The study of the effects of some Kenyan soils on the corrosion of the underground pipes

Abstract

Corrosion of metallic structures buried in the soil, or in contact with soils, has been a serious engineering and economic problem in most par ts of the world. A research programme was conducted to investigate the corrosive properties of the soils in which under ground pipes were laid. Domestic water-supply pipes were laid both in the field and in the labor e tor y . Soil samples were collected from various sites in Nairobi City. The soils were placed half-way in plastic containers measur inq 153mm x 245mm x 225mm. Three preweighed metal pipes, cut to size of 15311lm long, were laid horizontally in each of the containers. Soil was added to cover the pipes and an al r+t iqht lid was used to cover each container. The pipes were unearthed at intervals of four months, cleaned and examined for the ex tent and type of corrosion that had taken place. After dyinq , each pipe was weighed and the mass lost during burial calculated. After one year of laboratory experiment six soil samples were selected according to their respective corrosivity. Metal pipes, cut to lengths of one meter were buried half a meter deep in selected sites in the field and unearthed three times at intervals of six months. The pipes were cleaned and assessed for the extent and nature of corrosion that had taken place. The observed nature of corrosion was compared with that observed for the laboratory-buried pipes. The difference in mass in the Iaboratorv -butted pipes gave an indication of the extent of corrosion that had taken place. The soil samples were analysed in the laboratory for both chemical and physical properties. These included soilmoisture content, pH, acidity and alkalinity, Redox potential, electrical conductivity, sulphate and chloride ions concentratlons , . - presence of trace metals and the mechanical analysis. The observed corrosion was correlated with the properties analysed and the corrosive nature of each soil determined. The rate of corrosion was also electrochemically determined as corrosion current. This was conducted by various methods that included Tafel extrapolation, polarization resistance determination, electrical impedance determination and electrochemical noise analysis. The corrosion rate- has been found to be high in moist soils rather than dry or water-saturated. The same observation was made when the chloride and sulphate ions concentrations were low rather than high. Soils with low electrical resistivity showed high corrosion rate, as was also observed for those with high acidity and low pH. The laboratory-buried pipes showed a decrease in mass which ranged between 0.06% and 3.26% in one year of investigation. The observed loss in mass decreased with time of burial due to formation of corrosion products on the surface of the corroding pipe. However the field-buried pipes showed proqresivel y increasing deterioration with time. This was mainly due to washing away of the corrosion products from the surface of the pipe, by rain. Electrochemical methods were used for quick determination of corrosion currents which varied slightly with the technique used. The Tafel Extrapolation technique showed corrosion currents ranging between 6.0 and 17.0 ± 0.5pA while the Polarization Resistance technique showed corrosion currents ranging between 7.0 and 29.0 ± 0.5pA. The values for electrical impedance analysis showed just about the same values as those obtained for Polarization Resistance technique, and the electrochemical noise determination. This showed t hat each of the last three techniques was good enough for fast determination of corrosion r ate , The electrochemical determination of corrosion rate for both aerated and deaerated soil-water extracts showed a hiah corrosion current for aerated electrolyte. This indicated that oxygen was an important parameter in the corrosion process. Observation of the unearthed pipe specimens showed that highest corrosion was on the upper part of the specimen. This is due to mixed anodic and cathodic areas close to one another forming microscopic corrosion cells. The physical and chemical properties listed above were all found to be important parameters on the corrosion of buried underground pipes.