The corrosion resistance of 316L stainless steel coated with a silane hybrid nanocomposite coating

https://doi.org/10.1016/j.porgcoat.2011.08.001Get rights and content

Abstract

The present work aims at evaluating the corrosion resistance of 316L stainless steel pre-treated with an organic–inorganic silane hybrid coating. The latter was prepared via a sol–gel process using 3-glycidoxypropyl-trimethoxysilane as a precursor and bisphenol A as a cross-linking agent. The corrosion resistance of the pre-treated substrates was evaluated by neutral salt spray tests, linear sweep voltammetry and electrochemical impedance spectroscopy techniques during immersion in a 3.5% NaCl solution. In addition, the effect of the drying method as an effective parameter on the microscopic features of the hybrid coatings was studied using Fourier transform infrared spectroscopy and scanning electron microscopy. Results show that the silane hybrid coatings provide a good coverage and an additional corrosion protection of the 316L substrate.

Highlights

▸ Evaluation of the corrosion resistance of 316L stainless steel with hybrid coatings based on 3-glycidoxypropyl-trimethoxysilane (GPTMS) as the precursor and bisphenol A (BPA) as the cross-linking agent. ▸ Study of the effect of the drying method as an effective parameter on the microscopic features of the hybrid coatings. ▸ Results indicate the formation of a denser siloxane network upon thermal curing. ▸ Electrochemical analyses as well as salt spray test highlight the good barrier properties of the deposited silane film.

Introduction

One of the most effective metal corrosion control techniques consists of the electrical isolation of the anode from the cathode [1], [2]. A chromium oxide (Cr2O3) passivation layer formed on the surface of stainless steel in oxidizing environments is a typical example of this and guarantees the durability and the corrosion resistance of this particular metal [2], [3], [4]. However, chromates are now heavily restricted in corrosion control procedures due to the Cr(VI) toxicity and their carcinogenic nature [5]. For this reason, a number of promising candidates, so called “green inhibitors”, have been explored with the hope of replacing chromates. Among them, silanes, a group of silicon based organic–inorganic chemicals, have emerged as a very promising alternative [6].

Silane films not only ensure the adhesion between metal substrates and organic coatings but they also provide a thin, but efficient, barrier against oxygen diffusion to the metal interface [7], [8]. Recently, silane coatings have attracted the attention of the nanotechnology industry because they provide a highly uniform, robust and reliable coating with lateral resolution on the nanometer scale [9]. A general silane structure is (XO)3Si(CH2)nY, where XO is a hydrolysable alkoxy group, which can be methoxy (OCH3), ethoxy (OC2H5) or acetoxy (OCOCH3). Y is an organofunctional group such as epoxy, vinyl (Cdouble bondC) or amino (NH2) which is responsible for a good adhesion of a silane treated metal surface [6], [10].

3-Glycidoxypropyl-trimethoxysilane (GPTMS) is one of the organofunctional silane molecules that have been used as an effective coupling agent or adhesion promoter in glass/mineral-reinforced polymeric composites for decades [6], [11], [12], [13]. Also it has been used in transparent abrasion-resistant hybrid coatings for polymers [14], [15], [16], [17], metals [18], [19], [20] and for gas separation membranes [21], [22].

GPTMS can undergo a variety of reactions during the preparation of a hybrid by a sol–gel route. Hydrolysis of the methoxy groups gives silanol groups, which can subsequently condense to form the silane network. The silicon atom in GPTMS is tri-functional in terms of the reactive methoxy groups and is therefore able to form a three-dimensional branched siloxane silane network with a nominal stoichiometry SiO1.5. The epoxy rings can be opened and polymerised to form a linear poly(ethylene oxide) organic network [22], [23].

Cross-links between the two networks arise either from the pre-existing link in the GPTMS molecule, by direct reaction of silanols with epoxy rings, or by condensation of silanols with hydroxyl of the opened epoxy rings. The uncatalysed ring opening reaction occurs at a useful rate only at elevated temperature and so thermal curing is required [22], [23].

The use of the sol–gel process to prepare highly intermingled inorganic–organic hybrid polymer networks is of current scientific interest since it offers the possibility of tailoring the material properties by variation of the relative composition of the inorganic and organic phases. With such systems, the inorganic and organic networks are formed together (often simultaneously) to achieve homogeneous phase morphologies, which are impossible to produce by the other routes [22].

The microscopic features of the hybrid coatings obtained with sol–gel process depend on those hydrolysis and condensation reactions that are generally controlled by the pH of the solution [24], [25]. In the acid catalyzed reaction, the hydrolysis step is faster than the condensation step, resulting in a more extended and less branched network structure. In the base catalyzed reaction, condensation is faster than hydrolysis, resulting in highly condensed species that may agglomerate into fine particles [24], [26], [27], [28]. Furthermore, the sol–gel process includes a very complex reaction involving many variables such as pH, type and amount of solvent, water/alkoxide ratio [28], [29], concentrations of organic [26] and inorganic [23] reactants, aging [30], cross-linking agent/alkoxide ratio, and drying methods. We investigated some of these factors in previous studies on a 1050 aluminium alloy [23], [26], [28], [29], [31].

As a follow-up of these results, the present work aims at evaluating the structure and corrosion protection performance of silane sol–gel hybrid coatings deposited on austenitic 316L stainless steel. This steel combines properties such as acceptable biocompatibility and excellent mechanical resistance with easy fabrication at low cost, which permits its use in many industrial applications, consumer products and temporary orthopedic devices [32], [33]. In general 316L stainless steel presents good anti-corrosion properties, however its corrosion resistance is weakened when it is subjected to a medium containing chloride or moisture environment [34], [35], with a tendency to suffer localized corrosion (e.g. pitting corrosion). The corrosion may result in the loss of aesthetic appearance and structural integrity [36], and can be accompanied by the release of potentially toxic ions, which can be reduced by protective coatings [37], [38], [39]. Furthermore, the corrosion of stainless steel welds and joints occurs in the transpassive potential region in highly oxidizing environments of many industrial processes, especially in sulfuric acid media releasing Cr(VI) ions [40]. To resolve all these problems, the application of hybrid coatings has been increasingly investigated in the last years [38], [41], [42], [43].

Sol–gel coatings were prepared from hydrolysis and condensation of 3-glycidoxypropyl-trimethoxysilane (GPTMS) as the precursor and bisphenol A (BPA) as the cross-linking agent in acid catalyzed condition. The effect of the drying method as an effective parameter on the microscopic features and morphology of the silane hybrid coating was examined using Fourier transform infrared spectroscopy (FTIR). In order to study the corrosion protection efficiency, the coated and uncoated steel samples were investigated using potentiodynamic polarization curves, neutral salt spray tests and electrochemical impedance spectroscopy. In addition, scanning electron microscopy (SEM) analysis was performed to visualize the surface.

The results show that pre-treatments based on silane hybrid solutions present improved corrosion resistance for the 316L substrates. Furthermore, the methodology proposed in this work is simple to apply and it is compatible with actual environmental concerns.

Section snippets

Materials

All chemicals and reagents used were purchased from Merck, including 3-glicidoxypropyl-trimethoxysilane (GPTMS), bisphenol A (BPA), hydrochloric acid, sodium chloride and 1-methylimidazol (MI).

Preparation of the sol

The sol was prepared by adding stoichiometric amounts of the silane precursor (GPTMS) and the organic cross-linking agent (BPA) into HCl-acidified water (pH = 2). The H2O/Si molar ratio was chosen to be 0.5, relying on results of a previous study [26]. The solution was stirred at ambient temperature for 80 

Characterization

Fig. 1 shows the FTIR spectra of two series of hybrid films (A-SHC and T-SHC). The most resolved bands are attributed to the vibration frequencies of the glycidoxypropylsiloxane structural fragment. They include the oxiranes methylene bending at 1480 cm−1, the epoxide ring breathing band at 1250 cm−1 and the antisymmetric epoxide ring deformation bands at 758 and 902 cm−1 [44]. The band of the antisymmetric epoxide ring deformation at 758 and 902 cm−1 in the magnified region from 729 to 950 cm−1 in

Conclusions

In this study, the corrosion protective behavior of hybrid coatings based on 3-glycidoxypropyl-trimethoxysilane (GPTMS) as the precursor and bisphenol A (BPA) as the cross-linking agent as an “environmentally friendly” corrosion resistant protection system on 316L stainless steel substrates were studied. Additionally, the effect of drying method on microscopic features and morphology of the hybrid coatings was evaluated. The results of the FTIR analysis indicated the formation of a denser

Acknowledgements

The authors wish to acknowledge Ghent University for financial support of this study. The authors would also like to thank Peter Mast, Michel Moors, Christa Sonck and Veerle Boterberg for their technical help.

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