CRITICAL INTERFACIAL CHARACTERISTICS OF EPOXY COATING ADHESION ON STEEL SURFACES

Abstract

Protective coatings are used for external and internal corrosion protection of gas/oil storage, transmission and distribution. The key factors for corrosion protection are adhesion of the coating to the steel surface, and resistance of the coating to permeation of water, oxygen and/or ions. External three layer – Polyethylene/Hotmelt/Fusion Bonded Epoxy (FBE) coating is being used for corrosion protection of gas/oil pipelines buried under soil. FBE, being in direct contact with steel surface, has immense bearing on corrosion protection and hence service lifetime of the pipeline. The corrosion protection properties of the two FBE materials have been assessed on the basis of inherent chlorine contamination, reaction kinetics, dimensional stability after cure, water absorption etc. Analytical techniques like Thermal Analysis (TA), surface characterization techniques like Fourier Transform Infrared Spectroscopy (FTIR), Time of Flight Secondary Ion Mass Spectrometry (ToFSIMS) and electrochemical techniques like Electrochemical Impedance Spectroscopy (EIS) have proved to be extremely helpful not only for performance evaluation of coating materials but characterization of the coating-substrate interface as well. ToFSIMS has been extremely useful in elucidating the detrimental effect of contamination like oil, grease, fats etc at the FBE–substrate interface on the coating disbondment. The characterization of the interface has helped to predict service lifetime of a 3 LPE coating. Our analytical results have been validated by field tests. The most important factor influencing the service lifetime of a coating is the steel surface preparation by minimizing residual contamination. Clean interface should contribute towards alleviating the need for Cathodic Protection (CP) application and hence reducing maintenance costs. The ionic chlorine contamination is one of the most important critical interfacial characteristics of epoxy coating adhesion on steel surfaces. Incomplete cross-linking of the resin has turned out to be one of the contributory factors towards coating disbondment on steel pipeline buried under irrigated pathways. The strength of coating adhesion has been found out to be damaged by molecular moisture which diffused from the soil and surrounding atmosphere through the free volume in polyethylene and hotmelt. Quality of wash water for steel substrate turned out to be one of the characteristics for better performance of epoxy coatings on steel substrates. Furthermore the results confirmed that special attention must be accorded to periodic removal of settled water from fuel storage tanks at regular intervals.