Vapor pressure measurement of zn-fe intermetallic compounds
- PDF / 252,408 Bytes
- 6 Pages / 606.24 x 786 pts Page_size
- 76 Downloads / 179 Views
4/27/04
19:42
Page 487
Vapor Pressure Measurement of Zn-Fe Intermetallic Compounds KAZUAKI MITA, SHU YAMAGUCHI, and MASAFUMI MAEDA Most galvanized steel sheets are produced by a continuous galvanizing line. Some of the operations include a subsequent heat treatment termed galvannealing to form intermetallic phases on the surface. These galvanizing reactions are essentially regarded as “making intermetallic compounds” in the Fe-Zn-M system. This makes it important to know the thermodynamic properties of the system for any detailed discussion on the formation of these intermetallics; however, experimental difficulties have limited the number of studies. In the present study, two-phase regions of these intermetallics were examined, and the following two-phase mixtures were prepared at 723 K: Fe(␣) ⫹ ⌫, ⌫ ⫹ ⌫1, ⌫1 ⫹ ␦1 ⫹ . The double cell–type Knudsen mass spectrometer system was developed and employed, in which two sets of Knudsen cells were installed in the same cell holder. The reference material (pure zinc) was placed in one cell and a sample in the other. The cell holder was rotated to the reference and sample positions to measure zinc vapor effused from two cells by a quadrupole mass spectrometer. Material containing the two phases was placed in a Knudsen-type effusion cell and the mass spectrum studied to evaluate the vapor pressure of zinc. The activity of zinc in the intermetallics was determined by comparing the intensity from pure zinc and that from intermetallics. Results for Fe(␣) ⫹ ⌫ were 0.46 to 0.48; for ⌫ ⫹ ⌫1, 0.51 to 0.54; for ⌫1 ⫹ ␦1, 0.62 to 0.66; and for ␦1 ⫹ , 0.80 to 0.83, between the temperatures of 623 and 698 K.
I.
INTRODUCTION
MOST galvanized steel sheets are now produced by a continuous galvanizing line, where a zinc coating layer is formed on steel sheets by dipping them into a molten zinc bath. Some of the operations include a subsequent heat treatment termed “galvannealing” to form intermetallic phases on the surface. In the actual operation, aluminum is added to a zinc bath to control the reaction between molten zinc and iron by forming an inhibitor layer of Fe2Al5. During the galvannealing treatment, the Fe2Al5 phase decomposes and dissolves into the molten zinc layer in the early stage of the reaction, and Fe-Zn intermetallic compounds are formed. The galvanizing reactions are, therefore, primarily regarded as the Fe-Zn binary system. It is still controversial whether the reactions between iron and zinc are governed by a simple solid-state diffusion or whether the transport through liquid zinc has an important role. In either case, it is important to know the thermodynamic properties of the iron-zinc binary system for any detailed discussion on the formation of intermetallic compounds. Another practical aspect of the vapor pressure measurements is the zinc recovery from the galvanized steel sheets by the vacuum evaporation process proposed by Okada et al.[1] Because of the experimental difficulties, however, there are only a few studies[2–6] available on the thermodynamic prop
Data Loading...
