Deterioration of electromotive force of chromel-alumel thermocouples in reducing atmospheres at high temperatures
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I.
INTRODUCTION
THE problems of measuring temperatures in corrosive environments which have low oxidizing and/or high carburizing potential have received attention in recent years. 1,2In such environments, alloys are preferentially oxidized and/or carburized. The chemical composition of the alloys is altered, with the result that emf (electromotive force) of the thermocouple can drastically change.l'2 In petrochemical engineering plants, temperatures must be measured in high temperature reducing atmospheres, where deterioration of emf of thermocouples, especially that of the Chromel-Alumel* (CA) thermocouple, has been well *"Chromel-Alumel" is a registered trade name of Hoskins Mfg. Co. Ltd.
known and is one of the most important problems to be solved.l The origin of this decrease of emf of CA thermocouples has been known to be depletion of chromium in Chromel, which is caused by preferential oxidation and/or carburization of chromium. 2 This deterioration of emf can be improved by sheathing thermocouple with heat resisting superalloys. In addition, tantalum or titanium would be included within the sheath to getter or scavenge residual oxygen. 2 In Japan, a direct steel making system utilizing heat from HTGR (High Temperature Gas-cooled Reactor) has been developed in the past ten years; temperatures must be taken in the high temperature hydrogen-base gas mixtures required to reduce iron ores .3.4The National Research Institute for Metals (NRIM) in Japan has been conducting the research and tests of candidate heat resisting nickel base superalloys for heat exchanger of the HTGR, as has been TATSUE SHIKAMA is Researcher, TATSUHIKO TANABE is Senior Researcher, MASAKAZU FUJITSUKA is Researcher, HEITARO YOSHIDA is Division Head, Nuclear Materials, and RYOJI WATANABE is Division Head, Nonferrous Materials; all are with the National Research Institute for Metals (NRIM), 1-2-1, Sengen, Sakuramura, (Tsukuba Academic City) Niiharigun Ibarakiken, Japan, 305. Manuscript submitted January 21, 1980. METALLURGICAL TRANSACTIONS A
reported. 3 Temperature measurements have been knotty, but are an important problem in prosecuting creep rupture tests and corrosion tests in high temperature, hydrogen-base gas mixtures. 5 Sheathed thermocouples could be used on corrosion tests, but thermocouples have to contact specimens in case of creep rupture tests, so that large thermocouple sheaths could not be applied. Emf of the platinum-platinum/rhodium thermocouple is well known to be the most stable in clean atmospheres, but deteriorates in reducing atmospheres, and its price is relatively high. Emf of the tungsten-tungsten/rhenium thermocouple is known to be relatively stable in reducing atmospheres; however, its ductility is quite poor, and it could not be used on creep rupture tests, contacting specimens. Here, present authors have tried to apply CA thermocouples because of their low price and of their high emf. The present paper will analyze the deterioration process of emf of CA thermocouples in the atmosphere mentioned above, and wil
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