Corrosion Study of Protective Coatings Used in Sewer Concrete Pipelines

Abstract

Microbiologically Induced Corrosion (MIC) induced by the activity of various microorganisms including bacteria and fungi is a major problem facing water and wastewater treatment utilities around the world. Different mitigation strategies are used to combat the effect of microbial corrosion. Various polymeric coatings such as epoxy and epoxy mortar are used widely for this purpose. However, their performance is variable. More rigorous specifications are therefore required in selecting appropriate coatings. This would require an understanding of how coating properties affect their ability to resist acid permeation and structural and chemical integrity of the coating after an acid attack. The aim of this study was to obtain an understanding of the interactions between the industrial epoxy A, epoxy F and epoxy Novolac coatings and organic and inorganic acids which are found in sewer environment. In addition, the experimental findings of this study were used to develop semi-empirical models for the prediction of coating service life. The gravimetric method was used to test and understand the interactions between acids and epoxy coatings, establish the changes in epoxy properties due to acid attack and to find out the subsequent effect of acid attack on future acid permeation. A range of organic and inorganic acids and varied experimental parameters were selected for this purpose. By monitoring mass uptake, swelling and reduction in hardness combined with some instrumental analysis such as FTIR, TGA and SEM the acid-epoxy interactions and changes in epoxy structure were established. The results demonstrated the deteriorating effect of acid on physical and chemical properties of the coatings. The mechanisms of epoxy deterioration resulting from acid attack were identified as plasticisation & post-polymerisation, increased hydrophilicity and salt formation. It was found that the occurrence of these phenomena during acid attack increases the acid permeation rate and contribute to higher degradation rates in epoxy coatings. Also, the empirical models developed based on experimental data successfully predicted the acid permeation depth with coefficient of determination higher than 90%.