Anticorrosive Performance of New Epoxy-Amine Coatings Based on Zinc Phosphate Tetrahydrate as a Nontoxic Pigment for Carbon Steel in NaCl Medium

Abstract

Epoxy resin is known to react with a hardener such as polyamine to form a thermoset 3D polymer net with an outstanding physical and mechanical properties. They are widely used in coating and adhesives. In this study, we present a new epoxy resin material useful for making an anticorrosive formulation for carbon steel. The epoxy resin presented in this study is diglycidyl ether 4, $$4^{\prime }$$ -dihydroxy diphenyl sulfone (DGEDDS). It was prepared in a two-step process that involves reacting epichlorohydrin with 4, $$4^{\prime }$$ dihydroxy diphenyl sulfone then with sodium hydroxide. The structural elucidation of DGEDDS was carried out with Fourier transform infrared. The anticorrosive formulation DGEDDS–MDA–ZPH was prepared from DGEDDS and the hardener 4, $$4^{\prime }$$ -methylene dianiline (MDA) in the presence of the anticorrosion pigment zinc phosphate tetrahydrate (ZPH). Another standard formulation (DGEDDS–MDA) was prepared without ZPH. The physicochemical and anticorrosive performance of the coated carbon steel was evaluated using electrochemical impedance spectroscopy (EIS). The coated surface was subjected to morphological characterization by SEM before and after immersion in the corrosive medium and exposing it to the UV radiation. The value of the polarization resistance ( $$R_{\mathrm{p}}$$ ) obtained by the EIS method for the standard coating DGEDDS–MDA and epoxy composite coating DGEDDS–MDA–ZPH was 31898 and 72611 $$\Omega \,\hbox {cm}^{2}$$ during the 1 h of immersion in 3 wt% NaCl, respectively. After aging by exposing the coatings for a 2000 h to UV radiation the values were dropped to 2596 and 5189 $$\Omega \,\hbox {cm}^{2}$$ , respectively. The values show the high stability and resistance of the epoxy resin coating to electrolytes and UV radiation. The coating even showed higher stability in the presence of ZPH pigment. As shown in the results, the tricomponent composite showed an outstanding stability in protecting carbon steel form corrosion in an aggressive marine environment where UV is very intense and the humidity and salts are very high.