Effect of carbonate and phosphate conversion pretreatments and optimization on corrosion behaviour of subsequent electroless nickel coating on AZ91 alloy

The study reports a simple two steps chemical conversion treatment to enable electroless nickel (EN) coating on AZ91 alloy without using chromium or fluoride. The process involves calcium dihydrogen phosphate deployed in step 1 followed by the transformation into more insoluble product, for better corrosion and adhesion resistance, via alkali treatment in NaOH or Na2CO3 during step 2. The latter is attempted by varying the concentration in the range 0.25 to 2 M. The performance of conversion treatments is critically evaluated by the quality of the EN coating, deposited subsequently, with respect to morphology, corrosion, and adhesion resistance. The X-ray diffraction (XRD) and Raman analysis of the pretreated surface exhibits complete transformation to tri calcium phosphate or calcium carbonate after treated in NaOH or Na2CO3 solution respectively at an optimum 1M concentration. The crack dimensions on the surface of thus pretreated AZ91 are largely suppressed, compared to those at < 1M or > 1M NaOH/Na2CO3 concentration, as revealed by scanning electron microscopy (SEM) and 3D profilometer. The EN coating on pretreated substrate at 1M alkali concentration shows higher corrosion and scratch resistance than that at other concentrations. The 1M Na2CO3 appears superior to NaOH pretreatment, as it imparts better corrosion and adhesion resistance of the subsequent EN coating. The electrochemical impedance spectroscopy (EIS) on EN coating after 1M Na2CO3 treatment shows no coating deterioration up to 300 h of exposure to 0.5% NaCl solution as opposed to NaOH (for 100 h only).