NOVEL SURFACE TREATMENT PROCESSES TO PROTECT METAL SUBSTRATES

摘要

A novel electroless process for depositing corrosion resistant silica layers on metal substrates has been developed. The silicate is deposited from N sodium silicate solution (3.22 weight ratio sodium silicate, 37.5 % solution in water from PQ corporation) in the presence of sodium borohydride. ESCA (electron spectroscopy for chemical analysis) studies of the coating reveal the formation of a very thin (5 nm) zinc disilicate followed by a much thicker (500 nm) silicon dioxide layer. An adsorption-condensation mechanism is proposed to account for the two layer silicate formation. Based on this mechanism, the deposition parameters such as the concentration of the PQ bath, the concentration of the reducing agent and the temperature of the operating bath were optimized. Accelerated corrosion tests show a higher barrier resistance and better stability when compared to chrome passivation. The improved corrosion performance of the silicate coatings by the electroless process shows promise as an alternative to conventional hexavalent chrome passivation.