Factorial Design to study the influence of silane ratio, silane concentration and hydrolysis time on corrosion behavior of galvannealed steel pretreated with silane hybrid film

摘要

The automotive industry has used galvannealed steel due to its anticorrosive properties. Galvannealed steel appears from the submersion of the metal substrate in hot chemical baths of zinc, followed by a heat treatment at high temperatures which form intermetallic layers on the carbon steel/zinc interface. Although good anticorrosion properties are present, the application of a hybrid coating based on silanes can promote an improvement in corrosion protection, in addition to provide good adhesion of the paint. This is because silanes have organic and inorganic components that act as coupling agents. Hybrid coatings based on silanes are being extensively studied because they represent a sustainable alternative to replace the pre-treatments commonly used in white line industry and in the automotive industry due to its low environmental impact and low toxicity. The aim of this work is to study the hydrolysis conditions to obtain the silane hybrid film APTES: GPTMS as pretreatment of the galvannealed steel for the protection against corrosion. The study followed a 33 factorial experimental design, with three independent variables (silane ratio, silane concentration and hydrolysis time) at three levels each variables (silane ratio = 1:2; 1:1 and 2:1 APTES:GPTMS, silane concentration = 2.0, 4.0 and 6.0 v% and hydrolysis time = 30, 90 and 150 min) providing 27 experiments performed using the real impedance at 31 mHz as a response (dependent variable) for each experiment. The cure time and cure temperature in an oven were fixed at 150°C for 60 min. The solvent solution used was water/ethanol (50 %v/v.)The corrosion resistance was evaluated by electrochemical techniques such as impedance electrochemical spectroscopy (EIS) and the surface morphology was characterized by scanning electron microscopy (SEM). The statistical model revealed that silane concentration is the only variable that shows significant effect on EIS results because it affects the thickness and barrier properties of the hybrid film. EIS results obtained in aerated 0.1 mol.L~(-1) solution, showed the best condition for galvannealed steel pretreated with the hybrid silane film for the 1:2 APTES:GPTMS, 6% silane concentration and 90 min hydrolysis time.