Corrosion Behavior of Pulsed Gas Tungsten Arc Weldments in Power Plant Carbon Steel

Abstract

Welding plays an essential role in fabrication of components such as boiler drum, pipe work, heat exchangers, etc., used in power plants. Gas tungsten arc welding (GTAW) is mainly used for welding of boiler components. Pulsed GTAW is another process widely used where high quality and precision welds are required. In all arc-welding processes, the intense heat produced by the arc and the associated local heating and cooling lead to varied corrosion behavior and several metallurgical phase changes. Since the occurrence of corrosion is due to electrochemical potential gradient developed in the adjacent site of a weld metal, it is proposed to study the effects of welding on the corrosion behavior of these steels. This paper describes the experimental work carried out to evaluate and compare corrosion and its inhibition in SA 516 Gr.70 carbon steel by pulsed GTAW process in HCl medium at 0.1, 0.5, and 1.0 M concentrations. The parent metal, weld metal and heat affected zone are chosen as regions of exposure for the study made at room temperature (R.T.) and at 100 °C. Electrochemical polarization techniques such as Tafel line extrapolation (Tafel), linear polarization resistance (LPR), and ac impedance method have been used to measure the corrosion current. The role of hexamine and mixed inhibitor (thiourea + hexamine in 0.5 M HCl), each at 100 ppm concentration is studied in these experiments. Microstructural observation, surface characterization, and morphology using SEM and XRD studies have been made on samples exposed at 100 °C in order to highlight the nature and extent of film formation.