INFLUENCE OF THE CAST IRON SURFACE TOPOGRAPHY ON PROPERTIES OF ZINC COATINGS

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1 INFLUENCE OF THE CAST IRON SURFACE TOPOGRAPHY ON PROPERTIES OF ZINC COATINGS SKOTNICKI Wojciech 1, JĘDRZEJCZYK Driusz 1, SZŁAPA Ilon 2 1 University of Bielsko-Bil, ul. Willow 2, Polnd, E-mil ddress: wskotnicki@th.ielsko.pl, djedrzejczyk@th.ielsko.pl, 2 BISPOL SA Bielsko-Bił, Towrow 30, E-mil ddress: iszlp@ispol.com.pl Astrct The pper descries the corrosion resistnce comprison of hot-dip zinc glvnized cst iron with flke grphite in reltions to ronze. The prcticl ppliction of investigted cst iron is plnned s mterils for fitting elements working in corroding medium of se wter. The surfce of cst iron ws prepred in different wy: crude, turned, sndlsted nd oxidized. Oxidtion effect ws evluted y opticl microscopic tests, surfce roughness mesurements nd corrosion resistnce potentio-dynmic tests. It ws stted tht oxidtion process influence oth on the level of surfce development s well s the zinc coting structure nd zinc diffusion depth in cst iron mtrix. The oserved influence cuses the essentil cst iron corrosion resistnce increse which level reches vlue chrcteristic to ronze. Structure of coting lyers lloyed nd zinc nd their thickness is in direct correltion with kind of surfce preprtion nd in consequence corrosion resistnce. Exmintion presented in the elortion confirmed the thesis tht in nlysed cse high temperture oxidtion is not corrosion process ut rther kind of surfce tretment tht mkes wider the cst iron ppliction rnge. Keywords: Cst iron, oxidtion, surfce preprtion, corrosion resistnce, zinc coting INTRODUCTION There re million metl mchinery prts nd devices withdrwn from the use of every yer ecuse of wering them out y oxidizing [1, 2]. The most susceptile to tmospheric corrosion (oxygen nd humidity) re prts mde of iron lloys. i.e. steel nd cst iron. In industril prctice there re lot of methods protecting ginst the corrosion. To the most populr elong: pinting nd vrnishing, luminizing, zinc coting or ppliction of luminium zinc lloys coting [3]. Zinc-cotings re the most effective nd economic wy of iron lloys protection ginst the corrosion. Aprt from the high resistnce to the tmospheric corrosion this kind of coting demonstrtes lso the high resistnce to the triologic wer, i.e. cused y the friction. Mny mchinery prts re exposed during the ppliction to sliding friction, e.g. elements of industril fittings. The coting protecting mchinery prts surfces ginst corrosion tht re dditionlly exposed to the friction should possess very specil properties. Therefore the zinc coting put on prts of fittings should e chrcterized y smll friction coefficient wht mkes esier the regultion of the working medium flow nd reduce the consumption during the long-term using. Although zinc-cotings re universlly used for mny yers, these re still conducted the exmintions to improve their properties [4-9]. The glvnizing process of the grey cst iron performed in industril conditions is encountering difficulties, cused y susurfce grphite prticles nd the higher silicon content which is min reson of zinc-coting defects [7-9]. Etching pplied efore steel glvnizing in the hydrochloric or sulphuric cid isn't such useful with reference to the cst-iron cstings. Worsening of the qulity of the creted zinc coting is the result of the cids rections products precipitting in this process. The clening of the cst-iron csting surfce efore glvnizing is very difficult tretment [7-9]. It follows from the ove tht universlly technologies used normlly in steel glvnizing process cn t e utomticlly trnsferred to cst iron cstings nd especilly to the surfces fter mechnicl working. The presented prolems cn e solved y surfce decrurising of the cst iron in the process of high-

2 temperture oxidtion. The influence of high-temperture decrurising of the grey cst iron on properties of hot dip zinc cotings will e discussed in this rticle. 1. OWN RESEARCH During the tests grey cst iron GJL-250 ws used with flke grphite nd typicl chemicl composition determined in the PN-EN 1560: 2001 stndrd nd B555 ronze with chemicl composition ccording the EN573-1 stndrd (Tle 1). Test smples were tken from prts pplied for the production of checks vlve, s one-wy vlves. They re used to protect pipelines nd devices ginst the ckwrd flow of the medium sustnce nd re instlled on pipelines nd continers where we should prevent the strem withdrwl of the flowing medium. Checks vlve re destined to flow of: wter, stem, oil nd other nturl liquid nd gses mediums nd the work in se conditions ( Fig. 1). Tle 1 Chemicl composition Cst iron Bronze Chemicl composition [%] C Si Mn P S Mg Ce Ni Cu Al Chemicl composition [%] Cu Sn Zn P Al Fe Mn Si P Ni ,01 0,3 0,2 0,01 0,1 2 Fig. 1 Check vlves 2. TEST RESULTS To nlyze the cross-sectionl structure of the creted zinc coting the metllogrphic specimens were prepred. For metllogrphic oservtion n opticl microscope Axiovert A nd scnning microscope Joel - J7 were used. Results of smples oservtions re shown in Fig. 2.

3 c Fig. 2 The cross-section of zinc coting: ) the sndlsted surfce, ) crude surfce, c) the surfce fter oxidtion. Bsed on microscopic oservtion nd results of roughness profiles mesurements (3D) n influence of oxidizing on the cst iron microstructure nd the stte of the surfce efore glvnizing ws determined. The verge vlues of roughness mesurements - the sr prmeter is presented in the form of grph (Fig. 3). Chosen roughness profiles of the studied mterils surfces re presented in Fig Fig. 3 The verge vlues of mesured roughness prmeter sr.

4 Fig. 4 Isometric surfce imge nd roughness profile fter sndlsting 3D() i 2D () Fig. 5 Isometric surfce imge nd roughness profile fter oxidtion 3D() i 2D () Fig. 6 Isometric surfce imge nd roughness profile fter glvnizing 3D() i 2D () To determine the corrosion resistnce of the tested prts the potentiodynmic tests were performed in ccordnce with PN-76/H stndrd in the erted solution simulting se wter of 3.5% NCl. These studies were performed with ppliction of potentiostt Solrtron SI 1286, nd the mesuring cell in the three-electrode system. The device ws computer-controlled nd equipped with modern control softwre (CorrWre, ZPlot) nd softwre to results nlyze (CorrView, Ywn), llowing conducting repetle experiments with the elimintion of interference. The test results re shown in Tle 2 nd Fig. 7. During the study the potentil of nodic-cthodic trnsition EK-A nd the corrosion current ikor hs een mesured. Tle 2 The results of mesurements of corrosion potentil nd corrosion current Kind of surfce E K A [mv] i corr [A/cm 2 ] Corrosion rte [mm/yer] crude sndlsted oxidized ronze

5 Fig. 7 Registered potentio-dynmic polriztion curves - hot-dip zinc glvnized GJL-250 cst iron - 1) crude surfce, 2) snd-lsted surfce, 3) oxidized surfce, 4) ronze 3. RESULTS ANALYSIS Bsing on the conducted tests it ws stted tht the corrosion resistnce of the zinc coting put on the cst iron fter the oxidtion process ws similr to this one which ws determined on investigted ronze. It results mong others from the fct tht the structure of cotings creted on cst iron the surfces with hightemperture oxidtion is similr to the model structure nd is free from grphite precipittions decresing coting tightness Fig. 2. The coting creted on oxidized se is composed of clerly visile diffusion lyer nd outside pure zinc lyer. In internl lyer it is possile to distinguish phses: Γ, δ1 nd ζ, however outside lyer consists of phse η. After putting zinc coting on cst-iron surfce which wsn't oxidised it ws oserved tht grphite prticles penetrte inside coting decresing lyer continuity. It proves tht grphite prticles could e the min reson of zinc coting nticorrosion properties decresing. It results from cotings thickness mesurements mde y opticl microscope ppliction tht the smllest thickness of the zinc coting μm ws chieved on the crude cst iron surfce. For the surfce fter sndlsting the thickness of the coting ws in the rnge of μm. The gretest coting thickness μm fulfilling the requirements of PN-EN-ISO 1461 stndrd ws creted on cst iron fter high temperture oxidtion. The reson of such the coting thickness differentition could e the surfce roughness - level of surfce development efore zinc glvnizing. The smllest mesured roughness vlues (sr = 4.3 µm, srp = 19.9 µm, srv = 24.8 µm) were determined for cst iron crude surfce. After mechnicl clening surfce roughness incresed to: sr = 9.7 µm, srp = 24.9 µm, srv = 25.5 µm. After oxidtion nd the scle removl the verge roughness vlues reched the level sr = 11.5 µm, srp = 21.9 µm, srv = 18.6 µm. It ws oserved tht the level of surfce development influence on the thickness of internl zinc coting sulyers: lloyed lyer nd pure zinc lyer thickness. Moreover, removl in the process of oxidtion the grphite remins prevents theirs further diffusion deep into of zinc coting. On the other hnd filling the post grphite emptiness with zinc cn contriute to increse the corrosion resistnce of erlier oxidised surfces (Fig. 2c). The results chieved in the form of polriztions curves/grphs cn e divided to two fundmentl prts: the cthodic nd nodic polriztion. From the point of view of conducted exmintions the prt descriing the nodic polriztion is more importnt. Oserving of the nodic course it is possile to determine mteril s ehviour in corrosion environment. The most importnt points of the grph re corrosion potentil Ecorr nd corrosion current icorr. On the sis of potentio-dynmic mesurements of polriztion of the hot-dip

6 glvnized cst-iron electrode importnt differences were stted in the potentil of zinc cotings. For the cst iron crude surfce the potentil ws mv. The reduction of the potentil y the hlf to the vlue -698 mv ws recorded for the surfce fter sndlsting. Further potentil decresing to -644 mv ws recorded for oxidised surfces. Lowering the potentil ws comined with decresing the corrosion current out one order of mgnitude (3.36 of 10-6 A/cm 2 ), nd in the process with slowing down the corrosion. For prts mde of ronze the corrosion potentil ws -242mV, nd corrosion current 6.73 of 10-6 A/cm SUMMARY AND CONCLUSIONS On the sis of preliminry tests the following conclusions cn e formulted: The high temperture oxidtion of cst iron surfce exerts the influence in three directions: increses the level of surfce development, elimintes grphite prticles from the zinc coting structure nd llows for deeper zinc diffusion inside the cst iron mtrix. The chieved results confirm tht the high temperture tretment (oxidtion) of gry cst iron cstings surfce, llows for hot-dip zinc cotings cretion with the correct structure nd thickness. The zinc coting creted on the surfce fter oxidtion revel the structure in ccordnce with Fe-Zn system. The enrichment of the metl mtrix nd fter grphite voids in zinc my e the reson of the corrosion resistnce increse of the mterils previously oxidized. The corrosion resistnce of oxidized nd hot-dip zinc glvnized cst iron is similr to the ronze resistnce tested in the sme conditions. REFERENCES [1] Porter F.C. Economics of Cotings for Steelwork Protection. Progress in the Understnd nd Prevention of Corrosion 10th Europen Corrosion Congress. Brcelon 1993, pp.687. [2] Sndelin R. W. Glvnizing chrcteristics of different types of steel. Wire nd Wire Products 1940, 11, pp [3] Ttrek A., Kni H., Lierski P.: Znurzeniowe powłoki cynkowo-luminiowe. Gdy wymgn jest wyższ odporność n korozję. Lkiernictwo Przemysłowe, no. 3 (59), 2009, pp D [4] Thiele M., Schulz W. Coting formtion during hot-dip glvnizing etween 435 C nd 620 C in conventionl zinc melts generl description. Proceedings Interglv, [5] Gierek A., Ttrek P., Lierski P. Bdni zjwisk rozpuszczni w ukłdzie żelzo Armco kąpiel cynkow w wrunkch stcjonrnych, Inżynieri mteriłow, 2005, 5, pp [6] Wesołowski J., Głuchowski W. Universl nd cost-sving zinc coting on products mde of rective steel. Metl 2006, 60, 5, pp [7] Jędrzejczyk D., Skotnicki W., Węgrzynkiewicz S., Hjdug M. Influence of the surfce stte on hot dip Zn glvnizing effects of iron lloys. Metl st Interntionl Conference on Metllurgy nd Mterils. Brno, Česká Repulik, pp. 62 [8] Jędrzejczyk D., Hjdug M. Cst Iron Zinc Glvnizing Improved y High Temperture Oxidtion Process. Journl of Achievements in Mterils nd Mnufcturing Engineering 2010 Vol. 43. pp [9] Jędrzejczyk D., Hjdug M. Effect of the Surfce Oxidtion on the Hot Dip Zinc Glvnizing of Cst Iron. Archives of Metllurgy nd Mterils 2011 Vol. 56. pp