Preparation of Protective Alumina Coating on Austenitic Stainless Steel by Sol-Gel Method

Aluminum iso-propoxide was used for alumina coating on Type 316 stainless steel. The optimum thickness of sol-coating was obtained by proper adjustment of the viscosity of the sol and by proper selection of the specimen removal speed from the sol. After gelation in air, baking the gel-coated specimen in vacuum provided the formation of adhesive alumina coating, while baking in air resulted in serious oxidation of the substrate steel in addition to exfoliation of flaky alumina. X-ray diffraction revealed that the alumina coating was mostly amorphous containing small amounts of microcrystalline γ-Al₂O3 when baked up to 900°C in vacuum but baking temperature increase up to 1000°C resulted in transformation into α-Al₂O3 in addition to growth of γ-Al₂O3 crystal. The protective coating can be obtained by repetition of coating including baking at the highest temperature where the coating is mostly amorphous. For instance, when coating including baking at 900°C for 1 h in vacuum was repeated three times, the alumina coating was protective against corrosion of the steel even after immersion for 2 months in 0.5M H₂SO₄. However, baking at 500 and 700°C was not effective in preventing corrosion of the steel. Growth of γ′-Al₂O3 and transformation into α-Al₂O₃ resulted in the formation of island-like coating and hence were detrimental for protective coating.