Effect of phosphating solution pH value on the formation of phosphate conversion coatings for corrosion behaviors on AZ91D

In this paper, the pH value effect of phosphating solution on the formed phosphate conversion coatings (CCs) has been studied for the corrosion behavior of AZ91D. The electrochemical tests have been processed on the CCs fabricated under different pH values of phosphating solution. Morphological evolution and composition of the coatings, as a function of pH value, were examined by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XRD). The width and distribution area of micro-cracks on the surfaces were measured and statistical analyzed by an image-pro-plus software. Moreover, potentiodynamic polarization test, electrochemical impedance spectroscopy analysis, and immersion test were used to evaluate the corrosion property of the phosphate CCs in 3.5% NaCl solution. The dimension of the micro-cracks decreased with the increase of the value of pH. The CCs fabricated under the pH value of 4.0 showed the best corrosion resistance and the width and distribution area of micro-crack on the CCs were the minimum (0.78 (± 0.08) μm and 1253 (± 140) μm 2 , respectively). The self-corrosion current of the CCs was the minimum (2.85 μA/cm 2 ), and the radius of capacitance arc in the electrochemical impedance spectroscopy was the maximum. The corrosion rate in immersion test also was the lowest. This study shows an alternative for the metal corrosion. Graphical abstract Different pH values of phosphating solution on corrosion behaviors of phosphate conversion coatings on AZ91D were investigated.