Microstructure and Corrosion Behavior of Laser Melted 304L SS Weldment in Nitric Acid Medium
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JMEPEG DOI: 10.1007/s11665-016-2488-3
Microstructure and Corrosion Behavior of Laser Melted 304L SS Weldment in Nitric Acid Medium Girija Suresh, P.S.V.R.A. Kishor, Arup Dasgupta, B.N. Upadhyay, C. Mallika, and U. Kamachi Mudali (Submitted June 16, 2016; in revised form October 14, 2016) The manuscript presents the effect of laser surface melting on the corrosion property of 304L SS weldment in nitric acid medium. 304L SS weldment was prepared by gas tungsten arc welding process and subsequently laser surface melted using Nd:YAG laser. The microstructure and corrosion resistance of laser surface melted 304L SS weldment was evaluated and compared with that of 304L SS as-weldment and 304L SS base. Microstructural evaluation was carried out using optical and scanning electron microscopes attached with energy-dispersive x-ray spectroscopy. Corrosion investigations were carried out in 4 and 8 M nitric acid by potentiodynamic polarization technique. From the results, it was found that laser surface melting of the weldment led to chemical and microstructural homogeneities, accompanied by a substantial decrease in delta ferrite content, that enhanced the corrosion resistance of the weldment in 4 and 8 M nitric acid. However, the enhancement in the corrosion resistance was not substantial. The presence of small amount of delta ferrite (2-4 wt.%) in the laser surface melted specimens was found to be detrimental in nitric acid. X-ray photoelectron spectroscopy studies were carried out to investigate the composition of the passive film. Keywords
304L SS weldment, corrosion, delta ferrite, laser surface melting, nitric acid
1. Introduction Austenitic stainless steel of AISI Type 304L SS is the prime construction material for various process units of the Indian nuclear reprocessing plants, where nitric acid is the process medium used from concentrations as low as 0.01 M HNO3 and as high as 11.5 M HNO3 and from room temperature to boiling conditions (Ref 1, 2). 304L SS is characterized by high corrosion resistance in nitric acid medium as it spontaneously passivates in nitric acid medium by forming a stable chromiumrich oxide film (Ref 3-6). But at concentrations greater than 8 M HNO3, Cr(III) in the passive film gets oxidized to Cr(VI), which dissolves in nitric acid causing an increase in corrosion rates. In addition, the weld region is also a concern in nitric acid medium. The weldment is a complex region comprising of compositional and microstructural heterogeneities in going from the base region through the heat affected zone (HAZ) to the weld region. The weld region itself has microcompositional and microstructural inhomogeneities due to microsegregation of trace elements and formation of secondary phases during solidification. Processes such as sensitization, recrystallization and grain growth occur at the heat affected zone (HAZ) during Girija Suresh, Arup Dasgupta, C. Mallika, and U. Kamachi Mudali, Indira Gandhi Centre for Atomic Research, Kalpakkam, TN 603102, India; P.S.V.R.A. Kishor, Department of Metallurgy, Andhra U
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