On the use of sol-gel/nanoclay hybrid coating as an effective protection against corrosion for galvanized steel

摘要

Aqueous organosilane-based surface treatments have been extensively studied as coupling agents to replace chromate or phosphate layers. In addition to the high improvement of paint adhesion, a barrier effect is also observed with this type of layer. Many developments were then further carried out in the field of alcohol-based sol-gel providing thicker layers with good barrier properties. In a previous study we showed that the incorporation of nanoclays into an aqueous silane solution leads to a significant increase of the barrier properties of the coating. This is however the case only if the particles are well dispersed inside the silicon matrix. Dispersion was thus optimized through the use of sonication allowing the exfoliation of the clays when sufficient power is used .The corrosion protection results obtained by applying this coating on galvanized steel (HDG) are rather promising for further replacement of the actual passivation treatments used industrially. The aim of the present work is to obtain a very effective corrosion protection of HDG with optimized properties. The sol-gel matrix used to embed the nanoparticles (sodium montmorillonite NaMMT) is an aqueous mixture of three different silane molecules: γGlycidoxypropyltrimethoxysilane (γGPS), Tetraethoxysilane (TEOS) and Methyltriethoxysilane (MTES)~2. After application of the layer by dip-coating, a curing step is necessary to create a coating network. For the application of a sol-gel without nanoclay, the curing conditions were fixed at 180°C during 15 min. However, preliminary experiments show that in the presence of nanoclay, curing at lower temperature or with shorter time already provides nice barrier properties. The influence of NaMMT particles on the condensation/curing step was thus further studied. Sonication parameters (time, power) and amount of NaMMT particles were also varied. The corrosion protection was characterized by electrochemical measurements (electrochemical impedance spectroscopy, potentiodynamic polarization curves) and by classical ageing tests (salt spray, hygrothermal cycle,...) with flat or bended substrates to check the ability of the coating to be further formed. The reticulation of the layer was evaluated by Fourier Transform Infrared Spectroscopy.