Effect of sintering temperatures on corrosion and wear properties of sol–gel alumina coatings on surface pre-treated mild steel G. Ruhi a , O.P. Modi a , A.S.K. Sinha b , I.B. Singh a, * a Regional Research Laboratory (CSIR), Hoshangabad Road, Bhopal 462026, India b Department of Chemical Engineering, Institute of Technology Banaras Hindu University Varanasi 222105, India Received 5 October 2006; accepted 3 October 2007 Available online 7 November 2007 Abstract Sol–gel alumina coatings were developed on the surface pre-treated (zinc-phosphated) mild steel substrate and subsequently sintered at 300 °C, 400 °C and 500 °C. The alumina sol was synthesised using aluminium iso-propoxide as a precursor material. FTIR of the boehmite (AlOOH) gel sintered at above-mentioned temperatures was employed to identify the presence of various functional groups. The microstructural features and the phase analysis of the sol–gel coated specimens were carried out using SEM and XRD respectively. The corrosion resistance of the sol–gel alumina coatings was evaluated by electrochemical measurement in 3.5% NaCl solution at room temperature. The abrasive wear behaviour of the sol–gel coated specimens was measured in two body (high stress) conditions. The exper- imental results revealed that the sol–gel coated specimen sintered at 400 °C has superior wear and corrosion resistance properties as com- pared to the sol–gel coated specimen sintered at 300 °C. However, the sol–gel coated specimen sintered at 500 °C has exhibited a very poor corrosion and wear resistance properties. Poor performance of the sol–gel coatings sintered at 500 °C could be explained to be due to (i) the presence of numerous cracks (ii) absence of organic groups in the coating. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Mild steel; B. EIS; B. X-ray diffraction; B. SEM; C. Sol–gel alumina coatings 1. Introduction Mild steel is widely used as a structural material in a number of engineering applications because of its good machinability, high thermal conductivity and superior mechanical strength. However, it is highly prone to suffer from corrosion and wear as a consequence of which its ser- vice life is limited. Therefore, a number of approaches have been adopted to improve its corrosion and wear resistance properties [1–4]. Development of sol–gel based protective coatings is one of the recent approaches [5–8]. Coatings developed through the sol–gel route have been reported to improve the corrosion and wear resistance of various metal- lic materials like mild steel, stainless steel, magnesium, alu- minium and their alloys [9–21]. Coatings developed through the sol–gel route do not require high processing tempera- tures and high vacuum conditions as in the case of CVD and PVD [22–25]. Further, this process is relatively less expensive, easily adoptable and applicable to coat on mate- rials having complex shaped geometry. Good adherence of a coating to its substrate is one of the most important aspects of a successful coating. It has been speculated that an excellent coating is obtained when bonds between the film and the substrate is produced before a strong bond within the film is formed [26]. Pres- ence of polar solvents and highly diffusive Cl  ions adversely affect the bonds between the metal and the coat- ing. Therefore, strong bonding between the coating and the substrate has a unique significance when the coated metals are used in aggressive corrosive environment like seawater. The sol–gel based ceramic coatings have been reported to suffer from delamination due to mismatch in thermal expansion coefficient between the coating material and 0010-938X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2007.10.002 * Corresponding author. Tel.: +91 755 2587615; fax: +91 755 2587042. E-mail address: ibsingh58@yahoo.com (I.B. Singh). www.elsevier.com/locate/corsci Available online at www.sciencedirect.com Corrosion Science 50 (2008) 639–649