Effects of laser shock peening post-treatment on microstructure and corrosion resistance of phosphate conversion coatings

Abstract

In this work, to enhance the corrosion resistance of Mg alloys and extend their application fields, phosphate conversion (PC) coatings were deposited by chemical conversion followed by laser shock peening (LSP) post-treatment. The microstructure, phase structure, surface residual stress, and corrosion performance of the PC coatings before and after LSP treatment were studied. The PC coating showed a porous and rough structure. By contrast, after LSP treatment, the phosphate crystals on the coating surface were refined, flattened, and densified under the action of the shock wave, while generating residual compressive stress of −30 MPa. The thickness loss of PC coatings after LSP increases with the laser energy due to the competition between densification and erosion of the PC coating. Electrochemical analyses showed the corrosion current of PC coating after LSP decreased from 1.48 μA/cm2 to 0.25 μA/cm2, which was attributed to the dense and flat coating. Meanwhile, the impedance of the PC coating after LSP was increased by 2–5 times compared with the untreated PC coating. Especially when the laser energy was 160 MJ, the internal layer resistance of the coating reached 30090 Ω cm2, and the area damaged by the corrosive medium was smaller. Overall, the LSP post-treatment process is expected to improve the corrosion resistance of PC coatings deposited on Mg alloys, further broadening the application of magnesium alloys in medicine.