Preparation of Protective Multilayer SiO₂ and Al₂O₃ Coatings on Austenitic Stainless Steel by Sol-Gel Method

A method of forming continuous silica and alumina coating films using a sol-gel dip coating and a protectiveness of the coating films are presented. The protectiveness which is related to the defects in the coatings is evaluated by electrochemical methods including polarization curve and open circuit potential measurements in deaerated 0.5M H₂SO₄ at 303K. The transparent, continuous and adhesive sol-gel silica coating films are obtained when the coatings are applied to single layer alumina-coated Type 316 stainless steel substrate, while the films directly coated onto the mechanically polished substrate tends to produce some defects. The appearance and protectiveness depend on the withdrawal speed and the baking temperature. For instance, the most suitable withdrawal speed and baking temperature are 1.07 and 1.80×10^-3m/s, and 673K, respectively. The high protective coating can be obtained by two-layer silica-coatings due to reduction of the defect densities in underlying coatings.