Corrosion Behavior of Welded Joints for Cargo Oil Tanks of Crude Oil Carrier

Abstract

E32 grade corrosion resistant steel was welded with welding wires with three different S contents. The microstructure, mechanical properties, inclusions, and corrosion behavior of welded joint were investigated. The joint coupon corrosion test and potentiodynamic polarization test were carried out under the simulated corrosion environment of the inner bottom plates of cargo oil tanks. The pitting initiation and propagation mechanism of the weld metal were studied by scanning electron microscopy and infinite focus. The results indicated that the microstructures of three kinds of weld metals are all composed of acicular ferrite, ferrite side-plate and proeutectoid ferrite. The microstructure of heat-affected zone is composed predominantly of bainite. Joint welded with low S filler wire has good mechanical properties. S can decrease free corrosion potential and increase the corrosion tendency. The pitting initiation is oxide inclusion or sulfide-oxide inclusion complex. S can induce the formation of occluded area and promote the corrosion propagation. The chemical compositions of weld metal is similar to base metal, which can limit the galvanic corrosion between weld metal and base metal, and avoid formation of corrosion step.