Properties and performance of spin-on-glass coatings for the corrosion protection of stainless steels in chloride media

Spin-on-glass deposition was investigated as viable alternative to increase the durability and performance of 316L steel in chloride environment. The buildup of a detrimental interface oxide was prevented by non-oxidative thermal curing of the coatings, which leads to a transformation to an inorganic, SiO₂-like material. The degree of polymerization was found dependent on the curing temperature; however, even curing at the maximum investigated curing temperature of 800 °C led to still incomplete transformation, showing less SiO₂-like character with respect to thermally grown oxide or fused silica. Electrochemical analysis by cyclic polarization indicated that the coatings behave as imperfect barrier coatings, which may enhance the passive properties of the substrates; however, there is still some statistical scatter in the quality of the coatings. While there is a tendency for an increase of the upper limit of the breakdown potential, there is also a decrease of the lower limit. It was found that such lower quality coatings showed, in association with substrate defects, unevenly distributed coating flaws, which may act as initiation points of pitting corrosion and decrease the corrosion resistance of coated substrates. Further, the films showed instability in aqueous environment due to imperfect polymerization.