Silane with cerium obtained by sol-gel process for post-treatment on ZnNi coatings

摘要

A system constituted by a xerogel film on zinc-nickel electrodeposited coating (ZnNi/sol-gel) for corrosion protection of steel was studied. The aim of this work was to evaluate the effects of withdrawal speed substrates from the sol and addition of corrosion inhibitors to silane films on ZnNi coating obtained by dip-coating process. ZnNi (12 to 16 wt% of nickel and constituted of γ cubic phase) alloys were obtained by electrodeposition on steel (A-1008) substrate from alkaline electrolytes. The electrodeposition was carried out with at 2 A.dm-2 current density what resulted in a 13 ?μm thickness of ZnNi coating. Xerogel films were obtained on ZnNi coating by dipcoating process in a sol constituted with silane precursors as γ-methacryloxypropyltrimethoxysilane (MAP), tetraethoxysilane (TEOS) and 0.01 mol.L-1 of cerium. It was used three withdrawal speeds of substrates from the sol: 50cm.min-1, 20cm.min-1 and 10cm.min-1. The corrosion performance of the Sol-gel/ZnNi system has been evaluated by measurements of open circuit potentials with immersion times and polarization methods in a 5% NaCl solution. The ZnNi coatings were analysed for chemical composition by flame atomic-absorption spectrometry (FAAS) and for phase structure by X-ray diffraction. Scanning electron microscopy was used to study the morphology of the alloys. After drying in air under controlled temperature, the xerogel films was analysed from SEM. The increase in withdrawal speed involves a thickening of films and some cracks appears for the thicker coatings. The electrochemical analysis showed that the xerogel films have an inhibitory action depending on the operating conditions of dip coating. The measurements of open circuit potentials with immersion times noticed that Sol-gel/ZnNi/steel system has a better electrochemical performance than ZnNi/steel system and also superior to ZnNi post-treated with Zn-Ni coating post-treated with Cr (VI) conversion layers. The addition of small amounts of cerium salts to the solution (0.01 mol.L-1) promoted the corrosion potential (Ecorr) displacement in direction to more positive potentials but as a function of the withdrawal speed of substrates from the sol. The corrosion rate (icorr) was also affect by the withdrawal speed of substrates and the best corrosion performance was observed for ZnNi/sol-gel system with 10cm.min-1 of withdrawal speed of substrates from the sol.