The Efficiency of Hexamethylenetetramine ((CH2)6N4) and Thiourea (CH4N2S) in Protection of Coating Layer

Abstract

In the present study, a synergistic effect on the corrosion protection properties of plasma electrolytic oxidation and thiourea as inhibitor has been investigated by microstructure surface, polarization, and basic computational. To achieve this purpose, a PEO coating was performed under an alternating current condition in aluminate electrolytes containing hexamethylenetetramine. The structural observations on the coating layer exhibited that chemical additive would generate lower level of porosity than that without this additive, which was attributed to favorable formation of MgAl2O4 and carbon with the additions of (CH2)6N4. On the other hand, when CH4N2S was added to the coating surface, the results revealed that the structural surface defects of the PEO coatings were sealed by inhibitor-treatments resulting in decreases in micro-cracks and open micro-pores. Thus, it was found that the CH4N2S acts a good corrosion inhibitor for magnesium corrosion in aqueous solution. This adsorption of thiourea on the surface may be due to a weak interaction between an electron donor from thiourea that has a sufficiently low ionization potential and an acceptor such as oxide coating surface that has a sufficiently high electron affinity. Besides that, the inhibition efficiencies and the global chemical reactivity relate to total energy, EHOMO, ELUMO and gap energy (∆E). Potentiodynamic polarization method was used to evaluate the corrosion behavior of these coatings in 3.5% NaCl solution. On this basis, the results revealed that electrochemical corrosion results indicated that the treated PEO coatings with inhibitor showed better corrosion resistance than the untreated PEO coatings.