Measurement of Cr Evaporation in Several Nickel Based Alloys at Moderate Velocities (original) (raw)

Abstract

Chromium is the principle basis of corrosion protection in most moderate-to-high temperature oxidation resistant alloys. The loss of Cr from an alloy is significate in its expected operational lifetime, as its loss can lead to beak-way oxidation. Gas flow rate is an important parameter in the loss of Cr. Most laboratory Cr evaporation experiments have used relatively low gas flow rates. In this research the Cr loss rate was investigated at 760 °C, in an air plus 57 vol pct water environment with an apparent gas flow rate of 26.5 m·sec-1. This speed was achieved by spinning the samples on the end of a disc in a gas filled, heated retort. Since secondary alloy additions can have an effect on chrome activity and diffusion, three nickel based alloys with similar (~21 wt pct) Cr levels were investigated. The alloys chosen were HAYNES 230, 282, and Inconel 625. Moving and static samples (hung near the moving samples) were exposed for times up to 1000 hrs. Samples were first measured gravimetrically and then the surfaces examined via SEM. Finally the samples were cross sectioned and examined via SEM with EDS to determine Cr depletion. Results show a noticeable difference for the IN 625 surface, as viewed via SEM, between the surface scale formed on the respective static and moving samples. The viewed surfaces of the HAYNES 230 and 282 samples did not show as substantial of a difference between each’s respective static and moving sample. The moving IN 625 sample showed both higher mass loss and deeper Cr depletion than the static sample. The HAYNES 282 showed the least mass change, both in the static and moving conditions. At 1000 hr the HAYNES 282 had near zero total mass change while the static sample had some mass gain. At 1000 hr both HAYNES 230 and the IN 625 moving samples had gained about twice as much mass as their respective static samples. Analysis showed the Cr depletion was deeper in all the moving samples, as compared to their respective static ones. The IN 625 had the deepest Cr depletion, followed by HAYNES 282, and then the HAYNES 230.

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