The Open Corrosion Journal

2011, 4 : 27-33
Published online 2011 October 21. DOI: 10.2174/1876503301104010027
Publisher ID: TOCORRJ-4-27

Chromium Deposition on Cobalt-Based Alloys by Pack-Cementation and Behaviour of the Coated Alloys in High Temperature Oxidation

Gregory Michel , Patrice Berthod , Stephane Mathieu , Michel Vilasi and Pierre Steinmetz
Institut Jean Lamour (UMR CNRS 7198), Department 2 - Chemistry and Physics of Solids and Surfaces, Nancy - University, B.P. 70239, 54506 Vandoeuvre-les-Nancy, France.

ABSTRACT

Two TaC-strengthened cobalt-based alloys, with compositions Co-10Cr-0.25C-4.4Ta and Co-10Cr-0.5C-8.7Ta (wt.%) underwent Cr-deposition by pack-cementation, followed by heat-treatment to allow Cr diffusing deeper in their sub-surface. Thereafter they were tested for 50 hours at 1200°C in a thermobalance. These alloys were successfully enriched in chromium, with a maximal content on surface of about 30wt.% and a depth of enrichment of several hundreds of micrometers. This allowed a chromia-forming behavior of the cemented alloys despite the 10wt.%Cr of the bulk. This was especially true for the {C,Ta}-richest alloy the oxidation kinetic of which was wholly parabolic and analogous to a Ni-30Cr alloy known for its good behavior in oxidation at 1200°C. The parabolic constants and chromia-volatilization constants were at the same levels of the Ni-30Cr ones. However, the chromium enrichment step, especially the diffusion heat-treatment following the cementation, causes fragmentation and surface fraction reduction of the tantalum carbides which strengthen the alloy.

Keywords:

Pack-cementation, chromium, cobalt alloy, tantalum carbide, high temperature oxidation.