Performance of different organic coatings on steel substrate by accelerated and in atmospheric exposure tests

Abstract

Organic coatings act as a barrier to a corrosive solution and check the transfer of electrical charge from the corrosive solution. High performance organic paint systems were applied on plain carbon steel substrate with two different primers viz., epoxy Zn phosphate and epoxy Zn rich. Identical intermediate and top coats were applied on them having equal dry film thickness. Both uncontrolled long term testing and controlled laboratory testing were conducted to evaluate the performance of the coatings. Salt spray, and electrochemical impedance spectroscopy (EIS), were carried out on fresh coated panels whereas both EIS and DC polarisation studies were done on the panels withdrawn from field exposure racks using different electrolytes. Polarisation resistance, coating capacitance, water absorption, and corrosion potential were studied to understand the causes of deterioration and to have faster assessment of durability of these paint formulations. Corrosion products were characterised by Scanning Electron Microscope (SEM) through morphology and Raman spectra through phase identification. The performance of epoxy Zn phosphate base primer is very much comparable with epoxy Zn rich base primer. The findings were supported from the tests conducted on the panels exposed for two years under the industrial climatic conditions. The rust formed under the atmospheric condition is non protective lepodocrocite.