Effects of Laser Surface Melting on Corrosion Resistance of Stainless Steel and Nickel-base Alloy Clad Layers in Cast Bi-metallic Pipes.

Abstract

Laser surface melting for the inner layers of cast bi-metallic pipes has been investigated. Three kinds of centrifugally cast bi-metallic pipes of which inner layers are made of the stainless steel 904L, the nickel-base alloys 825 and 625, respectively were used. Laser surface melting was performed using a 2.5 kW CO2 laser unit with the beam traveling velocity being varied in order to achieve different cooling rates. Corrosion resistance was evaluated by the critical current density for passivation, the pitting potential and the density of pits after the FeCl3 solution immersion test. Corrosion test results indicate that laser surface remelting has a remarkable effect to improve pitting corrosion resistance in the alloy clad layers. EPMA analysis has revealed that element distributions across the solidification cell in the laser-treated alloys become more homogeneous than those in the as-cast alloys. Consequently, the concentrations of molybdenum and chromium at the dendrite cores which decrease in the as-cast alloys, enhance with laser surface melting, which results in the improvement of corrosion resistance in the laser treated alloy layers of the bi-metallic pipes. The theoretical calculation using the Kurz-Giovanola-Trivedi model has demonstrated that the change in element distributions in the laser treated alloys can be attributed to the increased solidification rate by laser surface melting.