Hot Corrosion of Inconel 625 Overlay Weld Cladding in Smelting Off-Gas Environment
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TRODUCTION
KIVCET lead and zinc smelters are used in service as a modern, direct smelting process.[1,2] A schematic of the KIVCET furnace and its 30-m-tall radiant boiler, as well as a section of the water wall tubes, can be seen in Figure 1. High pressure water flows inside the tubes to extract heat from the high temperature off-gas of the smelter. The off-gas is in contact with the fireside (hot side) of the tubes. The large radiant shaft boiler and a subsequent convection boiler to cool the off-gas of the smelter are two parts of the smelter. Based on Figure 1, the off-gas from the reaction shaft at 1648 K (1375 C) enters the 717 m2 vertical radiant boiler of the membrane wall construction by means of an uptake shaft, constructed with water-cooled copper elements. In the radiant section of the boiler, the off-gas is cooled to 1073 K (800 C) prior to entering the down-coming section of the boiler. The total surface area of the downcoming section of the boiler is about 486 m2 and the temperature of the off-gas decreased to 873 K (600 C) in this section. Afterward, the off-gas enters the convection section of the boiler with a total area of 1012 m2 and reduces the final gas temperature to 598 K (325 C). The final stages of cooling occurred in an adiabatic E. MOHAMMADI ZAHRANI, Ph.D. Candidate, and A.M. ALFANTAZI, Professor and Associate Dean of Research and Graduate Studies, Faculty of Applied Science at UBC, are with the Corrosion Research Group, Department of Materials Engineering, The University of British Columbia (UBC), 309-6350 Stores Road, Vancouver, BC V6T1Z4, Canada. Contact e-mail: ehsanmoh@ mail.ubc.ca Manuscript submitted December 5, 2012. Article published online May 29, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
sprayed tower and cooling tower, where the temperature of the gas lowered to 338 K and 291 K (65 C and 18 C), respectively.[3] In the radiant section of the boiler, the original boiler tubes were constructed of carbon steel. These tubes were experiencing unacceptably high corrosion rates because of the severely aggressive working conditions in the boiler environment. Consequently, significant wall thinning occurred in the tubes. Strength and structure integrity of carbon steel tubes, resistance of the tubes to high temperature and high pressure water and steam (used in heat transfer), and relatively low cost of the carbon steel are the key benefits of using the carbon steel in manufacturing water wall tubes of different types of boilers.[4] However, carbon steel does not have satisfactory resistance to corrosion at elevated temperatures. An alloy 625 (UNS N06625) was applied as a weld overlay on the water wall tubes of the radiant boiler by an automatic gas metal arc welding (GMAW) process to control/reduce the corrosion rate of the tubes. This process was done on-site in the boiler. In order to achieve a uniform coverage of the water wall (i.e., membranes and tubes), the weld beads were deposited by the GMAW technique in a vertical down mode starting from the membrane and then moving to the tube
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