EFFECTS OF ARC WELDIND CURRENT AND HEAT TREATMENT ON CORROSION OF WELDED MILD STEEL PLATES FOR UNDERGROUND TANK

Abstract

Corrosion is a major concern for owners and managers of petrol stations. This is because corrosion failure will adversely affect the lifespan of tanks. Leakages or ruptures caused by corrosion are commonly-identified problems in buried mild steel tanks which can lead to catastrophic failures, causing significant socio-economic losses for consumers and water utilities. Unlike in the pipelines where periodic inspection of the buried pipelines is vital, in the case of underground tanks appropriate mitigating measures via construction is preferable. The development of corrosion mitigation techniques, corrosion-induced deterioration of tanks remains a major problem globally, and billions of dollars are spent every year on the repair and cleaning of leakages into water/soil. Corrosion may alter the mechanical properties of metals due to chemical and micro structural changes at macro, micro or nano levels which impact their service lives. Up to present time, there has not been sufficient measured to mitigate corrosion for underground storage tanks (UST). Researchers have not explored this area comprehensively. This study showed that Corrosion mitigation in UST can best be achieved through proper development of production procedure in the joining of the mild steel plates. This was achieved through variation of arc welding current with electrode types and subsequent post heat treatment. A current of 80A, 90A and 100A were varied with Electrodes E6010, E6013 and E7018 respectively. A total of 54 coupons were gotten in all. Twenty-seven (27) coupons were subjected to post heat treatment. Nine coupons in each case of the post heat treated and non-heat treated ones were taken for micro structural examination and the rest were subjected to corrosion in aqueous acidified sandy soil in in bath for 60 days. It was observed that the heat treated coupons has a more refined grain sizes than non-heat treated counterparts and equally more corrosion resistance. Thus, it was established as a framework for UST construction using 5mm plate that the following stages be followed: preparation of 5mm plate; measurement and cutting of plate; and carrying out butt welding using current of 100A and E7018 electrode. Then, heat treatment of weld and HAZ to a temperature of 900oC,application of appropriate coatings, burying in a well prepared underground, covering with final backing sand (sandy soil only).