DROSS FORMATION MECHANISMS ON STAINLESS STEEL HARDWARE IN A ZINC-55%ALUMINUM BATH

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1 DROSS FORMATION MECHANISMS ON STAINLESS STEEL HARDWARE IN A ZINC-55%ALUMINUM BATH Ashok Vrdrjn 1, Bruce Kng 1 nd Mrk Bright 2 1 West Virgini University, Deprtment of Mechnicl nd Aerospce Engineering Morgntown, West Virgini, U.S.A. 2 Pyrotek Incorported, Metullics Systems Division Auror Rod, Solon, Ohio, U.S.A. ABSTRACT Dross formtion on stinless steel pot hrdwre in glvlume th (43.4wt% Zn, 55wt% Al, 1.6wt% Si) cn led to significnt production downtime nd mintennce costs. A series of immersion tests were undertken to understnd the dross formtion mechnisms nd surfce rections. The vrious steps involved in the formtion of the intermetllic dross lyer nd the ehvior of silicon during this process were studied. Growth of the intermetllic dross lyer nd its composition were nlyzed using SEM/EDS. The results indicted tht the initil rte of dross formtion ws retrded due to the formtion of silicon-rich inhiition zone on the stinless steel sustrte surfce. However, smples immersed longer thn 40 minutes showed rekdown of the silicon-rich lyer nd ccelerted rte of dross formtion. Growth of the intermetllic dross lyer continued to increse over time due to gglomertion of the dross prticles formed on the roll surfce. These results suggest tht initil dross removl on sink roll (y scrper) should e crried out within minutes fter the roll is plced in the glvlume th, nd strtegy of more frequent (or even continuous) dross removing ction with less scrping pressure my e eneficil. Keywords: Glvlume, sumerged pot hrdwre, dross mechnism, silicon inhiition INTRODUCTION One of the min prolems fced y the glvnizing steel industries is the formtion of intermetllic compounds (dross) on the surfce of the rolls nd frequent stoppge of the lines, resulting in huge energy nd mintennce costs. In the pst three decdes, steel sheets coted with Zinc-55Al-1.6Si (wt pct.) hve incresingly replced the trditionl glvnized construction products due to their high temperture corrosion resistnce compred to other coted steel sheets with sme thickness 1). Dross prticle formtion is controlled y the th, the steel sheet nd lso the pot hrdwre nd the rte of dross formtion is controlled y the th composition nd the th temperture. However, in generl, the th temperture is mintined depending on the specified luminum content in the th 2). The rection mechnisms of the cotings formed during hot dipping of iron in zinc ths with 0-10% luminum were originlly studied y Ghumn nd Goldstein 3) The hot dipping process of coting steel with Zn-55Al-1.5Si is typiclly performed t 600 o C nd silicon is dded to the molten lloy th to control the growth of the intermetllic lyer on the steel sheet, which, ccording to Selverin, et l. 4), improves the dhesion etween the lloy coting nd the steel sustrte. Silicon ddition lso inhiits the rpid exothermic rection etween the iron sustrte nd molten th constituents 4). Numerous reserchers hve developed severl sttic nd dynmic tests to determine the corrosion of pot hrdwre mterils in the glvnizing th 4-13) ut informtion relting to the mechnism of dross formtion on the roll surfce hs not een fully explined. Hence, the im of this reserch ws to investigte intermetllic prticle nd coting formtion mechnisms on the surfce of pot hrdwre mterils. Advnced understnding of these coting rections my eventully contriute to reduced downtime, decresed mintennce costs nd enhnced productivity for continuous coting lines. EXPERIMENTAL PROCEDURE Corrosion rod smples (13mm dimeter) of 309 stinless steel (0.08wt% C, 1.0wt% Si, 23.0wt% Cr, 14.0wt% Ni nd Fe lnce) were rotted (110 RPM) in molten th of Zn-55Al-1.6Si mintined t 649 o C (1200 o F). Also, to void potentil mteril chnge from sudden therml differentil, ech smple ws preheted to 538 o C (1000 o F) efore immersing into the molten th. The experimentl setup is shown in Figure 1. The durtion of ech test ws in 20 minute increments up to 140 minutes.

2 Figure 1: Experimentl setup of the dynmic dross uild up test. EXPERIMENTAL RESULTS AND SUMMARY After ech test, the 309 stinless smple ws removed, cut nd polished (to 0.3µ) using dimond pste on micro-cloth then ultrsoniclly clened with cetone for 5 minutes to remove ny residul polishing gents. The cross-section morphology of ech smple ws susequently nlyzed using Hitchi S-4700 Scnning Electron Microscope fitted with n EDAX Energy Dispersive X-ry Anlyzer (EDS). Elementl nlysis ws crried out t the smple-th interfce to identify nd mesure the compositions in ech region. Smple-Bth Interfce After 20 Minutes Upon creful inspection of the smple-th interfce fter 20 minutes of immersion, rection lyer of uniform thickness ws oserved (t 250X). However, t higher mgnifiction (t 1000X), the rection lyer ctully showed two distinct lyers s displyed in Figure 2. EDS mpping (Figure 3) reveled the outwrd diffusion of iron (Fe) nd uniform lyer of silicon (Si) on the smple surfce. c Figure 2: Microgrphs of the roll-th interfce () 250X mgnifiction. () 1000X mgnifiction. (c) 8000X mgnifiction.

3 Figure 3: EDS mp of the roll-th interfce t 1000X mgnifiction Smple-Bth Interfce After 40 Minutes As displyed in Figure 4, the interfce on the smples showed two min chrcteristic fetures susequent to 40 minutes immersion: (i) (ii) Micro-crcks in the rection lyer Smll discontinuous dross prticles on the th side of the rection lyer (verge size = 12µm) Figure 4: Smple-Bth interfce fter 40 minutes () Microcrcks on the roll-th interfce (2000X mgnifiction). () Smll dross prticles (~12µm) (1000X mgnifiction). Smple-Bth Interfce After 60 Minutes The smple-th interfce fter 60 minutes of testing hd uniform lyer of dross formtion. Agglomertion of smll dross prticles over the dross lyer ws lso oserved in certin res under higher mgnifiction (1000X) (s shown in Figure 5). Averge thickness of the dross lyer t the end of 60 minutes ws 40µm. The EDS spectrum of the dross lyer nd the th is shown in Figure 6. In Figure 6-1 the composition of th is clerly identified, while in Figure 6-2 the dross lyer is composed of Fe-Si-Al ternry intermetllic compound. From this nlysis, etter understnding of the effect of silicon in forming stle ternry intermetllic compounds my e scertined.

4 2 Bth Dross lyer c 1 Figure 5: Smple-Bth interfce fter 60 minutes () structure of the dross lyer formed t the interfce (250 X). () 1000X mgnifiction (c) gglomertion of smll dross prticles on the continuous lyer (4000X). Figure 6: EDS Spectrum showing the composition of th (loction 1) nd dross (loction 2) s indicted in Figure 5 (c). Smple-Bth Interfce After 80 Minutes The microgrphs of the 80 minute smple (Figure 7) showed two distinct lyers of dross with slight chnges in the luminum nd iron content. The inner lyer (closest to the roll) ws continuous nd hd uniform thickness, wheres the outer lyer ws irregulr nd contined smll dross prticles gglomerting to it. Dross Prticles Dross Lyer Figure 7: Dross lyer formed on the roll surfce fter 80 minutes () 3000X mgnifiction, () 1000X mgnifiction. Smple-Bth Interfce After 100, 120 nd 140 Minutes After 100 minutes nd eyond (Figure 8) the sme mechnism of dross formtion ws oserved s with the 80 minute smple except with incresing dross lyer thickness. Discontinuity of dross lyer towrds the th side ws oserved indicting gglomertion of the dross prticles over time. The size of the dross prticles on the outer lyer ws lrger compred to the 40 minutes smples. This increse in size ws possily due to gglomertion of smller dross prticles formed in the th nd moving towrds the smple s result of the dynmic motion of the th. In ctul pplictions, these lrger dross prticles would mke the overll roll surfce rougher, leding to poor qulity nd surfce imperfections on the glvnized sheets.

5 Dross lyer Dross Prticle c Dross Lyer Figure 8: Smple-Bth interfce fter () 100 minutes (1500X). () 120 minutes (1500X). (c) 140 minutes (1500X). DISCUSSIONS Bsed on the time series tests, the overll dross formtion mechnism on the sumerged pot hrdwre in glvlume cn e schemticlly represented s shown in Figure 9. Si Rich Lyer Avg. ~5 µm Fe-Al Prticles Avg. ~12 µm Fe-Al Dross (Fe 2 Al 5 ) Fe-Al Dross (FeAl 3 ) Fe-Si-Al Lyer Figure 9: Schemtic representtion of dross formtion on roll surfce in glvlume th. After 20 minutes, uniform protective lyer of silicon ws oserved etween the smple nd the th interfce (Figure 9-B). The source of Si-rich lyer formed fter 20 minutes my due to either the Si toms lierted from the roll surfce s result of dissolution of the steel or from solidifiction of the metllic phse silicon, which hs higher melting point (1414 o C) thn tht of the molten th (649 o C) 14). Susequently fter 40 minutes (Figures 9-C nd 9-D), crcks were oserved over the uniform lyer of silicon nd gglomertion of smll dross prticles were identified. These dross prticles were proly formed elsewhere in the th due to the rection etween the iron nd luminum nd the dynmic motion of the th cused the prticles to gglomerte on the smple surfce. After 60 minutes (Figure 9-E), uniform dross lyer ws oserved on the smple surfce nd silicon-rich res were oserved in the outer regions of the uild-up. After 80 minutes (Figures 9-F nd 9-G), two lyers of dross were oserved, the inner most thin lyer consisted minly of Fe nd Al nd the outer non-uniform lyer contined Fe-Si-Al. At indeterminte loctions, lrger dross prticles (Fe-Al) were oserved. After 100 minutes (Figure 9-H) the dross lyer nd the dross prticles oserved were lrger in size s compred to the 80 minutes smple. 15) It hs een reported tht the silicon phse present in the th converts the Fe-Al prticles to more stle ternry Fe- lyer. The increse in size of the lyer nd prticles were oserved on the remining smples. Bsed on Si-Al intermetllic these tests, the mechnism of the dross formtion cn e ctegorized under three dominnt stges:

6 1. Formtion of dross (Fe, Al nd Si) in the th 2. Agglomertion of smll dross prticles on the roll surfce due to the dynmic motion of the th 3. Formtion of continuous dross lyer over the roll surfce Furthermore, EDS nlysis of the smple-th interfce reveled the intermetllic compound formed ws ternry phse contining Fe, Al nd Si in ddition to the typicl Fe 2 Al 5 nd FeAl 3 phses. The dross lyer formtion t ech stge ws mesured nd the lyer thicknesses were plotted reltive to time. As outlined in Figure 10, the initil growth ws slower due to th e formtion of silicon inhiition lyer. However, fter 60 minutes, the dross growth rte settled into prolic trend G Thickness (microns) E, F 10 A B C, D Time (mins) Figure 10: Dross lyer growth pttern. SUMMARY The mechnism nd the rte of dross formtion re explined sed on the dynmic tests nd the effect of the silicon in the 55%Al-Zn th to convert the Fe-Al intermetllic compounds to stle ternry intermetllic compound hs een shown. ACKNOWLEDGEMENTS The uthors like to thnk the US-DOE (wrd # DE-FC36-04GO14038) nd the Industries of the Future-West Virgini for sponsoring this project nd re grteful to Mr. Liviu Mgen nd Mrs. Adrienne McGrw in the WVU-Chemicl Engineering Anlyticl Lortory for their ssistnce. Specil thnks to Pyrotek Inc. nd Wheeling-Nisshin, Inc. for their ctive prticiption in this project. REFERENCES 1. GlvInfo Notes, 2. S. Shwki nd Z. Adel Hmid. Surfce nd Interfce Anlysis, 35(2003). 3. A.R.P Ghumn nd J.I. Goldstein. Metllurgicl Trnsctions, 2(1971), J.H. Selverin, A.R. Mrder nd M.R. Motis, Metllurgicl Trnsctions, 20A(1989), K. Tni, T. Tomit, Y. Koyshi, Y. Tktni, Y. Hrd, ISIJ Interntionl, 34(1994) M.S. Brunnock, R.D. Jones, G.A. Jenkins, D.T. Llewellyn, Ironmking nd Steelmking, 23(1996) F.E. Goodwin, K.-M. Chng, V. Sikk, Glvnizer s Assocition Proceedings, Derorn, MI (2002). 8. K. Zhng, N.-Y. Tng, M. X. Yo, Glvtech Conference Proceedings, Chicgo, IL (2004) X. Liu, E. Brero, V. Sikk, Glvtech Conference Proceedings, Chicgo, IL (2004) X. Liu, E. Brero, J. Xu, M. Burris, K.-M. Chng, V. Sikk, Metllurgicl Trnsctions A, 36A(2005) M.X. Yo, J.B.C. Wu, R. Liu, Mterils Science nd Engineering, 47(2005) K. Zhng, N.-Y. Tng, F. Goodwin, S. Sexton, Glvnizer s Assocition Proceedings, Columus, Ohio (2006). 13. M. Bright, Doctorl Disserttion, West Virgini University, Morgntown (2007). 14. R.Y. Chen nd D.J.Willis. Metllurgicl nd Mterils Trnsctions A, 36A(2005), N.-Y. Tng, Glvtech Conference Proceedings, Chicgo, IL (2004) 683.