FARKAS, J.

NAGY, L. G.

VARSÁNYI, G.

English

Periodica Polytechnica (Budapest University of Technology and Economics)

HETEROGENEOUS ISOTOPIC EXCHANGE AT THE INTERFACE OF STEEL SHEETS IN CONTACT WITH IRON CHLORIDE SOLUTION I. QUALIFICATION AND PREPARATION OF THE METAL SURFACE By 'J. FARKAs*, L. G. NAGY** and G. VARS . .\.NYI** L • Department of Physical Chemistry, Technical University, Budapest (Received October 7, 1968) Presented by Prof. Dr. G.' VARS.tl';"n Introduction The purpose of the iuvestigatiou iuto heterogeueous jsotopic exchauge processes, aud withiu this scope, iuto exchauge processes ou metal surfaces (iu coutact with a solutiou coutaiuiug the tracer ious of the metal) is to study both kinetics and mechanism of the exchange processes, and reactivity, structure, self-diffusion etc., by applying these processes (as a method). It can be hoped that the investigation into isotope exchange processes on metal surfaces "will result in a method suitable also for investigating the kinetics of hetcrogeneous reactions on metal surfaces, with special regard to the catalytic action and the corrosion behaviour of metal surfaces. (For this vcry reason, it is no wonder that in our days the exchange between metals and their ions is the most extensively studied field of the isotopic exchange processes.) PANETH [1] and HEVESY [2] can be considered as the pioneers of these investigations. A survey of relevant investigations published in the literature is given by HAISSINSKY [7] and WAHL [12]. It is very difficult to compare the results of the various authors, because often the experimental parameters are not known, or the role of some factors (thus e.g. that of the quality of the metal surface) is not properly evaluated. Literature states a relatively great number of atomic layers of the surface to participate in the isotope exchange on metal surfaces, although in fact, the self-diffusion of metals is negligible at the given temperatures. This is sho"wn by the investigations of HEVESY [2, 3], ROLLIN [4] and COFFIN and TINGLEY [5], 'who have studied exchange processes between Pb, Ag, Bi, Cu, Sb, Mn, Te, La and their ions at different concentrations, temperatures and pH values, for various durations. L. hIRE and co-workers [6] have been carrying out investi-gations on the aqueous solutions of carrier-free radioactive metal ions. Our own investigations concern isotope exchange on the surface of com-mercial carbon steel. The ob,iect of our investigations is the elucidation of the * Ministry for Heavy Industries ** Technical University, Budapest, Dept. of Physical Chemistry 6 Periodica Polytechnica Ch. 13/3 268 .T. FARKAS et al. theoretical and practical problems of metal corrosion by the study of isotope exchange. Essentials of the investigated metal surface In all the experiments in which the physical and chemical properties of the surface of a substance are to be studied, a contaminant free surface must he studied, a contaminant free surface must he prepared, and maintained in this state throughout the experiments. According to an earlier definition, the clean surface is an atomically clean one, which contains only a few percent of monolayer foreign atoms that may either be adsorbed, or substitute the surface atoms of the original lattice. Practically, metal surfaces may he contamined by foreign, primarily organic suhstances (yarious fats, oils etc.) of extraneous origin or hy oxide layers of various thickness, due to atmospheric oxidative agents (oxygen, stem etc.). ROBERTS [8] describes several methods for the preparation of a clean metal surface, particularly methods using high vacuum. A clean surface according to the ahove definition can he prepared and maintained at a vacuum of the order of 10-9 torr. The cleanness of the surface can be crrecked by the surface propertics of the suhstance. Many approximations were used with various success. (The surface phenomenon tested can he electron emission, the ionization of gases, e.g. of helium (GOOD and MILLER, 1956), the scattering of low energy electrons (DAVISSON and GER.\IER, 1927), emission of photoelectrons (EISINGER 1958), secondary electron yield (WA:'iDERSLICE and WHETTEN,1963), Auger electron emission (MAGSTRU:.u, 1954) and catalytic reactivity (ROBERTS, 1962). The clean surface can he prepared either by vapour treatment of a split glass or mono crystal surface (in this casc, the actual surface can he multiple of the geometrical surface, if the layer is porous), by the chemical ignition of a "wirc or strip in depression (e.g. an oxidized iron surface can be reduced in hydrogen hy ignition for two hours at 850°C), or by crystal splitting or breaking in vacuum. The application of any of these methods is equipment exacting, and yields in many cases complicated surfaces, ·which are different from those met in practice, and in our case, unsuitable for the study of isotopic exchange on the surface. Almost without exception, these methods change the original structure of the metal surface, and result in a metal structure other than usual in practice (e.g. a spongy structure). It is principally due to this latter reason that ·we did not use metal surfaces prepared in the above ·ways for our investi· gations. Besides of the high purity requirements emphasis was layed on the intactness of the structure of metal surfaces, as used in practice. Besides the purity of the surface, the problem of the surfaee roughness the key issue in the subsequent evaluation of the test results, has been raised. HETEROGENEOUS ISOTOPIC EXCHANGE 269 Theoretically, for determining the surface area, a mathematical plane fitted to the ideal plane of some ideal crystal at absolute zero degree of temperature is considered as the reference surface. In this case atomic stumps, imagined as spheres of the order of 1 A, give the "unevenness" of the surface, the determi-nation of which - even at room temperature, involving amplitudes of thermal motion - is theoretieaily possible with the aid of complicated space functions. In practice, the geometrically confined part of the mrfaee (the so-called geo-metrical surface) is considered usually as the reference surface. The surface can be characterized hy the roup:hnpss fr:ctor. i.f'. ]'ntio of the actual to the R ::: T, '- T , •.•• V. 0 2 ' b2 ' ~2 'd2 ' a· n /j.1m/ Fig. 1. The cross section of metal surfaces and the concept of average roughness, characteristic to the surface geometrical surface. AccOTding to ROBERTS [8], the roughness coefficient of e.g. an aluminium layer of 200 A thickness, vaporized on a glass plate, is 2.5, while that of a radium layer of 200 A thickness 9. hIRE [6] did not determine thc surface of the silver plate used for the isotope exchange investigationE', but used in his calculationE' a roughnes5 factol' of 1.5, "estimated on the basis of general expericn cc". In Hungarian practice, the "surface" of metal objects has heen character-ized by the "mean roughness (Ra)" of the cross section of the mrface (profile), (Fig. 1) given in pm (10- 3 mm), (Hungarian Standard lVISZ 4721-58.) (For sale of illu;,:tration let UE mention that permissihle surface Tough-nesscs of machine parts are as follows: pistons 1.0 I'm, crank arms 0.6 ,urn, roller bearing slats 6.0 pm. File finishing can produce a surface of 0.4-10 finest grinding 0.16-0.6, and most careful lapping 0.04-0.1 pm roughness.) The ayerage roughness giye;:- the macroscopic roughness of the surface, which can be measured hy various "scanning needle" instruments. Even the grooves producing the macroEcopic roughness are not of "geometrical smooth-ncss". Their surface confined by crystal faces broken during processing, gives the microscopic roughness, ranging from 0.03 pm - the linear dimension of a micro-crystal - to the lower limit of macroscopic roughness. According to the aforesaid, the mean roughness (Ra) describes the groov-ing of the surface, while the roughness factor is indicative of the increase in 6* 270 J. FARKAS el al. surface area, produced by the roughness. Obviously, these to values are strictly correlated. Plate No. 1 :2 3 .1-5 6 '7 I 8 9 10 11 12 13 1-1 15 Table I Increase in surface and roughness factor calculated for surfaces of various roughnesses according to Fig. 2 i Number of ! peak:; on 10 f..Lill of i base edge .) :2 ,~ 1 .) 4· 1 ., ·t :2 .) :2 ;:; Pitch height (u.m) -2 2 0.5 0.5 0.1 0.1 Increase through peaks in the mi~ro regiou (macro + i Increase of the base ed!.!c through peaks in the macr'O region micro base edge (f..L0l) , edge of micro-peak: O.O:I.um ) 15.6 23.4 26 39.0 2).6 38..1 13.8 23.2 33.6 5(1.5 10.8 16.2 12.8 19.2 19.2 28.8 10.2 1:;.3 10.8 16.2 12.8 19.2 10.0 15.0 11.0 16.5 10.0 15.0 10.2 15.3 Surface area (pm') 550 1520 14·75 795 2550 260 370 830 235 263 368 225 270 :?:2~ 23:; 5.5 15.2 14.8 8.0 ·r -~;).;) 2.6 3.7 3.3 2.4 2.6 3.7 2.3 0) ~ -01 2.3 2A On the hasis of theoretical considerations, calculations for a model simulating surface roughness gave for various combinations roughness factors bet'ween 2.3 and 25.5 (Fig. 2 and Table 1). With a reference surface of 0.03 .um mean roughness (Ra) (microscopic roughness, i.e. the linear dimension of microcrystals), taking the increase in surface area proportional to the actual mean roughness (Ra) (Table 2). For a surface of a mean roughness of ego Ra = 1 ,!.ill a roughness factor of x = 33 will result. (This increase in surface area is illustrated by the profilograms, Figs. 3 through 7.) The surface of the tested steel sheets was ground 'with emery paper or grinding wheel of various finenesses. This operation aimed either at removing the surface oxide layer or to "adjust" the surface roughness. The surface roughness of the test sheets has heen measured 'with a GAlHl\IA profilograph, at a needle pressure of 0.1 g. The mean roughnes8es of sheets ground 'with HETEROGENEOUS ISOTOPIC EXCHA-'''GE Table 2 Axerage roughness values measured with profilograph, and roughness factors calculated by proportionality Plate ::\0. 3 -I 5 6 7 9 10 !Avcrage roughness! Roughness fuctor I measured Ra.! calculated 0.85 28 0.73 24 0.66 22 0.45 15 o "'i .. ~ 2-J. 0.66 22 0.71 24 0.81 'i~ -I 0.93 31 0.98 32 0.46 15 0.45 15 0.65 22 0.23 8 0.30 10 0.37 12 0.75 25 0.35 12 0.45 15 0.37 12 0.62 20 0.58 19 0.43 14 0.50 17 0.50 17 0.42 14 0.25 8 0.30 10 0.27 9 0.41 1-1 2.2 73 3.3 no 3.2 107 271 2:72 J. FARKAS et al. I Plate ,AveragerOUghnessl Roughness factor "0. I measured Ra calculated I 3.5 I 117 13 4.0 133 2.3 77 2.2 73 2.5 83 l5 0.61 20 0.67 22 16 0.61 20 0.48 16 20 0.37 12 0.38 13 0.33 11 0.13 4 23 1.9 63 emery paper 320 and 400 (grain size: 0.063-0.050 and 0.040-0.028 mm ranged between 0.13 and 0.85 ,um respectively, Table 2), while the Ra values of sheets ground with a grinding wheel vere 1.9 to 4.0 ,um. Roughness factors were calculated by proportionality from Ra values measured (Table 2) for the evaluation. Since the profilograph recorded Ra values differing by as much as 50 per cent even on identical surfaces, considered as homogeneous, in the evaluation of the results a relevant error of 50 per cent must be reckoned with, leaving other methods applied in our tests without consideration. 6 .um 5 '" '" " .. c:: -<: 3 '" ::, 0 2 '-. /, / , , , 3 \ 5 , , \ \ \ \ , \ \ :\ ,., . 10 "um Fig. 2. Symbolization of the surface roughness for different variants, in accordance with numerical values in Table 1 HETEROGESEOUS ISOTOPIC EXCHASGE 273 Fig. 3. Profilograms of ground (polished) surfaces 274 J. FARKAS et al. Fig. 4. Profilograms of ground (polished) surfaces HETEROGENEOUS ISOTOPIC EXCHASGE 275 Fig. 5. Profilograms of ground (polished) surfaces 276 J. FARKAS e' al. ® Ra= /1=1 Ra = 0, 4 fJim ni! 11 - I ! i i-r- i, 1 : I, 1 I \ I I \ I \ \ I I \ ! : ! I ] i ! I , i 1 , 1 I 1 I i 11 1 J i ! , j ~ : I i I i I I I i i I ! I ~ , {,'1WI 1fr1! I : ,,~ 'W' i Io.~W~ j i : I ! I I : L :/ In' 1~'1 \11 ~Hi ('\";,11,1 1\1\/11 ~I _,a 1fflt1lf1 HlIl'I"r i'lI"" , 1 I I I [ I i i, 1 I, i'vr' \ ~ ! ~ \ I 'rm{\..- ,\\11 j", , i I" ! ., I UIU I'i\--\ I I ! ' : : i ' I : i i, HH ! fr~ I I ! i I ! I i : TIll (11 I I I L j ! i I ! J ! 1 I iTl ! , i I I 1 I I I i I 1 I I I , I @) Ra =2,2 11 = 10 ® Ra<l,3 11=10 I I I I I 1,[ I I i ! ! I I I -1 i I ! • 1 i ! i : • I ! i I ~. I i /1.\ '>\I, I c<--.i.tA- .1 I Id A- ,.1 •• 1' i'vh, Y'- ""'-I J j AIM;\I~l V'f' I 'I '1'>4/'1.1 "b~ A, ,41 .", \i\r'~ it>-. lJI.~Y',i ; 1 i '1'Ilil(' Hii\' I i : : i : ; ! i 1 i i i , 1 i I I 1 i I 1 @) Ra=3,2 11 ,10 'J2) Ra '3,5 /1=10 ! ! ! i : I i ! i I i I I I I i'~;-: ! i i i i 1 i'l .• .vI'.." ! : i , ! , I I ,)t/~( , .. ~ r~1~Jt± I 1'" 'n., !it~ -+~! ' , f1J'J )-, "i '1'1; tv.i- "",~'l i i i , ... ii i I If /\t"Y "\ : i ! 1 .. • 1 1 I i] "~;-'- ! i i, I I 1 " I 1 Fig. 6. Profilograms of ground (polished) surfaces HETEROGESEOUS ISOTOPIC EXCHASGE 277 @ Ra= 0,6tpm 11=1 @ Fla= 0,67 pm I +- - - l-c-r) I IV It\. TlTT~nfH I I I I I I! I I i I 11 J IV _ - I I 1\, I l-L I 1/\ I I 111 lrl-Ll-t+n Jil/\, .~1"Il V~, \ A 11 I Y\ 11 I L J \ n I V I i I I I r + IJq f\f\14'-1 I If Ill! I 1Nl I I' 11'1 UI I 1 .I\~ I I I I I I --,- I I 1I I , V I I I [ I I V I 11 JUI ~I VI h I I I I I I 1" I I I . I I I I I I 111'1 I IT l'lfl In 1 I I i ! I i~ , I I I I I I I I i I ! i I 1111 I Ji I I V I I I i I , I l~i f-! i I I I I I I I I I I I 11 I I I I I I I I I V\( r --Ra= 0,61 Ra= 0,t,8j1m I ! I I I \/1 [\-{I ! I 1 I I i ! I I I , I -~ 11 I !~ I I I I I~ I •. , I I I I I I -.J-7\ JL -++ 1\, I , 'I~ rml I l ! J I I --1--- -i- I 'fr'1' 1\ PI- I I. .r ~'.r .I I . ( I I , VI 11 \ /" I ~ 1\ IV'~V \.oJI 11 1"-If' : I -; , --- :v- I I fI 1I I i i , ,L,_ \, IV L~ v I IJ I i --I I I @ Ra = 1,9 11=10 ;r---~=c ! ! I I I I I ! ;-;-t- --i- 1-._- --- , j i I I I 1 ! I ' i i I i ! -----I ,;-1- "j,f'JVI !\ r-\1),(I r~1 I v h. ~! 'rYI 1'11'1" Y I ! ! , I ; I , : ~-;-l H----- - - , ,---- --h- ---- ! I 1 ; i I I I I I I I Fig. 7. Profilograms of ground (polished) surfaces 278 J. FARKAS et al. For when investigating metal surface phenomena, the surface roughness is seen to be of a great importance for the evaluation of the results. There is no reliable measuring method, however, for such relatively small surfaces. Tested sheet surfaces were cleaned after grinding from eyantual fatty substances and residual emergy grains in an alkaline solution, by electrolytic defatting. The test specimens were connected as cathode, so that the dis-engaged hydrogen should remove the oxide layer that might form during grinding. According to our earlier investigations, a steel surface purified in this way 'would contain not more than 10-7 gjcm2 of residual impurity [9], hence the surface might be considered practically clean [10]. The plates were subsequently rinsed, and tested without contacting air. Chemical composition of the metal snrfaces As concerns chemical composition thcre is a great variety of commercial steels. Contaminants and alloying elements considerably influence the chemical and physical behaviour of steels. Similar investigations reported of in the literature, have mostly been made on polycrystalline (e.g. Cu, Ag) (seldom on monocrystalline) pure metals ones. A novel feature of our investigations was to test a commercial cold rolled steel. The chemical composition of the steel plates is shown in Table 3. Table 3 The chemical composition of the tested cold rolled steel plates C Si )In s ---------------~-----------.,--.- -.- - -_ .. Plate I (0.75 mm) Plate II (1.2 mm) Cu 0.10 0.19 <0.10 <0.10 <0.10 <0.10 0.035 0.029 )10 <0.05 0.09 0.04 0.03 Summary 0.39 0.36 0.05 ny 0.015 O.O:!O .-1.1 p O.OH 0.017 Ti 0.01 ny A definition is given of the surface of steel plates used for the investigation into hetero-geneous isotope exchange processes. Surface purity and the increase in surface area due to surface roughness have been determined. Methods for preparing the surface and control tests of the surface have been presented. HETEROGENEOFS ISOTOPIC EXCHAiYGE 279 References 1. PA2'ETH, F.: Z. Elektrochem. 28, 113 (1922). 2. HEYESY, G.: Phys. Z. 16, 52 (1915). 3. HEYESY, G.-BILTZ, M.: Z. Phys. Chem. )1.3, 271 (1929). 4. ROLLI2', B.: JACS. 63, 86 (1940). 5. COFFIN, C.-TI:'i"GLEY, 1.: J. Chem. Phys. 17, 502 (1949). 6. hIRE, L.: Acta Physica et Chimica Debrecina 21, 7, 47 (1965). 7. HAISSINSKY, M.: A magkemia es alkalmazasai (Nuclear chemistry aud its applications). Akademiai Kiad6, 1963. 8. ROBERTs, R. W.: British Journal of Applied Physics 14, 537 (1963). 9. K-I.GY, L. G.-FARKAS, J.-ZOLD, E.: Periodica Polytechnic a 9, (1965) 10. NAGY, L. G.-FARKAS, J.: Periodica Polytechnica 10, (1966). 11. SCHWABE, K.: Chemische Technik 13, 275 (1961). 12. WAHL, A. C.: Radioactivity Applied to Chemistry (J. W. N. Y. 1958. Prof. Dr. Gyorgy VARSJNYlj Dr. ,:yorgy Lajos NAGY Budapest XI., Budafoki u. Jeno FARKAS 8, Hungary 

HETEROGENEOUS ISOTOPIC EXCHANGE AT THE INTERFACE OF STEEL SHEETS IN CONTACT WITH IRON CHLORIDE SOLUTION I. QUALIFICATION AND PREPARATION OF THE METAL SURFACE

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HETEROGENEOUS ISOTOPIC EXCHANGE AT THE INTERFACE OF STEEL SHEETS IN CONTACT WITH IRON CHLORIDE SOLUTION I. QUALIFICATION AND PREPARATION OF THE METAL SURFACE

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