Organic-inorganic sol-gel coating for corrosion protection of stainless steel

One of the most effective corrosion control techniques is the electrical isolation of the anode from the cathode [1, 2]. The chromium oxide (Cr2O3) passivation layer formed on the surface of stainless steel in oxidizing environments is one example. This is the main reason for the durability and corrosion resistance behavior of this particular metal [2, 3]. A more generic approach to enhance corrosion resistance is to apply protective films or coatings. Through the modification of chemical composition of the coatings, such protective coatings can also permit the introduction of other desired chemical and physical properties, such as mechanical strength and hydrophobicity. Various organic coatings have been studied for corrosion protection [4–6]. Specifically, various oxide coatings by sol-gel processing have been studied extensively for corrosion protection of stainless steel [9–13]. In spite of all the advantages of sol-gel processing, sol-gel oxide coatings suffer from several drawbacks. In general, sol-gel coatings are highly porous with low mechanical integrity; annealing or sintering at high temperatures (>800 ◦C) is required to achieve a dense microstructure [14–17]. Consequently, sintering at high temperatures might introduce cracks and/or delamination of sol-gel coatings due to a large mismatch of thermal expansion coefficients and possible chemical reactions at the interface. Sintering at high temperatures also limits application of sol-gel coatings on temperature sensitive substrates and devices. One viable approach to dense, sol-gel-derived coatings without post-deposition annealing at elevated temperatures is to synthesize organic-inorganic hybrid coatings. When appropriate chemical composition and processing conditions are applied, relatively dense organic-inorganic hybrid coatings can be developed for applications, including wear resistance [18, 19] and corrosion protection [20–22]. Messaddeq et al. [21] studied corrosion resistance of organic-inorganic hybrid coatings on stainless steel. The coatings were made by dispersing various amounts of polymethylmethacrylate (PMMA) into zirconia (ZrO2) sol and fired at 200 ◦C for 30 min. PMMA-ZrO2 coatings demonstrated promising corrosion resistance and increased the lifetime of the stainless steel by a factor 30 [21]. However, phase segregation, incomplete coverage, and delamination were observed when the coatings consisted of a high content of organic components. In this paper, we studied the corrosion resistance of sol-gel-derived, organic-inorganic hybrid single-layer coatings on two types of stainless steel. Sol-gel-derived coatings were made from tetraethylorthosilicate (TEOS) and 3-methacryloxypropyltrimethoxysilane (MPS) using a two-step acid catalysis process, and were annealed at 300 ◦C for 30 min. It was demonstrated that sol-gel derived hybrid coatings could significantly enhance the corrosion protection of both 304 and 316 stainless steel substrates. Furthermore, the corrosion resistance behavior of the hybrid coatings on both types of stainless steel was compared and possible mechanisms were discussed. The silica-based organic-inorganic hybrid sol was prepared with an acid-catalyzed, two-step hydrolysiscondensation process. The hybrid sol was prepared by admixing a silica precursor, tetraethylorthosilicate (TEOS, Si(OC2H5)4), and an organic component, 3-methacryloxypropyltrimethoxysilane (MPS, H2CC (CH3)CO2(CH2)3Si(OCH3)3), to control the flexibility and density of the sol-gel network. Silica (SiO2) sol containing 10 mol% MPS with a TEOS : MPS ratio of 90 : 10 was used for analysis. An initial stock solution was made by adding amounts of TEOS and MPS in a mixture of ethanol (C2H5OH), deionized water (DI H2O), and 1N hydrochloric acid (HCl), resulting in a TEOS : MPS : C2H5 : DI-H2O : HCl nominal molar ratio of 0.90 : 0.10 : 3.8 : 5 : 4.8 × 10−3. The mixture was vigorously stirred at a rate of 500 RPM for 90 min at a temperature of 60 ◦C, and further processing of the sol required an additional 3.6 mL 1N HCl and 1.2 mL DI H2O to 30 mL of the stock solution. The sol was stirred again at a rate of 500 RPM for 60 min at a temperature of 60 ◦C. Ethanol was added to dilute the sol in order to obtain a volume ratio of 2 : 1 ethanol to solution. The substrates (10 mm × 40 mm in dimension) used for the analysis of the sol-gel coatings were 304 and 316 stainless steel that had been electropolished. The exposure of the substrates to nitric acid (HNO3) decreased the iron content and increased the chromium content