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Huygens Institute - Royal Netherlands Academy of Arts and Sciences (KNAW) Citation: Reinders, W., The systems Iron-Carbon-Oxygen, in:KNAW, Proceedings, 19 I, 1917, Amsterdam, 1917, pp. 175-188 This PDF was made on 24 September 2010, from the 'Digital Library' of the Dutch History of Science Web Center (www.dwc.knaw.nl)

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175

Chemistry. - "The system hon-Crt1'bon-o."Cy,qen". By Prof. W.

Rl<}INDERS. (Communicated by Prof. J. BÖl!:sEKlm).

(Communicated in the meeting of May 27, 1916)

Tn two previous communications 1) it has been shown what disso­ciation equilibria in a few temary systems metal-sulphur-oxygell may be _thought possible and the resu!t has been given of the investigations that have decided which 'equilibria ?>l'e really stabIe. , In a similar manner the r.lissociation equilibria in the systems metal-carbon-oxygen may now also be tl'eated. Without entE'ring into the discussion of the I1uroel'OUS possibilities th at are conceivable according to the nature of the meta! we will t1'eat in the following thè equilibria to be expected with a single metal, Jlamely iron. Sirnilar considel'ations apply m'utatis IIrntandis also to othel' metals.

The tel'nal'y system Fe-O-O is based on 3 bmary sJsteros, namely C-O, Fe-O and Fe-C.

The fiTst ha,> al ready been studied in 1864 by ST. CLAIRE D.mVU,LE 2) and later by BOUDOUARD 3), MAYER and JAeOBY 4), RHEAD and WUl'}EJ,ER5) in a roOl'e accurate marmel'. The propol'tiol1 of the two oxides CO and CO 2 in a gaseous mixture in equilibrium with carbon is conse­quently now known with a fair!,}' great certainty.

The stabIe oxides of iron are Fe20 g , Fe30 4 and FeO. 1 The first two form cel·tainly phases apart. Between Fea0 4 and FeO, howevel', a mixing in the solid conditiön might be possible, aceording to a more .recent research -of HILPERT and BEYl!lR 6). We will disl'egarn this posslbiHty, which has found no confil'mation dlll'ing the investi­gations in the ternary sj"stem with cat'bon 7).

As to the system iron-carbon a number of papers have appeal'ed, ever since )900 when BAKHUIS ROOZEBOOl\1 8

) published his \'iews

1) W. REIt.DERS, EquiJilJria in the system Pb-S-O. Proc. 17, 703 (1914) and W. REINDERS and ~'. GOUDRIAAN, EqUilibria in the system Cu-S-O. Proc. 18, 150 (1915).

2) C. R. 59, 873 (18M).

3) Ann. d. chirn. et d. phys. (7) 24, 1 (1901). 4) Journ. f. Gasheleuchtung 52, 1909.

6) Journ. Chem. Soc. 97, 2178; 99, 1140 (1911).,­

ol Bel'. 'do D. chern. Ges. 44, 1608 (1911).

'i) See, for instancè V. l~ALGKE, Z. f. Elektrochem. 22. 121 (1916).

8) Z. f. physik. Chem. 34, 437 (1910).

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thereon allel gave the fil'SL compl'ehensive melting poinl diagram, withont, llowe\'er, a, complete nIUwimit.y of opiniol1 being atlained. TbeJ'e has been a diffel'ence of opi niol1 parti('nlal'ly as to tbe q nestion ~

wbethe)' irOll eal'bide FeaO is a stable Ol' more likely a metastable -componnd. BAKHUIS ROOZlmOOl\I took it io be stabIe below 1000°·and in agreement iherewith E. D. OÁilIPBEI,L I') fonnd by means ofthel'lnic determinations a posWve heat of fOl'matioJl. OHARPY, BENEDTCI\S ~l1d

othel's have, howevel'. al'gned that cementite must be metastabIe, which conclusio!l is confil'med by (he experiments of ROYSTON 2). ~

~101'eoYer tlle subseqnent measul'ements of' RUFF and GEHSTEK 3)

~/ • .t' ~c _ •.............•.. ····>Fe C

, 3

"\, 1'00

. c 7°0 ······ö.., .......... -.... _ ....... _ ... _ ...... ;. Fe

3C

Fe --;.C

Fig. 1.

bave tallght ns that Fe2G is .eJldo-· I

(he1'mi(' (-15.1 caL). Henee, we ma,)' assnme with a fair degl'ee of cel'tainty tbat the soUd phase FeaO is me/astabie in l'egard to iron anel g'l'aphite and pl'obabl.l aIso in regal'd Lo iron anel amorplwlls carbon. In Fig. 1 we thus have tbe eliagmm of' condition. In lllan)' changës in conditiolJ, however, snel! as In a not exceedingly slow cooling, the eaJ'bide (cemen­tite) is of ten fOl'meel instend of gl'aphite + il'on which, once it

is fOl'lned, passes bnt exceedingly slovl'ly into the stabie phases 4). In considering the tem ar)' system we will, thel'efore, have to take

into account tbe posslbility of fOl'mation of FeaG.

Tlte isother1ns f01' temperatures ~elO1O 700°.

The equilibl'ia in the tel'nm'y system at constant temp81'atlll'e may - as it has been done' vvi tIJ the systems metal-suJphm-oxygen ó) -­

be l'epl'esenied in nn equilatel'al tI'iangle of wbich Fe, G anel 0 are the apexes.

Below 700° mal'tensite, the solid soIution of carbon in iron, is J10t yet slabIe. Oarbon nnd iron are thns III equilibl'il1lIl wijl! each

1) Journ. Iron and Steel Institute 59, 217 (1901). 2) Joum. h:On and Steel lnstitute 1, Hi6 (1897). 3) Bel'. d! 'D. chem. Ges. 45, 63 (1912).

1) Alsésee A. S~IITS Z. f. Elektrochemie 18, 51 (1912). IJ) REINDURS anel REINDERS anel GOUDRIAÁN, 1. p.

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177

othet' as solid phases. On addi­tioIl) of oxygen the lowest oxide, ferl'ous oxide, may, occU!' side by side. These thl'ee solid phases form

t ben with (he gaseous phase the

monoval'ian t eqllilibl'ill m lIL Besides this monovariant equili­

brium thel'e are still possible two olhers, namely thaL uetween

'-t-----=~~~--~O FeO, Fea0 4, C aud gas (II) and that uetween Fea0 4, Fe20 a, C and

gas (n. It' we start ft'om a mixture of. C and .B\ 0 3 of the compositiori q,

on withdrawal of gas, these mOlloval'iant eql1ilibria wilt be atlained Ül the order I, lI, IU, whel'e the eompositlOJ1 of thp solid phases mixlm'e changes in the dirertion q-I'-s-t and Ihe fo!lowing l'eactions take p/ace (the empirical composition of the gaseous phase to be

taken as CO~):

Il, " "

IJL "

C,FeO,Fe, gas "

In addition, wi th the r1letastable cal'bide as paJ'ticipating plmse, other metastable equilibria ma)' he thougbt possible, of which the principal Ol1es 'are I):

IV. Phases: C,FeO,Fgü, gas. Reaetion: 3FeO + 1! U ~ Fe80 t-~OO:q X4 X40

V. " FeO,FeaO,Fe, gas " x.FeO j FeaO~(3+x5)Fe 1 OO~6 Tbese 1ll01l0Val'iant equilJbria are to be ('onsidel'ed as the lil11ils

witbin whic/l tlle following dival'iant equilibria are stab/e:

a. Phases: Fe2 0 a, Fe30 4 , gas. Reaclion: 00+3Fe203~2Fe304+C02 b. Fe80 4 , FeO, gas. " 00+ Fe30 4:;:3 FeO +00, c. FeO, Fe, gas. ,,00+ FeO ;: Fe +OO~ d. 0, gas. ,,0 + CO 2 :;:2 00.

1) ft might aJso be possible that aftel' the equilibrium 1 followed at once an equilibrium with FeaO, hence between the pllases FeJ04' a, Fe3a and gas, then the equilibrium belween the phases l<'egO, ~'eg04, lo'eO aud gas and finally that between ~'eas, FeO, lo'e and gas. It would, ho wever, lead us too fal' to discuss a1l these eventualities separately.

"­Proceedings Royal Acad. Amsterdam. Vol. XIX.

:12

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Metastable aI'e the equilibria: _

e. Phases: FeO, FeaO, g·as. Reaction: 3FeO+500;:Fe3 °+40°2 _

f. " FeaO, Fe, gas. "Fe3C +°02 ;: 3Fe + CO. _

According to the law of mas~ action, wh en lhe gas C'onsists -of tIJ

mols. of UO and (1-,v) mol. of -002 alld wllen P is the total pl'eSSllre. the following l'elations should apply 10 these equilibria:

peo a: a.

peo2 l-.'v

peo a: b.

pco2 I-iv

peo ,v (J.

peo2 l-.v

p 2 eo ,v~

d. - 1-.v P=kd

PC02

p5 CO .v5 e· --= --P=1ce p4 e02 (l-a:)4

p~CO 1,2

j. --- P=1ct PC02

I-,v I

lf in accol'dance wilh R SOHENOK 1) we repl'esent the 1'e1ation, bet ween te

and P graphically there are formed a series of lines as indicated in F!g. 3.

The lines a, band care stmight; in these equilibria .x is illdependent

=ka

=!Cb

= ICe

of the pressul'e. Tbe lines cl and f CO%.

A.. ~

JJ

c:

are cnbic hJperbo]es alJd e is a simi~ l~ig. 3.

I

I

i ~ , . \ . ,:

lar cnI'ved line of hig'het' deg'l'ee. The monoval'iant equilibria are now found as points of intel'section of these lines, namelJ I, 11 and I11 as points of' intel'section of a, b or c with d, 1 V as point of intel'section of d and e and V as [hat of c, f and e.

Tbe equilibria IV and' Vare metastable because Ihis is tbe case with one of the participatillg phases, Fe30. From th is it may be dü'ectly concluded that V must 1ie above 111 and ] V below the same. As V and 111 are both bituated on the line c the composi­tion of the gaseous phase fOl' these equilibria is the same. When . now this:gaseous phase comes a1tel'nately in contact with the solid

1) R. SCHENCK, Zeil. angew. Chem. 17, 1077 (1904); Z. f. Elektl'och. 15, 584 (1901); Physikalische Chemie del' Metalle, Hálle a. S. (1908).

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phases of TT and of TIl the metasiabJe cal'bide will bave to disappeal' by tbe foIlowing tmnsformations:

x FeO + ~'e3C ~ (3 + x>, Fe + COx Pv

CO" + x Fe ~ C + x FeO . Pm

Total FeaC ~ 3 Fe + C

These transformations, however, ollJy then take pJare iJl 111e sen se indicated above if Pv>Pm.

The sitllation of the poin ts V, 1 TT and 1 fJ in l'egard to each other must, therefol'e, be that indieated in Fig. 3.

From the equilibria indicated hy the lilles a, b, c etc. we can dedllce the reactions that take plaee at tile Jeft and the right of these lines and from this fine! t he phases that are stabIe in the different fields of Fig. 3.

From this it appears that the carbon is only stabie with gaseous mixtnres frorn the' regioll A at the right of cl; at the left of d A l'eacts with the carbon dioxide with CO-fol'malion, until the romposition of tile gas is so tal' rnodified that it can be illdicated by a point of the line cl.

In~ l.he l'egion E only :&~203 is btabIe, in D FeaO 4' in C FeO and in B Fe. .. ..

In! the point TT the 3 Iines c, .f and e lIleet and separate from each~ othel' 3 regions, a fil'st orÏe Al whel'e FeaC should be stabIe, aserond l)11e A 2 whel'e Fe and t a third one ASI ,~l;'el'e FeO should be stabie. Henre,) the li~e e should be stabJe onIj, from TT to the

I .' región of I1ighél' IJi'éssnres, the lines c and .f onlJJ I from. rT to lower

l I rl

pressures. "

TJw equilibria ((I'ound the' pbint TT, mie Iwtvevè1', all 7~fstastable in 1'fqm'd to t!te ca7'bbn because all CO:concenti'ations 10 t11e l'ight

I ~ t I \

of d al'é metastable. Above the line cl onl)' C and one or two of the solid phases Fe 20 a, Fe304,J~eO Ol' Fe a're sta~le aud the gaseonR phasé is metästable.

If now we start from a mixtlll'e of Fe20 a and C of the tota1 composition q (Fig. 2) and 10 wel', at a conslant Lemperature, tlle pl'essul'e abo\'e, ga., will fortll of the composition I when the pres­sure 1 (Fig. 3) is attained. If Ihis gas is l'emoved by suction the monovariant equilibrinm I is l'etained so long as Fe20 a is still present. The composition of the ferricoxide-carbon-mixlure thel'echanges fl'om q to l' (Fig. 2). WÎ1en all the Fe2ÓS is, IIsed up, the pl'essure of the divariant eqnilibl'ium Fea0 4 , C, gas chang'es accol'ding 10 the line [-[f. until at this last pressllre tlJe redurtiol1 to FeO sets in and when more g'as is withdl'awn, tbe pressllre remains constant;

12*

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HsO

the composition of the soJid-phase mixture then changes from r to §.

Subsequently, in the monovariant equilibl'i1~m lIl, the composition ~

changes ti'om s 'to t so that finally a mixture of Fe and FeO remains. If the ol'iginal mixture q had been a little richel' in carbon a mixtUl'e of Fe + C might have been left /behind at the lermination.,

Conversely, by interar'tion of a gaseous mixtnre of 00 + 00 2 on ü'on at a snfficient pressure, this gas will be absorbed wilh sep:l,. ration of 0 and formation of iron oxides and in succession the eqnilibrium lIl, Ir and I can be attained.

The isothe1'm f01' tempemttwes between 880° anc! 1100°. At tbese tempel'atures iron forms with carbon solid sohitions,

martensite, of which the C-content varies fl'om 0 to a_bout 2%' The projection of the spacial isotherm on the x-plane then becomes as indicated in Fig. 4.

Aftel' tlle 2 monovariant equilibria 1 and II JlOW follows VI between FeO, 0 and the C-satnrated solid solution a. Aftel' tbis comes tlle divariant equilibrium between FeO, the unsaturated solid 801utio115

\

c

E

co" Fig. 4. ~'ig. 5.

and the gaseous phase. The ral'bon content of the unsaturated solut.ions can vat'y from a to O. l.f we repl'esent the empil'icaI composition of these solulions by the fo!'mula FeOy, tile reartion, applying to the dival'Îant equilibria between FeCy and gas, wil! be represented by

ft. FeO + (1+2y)OO;::FeOy + (1+y)002' Fo!' this equilibl'ium exists the relation

p~t2y 1!)1+2V

-+ = ---+ . Py = COllstant, P~'(J~ (1-m)1.1/

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181

if x indicates the part 00 in het gaseous mixture of 00 + 002 ,

hence I -+2

[IJ '/

----1- . P = kl!. -+1

ll-,v).1f

The graphir representation of this J'elation is again a similar curve 1

as line e and - as '!I will be llsually smalI, therefore - large -y

of a very high degree. The limits, between which y can mry, is on the one side the value

of the saturated sollltion rr, on the othel' &ide O. In this latter case the above relation .passes into the equation ;

al -- =lcc, that is the relation fol' the reaction FeO + CO ;:Fe + CO 2 l-,v which in Fig. 5 is indicated by the straight c.

Tbe lines indicating the equilibria of tbe different solid solutions \ FeO)', each with FeO and the gaseous phase, th us form a collection of curves of increasing üighel' order the latter of which, for y = 0, passes into tbe straight line c.

The fh'SI, rehtting 10 tbe solid solutiun saturated with carbon is indirated by the line !ti' The point of intersection of this line with the line cl gives the monovariant equilibrium VI.

The points of intel'section of the other lines ft with the line cl, also the point of intel'sertioIJ of (' with cl (lIl) are metastable equi. libria because they relate to solid solutions unsaturated with carbon. They are, the,'efore, situated at the l'ight fl'om VI, between VI and JII ..

Below VI the surcessive curves cut eacl! other and the envelope I formed by these intel'sections now forms the 'eqnilibrium line of FeO with the different unsalurated solid solutions. It runs fl'om VI (equi. librium of the saturated sollltion) to J( (equilibrium with pure Fe).

Fl'om VI, the monovariant equilibrium between 0, FeOy, FeO and gas,. thus run 3 lines of divariant equilibrium, namely

dj between 11 and VI for the phases 0, FeO and gas. cl, " VI and CO "" " 0, solid solution anä gas. l, " VI and K "" ,. FeO, solid solution and gas.

They enclose in the plan es

F where are stabIe solid solutions + gas.

a " " " FeO + gas. A " " ,,' 0 + solid solutions (pressure < p VI).

or a + FeO (pressure > pVl and < pIl)

whilst in both cases tre gaseous phase is metastable.

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182

When instead of the stabJe cal',bon the metastabie Fe3Û occurs, -met~tstabJe eqUIlibria are formed in the fieJd A. The Jine l does not then lerminate aJl'eady in VI. but ends in the point VIn whel'e lt

cuts the line e alld wherc there is consequently mOJloval'iant equi­lIbrium betweell FeO, Fe~O, FeCy anc! gas 'Fhe region of the mixed crystals F thus becomes largel' also anel IS limited on the one side by the line I, on the other side by a cubic hypel bole m, which passes tlll'ough 00 and VlIr and indicates thf' eq11llibrium of the l'eaction

(~-) l?eOy + CO 2 ~ (_1_) Fp3r. + 200 3y-1' 3y-l

p 2 co ,'1]2

--=--P=km pr02 1-.'11

hence,

To the right of this line Fe30 is then stabIe.

The isothe1'Jl~ f01' tempemtu1'es between 700° rtncl 880°. Whel'eas above 8800 all mixed crystc.lls from pure iron to the

on es most l'Îch in carbon are stable, this is no longer the case below 880°. Those pOOl' in c<tl'bon become metastable anel onl)' those whose composition is situated between two limits - indlcated in Fig. 1 by the letters a and bare stabie.

Hence, the isotherm gives 4 monovariant equilIbria, namely:

C

Fig

"

J. Fe20 3, Fea(\, C and gas Il. Fe30 4, FeO, 0 and gas

VI. FeO, (FeUY)b, 0 alld gas VII. FeO, (FeOY)ll> Fe and gas.

E :IJ

co"

. , 'o', "'-\'!-" \'l(mr)

I ~\

\ 'f t \\ \ \ \

I ' \ . \

.!iJ, 1

::0: ,hl

r;\

C

6 Fig. 7.

co

r'

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183

Between VI and VII is situated the region of the dival'iant equilibria of FeO with mixed crystals a to b.

The p-x-chagram (Fig. 7) much resem bles that fol' temperatlll'es above 880° (Fig. 5) with this difference, however, that the line l is not stabie as far as the foot of the line c, but only as far as VII, wheL'e it cuts the line c. Below this point of intersection, the line c, the equilibt'Ïum FeO -Fe, becOlnes stabIe.

Below the stability reg ion of the mixed cl'ystals F thllS appears a region for _ pure iron, B. The demarcation bfltween the l'egiolls is given by' the 1ine i which indicates the equilibrium

FeOy + y002 ~ Fe + 2y002 to which applies the relation:

• P~co .1]2

--=--P=kz pco2 1-.11

The line i is, lherefore, Iike cl and m an ordinary cnbic hyper­bole of which the parameter let changes with the temperature in that sense that it becomes nought at 880° (the isolated region B disappeal's) and eql1al to led at + 700° (the region F whel'e the mixed cl'ystals are stabIe disappears).

The metastable equilibria, in caee Fe3U does not separate instead of carbon, are analogolls to those for tempern.tul'es above 880°, Instead of VI we thus obtain the equilibrium vlir and a demarcation of the mixed-crystal l'egion not by the line cl but by m.

Injl1tence of the temlJemtUl'e,. the p-'I'-lines. An incl'ease of the tempel'atUl'e causes the equilibl'ium to S~llft

very strongly to the right. The constant led of the reaction tbus becomes greatel' and 1 he line cl much steeper.

The temperature has comparatively Iittle influence on the equi­libria of the iron oxides with 00 and 002, I) The conseqnence is that the points of intel'section I, II and lIl, which indic~te the pressures of the different monovariant equilibria, stl'ongly rise with the temperatul'e. Tbe lines indicating this l'elation have a similar course as the weil known dissociatioll lines for hydl'ated salts, (,aI'­

bonates etc. TfleY also can, as referrmg to the monovariant equilibrium beiween a gaseoLls and 3 solid phases of constant composition, be l'epl'esented by the equation:

A lonp = - - + B.

,7 T

1) BAUR and GLAESSNER, Zeitschr. f. physik, eh. 43, 354 (1903),

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184

AIso the metastable equilibrium V will give a similar p-T-tine_ which, bowe"er, lies above line lll.

The equilibrium VII in which the mixed cl'ystals pal'ticipate has a totatly different COllrse, whicb, llOWeVel', is quite comp,urable -to tbe mono\'ariant equilibrium liquid-solid-ga~ in a binal'y system, The pressure which at the starting point at ± 700° has a finite value, falls to 0 at 880°, where the existible region of iron termi­nales. Accol'ding to the thel'mic effeeL Q Vll of tbe l'eaction between tlle different phases of the equation VII

(x + ~) Fe + COx;:: ; FeOy + xFeO

which fot' small valueR of y (close to 880°) is negative, is also' negative for large values of y (at 10wel' tempel'atnl'e) Ol' th en becomes positive, tbe p-T-line VII will on J'aising -.tl1e tempel'atul'e alwaJ's presenta falling course or attaill 1) a maximum between 700~-880°.

Of the eqnilibl'lL1m VI l'epresented by the l'eaction: X Fen + f1" + xy) c ~x FeOl + 00,- + QVI

the heat of l'eaetion is SUl'e to be al ways negative 2). In eonnexion tllel'ewith the p-T-line of Vl is continuonsly rising.

The lino VlIr indicating the equilibrium bel ween FeaO, FeO)', FeO and gas al80 bas a com8e similar to VI.

·The entire p- T~pro,jee­tion I10W becomes as indi­cated ill Fig. 8 3).

The lines IU, VI and VII meet in the quintuple point 0, where there is eqllili-

~. brium between the phases Feey , 0, Fe, FeO and gas.

'-----------:1:--.----:+-:-- T Besides tl1ese three lines 700"

Fig. 8. there also meet the lines for two other monovariant eqnilibria, namely

1) We dlsregal'd here the trallsformation ~ iron ~ ,.,·il'on at 7800. The !ine VII thus consIsts in fact of 2 pal'ts, one relating to the equilibrium with _:.(-iron and one relating to p-iron.

2) The li.eat of formatiun of FeO is about 67,3 [BAUR au;! GLABSSNER, Z. phys. Ch. 43, 368 (1903)], that of CO 29,3 and of CO2 97 calories. TlJe heat of forma­tion of l~eCy wil! be but tdling In all cases where y is vel'y smal!. If we neglect it. then QVI becomes -67, 3x + 29,3 (2-x) + !:l7,0 (x-I):::: (-38,4 + 0,4x) ca!., that is about - 38 cal.

3) Jn Lhis figure VI should be drawn steeper than III (and VlIl steeper than V), as fol' tempemtures above 700" the pl'essure in Lhe metaslabIe equilibrium III is lower than that in the stable equilibrium VI. (See Fig. 7).

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185

IX between tbe phases Fe, FeC\" 0 and gas

and X " " " Fe, FeOy, C, FeO

As these ternary equilibria are su bOl'dinate to the binary eq uili­brium between Fe, FeOy and 0 and as this alters but very little with the preSSlll'e, the equiliul'ia IX alld X vvill' also be alrpost independent of the pressllre.

Beside the stabie quintnple point 0 with 0 as a participating phaE>e there also exists a metaEltable qUll1luple point 0', in which, instead of carbon, FeaC participates and whel'e V and VIn come in contact with the prolollgation of VIL

Of these different lines II has been determined expel'imental1y by R. SCHENCK and V. FALen: 1) and the line III by different investi­gators and that- l'epeatedly 2). The equilibria ah'Vays set in but ex­ceediugl.r slowl,v and al'e, as is to be expected, dependent 011 the kind of eal'bon that i~ used. With gl"aphite are obtained 10we1' pressures than with amorphou'3 carbon. Looking at the fairly con­cordant t'esults obtained by the different in vestigatol's we find, with graphite 6800 as the temperatnl'e whel'e ]Jm becomes = 1 atm.

0' is situated at about 7000 and 0 therefol'e certainly above 700°. From this it follows that tbe presslll'e in the quintuple points wiII be greatel' than 1 atmosphere.

As to thc' othel" Jin,es of Fig. 8 nothing is known with certainty. I probably liys at such low temperatUl'es that the reaction velocity is too smal! to obtain stabie eq uilibJ'ia; VI and VII will lie at pressures > 1 atm.; V and VII, howevet shonld, withont greàt tl'ouble, be acces~ible to the experiment. As to V, some indicatiolls at'e to be fonnd in the different expel'iments. SCHI!lNCK a) obsel'ved that on intel'action of much 00 with comparatively little iron th ere was formed a lowel' .equilibrium pl'essul'e aml a OO-rirher equilibrium gas than on the intet'aClion of little 00 with muciJ iron. He sup­posed that in the first rase the equilibrium IV between FeaO, FeO, amol'phous 0 and gas is obtained and then ealcnlates from the relation: CO: 002 and the vàlue of kc that fot' tbe equilibrium V at 6500 p = 51.92 atm., at 7000 p = 166.3 atm.

A p1'iori, this assurnption anti the conelusion drawn thel'efl'om is not very acceptable. As shoWJl from Fig. 3, the equilibrium IV, as the intel'secting point of cl with e, which at th at place is donbly

1) Bel'. d, Deutsch. chem. Ges. 40, 170S (1907). 2) SCHENCK, SElIHLLER and FALCKE, Her. 40, 1704 (1907); VAN ROYEN, Dissel'­

tation, Bonn 1911, H. NIPPERT, Disserlation, Breslau 1913: V. l<'ALCKE, Z f. Elek­troch. 21, 37 (1915); 22, 121 t19J 6).

3) R. SCHENOK, H SEMILLER and V. FALCKE, Bel'. d D. chem. Ges. 40: 1710 (1907).

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186

metastable, is vet'y little stabie. Also all eqUilibl'inm of FeaU with 0, wbicb is founded on IV has never been observed. MOl'eover the assnmplion impIies tbat a stabIe equilibrium all'eady forroed by a shol'! mteraction of 00 with Fe wonld on pl'olonged action of 00 pa&s into anothet· equilibrium metastable to,~ard~ the fit'st and ex­hibiiing alowel' eql1ilibt'Îl1m pl'essul'e. This is contl'ary to the ideas of stabie and metastable.

The later reseal'ches of FALOKE I) have indeed bnt partly confit'med these first obsel'\'ations of SCH1~NOK and his co-worlrers. I t was con­fil'med thai on short interartion of 00 witlJ Fe asolid phase mixture was obtained tbat on hearing in vnCll u m yields higher pl'eSSlll'eS_ than the mass fOl'med by prolonged intel'action of CO with Fe. The first pressl1l'es measured at different temperatures givé a p-T-line situated at about 10-120 10wer than the second one, whereas the latter gets very close to that of the equilibriu'lll FeO, Fe, gl'aphite, gas. As to the composirion of the gas, it appeared, ho wever, that this III both cases does not materialJy differ anel, acrol'dmg to the tempemture, varies from 52-61 0/0, " MOt'eovel', F ALOKE l'emarks tbat the fil'st. reaction product yields with HOI plainly hydl'ocal'bons and leaves no residllal carbon, whel'eas the ü'on cal'bonised fol' a long time with 00 yields little Ol' no hydl'ocal'bons and lea\'es rnllch carbon. Also HIIJPlmT and DmOKl\IANN 2) founel tbat on hea.ting fe 1'1' ic oxide in a CUl'l'em of carbon monoxide at tempel'atm'es from 720-8000

, free carbon did not form until the prepal'ation had taken up 6% of' 0 (Fe3C contains 6.6% 0.) and was completely l'educed.

Hence it looks to me very probable Ihat the higher pressures yielded by the iron carbonised fot· a short time, l'elate to the meta· stabie equilibrium V (FeO, Fe, FesO, gas) and the lower lJl'essures to the eqnilibrium III (FeO, Fe, rarbon, gas).

The }J1'ojection of the fmt1"phase lines on tlte 1 .... ,v.plane.

The change in the l'elation 00: OO~ with the temperatlll'e in the diffel'ent equilibria is schematically inellCated by Fig. 9.

Of this the line III between 6000 and 7000 has been detel'minecl

CO .. 52 61 expel'imentally 3); the pl'opol'tion Val'leS trom 10 . CO + CO,

Of the line III SCHI<1NOK anel FALCKE announce the pl'essure but not

1) 1. c. J) Ber. d Deutsch. chem. Ges. B, 1281. 3) SCHENCK and co-workers allq f A~CI\E l. C.

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187

7~. 980 7' CO~--.--~----~~--------~--

na

.1' ... 0 ..

~D~~------~---------------------FIg. 9.

the pl'oportion 00 ='00 ~ 1). As to the other lines nothing is known.

Tlte eqwlibl'ia at con.~tant p7'eSSU?'8. Of particulal' importance is also the progl'essive change of the

different dival'iant eqnilibl'Ïa with the temperature at a constant pl'essul'e. They form as it were the section 1. t!Je p-axis of the spacial p-T-x-Figure. In the case that tbis constant pl'eSSUl'e is smaller than the pl'essllJ'e of Ihe l}uintuple point 0, fol' insrance as PI in Fig. 8, this seetion becomes sueh as schemaLlcally J'epl'esented in Fig. 10. Wllel'e on page 12 it is demonstraled tllat the quilltllple poillt pl'esslll'e exceeds L atmosphel'e, Fig. 10 thus also relates to 1 atm, pressul'e,'

~~--------------------------

c !i FeO

__ ... 7 t~~ ____ ~~~~~~O~ ________________ T

Fig. 10. Tlle divariant equihbl'ia ct, b, C l'eappeal' in ibis Fig. as Iines

enclosing t.he fields whel'e the different solid phases are stabie. 'rhe lines a, band c are independent of the pl'esslll'è ::tnd coincide

with the Hoes 1, IL and lIL + Vi l from Fig. 9. 'fhe lllles d and

1) Whel'e the equilibrium CD: 002 with FeO + l"e304 (b) and of CO: CO2 with l?e + l"eO tc) is independent of the pl'essure and of the pt'esence of carbon, the projectioJl of Ir wiII coincide wilh lhe line b in the section fOl' constant pressure and those of lil with c. These lines have been determined by BAUR and GLAilSSNER.

~

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188

i are, however, veLT independent of the pressut'e nud shift at a larger_ pl'essnre, tbe first towards the left, the second to above.

Lowering of the pressul'e therefore causes extension of the existable iron region inclosed by the lines c, i and d. Increased pl'essure causes this region to shl'ink.

Of the lines present in this figul'e cl is lu)'own f/om the research of BOUDOUARD a,nd othe1's 1), band e ,have been determined b} BAUR

and GLA F.SSNl!lR~) bet ween the tempel'atures 400° and 9))0°. The line b has accol'ding to 'these meas1.1l'~ments a maximum of 4:6°/0 CO at 500° and falls on increase of tempel'atnre to 23° ° CO at 950°. The line c ha,s a minimnm at 675° and 58°/0 CO and i'ises afterward!, to 940° anel 75°/0 ,CO, The point of intel'seetion i! is fol' 1. att1los­phere at 647° allel 37°/0 CO, III at 685° anel 59" ° CO.

As . to the points V [I anel IX and ([Ie lines i and l no mention is made thel'eof by BAUR and GLAI~SSNER. Probab1y a part of fhe points detel'minecl by them will belong Ilot to e, bnt 10 1ine Z. As ho wever they do not state any analysis of the solid phase, rhis can"not now be decided.

If the pressllre is increased to above that of the qllintuple point 0, the lines 111 anel VII (Fig. 8) are no longer cut but instead thel'eof VI. The existable region of Fe then eli sappe ars and the section beeomes as shown in Fig. 11.

C~r--------------------------

Co1..

c

mené" Kr! staefen

ä FeO

/------'--/ l'() -~ "31 ...,..

---------Fig. 11.

The lines c and i have disappeared in this section anel wirh the~e the eqnili­bda UI, IX and VII. In tb epIace Ih ereof arri yes

'tlJe equilibrium VI. FeO 1l0W does not pass into Ïl'on on l'eduction in contad with carbon, but elireclly forms mixed cl'ysta1s.

An investigalion to deeide the most impol'taJlt points in these eqnilibr!t. is now in progress. The l'esults wil! be communicateel in dlle 1.,{J LU ;:,e.

Delft. \

1) J. c. p. 1.

Laûomtory Z'IlO1'g" and pltys. cltemistry Tee/mical Univel'sity.

2) Z. f. physik. Oh. 43, 354 (1903).

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