New Silica-Based Coatingfor the Protection of Electrogalvanized Steel against Corrosion

摘要

In order to replace the phosphate or chromate conversio n treatments, which arerncurrently applied on zinc surfaces, a new kind of silica based protective coating was derived.rnThe layer is deposited on the metal surface by a simple immersion into an aqueous solutionrncontaining metasilicate and a nanometric silica sol, followed by a post treatment dryingrnprocess. The composition of the solution and the experimental conditions of the layerrnpreparation have been optimized on the basis of salt spray tests. After 100 hours of testing,rnzinc electrocoated steel plates covered with such silica layers conserve an excellent aspectrnwithout white rusting, whereas non-protected samples are strongly corroded and covered byrnred rust.rnAccording to SEM investigations, the silica layer is dense and homogeneous withoutrncracks, and perfectly adherent on zinc coatings prepared from alkaline plating bath. Therncorrosion protection was also studied by electrochemical impedance analysis during longrnterm immersion at rest potential in a dilute sulphate solution. At first, the impedance diagramrnexhibits the characteristic feature of a strong barrier effect with a polarization resistance Rp,rnwhich may reach several hundred kilo-ohms. When time id elapsing, Rp decreased until arnstationary value in the order of 10 to 20 kilohms, without apparent corrosion of zinc.rnAccording to these results, this new kind of protective layer has a large potential field ofrnapplication for the protection of galvanized steel.