High Temperature Corrosion of a Pt-30 wt.% Rh Alloy in a Phosphorizing Gas
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High temperature corrosion of a Pt-30 wt.% Rh alloy in a phosphorizing gas was isothermally investigated at 1285 K using a gas switching technique. Diffusion of P into the alloy created an outer layer of Pt-rich liquid and blocky (Pt, Rh)2P precipitates along with an inner layer of fcc and (Pt, Rh)2P plates in a cellular microstructure. Concentration profiles measured by SEM-WDS and EPMA across the layers at room temperature showed that there were three fcc phases: first was a 12 at.% Rh phase in the outer layer; second was a 37 at.% Rh phase in the cellular microstructure; and third was the initial 43 at.% Rh alloy. Also, the EPMA data registered approximately 0.1 at.% P in fcc of these layers. Based on the surrounding binary phase diagrams and the experimental data obtained in this study, a partial Pt-Rh-P phase diagram was constructed. A diffusion path for the corrosion microstructure was drawn on the partial phase diagram to help develop a step by step model for how the microstructure evolved. Growth kinetics of the inner layer were used to calculate a P diffusivity of about 10−12 m2/s in the Pt-Rh alloy at 1285 K, suggesting rapid diffusion by either an interstitial or interstitialcy mechanism.
Keywordsconcentration profiles diffusion diffusion paths electron probe microanalysis (EPMA) equilibrium diagram
This work was performed in support of the US Department of Energy’s Fossil Energy Advanced Gasification Program. The Research was executed through NETL Research and Innovation Center’s Advanced Gasification effort. Research performed by AECOM Staff was conducted under the RES contract DEFE0004000. Authors acknowledge Mr. R. Krabbe (NETL) for experimental set-ups, Dr. R. Chinn and Mr. M. Fortner (NETL) for metallography, Mr. K. Collins (NETL) for SEM-WDS/EPMA and Ms. J. Chouinard (CAMCOR) for EPMA.
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