique on the surface of S355 offshore steel, and the composite coating structures were analyzed using scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction. The corrosion behavior of the composite coating in 3.5% NaCl solution was investigated by polarization curve, electrochemical impedance spectroscopy, corrosive wear test, and immersion corrosion test, and compared with that of the cladding layer and substrate. The results show that the composite coating is mainly divided into inner dense layer and outer loose layer. The loose layer is mainly composed of γ-Al2O3, and the dense layer is mainly composed of α-Al2O3, and the surface hardness of the composite coating reaches the maximum value of HV0.2 1423.3, which is 47.6% higher than that of the cladding coating. Moreover, the surface hardness of S355 offshore steel is significantly improved. The interaction between corrosion and wear in the substrate is mainly corrosion-accelerating abrasion, whereas that in the coating is wear-accelerating corrosion. After micro-arc oxidation treatment, the corrosion potential of the composite coating moves negatively, the passive current density increases, the scale factor of wear-accelerated corrosion gradually decreases, and the corrosive wear resistance of the coating significantly improves. The immersion corrosion method of the cladding coating is mainly pitting corrosion, the composite coating is slightly corroded, and the maximum impedance modulus reaches 105.3 Ω·cm2, which is two orders of magnitude higher than that of the cladding coating. This finding indicates that the corrosive wear resistance of the coating can be further improved after composite treatment.