Determination of the protective properties of electrodeposited organic coatings

Abstract

Using electrochemical impedance spectroscopy, thermogravimetric analysis, gravimetric liquid sorption experiments, differential scanning calorimetry and optical microscopy, the corrosion stability of organic coatings was investigated. It was suggested that electrolyte penetration through an organic coating occurs in two steps. The first step is related to water uptake, when molecules of pure water diffuse in the micropores of the polymer network according to Ficks law, and it is independent of the type and dimensions of ion in the corrosive electrolyte. The second step is related to water and ions penetration through the macropores which leads to contact between the electrolyte and the metal surface and the beginning of electrochemical processes at the metallic interface. Statistical analysis of different layers of the coating through its thickness using optical microscopy and image analysis enabled determination of the pore number, pore size distribution, mean pore diameter, mean pore area and mean percentage of the surface covered by pores. These results suggested the dimensions of the pores through the coating and, knowing the number of the pores on each layer, the quantity of electrolyte inside the macropores was calculated. It was shown that conduction through the coating depends only on conduction through the macropores, although the quantity of electrolyte in the micropores of the polymer network is about one order of magnitude greater than that inside the conductinq macropores.

Na osnovu rezultata dobijenih iz spektroskopije elektrohemijske impedancije, termogravimetrijske analize, diferencijalne skenirajuće kalorimetrije, sorpcionih merenja i optičke mikroskopije, ispitivana je koroziona stabilnost polimernih prevlaka i pretpostavljen je mehanizam prodiranja elektrolita kroz njih. U prvom stupnju dolazi do absorpcije molekula vode koji difunduju u mikropore polimerne mreže prema Fikovom zakonu difuzije. U drugom stupnju molekuli vode i joni prodiru kroz makropore u prevlaci do metalne osnove, što dovodi do degradacije prevlake i procesa korozije na supstratu. Takođe su određeni oblik i dimenzije provodnih makropora i izračunat je sadržaj elektrolita u prevlaci.