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

Abstract

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.

Introduction

Ceramics are known to be promising coating materials because of their improved oxidation, corrosion, erosion and wear properties. Generally ceramic coatings are deposited on metallic substrates using conventional methods like chemical vapour deposition (CVD), physical vapour deposition (PVD), pack cementation method etc. However, these coatings can only be deposited at high temperatures and pressures. The major drawbacks associated with these methods are oxidation of substrate during film densification, arising tension in the film due to its shrinkage during heat treatment and difference in thermal expansion co-efficient between the film and substrate. Normally coatings applied by these methods suffer from defects like pores, fissures in the coatings resulting in the initiation of corrosion from these regions in wet corrosive environments.

Alternatively, efforts are being made to apply corrosion and wear resistant ceramic coatings by the sol-gel technique on metallic substrates [1], [2], [3], [4]. This technique is considered to be simple, easy to adopt and requires low processing temperature. An important advantage associated with this methodology is that corrosion controlling dopants can be introduced in the sol at a desired level. Coatings by this method can also be applied on glasses [5], [6], [7], [8], [9], [10], plastics, ceramics in addition to metals [11], [12], [13], [14], [15], [16] of simple as well as complex shaped geometry. Further, coatings produced by sol-gel methods are homogenous, free from defects and inexpensive. This technique has been successfully used for developing amorphous as well as crystalline structures of the coatings at ambient temperature. Extensive research has been carried out by several investigators on applying coatings of ZrO₂, SiO₂, CeO₂, Al₂O₃ [17], [18], [19], [20] by the sol-gel process to prevent chemical corrosion and gas oxidation of the metallic substrate. Among the various ceramic coatings tried, alumina is one of the important technical ceramics because of its useful properties like chemical inertness, better thermal stability, improved wear and corrosion resistance. Most of the alumina coatings have been applied on stainless steel substrate by hydrolysis and poly condensation of aluminium alkoxide using sol-gel method [21], [22], [23], [24]. It has been reported that the properties of the coated alumina film such as microhardness, corrosion resistance and electrical properties substantially improve with increasing film thickness [25]. However, a moderate wear resistance of alumina coating is reported due to the limited strength of sol-gel binder phases [26].

Recently, phosphate bonded composite sol-gel based alumina coating has been developed and characterized on stainless steel substrate to improve its abrasion and corrosion resistance properties [27]. It has been reported that the microstructure of the coating depends on processing variables, which have a major role in determining their mechanical and electrochemical properties [26]. To the best of our knowledge no work has been reported on applying alumina coating on a pre-phosphated mild steel substrate using the sol-gel technique and its influence on abrasive wear and corrosion resistance properties.

In view of the above, an attempt has been made to increase the corrosion and wear resistance properties of mild steel substrate by applying alumina coating using the sol-gel technique. Prior to alumina coating, the mild steel substrate has been phosphated to improve its bonding with alumina layer. As known, phosphating is the treatment of metal surface to provide a coating of insoluble metal phosphate crystals, which strongly adhere to the base metal [28]. The insoluble metal phosphates of the phosphating solution precipitate on the surface of mild steel substrate to form a highly interlocked crystalline structure. This provides an excellent base for secondary coating and helps in improving the bonding of sol-gel alumina layer with the base metal [28]. The sol-gel coating is likely to improve corrosion and wear resistance property of mild steel substrate which is generally used for handling of abrasive materials like coal, ash and slurries in different engineering applications. In the present investigation open circuit potential variation with time (OCP), potentiodynamic polarization and impedance measurements were performed for alumina coated (with and without pre-phosphating) and bare mild steel specimens in 3.5% NaCl solution at room temperature. Two body (high stress) abrasion studies of the specimens have been carried out. Efforts have also been made to understand the bonding characteristics of pre-phosphated alumina coated mild steel substrate by examining its cross sectional microstructure with the help of SEM. Compositional analysis of the coating is done by EDX analysis.