Surface modification, characterization and corrosion protection of 1,3-diphenylthiourea doped sol-gel coating on aluminium

Aluminium based materials undergo natural process of deterioration on exposure to corrosive environment. Among the strategies of corrosion control, sol-gel coatings have become popular as they have successfully replaced toxic hexavalent chromate coatings used hitherto. The adherent ability of sol-gel coatings combined with their ability to prevent the access of electrolyte thereby has made sol-gel coatings a promising and valuable method of corrosion control. In the present work, 3-aminopropyltrimethoxysilane (APTMS) was used as a precursor for silane based sol-gel coatings. In order to improve the active protection of metal, 1,3-diphenylthiourea (DPTU) was introduced into the sol-gel system. The nature of the coatings was studied using FT-IR, SEM and EDX. Thermogravimetric analysis (TGA) showed the thermal stability of the sol-gel coating. The inhibitor doped sol-gel coating increased the hydrophobic nature of the coating. For comparative studies, Thiourea (TU) was doped over APTMS sol-gel matrix and evaluated using electrochemical studies. EIS results of 0.1 M DPTU doped APTMS sol-gel coated Al surface showed better anti-corrosion properties than 0.1 M TU doped in 3.5 wt% NaCl medium. DPTU doped APTMS sol-gel coating forms an adherent layer over metallic surface which acts as significant physical barrier against the attack of corrosive ions in neutral medium which may be the reason for the improved corrosion resistance.