Evaluation of a sulfursilane anticorrosive pretreatment on galvannealed steel compared to phosphate under a waterborne epoxy coating

Abstract

Galvannealed coatings are produced by annealing zinc hot dip coated steel where iron and zinc undergo interdiffusion to form iron–zinc alloys. They offer some attractive features like better weldability, paintability, corrosion resistance, ductibility and formability, depending on the morphology, composition and structure of the coating phases. Silane films are a good choice to replace phosphate pretreatment, and in the case of bis-sulfursilane films, the sulfur chains also help to increase the hydrophobicity of the film and its protection against corrosion. Epoxy coatings are reasonably corrosion resistant and generally applied as primers or direct-to-metal (DTM) coatings in coating systems for different metallic substrates.Using the optimum conditions previously determined for obtaining a 3% bis-1,2-[triethoxysilylpropyl] tetrasulfide (BTESPT) solution in a water/ethanol solvent at pH 4 hydrolyzed in the presence of 50ppm Ce(IV) ions, the silane solution was applied on galvannealed steel samples by dip coating process with samples remaining immersed for 15min before being withdrawn and cured for 40min at 150̊C. The bare galvannealed samples were previously treated, first by sonication in acetone and then by a commercial alkaline degreasing solution. The phosphate samples were prepared in an industrial set using the tricationic bath containing Mn, Zn and Fe phosphates.The waterborne epoxy coating was applied with a brush to reach a 60±6μm dry film thickness layer. The evaluation was performed by electrochemical technique: EIS, electrochemical impedance spectroscopy, and also by accelerated tests in a salt spray cabinet (SSC). The silane, phosphate and non-treated samples were tested in 3.5wt% NaCl solution in a cell of 20cm2 after 3h, 24h, 96h, 264h, 360h, 528h and 720h of immersion in the electrolyte solution. The results indicated that the silane pre-treated samples have shown the highest impedance modulus value even after 530h of immersion.Adherence and flexibility were also evaluated by tape test–X method–and conic mandrel test. The results have evidenced the superior adherence of the coating for the silane pre-treated samples. The SSC test revealed that the silane pretreated samples showed less blistered area around the scratch.In summary, the sulfursilane pretreated samples presented an overall better performance concerned to the corrosion resistance compared to phosphate and waterborne epoxy coated galvannealed steel samples.