Effect of laser surface modification on the corrosion resistance of Alloy 600

Abstract

To improve the corrosion resistance of Alloy 600, a typical alloy for steam-generator tubing in nuclear power plants, the surface of the alloy was modified by melting or alloying with a continuous CO, laser beam. In laser surface melting (LSM), a rapidly solidified surface layer of 300 mu m in thickness was obtained. Anodic polarization measurements showed that the corrosion resistance of the laser-surface-melted specimen was improved slightly. This might be attributed to the elimination of microstructural inhomogeneities, such as precipitates, inclusions and segregated impurities pre-existing at grain boundaries. For laser surface alloying (LSA), chromium was electroplated on the surface of the specimens before laser-beam irradiation. The alloyed layer showed a pore- and oxide-free and very homogeneous microstructure, with a chromium composition of 28-30 at%. The corrosion properties of the alloyed specimen were examined by anodic polarization, electrochemical potentiodynamic reactivation, the modified Huey and slow-strain-rate tests. The properties, in particular intergranular corrosion resistance, were significantly improved compared with those of as-received or LSM specimens. The improvement in corrosion resistance might result from the easy formation of a more stable passive film on the alloyed surface layer with high chromium content. (C) 1998 Elsevier Science S.A. All rights reserved.