Abstract
Planning the renewal and replacement of historic cast iron sewerage assets is key to the continued collection of wastewaters from thousands of homes, and the prevention of pipe failures potentially leading to pollution and flooding. The graphitic corrosion of cast iron is a relatively poorly understood process, and there appears to be little published data on the rate of this process in wastewater. Consequently, the aims of this research have been to improve the current understanding of cast iron corrosion in a sewage environment, and to understand further the process of graphitic corrosion. This has been achieved via three extensive experimental investigations. Firstly, multiple microstructural characterisation techniques were used to investigate the physical and chemical characteristics of graphitic corrosion, and these have revealed graphitic corrosion to be a more complicated process than current literature suggests, with composition, specifically silicon content, and microstructural aspects, including graphite flake distribution, playing key roles. Secondly, electrochemical and weight-loss methods have been used to perform corrosion studies of cast-iron in multiple wastewaters. The short-term (<1.5 years) corrosion rates attained from these studies, are very similar to those seen in the literature for cast irons which have corroded for >100 years in a soil environment, and this suggests that submerged sewage environments are no more aggressive than most soils. These studies have also shown that wastewater corrosion rates vary depending on the environment, e.g., septicity, flow conditions and biological activity. Thirdly, multiple electrochemical, weight-loss and characterisation studies have been used to understand the effect of microstructure on the progression of graphitic corrosion. It has been found that cast irons with smaller graphite flakes tend to exhibit greater volumetric metal loss via graphitic corrosion, than cast irons with larger flakes, and hence the graphitic corrosion process is affected by the graphite flake size.