Impedance spectroscopy study of anticorrosive properties of epoxy compositions
Although metals and their alloys are important structural materials, electrochemical or chemical interactions between metal structures and the environment leads to their spontaneous destruction. In order to protect metal products from corrosion, epoxy resins and polymer composite materials can be applied. However, polymer-coated metals may degrade under operating conditions due to electrochemical reactions at the polymer-metal interface caused by water absorption and diffusion in epoxy coatings. The present study is aimed at an investigation of the anticorrosive behaviour characteristic of epoxy compositions following exposure to sulphuric acid. The method of impedance spectroscopy was applied to evaluate epoxy coatings on a steel base. The composition of the epoxy binder included bisphenol A resins cured with various amine curing agents. In order to identify structural changes in materials, as well as the changes in their protective properties, the effects of an aggressive environment were simulated by means of exposure to concentrated sulphuric acid over a 30-day period. Impedance hodographs of the studied systems were obtained and equivalent schemes for the approximation of experimental data proposed. The high porosity of the NPEK-114L based epoxy system was established to result in an increase in the corrosion rate. Dissolution of corrosion products over a longer test period - and consequent higher diffusion of corrosive ions in the coating - was determined to cause a decrease in corrosion resistance. Epoxy compositions based on NPEL-128 resin (oligomeric product based on diphenylolpropane diglycidyl ether) demonstrated improved anticorrosion characteristics. The addition of alkyl glycidyl ether for NPEK-114L resin viscosity reduction was shown to affect the protective properties of the composite under acid exposure.