Salmon Sperm DNA As a Corrosion Inhibitor on Ductile Cast for Use in Water Distribution System Pipes

Abstract

Ductile cast iron (DCI) is a high carbon iron alloy with a composition of up to 3.7% C, 2.3% Si, 0.35% Mn and various smaller constitutes. Upon casting of DCI, the high carbon content results in solidification in a secondary phase in the form of graphite nodules. These graphite nodules form around a matrix of ferrite (α-Fe) and pearlite (α-Fe + Fe3C) and impart desirable properties such as high strength properties and corrosion resistance due to graphite’s cathodic potential. DCI is commonly used for water distribution pipework and contribute to 40% of all drinking water distribution pipes in Wales. Several different corrosion inhibitors are used to prevent corrosion of DCI in drinking water distribution applications, such as sodium silicate (Na2SiO3) and ortho-polyphosphates. However, the use of deoxyribonucleic acid (DNA) could provide a novel alternative to conventional corrosion inhibitors currently in service. The DNA molecule is formed by a double helix polynucleotide strand of phosphate and sugar groups via covalent bonds and base pairs bonded by hydrogen bonds to the molecule. The main advantage of DNA as a corrosion inhibitor is the surplus of phosphate molecules available to be chemisorbed to the surface of DCI. Alongside the potential of aromatic heterocyclic compounds found in base pairs, which could also provide corrosion inhibition via physicochemical adsorption.This work aims to assess the use of deoxyribonucleic acid (DNA) on the corrosion of ductile cast iron (DCI) in a corrosive electrolyte. DNA is considered a green, non-toxic inhibitor that shows potential when used in immersion corrosion conditions. Literature has demonstrated the use of DNA as a corrosion inhibitor for other materials such as Magnesium alloys, Copper and X80 steel. However, there is little published work on the use of DNA as a corrosion inhibitor on DCI. The DNA used in this study was extracted from salmon sperm (Sigma-Aldrich) and dissolved into a 0.17 mol dm-3 sodium chloride (NaCl) solution at the concentrations of 50, 100 and 500 mg/L adjusted to pH 7 via Sodium Hydroxide (NaOH). Electrochemical analysis and surface characterisation was carried out over a range of time periods. The corrosion inhibition afforded by DNA was determined through using a variety of electrochemical techniques such as: open circuit potential (OCP), anodic and cathodic polarisation and linear polarisation resistance (LPR). In-situ microscopy was used to study the corrosion mechanisms that occur on the surface microstructure of DCI. Post corrosion, the DCI surface was characterised using scanning electron microscopy (SEM) and FTIR (Fourier transmission infrared spectroscopy).Electrochemical results, performed over a 24-hour period, showed that as concentration of DNA increased an increase in voltage potential and decrease in corrosion rate (mpy) was observed. A decrease of 60% in corrosion rate after 24 hours is provided by 500 mg/L DNA compared to the control of 1% NaCl. However, after 30 minute’s immersion followed by cathodic polarisation studies showed an increased concentration of DNA can behave as a cathodic inhibitor. A theory for this inhibitive effect is that a passivating film forms on the DCI surface and that it is most effective after 30 minutes exposure due to the high efficiency observed during the longer-term studies. This could suggest that a film formed on the surface is most efficient after 30 minutes adsorption due to the high efficiency seen during the longer-term studies. After 72 hours of immersion there is a change in the surface of DCI, with a more robust scale appearing at higher concentrations. This change in coating surface may provide the increase in corrosion inhibition as the pathway for further corrosion attack is blocked by a protective scale. The protective scale is characterised using FTIR to examine the changes in chemical structure of the corrosion layer. Whereby PO-2 and base pairs are identified on the corrosion scale surface after 24-hours immersion in DNA. The corrosion inhibition observed using DNA on DCI in neutral immersion conditions is afforded to the adsorption of inhibitive species onto the DCI surface, providing an insoluble layer.