Improved Corrosion Properties of Hot Dip Galvanized Steel by Nanomolecular Silane Layers as Hybrid Interface Between Zinc and Top Coatings

Thin organic coatings (TOC) or paints on hot dip galvanized steel (HDGS) improve the corrosion properties and create visually pleasing surfaces. Delamination of these coatings leads to corrosion and peeling of the paints. Hence, a novel method for improved adhesion and corrosion properties for HDGS surfaces is introduced. It is shown how the fabrication of a nano-molecular silane film as an interfacial layer between the HDGS and TOC or paint improves the corrosion properties of HDGS in different pH regimes. Understanding the corrosion behavior of ultra-thin silane layers under differing pH is crucial, as subsequent coatings have different pHs. By varying the silanization parameters, two different nanomolecular surface structures of aminopropyl trimethoxysilane (APS) on HDGS were fabricated: well-ordered monolayers with approximately 1 nm thickness and highly clustered APS films with a thickness in the range of 5 nm to 8 nm. To verify the nanomolecular APS structures, photoelectron spectroscopy and contact angle measurements were used. The corrosion properties of HDGS and silanized HDGS were studied with linear sweep voltammetry and electrochemical impedance spectroscopy. It is shown that at pH 5 and 7, passivation behavior is observed on silanized samples, but the most significant improvement in corrosion resistance is found at pH 10, where the corrosion currents of silanized samples are up to two orders of magnitude lower than on uncoated metallic samples. Also, it is demonstrated that the corrosion inhibition of APS is not only dependent on the thickness of the silane film, but also the molecular ordering at the surface. The thin, well-ordered APS monolayer is more resistant toward corrosion in NaCl solution (pH 7) than thicker clustered APS layer. This indicates that the highly ordered nanomolecular surface structure protects the HDGS/silane interface from the Cl -adsorption better than the thicker, but more randomly ordered, APS layers. Nanomolecular interfacial silane films for enhanced corrosion and adhesion properties on HDGS are transferrable to industrial production lines providing a low cost and environmentally friendly method for improved HDGS products.