Electric pole failure: A safety concern

Abstract

A galvanized electric street lamp pole with expected life of 25 years, near a slag dumping area failed from its base after 5 years of service. The base was found to be severely corroded and pole completely covered by mud. Visual inspection revealed no pitting at the inner surface but thinning at the base. 200 μm zinc coating was absent at the fractured region but remained intact at the inner surface. Finite Element Analysis showed that the corroded pole with 1.5 mm thickness (measured in failed zone) would start failing due to high stress exceeding yield strength at a wind speed of 86 km/h, much lower than that prevalent confirming that the pole failed due to corrosion. The soil pH was 8.0–9.0 at which the zinc coating was stable. Resistivity and moisture content of the soil were measured to be 333 Ω and 56% respectively. In order to investigate soil corrosivity for buried structural materials the oxidation reduction potential was measured. At this combination of resistivity, moisture content and ORP the soil was highly corrosive. The decreased resistivity, possibly due to the presence of ions from condensed CO gas hanging and dripping just above the failed pole, enhanced flow of ions thereby making the soil electrolytic resulting in faster formation of Zn2+ ions. The pole thus failed from the base due to accumulation of mud creating an oxygen concentration cell at the interface. Tar based cathodic coating was recommended to combat oxygen concentration cell corrosion along with proper maintenance and mud removal in order to avoid such kind of failure.