The factors affecting the corrosion of ductile cast iron
Authors:   , Latty Monique 
Published: June 2024

The ability to reduce the corrosion of ductile cast iron pipe material is imperative, as it is widely used within many drinking water distribution systems. Reducing ductile cast iron corrosion can diminish the amount of iron particles released into drinking water. The aqueous interactions between ductile cast iron and various environments are investigated throughout this work. Corrosion mechanisms and results are evaluated to assess the impact of both common and novel inhibitors on ductile cast iron pipe material. Chapter 3 investigates the impact of sodium silicate inhibition on the corrosion response in both hard and soft drinking water provided by Welsh Water. Results suggested that the softer water gave optimum inhibitor concentration at lower levels compared to harder waters. Whereby harder drinking water collected from Whitbourne water treatment works (WTW) required 4 ppm Na2SiO3compared to Pontsticill WTW, showing an optimum corrosion response at 2 ppm Na2SiO3 after2 hrs immersion. This study suggested that increased calcium ions within hard drinking water can impact the concentration of sodium silicate inhibitor required for optimum corrosion resistance. Chapter 4 assesses the response of ductile cast iron immersed in drinking water collected from Pontsticill water treatment works when dosed with a range of phosphate inhibitor concentrations. At the time of this study, phosphate was dosed into Pontsticill drinking water at a concentration of1.8 mgL1 for plumbosolvency mitigation. However, studies suggested that over 12 hours, 9.2 mgL1can provide optimum corrosion rate resistance. Studies also showed that Pontsticill drinking water is less corrosive without the presence of phosphate compared to the addition of sodium silicate. It was determined that at high phosphate concentrations at pH 7, the phosphoric ions dissociated in the solution could provide sufficient coverage to the exposed ductile cast iron surface. Chapter 5 introduces the first novel inhibitor investigated within this work. L-tryptophan (L-Trp)is an amino acid inhibitor and was dosed into a 0.17 mol dm−3 NaCl solution at pH 7. Immersive experiments took place with ductile cast iron, and results presented a linear relationship of increasing corrosion resistance with increasing inhibitor concentration when examined over 2 hours. However, the optimum corrosion resistance is only obtainable over 24 hours via the highest concentration of1 x 102 mol dm−3 L-Trp. It was suggested within this chapter that the zwitter ion nature of this molecule provides corrosion resistance. In localised areas of increased pH, the zwitter ion form can provide insoluble complexes on the ductile cast iron surface. Chapter 6 investigates the influence of salmon sperm deoxyribonucleic acid in solution with a0.17 mol dm−3 NaCl solution on the corrosion response of ductile cast iron. DNA was found to be a valuable inhibitor for preventing ductile cast iron corrosion over short- and long-term conditions. It was concluded that DNA could act as an anodic inhibitor to provide barrier protection to the contribution of ductile cast iron corrosion. Chapter 7 concludes this work by assessing artificial drinking water compositions with varying calcium hardness on the inhibitors previously featured. The artificial drinking water composition was created based upon average water compositions with ’hard’, ’medium’ or ’soft’ hardness. Sodium silicate, phosphate, L-Tryptophan and DNA were added to the artificial drinking water compositions and the electrochemical response was recorded. The control electrolyte showed that the hardest artificial water presented the lowest corrosion rate and open circuit potential. The optimum inhibitor concentration was examined by water hardness. Within soft and medium artificial drinking waters, the sodium silicate inhibitor concentrations required were similar to that of the drinking water samples obtained. Using artificial drinking water, novel inhibitors such as L-tryptophan and DNA can be investigated in relation to the response of ductile cast iron. The inhibitors can provide some knowledge of potential corrosion resistance performance before administering into circulation within water distribution systems