Epoxy-silane hybrid coating enhances corrosion protection on galvanised steel

Silanes with alkoxy group, epoxy group, amine group and thiol group were chosen to understand the role of functionalities in the performance of designed hybrid coatings. Moreover, the silanes were added at three different concentrations into epoxy polymer to asses the effect of concentration on anti-corrosion and adhesion properties of the coating films.

Evaluation methods and tests

From scanning electron microscopic (SEM) images, the scientists observed a uniform coating without any agglomeration of coating particles over galvanised steel substrates. The corrosion performance of casted and cured films was evaluated by using potentiodynamic polarisation and AC impedance spectroscopy method. The physical properties, such as thermal and thermo-mechanical behaviour, were studied using differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) respectively. The adhesion strength between coating films and galvanised steel substrate was checked by “pull off” adhesion test.

Silane 3 wt% causes best improvement

It was found that due to the grafting of sol-gel coatings onto organic polymer backbone, the adhesion property and anti-corrosive performance has improved remarkably as compared to non-grafted epoxy polymer. It was observed that amino silane showed superior performance compared to thiol silane. The poor performance of thiol silane grafted epoxy coating could be attributed to some chemical incompatibility of hydrophobic and non-polar sulfur silane moiety and hydrophilic and polar waterborne epoxy polymer backbone. Addition of silane by 1 and 3 wt% into epoxy polymer backbone caused improvement in both anti-corrosive property and adhesion strength but further increase of the silane concentration to 5 wt% led to deterioration of protective property of the films. This drop in performance can be attributed to excessive consumption of epoxide groups in epoxy resin by amino and thiol functionalities present in silane.

The study is published in: Progress in Organic Coatings, Volume 101, December 2016, Pages 24-44.