Oxidation of dissolved iron in platinum Oksidacija železa, raztopljenega v platini Grega Klančnik,1, *, Jožef Medved1 University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Materials and Metallurgy, Aškerčeva cesta 12, SI-1000 Ljubljana, Slovenia Corresponding author. E-mail: grega.klancnik@omm.ntf.uni-lj.si Received: February 10, 2010 Accepted: July 20, 2010 Abstract: Platinum is used as material for high temperature applications as sensors and heating elements. For example, the most vital parts of the simultaneous thermal analysis device (STA) are commonly made of platinum: thermocouples and platinum sample holder. STA is a combination of two thermal analysis techniques: thermogravim-etry (TG) and differential scanning calorimetry (DSC). An uncon-taminated holder is needed in TG analysis by which mass change of examined sample is determined as function of temperature and time. When holder is exposed to various steels (especially low alloyed steels) at higher temperatures there exists some risk of contamination of measuring parts (made of platinum) with elements, especially if measurements take place without a proper protection of the sample holder (cover on crucible, protective atmosphere etc.). When a pure melt of elements like iron is in contact with platinum sample holder, local contamination occurs. Dissolved iron can oxidize in the platinum holder. Heat transfer from heating elements to platinum and further to examined sample is changed. Contamination also affects obtained termogravimetric curves. Izvleček: Platina se večinoma uporablja v visokotemperaturnih aplikacijah, v senzorjih ali grelnih elementih. Večina vitalnih delov simultane termične analize (STA) je narejena iz platine: termoelementi in nosilec. STA je kombinacija dveh termičnih analiz: termogravime-trije (TG) in diferenčne vrstične kalorimetrije (DSC). Za ugotavljanje sprememb mase preiskovanega vzorca uporabljamo TG-analizo v odvisnosti od časa in temperature. Meritve se morajo izvajati s čistim nosilcem. Izpostavljanje nosilca različnim jeklom (predvsem malolegiranim) pri višjih temperaturah lahko povzroči kontaminacijo merilnih (platinastih) delov, kadar zaščita platinastega nosilca ni ustrezna (pokrivanje lončkov, zaščitna atmosfera itd.). Taljenje železa v platinastem nosilcu povzroči lokalno kontaminacijo. Raztopljeno železo se lahko v platinskem nosilcu oksidira ter spremeni prenos toplote iz grelnih elementov na platino, obenem pa ima kontaminacija tudi vpliv na meritve termogravimetričnih krivulj. Key words: thermodynamics, oxidation, platinum Ključne besede: termodinamika, oksidacija, platina Introduction Fe3Pt). When mole fraction of iron is higher than xFe = 0.2, the liquidus tem- Knowing the activity of iron in the Pt- peratures are below 1550 °C which Fe binary system the calculation of usually represents the maximum tem- equilibrium partial pressure of oxygen perature for examination of steels with for metal oxides can be done. Calcu- the STA device. Result is possible pres- lation was done with the known data ence of iron in platinum sample holder. of iron activity aFe in the Fe-Pt binary phase system. Some data can be found in references (Gudmondsson and Hol- Thermodynamic calculations loway).m In this paper thermodynamic calculations were performed using Thermodynamics of iron oxidation aFe, at different temperatures, with the Pourbaix diagrams are known as the TCW4 software. Many authors were high temperature oxidation temperature studying oxidation (of iron) and the im- diagrams or the predominance diagrams portance of mass gain for protection of with multivariate equilibria between el- material itself by adding modificators. ements and their oxides or between two [2, 3] This paper deals with analysis of oxides as a function of equilibrium par- mass reduction of oxides due to oxide tial pressure of oxygen and the temper- reduction process. ature of system.[4] This diagram enables to study behavior of multivariate oxide Phase diagram in Figure 1 represents system, in our case system of iron ox- the Fe-Pt binary phase diagram. If ides (wustite, hematite, magnetite) that contamination of platinum holder with are formed on contaminated platinum. iron is rather high, some intermediate Equilibrium of each reaction (oxida- phases can be formed (Pt3Fe, PtFe and tion) is determined by: 77° C ;CC_A2+[ CC_L 102 -X-Fe Figure 1. Binary phase diagram Fe-Pt AG°=-RT\nK (1) AG0=AF0-r-M°=-/?r Inf+RThp0 (4) where is: R - gas constant (8.3144 J/mol K), T - temperature (K), K - equilibrium constant. For the reaction of oxidation: xR + O2 = _yP where: R - reactant and P - product, equation 1 can be rewritten as: at AG=-RT\n^ 1 „x _ aR Po, where are: apy, aRx - ac tant. Further: (2) where f = apy/aRx is the predominance ratio. When the ratio is f >> 1 product component in the reaction predominates over equilibrium, iff << 1, reactant predominates. According to this information, eq. 4 can be rewritten: AS0 i A//0 R (5) (3) apy, aRx - activity of product and reac- In our case the predominance ratio is 1 and the value of ln f is therefore 0. In this case the Pourbaix diagram is constructed with equal activity coefficients of product and reactant. This means that Pourbaix diagram is constructed only for oxidation of pure and undissolved iron. The construction of oxidation predominance diagram is done when all the possible reactions are col- lected with known values of enthalpies where: and entropies of formation, AH0 and aFe - activity of iron, AS0 (Barin & Knacke[5]). In this case aMuOv = 1 - activity of oxide, it can be assumed that AH0 and AS0 are p0i - partial pressure of oxygen. temperature independent. Calculations of these two values are based on the following oxidation reaction: xM O + O = vMO ah 2 ^ u v (6) The enthalpy of formation AH0 is calculated by applying the following equations: AH]=yAHfMo0-x-AHfM „ (7) J fMnOv J,M„Oh (8) Knowing data of enthalpies and entropies of formation based on the reaction of oxidation, calculation of p0 can be performed for the possible reactions of oxidation: 2Fe+02^2Fe0; K = a FeO aFe-Po2 2/3 *Fe + 02^Fe203; K = (10) 3 3 aFe -P0l 1/2 - Fe + 02 -> ^ Fe304 . K = 3 2 2 uFe Po2 4 FeO + 02 2Fe1Ol ; K = — (12) aFeO ' Po, The oxidation affinity in the platinum - iron system Platinum may be treated as an inert component. Active component in this case is only the dissolved iron in platinum. For calculation of partial pressure of oxygen for formation of oxides from dissolved iron, the activity coefficients are needed (lower values than 1). In this case the predominance ratio f is no longer 1 and it depends on real values of activities of dissolved iron. With the known value of AG0T for separate reaction of oxidation, the equilibrium constant of corresponding oxidation condition can be calculated from the eq.1: K _ -(AG0/RT) a MA (14) aFe'P07 (9) Knowing activities the calculation of the partial pressure needed for formation of an oxide can be performed for different temperatures and concentrations of iron dissolved in platinum. The affinity (further A) for oxidation of dissolved iron is calculated from the chemical potential of oxygen which depends on partial pressure of the system (furnace): 6FeO + 02-> 2FeJ04 ; K = (13) aFeoP 0, Mo, = K +RT\na„ (15) where: //02 - chemical potential of oxygen (depends on partial pressure in the system), , 0 - standard chemical potential of oxygen. If the reference state is 1 bar the activity coefficient of oxygen is equal to the partial pressure a0= p0,. The affinity is calculated from the difference of Gibbs free energies between equilibrium partial pressure of oxygen and the pressure in furnace (eq. 16). Negative affinity (A) indicates possible existence of an oxide. Positive affinity represents decomposition of oxide at existent partial pressure of oxygen in the system. A = RTln(p0 ) -RTln(p ) = RT In ^ (Po2) (16) platinum sample holder, where contamination was detected. Another TG measurement was done without sample or crucibles. Heating rate was also 10 K/min under inert argon protective atmosphere was applied. In the case when highly pure argon atmosphere was applied and content of oxygen was known, partial pressure of oxygen was determined to be po2 = 10-6 bar at the total pressure of p = 1 bar inside the furnace.[6] The effect of contamination of platinum with iron / iron oxide visible on TG curve can be calculated if thermodynamic properties (activities) of the platinum - iron binary system are known. In order to determine the activity of iron in the platinum-iron system a Thermo Calc for Windows (TCW4) with the TC binary solutions database V1 was applied. Experimental Measurements of characteristic temperatures were performed with iron 99.8 % pure. The STA 449-C device of Netzsch Company was applied. The maximal temperature reached was 1550 °C at heating rate of 10 K/min, followed by 15 min of holding at 1480 °C. An empty crucible was used as reference. Crucible was made of highly pure Al2O3. After experiment, both crucibles (for sample and reference) were removed from the Thermodynamic calculation to construct Pourbaix high temperature diagram for un-dissolved iron and its oxides was performed by using equations 9-13. The oxygen affinities for dissolved iron in platinum were calculated by eq. 16 using equations 9-11. Results and disscusion DSC heating curve, Figure 2, of iron revealed that characteristic tempera- tures slightly differ from the values listed in reference1. Additional peaks that appeared represented impurities in iron wire. TG heating curve shows drastic drop at the holding temperature 1480 °C which takes place also by further heating to temperature 1550 °C. Contamination of the platinum surface with iron is presented in Figure 3. Because of drastic mass decrease determined by TG curve it was expected that vapors could have contaminated the sample platinum holder. By analyzing TG curve and by removing the crucibles a local contamination was revealed (Figure 3). 0,2-1 0,0 -0,2 - -0,4- TO -0,6E > -0,8 - « -1.0-1 O -1.2 -1,4-1,6 --1,8 T =737°C Cune 600 —i— 800 TG (%) 1000 1200 trc Figure 2. DSC and TG heating curves for iron sample 1400 103,0 102,6 102,0 101,5 101,0 a b Figure 3. Contaminated platinum sample holder (a), Al2O3 crucible (b) 1.0- 0.9- 0.8- 0.7- >> 0.6- > 0.5- CJ 04- 0.3- 0.2- 0.1- 0- A 0 \ i ,/ V apt ÛFc i 1 ?i ?i pi J 22 2 i——"-i— —r-—: 1.0- 0.9- 0.8- 0.7- Ofi- ■»—1 0.5- > .—- 0.4 O CZ 0.3- 0.2- 0 1 - 0- \ \_2 V «Fe api h i i i ""--î*- t ? 22 X 0.2 0,4 0.6 o.a i.c (a) XFe (b) Xpe (c) 1.0- 0.9- 0.8- >> 0.7- fl fi- > ■S 0.5" O 0.4 - Oj 0.3- 0.2- 0.1 - 0- A 0 (d) XFe Figure 4. Activity of iron in the Pt-Fe binary system: at 50 °C (a), 600 °C (b), 1300 °C (c) and 1550 °C (d) Values of iron activity coefficients vary with composition and temperature (Figure 4). The activity of iron in the temperature range from 50 °C to 1550 °C is mostly lower than that of ideal solution where (aFe = xFe), and typical for systems with intermediate phases. Thermodynamic calculations indicate formation of oxides in the temperature range between 700 °C and 4000 °C, based on possible reactions of oxidation, Figure 5. Figure 5 shows that most probable reaction in oxidative atmosphere will take place by oxidation of iron to hematite at lower temperatures ( -800 °C) and further oxidation of wustite to hematite at higher temperatures (>-800 °C). Decomposition of the formed oxide to elementary iron is not possible at the system's partial pressure of oxygen (pO2 = 10-6 bar) and the maximum temperature 1550 °C. At least 1700 °C is needed for decomposition of the high temperature oxide FeO. At the temperature around 1700 °C, FeO decomposes to elementary iron and oxygen that is swept off with argon purge gas. At higher temperatures (>1700 °C), the less stable reaction in this system is further oxidation of FeO to the higher oxide of Fe2O3. Temperature of decomposition will be different if iron is dissolved in platinum. Partial pressure of oxygen that is needed for oxidation (colored regions) of dissolved iron in platinum is presented in Figure 6. The diagrams show that much lower partial (dissociation) pressures of oxygen in the system are needed for decomposition of formed oxides in pure iron and in the region of higher iron contamination. From Figure 6, the partial pressure of oxygen in the furnace, pO2 = 10-6 bar, is already low enough to achieve decomposition of formed iron oxide in the regions with small iron molar content. For complete decomposition of formed oxides in all concentration regions, at rather low temperatures, proper vacuum system should be used. The use of vacuum during heating has Figure 5. Pourbaix high temperature diagram for un-dissolved iron and its oxide also effect on evaporation of other elements in the investigated samples and as consequence possible contamination with new elements and formation of new and more complex oxides with the existing one. Nevertheless, when system's temperature is increased high enough thermodynamically calculated partial pressure of oxygen in the Pt-Fe system shows that all the oxides are less stable in some point and eventually they decompose. Figure 6. Pourbaix high temperature diagram for the Pt-Fe binary system at different temperatures: FeO (a), Fe3O4 (b) and Fe2O3 (c) Oxygen affinities for dissolved iron were calculated with the eq. 16 and the results are presented in Figure 7 for four different temperatures of the system. At 50 °C, the calculated oxygen affinities are negative for all the three oxides formed according to eqs. 9, 10 and 11 and as that all ther-modynamicly possible. Both formed oxides, Fe2O3 and Fe3O4 are thermo-dynamicly more possible as FeO. With increased system's temperature to 600 °C, Figure 7 b, the first dissociation can appear if FeO is present. From Figure 7 b, affinities of Fe2O3 is more negative than the affinity of Fe3O4 and FeO in regions of small iron content (*Fe < 0.05). That means that the Fe2O3 oxide is most probable and will began to decompose at 600 °C. Neverthless, first mass decrease is expected at 600 °C regardles on the type of existed iron oxide. From Figure 7 c, d, the decomposition of iron oxides will take place in regions with higher iron contaminations if temperatures are higher than 600 °C. When riching the maximum temperature in furnace, 1550 °C, the absolute disociation of formed iron oxide FeO in regions of higher iron content is impossible as a consiquence of rather low temperature reached inside furnace and low partial pressure, seen also in Figure 5, 6. Figure 7. Diagram of oxygen affinities at various temperatures : 50 °C (a), 600 °C (b), 1300 °C (c) and 1550 °C (d) TG/mg 200 400 600 300 1000 1200 1400 Temperature !"C Figure 8. TG heating curve of an empty and of contaminated platinum sample holder TG heating curve of an empty and of Conclusions contaminated platinum sample holder after exposure to iron containing sam- Process of oxide decomposition is ples is presented in Figure 8. First peak complex and not sudden. The partial appears at 183 °C as result of loss of pressure of oxygen in the system is humidity inside the furnace. At tem- high enough, that oxides will not de- perature 707 °C, the first reduction of compose to elementary iron in all con- mass was determined as a consequence centration regions. The first change in of reduction of formed iron oxides. the TG curve was expected to be at Figure 7b shows that most probable temperatures above 600 °C which is in decomposition takes place with the good agreement with our experimental hematite in concentrations under xr = results. Plotted TG curve shows first Fe 0.1. The second reduction of mass, at visible mass decrease as a result of de- 1380 °C, indicated a higher local con- composition of hematite to elementary tamination or longer reduction time of iron, at 707 °C, followed by continuous formed oxides. Figure 7 c shows that drop of the TG curve with decomposi- all three oxides can contribute to the tion of magnetite and wustite. When mass decrease in TG curve. At higher contamination with other elements is temperatures, 1550 °C the most prob- present, the characteristic TG curve is able decomposition of wustite takes changed again. And to eliminate phe- place in the regions up to xFe = 0.4. nomena that are not in relation to the measured sample, basic curve should be recorded without a sample. References [1] Gudmundsson, G., Holloway, J. R. (1993): Activity-composition relationship in the system Fe-Pt at 1300 °C and 1400 °C and at 1 atm and 20 Kbar. American Mineralogist; Vol. 78; pp. 178-186. [2] http://www.freepatentsonline. com/3811874.html [3] Buscail, H., Courty, C. & Larpin, J. P. (1995): Effects of Ceria Coatings on pure iron oxidation, comparison with extra low carbon steels. Journal de physique iF;Colloque C7, supplement au Journal de Physique III, Vol. 5. [4] DeHoff, R. (2006): Thermodynam- ics in material science; Taylor and Francis group. [5] Barin, I. & Knacke, O. (1973): Ther- mochemical properties of inorganic substances; Springer-Verlag. [6] http://www.aerogas.de/argon.html [7] Barin, I., Knacke, O., Kubaschewski, O. (1977): Thermochemical properties of inorganic substances; Springer Verlag.