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

: Santos, Claudia Beatriz dos; Bonino, Jean-Pierre; Ansart, Florence; Xuereb, E.; Salacruch, Celine; Bares, Pierre; Menezes, Tiago Lemos; Malfatti, Célia F.

Deutsche Gesellschaft für Chemisches Apparatewesen, Chemische Technik und Biotechnologie e.V. -DECHEMA-, Frankfurt/Main; European Federation of Corrosion -EFC-; Institute of Metal Finishing:
Managing corrosion for sustainability. EUROCORR 2008, the European Corrosion Congress : 7-11 September 2008, Edinburgh. Book of Abstracts
Frankfurt/Main: DECHEMA, 2008 (European Federation of Corrosion. Event 299)
European Corrosion Congress (EUROCORR) <2008, Edinburgh>
Fraunhofer IPA ()
sol-gel; Zn-Ni-Schicht; Cerium; Schutzschicht; Oberflächenbearbeiten; Korrosion

A system constituted by a xerogel film on zinc-nickel electrodeposited coating (ZnNi/sol-gel) for corrosion protection of steele was studied, The aim of this work was to evaluate the effects of withdrawal speed substrates form 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 gamma cubic phase) alloys were obrained by electrodeposition on steel (A-1008) substrate frome alkaline electrolytes. The electrodeposition was carried out with a 2 current density what resulted in a 13 µm thickness of UnNi coatinig. Xerogel films were obrained on ZnNi coating by dip-coating process in a sol constituted with silane precursors as gamma-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) 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 measurement 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 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 corrosin potential (E(corr)) displacement in direction to more positive potentials but as a function of the withdrawal speed of substrates from the sol.
The corrosion rate (i(corr)) 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.