Microstructure and corrosion performance of Zr-1Nb alloy laser welded joints treated by micro-arc oxidation

The protective coatings on laser-welded joints of Zr-1Nb alloy were prepared by the micro-arc oxidation (MAO) process. The morphology, microstructure and phase composition of MAO coatings at different regions of Zr-1Nb alloy laser-welded joints were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion behaviors of the coatings. The MAO coatings at the fusion zone (FZ) and heat-affected zone (HAZ) were smooth with some small holes, while MAO coatings at base metal (BM) were relatively rough with large holes. The cross-sectional topography at FZ coatings was quite dense with less distribution of pores and cracks, which can effectively improve the protective properties of the coatings. HAZ coatings were divided into outer loose layer and inner dense layer. The BM coating layer possessing many holes was a typical sparse layer structure. The phase compositions of welded joints treated by MAO were diverse at different zones of the joints, but all of them consisted of the monoclinic ZrO2 and tetragonal ZrO2 phases. The coatings of FZ had the best corrosion resistance, compared with HAZ and BM. The results of EIS indicated that the thickness and defects of the inner compact layer played a key role in improving the corrosion resistance of the MAO coatings on the surface of the Zr-1Nb alloy laser-welded joints.