Corrosion Studies of 12-Aminododecanoate Modified Zn-Al LDH/Epoxy Coating

Abstract

Addition of environmentally friendly layered double hydroxide (LDHs) to organic coatings can increase barrier properties within the coatings due to the platelet structure of the LDHs. A new approach combining silane coupling agents to enhance compatibility of the inorganic and organic matrix is used to form improved corrosion resistance coatings. For this work, Zn-Al-LDH are prepared via a co-precipitation method and modified by sodium 12-aminododecanoate through an anion exchange process. The organo-modified layered double hydroxide (LDH) is functionalized using 3-aminopropyltriethoxysilane (APTES) which is then incorporated into the epoxy matrix (EPON 828). The reaction between amine part of the amino dodecanoate as a strong nucleophilic group and epoxy ring part as an electrophilic group improves compatibility of LDHs layers and epoxy molecules in the composite. Differential scanning calorimetry is used to study this exothermic epoxy-amine ring opening reaction to determine the curing temperature of the coatings before and after modified LDH inclusion. The change in LDH structure, effect of incorporation of modified LDHs in the epoxy matrix, grafting of APTES on the LDH surface and elemental analysis are investigated respectively, by x-ray diffraction, thermogravimetric analysis, infrared spectroscopy and energy dispersive x-ray analysis. In the last step of preparation, Epikure 3571 as a curing agent, is added to the LDHs/epoxy system, then the coating is applied on steel substrates. After the thermal curing process, the anti-corrosion performance provided by this coating is assessed by multiple methods including, open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy in 3.5% NaCl aqueous solution. Further, scanning electron microscopy is used to study morphological changes on the coating before and after immersion test and to scan the localized corrosion spots. The results are compared with the coated steel samples with clear epoxy coating and none modified LDH/epoxy. Figure 1