Mechanism and Kinetics of Corrosion of Selected Iron and Cobalt Alloys in Refluxing Mercury
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION CLEVELAND OH LEWIS RESEARCH CENTER
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Five alloys of engineering interest, the cobalt-base HS-25 and H-8187 and the iron- base SICROMO-9M, AM-350, and AM-355, were corrosion tested in boiling mercury in the temperature range of 1000 degrees to 1300 degrees F 811 degrees to 977 degrees K for times as long as 6000 hours. Metallographic, chemical, and physical analyses were used to determine the extent and the nature of the corrosion. Corrosion mechanism and reaction kinetics were inferred through a comparison of experimental and theoretical kinetic constants. The experimental kinetic constants i. e., the time-law constant bsub Gamma and the activation energy Delta Eta Gamma were obtained from the test data by multiple regression analysis. The theoretical kinetic constants b1 and Delta Eta 1 for each of i steps of proposed corrosion models were obtained from transport theory relations and the pertinent boundary conditions. Changes of mechanism during the progress of corrosion were indicated for all the alloys tested except the AM-350 and AM-355 alloys. The shifts from one corrosion regime to another were associated with changes in the nature and length of the corrosion- affected zone. Three corrosion regimes were characterized, linear Iota, parabolic, and linear II. The probable rate-determining step in the two linear rate regimes is boundary-layer diffusion. The probable rate-determining step in the parabolic rate regime is liquid diffusion.
- Physical Chemistry
- Properties of Metals and Alloys