“Hybrid organic-inorganic sol gel coatings applied on phosphatized carbon steel: the effect of coating heat treatment

IANINA, SANTANA; SERGIO, PELLICE; SILVIA, CERÉ; ANDRÉS, PEPE

Praga

Congreso; 63rd encuentro anual del ISE; 2012

International Society of Electrochemistry

Carbon steel is widely used in industry; however its susceptibility to corrosion in many environments limits its applications. For many years, chromate conversion coatings have been widely applied for corrosion protection of metals. However, the carcinogenic effect of chromate has generated serious environmental and health problems, for that reason its use has been prohibited by new regulations. Sol gel process is a chemical method that allows the synthesis of glass and ceramic materials at lower temperatures than the conventional methods and can be used to obtain functionalized coatings. Dip-coating process is one of the most used methods for deposition of sol-gel materials; this consists in the immersion of the substrate in the sol followed by a subsequent withdrawal at a controlled rate ensuring an extremely smooth and homogeneous film. In this work nanostructured hybrid films were synthesized by the sol–gel route from tetraethoxysilane (TEOS) and 3-glicidoxipropyl-trimetoxisilano (GPTMS) using a colloidal silica suspension to improve mechanical and barrier properties, and Ce(NO3)36H2O as a supplier of Ce(III). Sols were prepared with molar ratios TEOS/GPTMS/SiO2 = 42/18/40 ¿? (for so-called TGS sol) and TEOS/GPTMS/Ce = 66.5/28.5/5 (for the TGC sol), both in acidic conditions using ethanol as solvent. Carbon steel plates, AISI 1010, were used as substrates. Prior to the application of the coating, samples were treated with a phosphoric acid 2 % v/v in order to improve coating adherence. After phosphoric treatment two layered films were deposited as follows: a first layer of TGS and a second one of TGC on the top of the first. According to thermogravimetric analyses (TGA) of sols, after each layer deposition, a thermal treatment was performed in order to densify the layers and provide a close structure without losing the plasticity given by the organic component of the coating. Two thermal treatments were tested: one at 120 °C and another at 250 °C. The coatings applied onto AISI 1010 carbon steel were homogeneous and cracks-free. The corrosion resistance of both kinds of samples was evaluated by means of potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) measurements using a 0.35 wt% NaCl solution. Microstructural characterization of substrates and sol-gel coatings was performed by optical microscopy and scanning electron microscopy (SEM/EDS) techniques. Electrochemical behaviour at various immersion times was modelled by equivalent circuits showing the effect of heat treatment on the corrosion processes.