Effect of phosphorous surface segregation on iron-zinc reaction kinetics during hot-dip galvanizing
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I.
INTRODUCTION
PREVIOUS characterization of zinc coatings has largely been dedicated to coatings deposited on iron and low-carbon steels; however, interstitial-free (IF) steels containing titanium and niobium have recently become important because of their formability. The IF steel grain boundaries are essentially carbon free due to the precipitation of Ti and Nb carbides within the grains. It has been postulated[1] that the chemical nature of the grain boundaries increases the thermodynamic activity at grain boundary sites on the steel surface during hot-dip galvanizing, resulting in the preferential nucleation and growth of Fe-Zn phases at these sites. Phosphorous is added to IF steels as a solid solution strengthening agent, in order to prevent denting of automotive steels. Previous work on galvannealed coatings has indicated that substrate chemistry can affect coating formation kinetics,[2] but specific information on the effect of phosphorus is not readily available. Allegra et al.[3] found phosphorous to segregate to ferrite grain boundaries in low-carbon steels (0.02 to 0.06 wt pct C) containing at least 0.04 wt pct P, blocking the diffusion of Zn along the grain boundary and lowering the thermodynamic activity. Mercer[4] proposed that the phosphorous enrichment at the grain boundaries impeded Fe and Zn interdiffusion, thus lowering the amount of Fe found in Zn galvanizing coatings on rephosphorized steel. Therefore, phosphorous in the base steel acts as an inhibitor to Fe-Zn alloy growth primarily due to grain boundary segregation of phosphorous.[3,5] Recent transmission electron microanalysis on thin foils has shown that P segregation actually occurs in a Fe-0.1 pct P alloy under simulated continuous annealing conditions.[2] In contrast, Lin et al.[6,7] proposed C.E. JORDAN is Physical Metallurgist, Knowles Atomic Power Laboratory, with Lockheed Martin, Schenectady, NY 12301. R. ZUHR is with the Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. A.R. MARDER is Professor with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015. Manuscript submitted March 7, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
an alternative mechanism, whereby P segregates to the steel surface during recrystallization annealing, stabilizing the inhibition layer and retarding the rate of Fe-Zn phase growth reactions during galvanizing. It was the objective of this research to systematically evaluate the effect of P surface segregation on Fe-Zn reaction kinetics during galvanizing. II.
EXPERIMENTAL PROCEDURE
The substrate material used for P-ion implantation was a low-carbon steel in which extremely large grains (10 to 20 mm) had been produced using a strain-anneal technique. Since it was not possible to procure an adequate number of large grain samples with identical chemistry for use in ion implantation, three alloys with similar chemistries were used as implantation substrates. The substrate chemistries are listed in Table I and were determined by both wet che
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