Wear and electrochemical characterization of sol-gel alumina coating on chemically pre-treated mild steel substrate

In the present investigation sol-gel based alumina coating was obtained using aluminium alkoxide as the precursor material. The coating was deposited on a mild steel substrate by a dip coating technique. Chemical pretreatment (phosphating) was done on one set of mild steel samples prior to deposition of the alumina coating in order to improve the bonding characteristics. After deposition of the coating, corrosion and wear properties of coated and uncoated mild steel surfaces were evaluated. In electrochemical characterization, open circuit potential (OCP) variation with time, potentiodynamic polarization and electrochemical impedance measurements were carried out in 3.5% NaCl solution at room temperature. Two body (high stress) abrasive wear tests were performed to measure the abrasion resistance of the coating. Microstructural and elemental composition of the coating was investigated by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDXS). The experimental results indicated the occurrence of significantly higher corrosion and wear resistance of pre-phosphated alumina coated samples as compared to alumina coated samples without pre-phosphating. Presence of phosphorus and zinc in the intermediate phosphating layer appears to increase the bonding and corrosion resistance properties of the outer alumina layer. Results have been discussed in terms of the observed data related to electrochemical and wear studies.