Corrosion Resistance of Bis-Silane-Modified Epoxy Coatings on an Al-Zn-Mg-Cu Alloy

A bis-silane prepolymer-modified epoxy coating was applied to an Al-Zn-Mg-Cu alloy to improve corrosion protection. The bis-silane prepolymer was synthesized from tetraethoxysilane and γ-glycidoxypropyl trimethoxysilane. To study the mechanism of protection failure by the silane–epoxy hybrid coating and to determine an appropriate coating for protection, various coatings with different silane prepolymer contents were developed. The corrosion behavior of the coatings was studied by electrochemical impedance spectroscopy in a 3.5 wt.% NaCl solution. In comparison with silane monomer-modified epoxy coatings, the bis-silane-modified epoxy coating had lower coating capacitance (Cc), higher charge transfer resistance (Rdl) and lower double-layer capacitance (Cdl) during long immersion times, indicating a higher resistance to water permeation and stronger protection against substrate corrosion. The lowest Cc and Cdl values were obtained by mixing epoxy and the bis-silane prepolymer at a ratio of 1:1.