Hydrophobic phytic acid conversion layers for corrosion protection of steel surfaces

Abstract

AbstractThe possibility for a good conversion protection based on phytic acid (PA) solutions is described many times in the literature.1,2 The latest results show that PA cannot improve the corrosion protective properties with an organic coating,2 although PA conversion layers without organic coatings have already successfully been applied on different surfaces and the development of layers with enhanced corrosion protective behavior was possible.3,4 The reason why PA does not work together with an organic coating is based on the hydrophilic surface and therefore the low contact angle of the PA conversion layer.2 One solution is to modify PA directly and/or change the solution properties to increase the hydrophobic properties. In this work, a new and innovative way to synthesize a new type of sustainable organic PA-based conversion layers on steel, which works completely without titanium or zirconium and is a new approach for hydrophobic conversion layers, is presented.5 The results prove the formation of a pure PA-based conversion layer on the surface. The infrared (IR), Raman, and nuclear magnetic resonance (NMR) spectroscopy verified the new synthesis products and by means of the vibration spectroscopy, the resulting conversion layers. To analyze the new physical properties, the contact angle of the conversion layers was detected. By means of the electrochemical impedance spectroscopy (EIS), the electrochemical stability of the conversion was studied and with cyclic voltammetry (CV), the solidity ratio was investigated. At last, the corrosion protective performance of the layers in combination with an E-coat in the salt spray test (SST) was examined. All modified PA-based conversion layers in combination with E-coats increased the corrosion protective properties in relation to industrial used phosphate conversion layers of steel surfaces. The overall result is a new kind of hydrophobic phytic acid-based conversion layer which shows advanced anticorrosion and coatability properties compared to other layers based on PA. The results if the EIS measurements show that the resistance of the coating significantly increased, and diffusion could be suppressed by coating the metal surface with PA conversion layers. The comparison of the SST results for the reference and the modified PA underline these observations. The overall SST rating increased by 2 and the delamination went down to 1.9 mm while the corrosion was 0.1, comparable to phosphated steel.