EFFECT OF LASER-SURFACE MODIFICATION ON THE CORROSION-RESISTANCE OF ALLOY-600

To improve the corrosion resistance of Alloy 600, a typical alloy for steam-generator tubing in nuclear power plants, the surface of the all oy was modified by melting or alloying with a continuous CO, laser bea m. In laser surface melting (LSM), a rapidly solidified surface layer of 300 mu m in thickness was obtained. Anodic polarization measurement s showed that the corrosion resistance of the laser-surface-melted spe cimen was improved slightly. This might be attributed to the eliminati on of microstructural inhomogeneities, such as precipitates, inclusion s and segregated impurities pre-existing at grain boundaries. For lase r 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 ch romium composition of 28-30 at%. The corrosion properties of the alloy ed specimen were examined by anodic polarization, electrochemical pote ntiodynamic reactivation, the modified Huey and slow-strain-rate tests . The properties, in particular intergranular corrosion resistance, we re significantly improved compared with those of as-received or LSM sp ecimens. The improvement in corrosion resistance might result from the easy formation of a more stable passive film on the alloyed surface l ayer with high chromium content. (C) 1998 Elsevier Science S.A. All ri ghts reserved.