Two layered silica protective film made by a spray-and-dip coating method on 304 stainless steel

The silica coating has attracted much attention because of its superior corrosion resistance with almost no harm to human health and to the environment. In this study, a two layered silica film was tried to get an enhanced corrosion resistance. The silica film was prepared on the hairline finish 304 stainless steel surfaces by-a-spray- and subsequent-dip-coating process. The spray coating solution was prepared by mixing sodium silicate solution, silica colloid, tetraethyl orthosilicate (TEOS), methyltriethoxysilane (MTES), ethanol, and distilled water. Then the solution was sprayed onto the stainless steel surface, and was dried and heat treated. The dip coating solution was prepared by a simple mixing of TEOS and acidic water into ethanol, and the prior spray coated sample was dipped into the solution. The outer dip coated layer was intended to cover spray coated rough and porous layer and hence to enhance the corrosion resistance. A homogeneous and crack free surface was successfully obtained after the dip coating. The prepared silica film was characterized using scanning electron microscopy, potentiodynamic polarization scan, and electrochemical impedance spectroscopy. The two layered film showed an enhanced corrosion resistance. The enhancement was attributed to a protecting effect of the dip coated layer where the diffusion of ionic species was successfully impeded.