Galvanic corrosion behavior of AZ31 Mg alloy coupled with mild steel: effect of coatings

For Mg alloy, it is important to develop effective protection coatings against galvanic corrosion. In this work, the effects of plasma electrolytic oxidation (PEO) coating with and without a polyurethane (PU) top layer on the corrosion and galvanic corrosion (coupled with mild steel) behavior of AZ31 Mg alloy are investigated. The microstructure and compositions of the coatings were studied by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffractometer (XRD) and Fourier transform infrared spectroscopy (FT-IR). The corrosion and galvanic corrosion behavior of uncoated, PEO and PEO-PU coated AZ31 Mg alloys were evaluated by electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) tests, respectively. Results showed that the PEO coating is characterized by many micropores in structure and the micropores are sealed by the PU top layer. The OCP, EIS and SECM tests revealed that the single PEO coating only slightly reduces the galvanic corrosion tendency and marginally improved the corrosion resistance, resulting in short-term galvanic corrosion protection for AZ31 Mg alloy. In contrast to the single PEO coating, the PEO-PU coating greatly improves the stability of the AZ31 Mg alloy to galvanic corrosion and provides much better long-term corrosion protection performance, which could effectively protect the substrate from galvanic corrosion. The galvanic corrosion processes of the AZ31 Mg alloy with PEO and PEO-PU coatings are also discussed. These results demonstrate that the sealing of PEO coating is essential to inhibit the galvanic corrosion between Mg alloy and other metals.