Quantification of Galvannealed Coating Phases Using the Galvanostatic Technique
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nnealed coating is produced by in-line heat treatment of the zinc coating, which is carried out by heating the coated steel immediately after zinc bath to approximately 550 °C and holding there for a few seconds. This leads to alloying of zinc with the iron by diffusion. After this heat treatment, the average iron content of galvannealed coating is about 10 wt pct, which varies from 7 wt pct (at the top of the coating) to 23 wt pct (near steel interface), and the coating becomes a layered structure of Fe-Zn intermetallic phases.[1,2] The optimum alloyed galvannealed microstructure consists of a thin iron-rich gamma (C) layer of about 1-lm thickness and an overlay containing delta (d) phase, interspersed with a small amount of zeta (f) phase, at the top. The ranges of iron content of the f, d, and C phases are 5 to 6, 7 to 11, and 23 to 28 wt pct, respectively.[2] Different Fe-Zn intermetallic phases in the galvannealed coating have distinct anodic dissolution potential. Therefore, at a given current density, dissolution of each alloy layer takes place at a particular potential. The length of the time (in the galvanostatic potential-time curve) for which this particular dissolution potential is sustained is the measure of the thickness of the same layer in the galvannealed coating.[3] Quantification of
R. MISHRA, Researcher, is with Research and Development, Tata Steel, Jamshedpur-831001, India. Contact e-mail: rajiv.mishra@ tatasteel.com Manuscript submitted October 19, 2007. Article published online July 11, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
galvannealed coating phases using the galvanostatic method is based on the aforementioned principle. The galvanostatic method is an indirect method for coating phase characterization compared to other metallurgical techniques such as scanning electron microscopy with energy-dispersive spectroscopy (EDS) and glow discharge optical emission spectroscopy (GDOES). However, once the relation between the morphological structure and galvanostatic curve is known, the method can be used in a much simpler way to characterize any layered metallurgical structure. The galvanostatic method does not require calibration or sample preparation unlike other methods such as GDOES (requires calibration) and EDS (requires sample preparation). This method is simple and therefore can be effectively used for characterization and quantification of galvannealed coating phases, because the microstructures of galvannealed coatings are known. The coating phases of three different galvannealed steels (named as S1, S2, and S3) were characterized by galvanostatic methods. The details of substrate steel composition and coating process parameters are listed in Tables I and II, respectively. The steels of different thickness and chemical compositions were coated from the same bath. The processing line speed was also different for each steel grade, which led to a difference in their coating weight. These coated strips were passed through the galvannealing furnace maintained at different powers. The coating mi
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