Analysis of early failure of galvanised borewell pipe

Galvanised pipes are used extensively for borewell application to tap underground water at various geographical and geological conditions. The depth of borewell varies from place to place depending on many factors such as, (i) the quantity of water need to be tapped out per unit time, (ii) the surrounding geological conditions and (iii) the depth of underground water level. Early failure of galvanised borewell pipe is very common phenomena and purpose of this work is to identify the probable causes for such failures. The present work details on the analysis of such corroded borewell pipes from south eastern part of India. The steels substrate, coating and the corrosion products from different depth of the borewell assembly as well as the soil and water samples were investigated under SEM/EDS, XRD. Two major parameters were identified for early failure of borewell galvanised pipe, such as, (i) higher level of chloride content (1065 ppm) & high hardness (1802 ppm) in the underground water and (ii) wrong design of the borewell pipe setting including dissimilar grades of steel used in the pipes. Primary borewell steel pipes are more electronegative than the sleeve steels. This resulted in corrosion at the surface of primary pipe near the sleeve joint. The ground water with elements like magnesium, calcium and sodium leads to formation of hard scale inside of the pipe. Such scale further enhances the deposition of chloride ions on the steel surface and triggers the pitting corrosion. The most susceptible region for early failure of galvanised borewell pipe is the sleeve joint region. Many bare (without zinc coating) and exposed joint threads are visible near the sleeve joint area with lower steel thickness. Crevice corrosion at the edge of the sleeve joint also plays important role in failure due to absence of any insulating materials at the joint.