Protection of 316L stainless steel against corrosion by SiO2 coatings

Abstract

Oxide films prepared by sol-gel methods and presenting high resistance to heat, corrosion, friction and wear, as well as excellent mechanical properties, have recently been developed and put into practical use as structuraI materials [1-5]. The process of preparation offers potential advantages for modifying the properties of surfaces by low-temperature treatment without altering the original properties of strength and toughness of the substrates. A number of reports on sol-gel coatings concerning the prevention of chemical corrosion and oxidation have been published [6-10]. All of these films increase the protection of metal substrates from air corrosion (tested up to 800 °C) and acid attack (tested up to 80 °C). The most promising corrosion prevention for stainless steel has been studied by our group using sol-gel films of ZrO2, SiO2, SiO2-TiO2 and SiO 2A1203 prepared by dip-coating using sonocatalysed sols [11-14]. The. properties of these coatings have been studied by electrochemical techniques in NaC1 and H2SO 4 solutions. Although preliminary measurements have shown that SiO2 films are not the best protective coatings [13], they provide a very adequate model system to correlate corrosion protection with the physical structure of the sol-gel films. In this work, amorphous coatings of SiO 2 were deposited on 316L stainless steel by the dip-coating technique using a sol preparation involving sonocatalysis. The films were prepared through hydrolysis polymerization of metal alkoxide solutions and conversion to an oxide layer by heating at relatively low temperatures. The effect of the time of heat treatment of the SiO2 films on the corrosion resistance of stainless steel was studied in 15% H2SO 4 through potentiodynamic polarization curves at 25 °C. The substrate used in the experiments was 316L stainless steel (SS 316L, Caseurop, France) with chemical composition (wt %): 67.25 Fe, 18.55 Cr, 11.16 Ni, 2.01 Mo, 0.026 Cu, 1.71 Mn and 0.028 C. The specimens were machined into dimensions of 30 mm × 15 mm x I mm, degreased ultrasonically in acetone, cleansed by distilled water then dried in air. For silica films, tetraethylorthosilicate Si(OC2H5) 4 (TEOS) was used as the source of silica, absolute ethanol (C2HsOH) as solvent and glacial acetic acid CH3COOH as catalyst. The silica sonosol was prepared by dissolving Si(OCzHs) 4 in absolute ethanol to which a small amount of acetic acid CH3COOH was added. The volume ratios of Si(OC2Hs)4/C2HsOH and Si(OC2Hs)4/CH3COOH were, respectively, 1 and 5. The mixture was submitted to intense ultrasonic irradiation (20 kHz) produced by a transducer (Heat Systems Ultrasonics W385). After 25 min the resulting sol was homogenized and remained stable for about five weeks at room temperature when kept in a closed vessel. Coating films were formed on the substrates by dipping into the clear sonosol and withdrawing at a speed of 10 cmmin -1. The resulting gel films were dried at 60 °C for 15 min and densified in a furnace with air atmosphere by increasing the temperature at a rate of 5 °C min -1 up to 450 °C when an isothermal holding of 1 h was applied in order to remove the organic residues. The temperature was then increased again at the same rate up to either 600 or 800 °C and maintained at that value for variable lengths of time to complete the densification and obtain adherent coatings. The average thickness of the heat-treated films at 800 °C was around 0.4/zm. X-ray diffraction (XRD) analysis of the substrate and coatings was done with a Philips diffractometer using CuKo, The diffractogram of SS 316L shows the existence of a crystalline phase which corresponds to the cubic phase of the alloy containing Cr, Fe and Ni [4]. When the steel was heated at 800 °C for 2 h in air, additional XRD peaks appear corresponding to a mixture of cubic and hexagonal CrzO 3 [4]. In contrast, samples coated with SiO2 analysed after oxidation tests in air at 800 °C for 2 h showed only the peaks of the original substrate, indicating that the silica coating remains amorphous and inhibits any oxidation of the base material. A Bomem Fourier transformation infrared (F-FIR) spectrometer was used to obtain high resolution spectra of the coatings in the 400-4000 cm -1 range; the measurements were carried out at room temperature by reflection at an incident angle of 30 ° . The spectrum of a coating deposited on SS 316L and