Ignition of nickel in environments containing oxygen and chlorine
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I.
INTRODUCTION
MIXEDoxidation refers to the oxidation of a metal in an environment containing more than one oxidizing species. Under most conditions only one condensed phase will be thermodynamically stable. The condition of greatest commercial importance is that in which the metal oxide is the thermodynamically stable phase but a small amount of a second oxidizing species is present which can affect the kinetics of the oxidation process. Small amounts of chlorine contamination can have this effect, and chlorine is a common contaminant in many industrial environments. 2 Because of the high vapor pressure of many metal chlorides, the vapor pressure of the chloride reaction product can be significant even under conditions where the condensed chloride phase is not thermodynamically stable. Volatile species have been shown to play a significant role in the mixed oxidation-chlorination of cobalt in argon-oxygenchlorine environments at temperatures between 900 and 1200 K. 3'4'5
Volatile species have also been shown to play an important role in the corrosion of iron, nickel, and alloys of these metals in environments containing oxygen and either chlorine or hydrogen chloride. 6-9 In the present study, the corrosion behavior of nickel has been investigated by thermogravimetric analysis in argon-oxygen-chlorine environments at temperatures between 900 and 1200 K. The results of these experiments have been combined with the results of scanning electron microscopic investigations of the morphologies of the corrosion products to characterize two regimes of corrosion behavior for the mixed oxidation-chlorination of nickel in these environments.
II.
APPARATUS AND PROCEDURE
The apparatus used in this study has been described elsewhere. ~~ It consists of a Kanthal wound furnace, a Cahn 2000 microbalance, and equipment for purifying and mixing gases containing argon, oxygen, and chlorine in any proportion. In each experiment, the metal specimen was suspended from a quartz fiber within a quartz reaction tube just above the furnace. The furnace was purged with argon which had been deoxidized by passage through hot copper turnings, and then was heated to the desired reaction temperature. When the furnace reached an appropriate temperature it was raised to heat the specimen. The specimen was heated to the reaction temperature in 10 minutes or less in purified argon gas. This heating was accompanied by an increase in mass of less than 0.1 mg cm -2, indicating that oxide formation during the heat-up period was minimal. Premixed corrosive gases were then introduced into the tube, passing upward past the specimen, and the mass of the specimen was recorded for 150 minutes or until the change in mass exceeded the range of the microbalance. The superficial gas flow rate through the furnace was fixed at 1.5 cm s -j at each reaction temperature. The metal specimens were prepared from 1.27 cm diameter 99.99 pct pure nickel rod. Disks approximately 0.7 mm thick were sliced from the rod using a low-speed diamond wafering saw. A 2 mm diamete
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