Corrosion resistance of stainless steel coated with sol-gel silica

Sol-gel silica films were deposited on AISI 304 stainless steel using tetraethyl ortosilicate (TEOS) as chemical precursor. The coated steels were heat-treated from room temperature to 600 degrees C. The structural evaluation of the coated steel samples was made by scanning electron microscopy, infrared absorption spectroscopy, and chemical deep profiles of Si, O, and Fe obtained by Rutherford backscattering spectroscopy. Corrosion experiments were carried out on samples with and without coatings, in 1 N H2SO4 and 3.5% NaCl solutions to measure corrosion potential, current density and pitting potential. The obtained sol-gel silica coatings were homogeneous, free of cracks and had thickness from 50 to 120 nm. Polarization curves of the coated samples indicated improvement of the corrosion resistance. For tests carried out in a 1 N H2SO4 solution, the corrosion potential increased from -341 mV (stainless steel) to +383 mV for coated sample heat-treated at 80 degrees C, and up to +508 mV for coated sample heat-treated at 200 degrees C. The corrosion current density decreased from 3.65 mu A/cm(2) to 0.027 mu A/cm(2) and 0.021 mu A/cm(2), for the 80 degrees C and 200 degrees C heat-treated silica coatings, respectively. For the corrosion experiment carried out in a 3.5% NaCl solution, the coated sample (heat-treated at 80 degrees C) increased the corrosion potential c.a. 230 mV and increased by one order of magnitude the pitting potential. (C) 2000 Elsevier Science B.V. All rights reserved.