Electropolymerisation of aniline on AZ91 magnesium alloy: The effect of coating electrolyte corrosiveness

Abstract

In this study, polyaniline was coated on AZ91 magnesium alloy using an electropolymerisation technique, and the effect of corrosiveness of the coating electrolytes on the polymerisation and the coating performance were evaluated. Two electrolytes, i.e., aniline + sodium salicylate (PASS) and aniline + potassium hydroxide (PAPH), with different corrosiveness, were used for polyaniline coating on AZ91 magnesium alloy. Potentiodynamic polarisation results suggested that salicylic acid (C7H5NaO3) was more corrosive for the alloy than potassium hydroxide (KOH), which can be attributed to the difference in the pH of the electrolytes. The PASS electrolyte coating formed on the alloy was relatively thick (~9 µm) and exhibited scattered pore-like morphology, whereas the PAPH electrolyte coating was thin (~3 µm) and uniform. Fourier Transform Infrared (FTIR) spectroscopy analysis revealed that the PASS electrolyte coating corresponds to polyaniline, whereas the PAPH electrolyte coating showed weak polyaniline bands. The corrosion protection performance of the coatings was evaluated in chloride-containing solution. The potentiodynamic polarisation results suggested that the corrosion rate of the alloy decreased significantly with the PASS electrolyte coating, whereas the PAPH electrolyte coating was detrimental. The degree of protection (DP) provided by the PASS electrolyte coating was ~83%. Post-corrosion analysis revealed higher corrosion attack in the PAPH electrolyte-coated alloy in comparison with the PASS electrolyte coated alloy. Thus, it can be concluded that the corrosiveness of the PASS coating electrolyte did not adversely affect the formation/performance of polyaniline on AZ91 magnesium alloy.