Sol-gel coatings for corrosion protection

The above excerpt from an English Translation of Pliny the Elder’s Latin book on Natural History dated 77 CE may perhaps be the earliest written record of the use of a sol-gel type anticorrosion coating on metals. The war of man against corrosion has been relentless. We have devised various ways to protect various metals from corrosion – through judicious materials selection, application of various kinds of inorganic and organic coatings, using corrosion inhibitors, cathodic protection, and design elements that prevent corrosion. However, nature likes to revert back to its lowest thermodynamic form as an oxide from which we extract almost all materials for industrial use. Although modern scientific literature on corrosion protection coatings can be traced back to the era of pack cementation and electrodeposition in general, followed by aluminizing of iron in the twentieth century, the systematic study of sol-gel type coatings mitigating corrosion is relatively recent. Sol-gel is a surprisingly simple process that involves dipping the substrate with a sol to form a tacky, adherent gel film on curing, which then may or may not be subsequently calcined to get a microporous and mesoporous protective organic, inorganic or hybrid corrosion protective coatings. The advantages of sol-gel coatings, vis., benign conditions of deposition (e.g. relatively low temperatures) and the ability to produce coatings on complex shapes without the need for machining or melting (hence no expensive equipment) has led to a significant amount of work on sol-gel based protective coatings primarily for metals. The ease of application led to a burst of research since the turn of the century (Figure 1) has also been driven by the need for creating environmentally friendly materials and processes to replace the traditional chromium-based and/or solvent-based anti-corrosion coatings. Sol-gel-based coatings can be brought about through an inorganic or organic route. The former, which was probably the technique described by Pliny the Elder at the start of the Common Era, involves the gelation colloidal suspension of nanometric particles of inorganic materials (e.g. lead oxide, lead carbonate, and calcium sulphate in Pliny’s antipathia) to form a network in a continuous liquid phase. This is however rarer than the organic route in which a prepolymer is polymerized into a gel to form a protective network. The alkoxide-based process – the formation of an oxide (usually silicon oxide) network by progressive condensation of a metalloid alkoxide in a liquid medium is a classic example (Figure 2). In this alkoxide route, subsequent sintering the gel-coated substrate to high temperatures (440–1200°C) for short heating times, about 15 minutes, leads to the hardening of the coating to various degrees due to the formation of the oxides. Alkoxide-based sol-gel corrosion coatings have many benefits such as ease and flexibility of the fabrication process, the abundance of commercially available precursor reagents with tailored functional groups, and their low environmental impact. Additionally, there is excellent control over precursor stoichiometry and the ability to integrate diverse components that introduce complementary functions. Alkoxide-based sol-gel-generated SiO2, ZrO2, Al2O3, TiO2, and CeO2, coatings. have all been studied at various times because of their excellent chemical stability and provision of effective protection to the metal substrate. However, alkoxysilanes, such as tetra-oxy silicate (Si(OR)4) and organically modified silicates (Ormosils, R’nSi(OR)4−n) have been the most frequently studied precursors for sol-gel generated SiO2-based coatings [2]. The thickness and nature of these coatings can be modified by controlling the rheology of the sol, by using additives, or by modifying the reaction conditions including drying time and calcining temperature. Multi-component oxide coatings have been developed to overcome the limitation of single-component oxide layers, broaden their application areas and improve the comprehensive protective ability of steel substrates. Composite coatings containing SiO2 and ZrO2 or Al2O3 have been shown to act efficiently as corrosion protectors of 316L stainless steel substrates in aqueous NaCl and acid media at RT [3]. Hybrid silica-titania hybrid coatings containing cerium (III)