The effect of soil resistivity on corrosion behaviour of coated and uncoated low carbon steel

Abstract

Underground pipeline and steel structure are usually expected to have a long working life. Pipeline made of steel normally corrodes in soil if remain unprotected. Thus the aim of this project is to determine the effect of soil properties and content on the corrosion behaviour of low carbon steel. Soil and water samples were taken along Bekok, 50km water pipeline in Yong Peng. The samples used in this research is low carbon steel both coated (with bitumen) and uncoated. Moisture, pH and the resistivity of soil are the parameters that involve in determining the corrosivity of the soil. The concentrations of the soil parameters are found to have the following properties: pH (1.76- 5.6), temperature (25-50°C), moisture contents (20 – 40%), and resistivity (<1000 O.cm). Corrosivity of the soil samples were evaluated using the American Water Works Association (AWWA) C 105 numerical scale. The resistivity measurement shows that the value decrease by increasing the water content and ionic concentration. The corrosion rate of low carbon steel in soil test solution has been investigated using electrochemical test method (Tafel slope). The result shows that the corrosion rate of the steel increases considerably at very low acidic pH environment (pH 1 to 2) and corrosion current density increase with temperature in the range of 25 to 50°C. Immersion test indicated that the highest corrosion rate of the steel sample was observed in soil containing the lowest pH value 1.76. Based on visual inspection, it is found that uncoated sample has higher corrosion rate than coated steel sample.