The effects of water vapor on solidification of galvanized coatings
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R. G. HART and H. E. TOWNSEND are Supervisors, Sheet Steels and Coated Products Division, Research Department, Bethlehem Steel Corporation, Bethlehem, PA 18016. N. S. BERKE, formerly Engineer, Sheet Steels and Coated Products Division, Research Department, Bethlehem Steel Corporation, Bethlehem, PA, is now with W. R. Grace and Company, Cambridge, MA 02140. Manuscript submitted June 14, 1983. METALLURGICALTRANSACTIONS B
the panels were allowed to drain without wiping and cooled to room temperature at a rate of approximately 38 ~ per second. The cooling rate was determined in separate tests of panels with thermocouples attached. No intermetallic layer was detected in metallographic examination at 500x. The oxygen and water vapor concentrations in the 30 pct H2-N2 atmosphere of the coating chamber were monitored with an anodized aluminum moisture detector (capable of measuring 0.5 ppm water vapor) and a H2/O2 fuel cell oxygen analyzer (capable of measuring 0.1 ppm 02). A 30 pet H2-N2mixture was maintained within the 20 m 3 chamber by mixing 99.98 pct H2 and 99.95 pct N2 gases. The pure environment was achieved by use of a Dow Chemical Company Ridox oxygen getter and a Vacuum Atmospheres Company molecular sieve. The trace-H20 environment was achieved by removing the lid from a 250 ml jar of distilled water within the pure environment until the higher level of water vapor was attained. Water and oxygen content for the two conditions are given below. Composition (Parts per Million)
Pure Trace- H20
HzO 0.6 75
0_2 0.3 1.3
The surfaces of the coated steels were analyzed (after transferral from the coating chamber to the spectrometer, with intervening exposure to ambient atmosphere) by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) using a Perkin-Elmer Model 590A/548 SAM-AES-XPS spectrometer. Areas of analyses were 0.2 and 4 sq. mm for AES and XPS, respectively. To obtain compositional information about layers below the escape depth of Auger electrons and photoelectrons, samples were sputtered in situ with 5 keV argon ions. The results given below are typical of those obtained from duplicate analyses of six samples from each condition. AES sputter profile analyses (Figure 2) show a high concentration of aluminum at the surface of all coatings. There is more aluminum at the surface of galvanized coatings solidified in 30 pct H2-N2 with H20 addition than coatings solidified in ultra-pure 30 pet H2-N2. The carbon at the outermost surface is typical of surfaces exposed to ambient conditions prior to analysis in the AES spectrometer. XPS spectra, Figure 3, show the presence of aluminum oxide and zinc metal near the surface of the galvanized coating produced in the atmosphere containing trace H20. In contrast, surface spectra from the coating produced in ultrapure H2-N2 show little aluminum oxide, considerable zinc oxide, and some zinc metal. These data indicate that a thin, continuous skin of aluminum oxide covers the surface of the coating produced in the H20-containing atmosphere, since the un
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