Keywords: Mass spectrometry; Volatile species; Protective coating; Steam oxidation
1. Introduction
Ferritic steels are characterized by its excellent oxidationresistance, while austenitic steels are characterized by its goodtoughness and strength properties. Martensitic 9–12% chromi-um steels are used for critical components in steam power plantssuch as rotors, piping and valve bodies due to their low thermalexpansion, good thermal conductivity and acceptable corrosionproperties. It is well known that creep strength of somematerials can be increased by reducing its chromium content,but it is deleterious for the corrosion resistance [1].
Many authors [2,3] have studied the oxidation kinetics offerritic/martensitic steels such as the P91 and P92 under differentconditions, including temperatures between 600–700 °C rangeand diverse atmospheres (O2, N2, Ar, water steam, etc.). All theseworks investigated the initial stages of oxidation but no attemptsof verifying experimentally the volatile species formed areperformed. F. J. Pérez et al. [4] presents early an experimentalwork on P91 and P92 steel oxidized at 650 °C and 1 atm in Ar+
10%H2O steam. They combined thermogravimetric measure-ments and mass spectrometry during oxidation process, deter-mining volatile chromium-hydroxides and oxyhydroxides speciessuch as: CrOOH(g), Cr(OH)3(g), Cr(OH)6(g) and CrO2(OH)2(g)in P91 steel and Cr(OH)2(g), Cr(OH)3(g), CrO(OH)2(g), Cr(OH)4(g) and CrO(OH)4(g) species for P92 steel. These results suggest
that evaporation of chromium oxides and oxyhydroxides occurswhen steam is present. In these oxidation conditions, F. J. Perez etal. did not find evidence of the presence of volatile speciesCrO2OH(g) in the ferritics steels P91 and P92 studied, but theydetected CrO2(OH)2(g) species in the P91. These authors alsoreported the formation of FeO(g) and FeO1.5(g) volatile speciesduring the oxidation of P92 steel. They finally conclude that thebreakaway oxidation begins earlier for P92 than for P91 steel.
The oxyhydroxide vapour species, CrO2OH(g) [5,6] and CrO2
(OH)2(g) [7,8] have been identified on volatile of chromium fromCr(s) and Cr2O3(s) samples oxidized in the presence of air andsteam atmosphere at high temperatures. Also, the species Cr3O9
(g), Cr4O12(g) and Cr5O15(g) have been identified from chromiumvolatile in air at temperatures below 500 K [9,10].
H. Asteman et al.,[11] reported the volatile species CrO2
(OH)2(g) during the oxidation of type 304 L stainless at 873 Kin the presence of and O2+10%H2O atmosphere and caused bychromium evaporation.
Several researchers [12–15] have attributed the formation ofvolatile species such as CrO3(g), CrO2OH(g) or CrO2(OH)2(g) tothe breakdown of chromia scales at temperatures near 1000 °C,and with the subsequent depletion of Cr in the scale formed.
The alloys based on Ni, (Co, Fe) with incorporation of Cr orAl have very beneficial effect for the formation of protectivealumina oxide (Al2O3), for example, additions of 10% of
Fig. 3
chromium they can favour the alloy alumina formation with lessof 5% of Al. Alumina does not suffer from problems of scaleloss at high temperatures and provides very good resistance tooxidation [16–20]. Previous works has looked at Ni-base andalumina forming alloys for surface recuperators to oxidation tohigh temperatures and in water vapour [21,22].
Slurry coatings are suitable for internal and externalapplication to small (e. g. blades) and large components (e. g.,steam pipe, casings, valve internals) and can be relatively easilyapplied to complex shape objects. Slurries of aluminium layercan be understood as diffusion coatings since the adhesionbetween the deposited layer and the substrate is of chemicalnature, i.e. during the heat treatment of the coating. Migration ofthe elements between the coating and the substrate takes place.The main mechanism of degradation during the oxidation ofthese coatings is the interdiffusion with the substrate on whichAl spreads towards the interior of the substrate and the Fe fromthe base material goes out, reaching a minimum criticalaluminium concentration in the surface of the coating makingit to loose the capacity to form protective oxide layer [23].
In our present work, oxidation of the P91 steel without and ofFeAl-slurry coating at 650 °C and +80%H2O atmosphere isinvestigated.
Our main objective is to find a direct relation between TG-MS (TG-Mass spectrometry analysis) experiments and the
formation of volatile inorganic oxyhydroxides species thatappear for these samples in their initial period of oxidationunder the mentioned conditions. Comparison with thermody-namic predictions for the system and previous published workswill also be discussed. All those data will give important inputsfor coating design based on experimental evidences and validthermodynamic calculations.
2. Experimental
Oxidation tests at high temperatures were performed on thesamples of ferritic/martensitic steel P91 without and with Slurrycoating of FeAl. The chemical composition of P91 steel:Fe=89.35, Cr=8.10, Mo=0.92, Mn=0.46, Si=0.38, C=0.10,Ni=0.33, P=0.020, V=0.18, Al=0.034, Nb=0.073, N=0.049and S=0.002 (all composition in weight percentage). Sampleswith a size of 10 × 20 × 3.5 mm were cut for each test andpolished to 600-grid with SiC paper before of the oxidation andcoating deposition. All samples were cleaned with acetone inultrasound bath for 10 min.
The painting used for the Slurries coatings of FeAl wasIPCOTE 9183 (supplied by “Indestructible Paint”): Cr2O3 (2.5–10%), H3PO4 (10–25%) and powder of Al (25–50%). Waterand diluted chromic acid was used as dissolvent of the painting.Previously, the painting is homogenized by thermal agitation,then placed in a stove at 85 °C for 15 min and then to a furnaceat 350 °C for 30 min to eliminate the dissolvent. Next, a
Fig. 5
diffusion treatment of the painting is made in furnace for 10 h at700 °C to produce the protective coatings phases. After thethermal treatment, the samples were slightly polished with SiCpaper (30 μm) for to eliminate the excess painting.
Identification of volatile species emanating from samplesduring oxidation was performed with the Quadrupole MassSpectrometer QMS 422 (PFEIFFER VACUUM instruments)placed inside of the oxidation system.
Oxidation experiments and thermogravimetric (TG) mea-surements were performed using a thermobalance (sensitivity of10− 7 g) of symmetrical thermoanalyser (SETARAM-TAG 16model). Initially, the furnaces were pumped by a membranepump to a pressure of 10− 2 mbar. A constant flow of 500 ml/min of high purity Ar gas (99.999% pure) is introduced in thesystem, filling both furnaces until reaching the atmosphericpressure, places where the sample in study and the referenceSiO2 crucible are located.
The steam was introduced to the furnaces by a constant fluxof argon carrier gas (16 ml/min) through a water flask(humidifier). The distilled water from the flask is heated togive the desired relative humidity percentage by volume of eachexperiment.
Routinely, for the oxidation tests of the samples, wepreviously must calibrate the thermobalance (verifying levelzero), measure the sensitivity of the Mass spectrometer andmake the measurement of the background of the system. This ismade with two quartz crucibles as calibration samples placed ineach furnace and also maintaining the same conditions ofoxidation.
Always in all the results of mass spectrometry we subtractedthe measures of the background respective. Thus, to the finalonly we would obtain the measures of the sample and not of thesystem (the apparatus or any gas contaminants). Theseconsiderations and conditions of calibration were reportedbefore in another work [4]. In this work only we reported thefinal results of each sample.
On the results of composition analysis by mass spectrometrypresented and to the diverse mass numbers assigned to thevolatiles species detected in the oxidation of the samples of thiswork it can be stated: first, we to find the probable speciesvolatile have made a series of simulations for materialassumptions that they will be as the bases of our alloys (Cr,Fe, Si, Al, Cr2O3, Al2O3, SiO2, etc.). The simulation conditions
of oxidation were the same as all the experiments. The Thermo-calc program (Software AB:Fundation of ComputationalThermodynamics 1995–2003, Stockholm — Sweden) and theHSC Chemistry for Windows (Software Outokon: version 3.02,Finland 1994) have been used for the simulations. All thetheoretical results that we obtained have been compared withothers of the bibliographies.
All specimens were oxidized at 650 °C in Ar+80%H2Oatmospheres and with exposure times of 50 to 200 h. The test ofoxidation of FeAl slurry-coated P91 steel was made in threestages and oxidized in the same conditions without having toremove them from thermobalance in all the experiment: the firststage was of 100 h, after an additional time of 50 h and finallyanother time of 50 h until completing 200 h of test.
We have observed an apparent noise in the TG spectrumswhen we worked with high percentages of water steam (greaterof 50%H2O) and when the samples were of small size.
Phase component of the surface scales of the oxidized sampleswas studied by the X-Ray Diffraction (XRD) of a Philips X'Pertdiffractometer, using Cu Kα (1.54056 Å) radiation and by meansof angle 2Θ and grazing incidence (GIXRD).
The microstructure observation by scanning electron mi-croscopy (SEM) and X-ray energy dispersive spectrometry(EDS) measurements were performed in a JEOL-JSM 6400scanning electron microscope operated at 20 KV.
Fig. 7
3. Results and discussion
The mass change as a function of testing time (continuouslines) of oxidized samples of the P91 steel with and without Al-slurry coating at 650 °C in Ar+80%H2O atmosphere and thetemperature profile (dash line) are showed in the Fig. 1. As seenfrom the figure, in the first 100 h of oxidation the coated sampleloses mass around of 3.5 mg/cm2 and while the P91 steelwithout coating gains mass around of 4 mg/cm2. The resultsdemonstrate in the initial stages of oxidation under theseconditions, the sample of FeAl-slurry/P91 loses mass byevaporation of aluminium or perhaps due volatile oxide ofaluminium by chemistry reaction of the coating with theenvironment. The loss of aluminium by evaporation of thecoating will finish until this element by diffusion reacts totallywith the substrate (P91 steel), to form protective coatings, asintermetallics or the alumina (Al2O3) that has formed in allsurface maintains the property of protective coating to theoxidation. The P91 steel without coating in the initial stages ofoxidation gains great amount of mass, this is due to that it beginsto lose chromium in form of volatile gaseous species as,chromium-hydroxides and oxyhydroxides, thus losing theprotection to the oxidation and then begins the breakaway ofoxidation with the formation of iron oxides in its surface.
Fig. 2 shows the results of the spectrum of mass spectrometrymade simultaneously during TG measurements of oxidation ofthe P91 steel without coating at 650 °C inAr+80%H2O for 100 h.The species were identified as, Cr(g), CrO2(g), CrOOH(g), CrO2
(OH)2(g), Cr(OH)6(g), FeOOH(g), Fe0.947O(g), Fe2O3(g), CO,CO2, Mo(g), and HNi(g). By theoretical calculations, theevaporation of Cr and Fe in form of volatile species is thermo-dynamically possible at this temperature in atmospheres with thedesired water vapour percentage. The presence of the volatilespecies CrO2(OH)2(g) during the initial stages of oxidation of P91steel indicated that the sample is in the breakdown of chromiascale dose occur. It is suggested that this chromium depletionmayhave important implications for the long-term stability of theprotective oxide scale formed on P91 steel, as other authors havebeen proposed in the case of ferritic steels which were oxidized inwater steam [7–15]. This may result in Fe2O3 (hematite)/Fe3O4
(magnetite)/FeO (wüstite) formation, and “breakaway corrosion”
that confirmed the mass spectrometer with the appearance of theother species as: oxides, hydroxides and oxyhydroxide of iron.This effect could explain the observation by Khanna and Kofstad[24]. Other investigators also have suggested these results bythermodynamic calculations for oxidations at higher temperature(1000 °C), but with smaller percentage of water steam [11,12]. Inaddition, we have detected the fragmentation of others volatilespecies of H2O, O2, N2 and Ar molecules. We make emphasisehere that, in our results of mass spectrums of the samples we willnot enter into details on these species because they are not directlyrelated with the oxidation process.
Fig. 3 shows the volatile species found by means of the massspectrometry of FeAl slurry-coated P91 steel sample oxidized at650 °C in Ar+80%H2O atmosphere for 14 h. These importantspecies have been identified as Al(g) and AlO(g). The detectionof these volatile species demonstrates us the loss of mass of thesample in the first hours of oxidation (until 100 h: Fig. 1) underthese conditions by the evaporation of aluminium of the FeAlcoating in form of gas and oxide.
Thermogravimetric and temperature measurements of anadditional time of 50 h until completing 150 h of test of FeAl-
Fig. 9.
slurry/P91 oxidized sample in Ar+80%H2O at 650 °C areshown in Fig. 4. In the first test hours of oxidation (6.5 h) thesample loses ∼ 0.05 mg/cm2 of mass. This is due to theevaporation of Al of the coating in form of gas and oxide, as wedescribed before. However, in the time interval between 6.5 hand 50 h we observed an increase of mass up to∼0.170 mg/cm2.
These results indicated that initial oxidation stages under theseconditions, FeAl Slurry on P91 steel shows higher oxidationresistance protection than the P91 steelwithout coating (substrate).The P91 steel with slurry FeAl coating during oxidation formed aprotective alumina scale on its surface.
Fig. 5 shows the volatile species found by means of the massspectrometry in FeAl-slurry/P91 sample in the second oxidationstage (sample of Fig. 4), whichwas exposed at 650 °C inAr+80%
H2O atmosphere for 110 h. These species have been identified as,Al(g), AlO(g), and FeOOH(g). The presence of these volatilespecies confirms that the initially evaporated species from thecoating are in form of aluminium oxide and gas.
The small mass gain observed by TG measurements of thissample (Fig. 4) during initial oxidation are related directly withthe presence of volatile species FeOOH and the iron oxidelayers that would have formed in the surface.
Fig. 1
Fig. 6 shows thermogravimetric results of the third stage of50 h of oxidation of FeAl-slurry/P91. The results demonstratethat the sample continues its oxidation due to the possible lossesof alumina and other protective oxides or intermetallics phasesthat had formed initially. Thus the sample gained an additionalmass of 1 mg/cm2 by the formation of iron oxides.
By means of these results we verified that the coating of FeAl-slurry plays an important role in the formation of protective layers
resistant the water steam oxidation, where the alumina protectivecoating and/or the intermetallics phases formed by diffusion orreaction of aluminium of the coveringwith environment avoid theloss of chromia and thus to avoid finally the breakaway oxidation.
The results of the study of mass spectrometry madesimultaneously during TG measurements of FeAl-slurry/P91sample that was oxidized at 650 °C, in Ar+80%H2O and by160 h are showed in the Fig. 7. These volatile species have beenidentified as Al(g), AlO(g), and FeOOH(g), and were the sameones that we detected to 110 h of oxidation (Fig. 5). Weobserved that in this time of test, the intensities of masses linesof Al(g) and AlO(g) have diminished respect to the same onesfound to smaller time of oxidation, and while the line of theFeOOH stays almost constant. This suggests that during thistime of oxidation the aluminium from the coating diffused orreacted inwards the substrate (P91 steel) and then formedprotective intermetallics coatings. Also, the depletion ofaluminium would be related to the loss of the alumina byevaporation of the species of Al that were detected during thetest and thus the sample would gradually lose its protectiveness.These results are verified by means of the TG spectrum (Fig. 6),where we observed as in this time the sample begins to gainmore mass due to the growth of iron oxides.
Fig. 8a–b show the results of the XRD analysis in the 2Θmode, of the samples of FeAl-slurry/P91 and to P91 steel that wereoxidized in the same conditions as we explained in the Fig. 1. Theresults indicate that both samples present two oxide phases,consisting of cubic magnetite (Fe3O4) and rhombohedral hematite(Fe2O3) [25,26]. The sample with FeAl coating presents anothermost predominant phase of monoclinic Fe2Al5 [27] whencompared to P91 steel (uncoated sample). By means of XRDpatterns of coated sample we did not observe other phase ofaluminium oxide and therefore we resorted to another techniqueknown as grazing incidence X-ray diffraction (GIXRD). Thistechnique helps us to determine the thin layers on the surface of theoxidized sample. The results of Fig. 9 indicated that the samplepresents another important phase that it belongs to therhombohedral alumina (Al2O3) [28]. In addition, the phases ofFe2Al5, Fe2O3 and Fe3O4 were identified, these phases werealready found before by means of the other technique of XRD(Fig. 8a).
SEM image of the cross section of the P91 steel without coating(sample of Fig. 8b) that was oxidized at 650 °C in Ar+80%H2Ofor 200 h is showed in the Fig. 10. Observation at this magni-fication indicates an oxide scale formed in the surface withthickness between 30 and 50 μm. EDS analysis of the crosssection (small area) confirmed that the composition of the oxide isa mixture of iron oxides and with atomic percentages approx-imately to hematite (Fe2O3) and magnetite (Fe3O4). Also, there isinternal oxidation layer of thickness between 10 and 20 μmformed between the oxides of iron and the substrate, a type mixedspinel of iron and chromium (Cr2FeO4).
Fig. 11a–b shows the micrographs at different magnificationof surface and the cross section of FeAl-slurry/P91 (sample ofFig. 8a) after being oxidized under the same conditions as forthe P91 steel sample. In all surface (Fig. 11a) we observed thatthe sample presents a granular morphology with a great
roughness. In the image of the cross section of the sample(Fig. 11b) “oxide nodes” were observed. The nodes presented adiameter up to 50 μm, approximately. The nodes may formduring the oxidation of the sample by the crack, pores and/orimperfections of the protective coating.
Elemental mappings of the cross section of the oxidizedsample and of a node by means of EDS analysis are showed inFig. 12. The mappings indicated that the aluminium is in thesuperior part of the sample and together with the oxygen formedalumina (Al2O3) as protective layer. In the oxidation node weobserved small amounts of alumina and in other cases it wasabsent. In the images of the node we observed that the iron andoxygen together with aluminium are the elements of great partof the node and that according to its atomic percentages byprecise analyses indicated two superficial layers: the hematiteFe2O3 and magnetite Fe3O4. Also, we have observed anothermajority Fe2Al5 intermetallic layer. In addition, in the inferiorpart of the node with substrate (P91 steel) we have observed azone with chromium, iron and oxygen, and of compositionapproached to the mixed spinel Cr2FeO4.
These complementary results found by means of thetechnique of analysis EDS were verified previously with thespectrums of XRD for both oxidized samples (Fig. 8a–b).
4. Conclusions
The oxidation behaviour of P91 steel with and without FeAl-slurry coating was investigated. These samples were oxidized at650 °C in Ar+80%H2O atmosphere for 50 to 200 h.
The following volatile species as: Cr(g), CrO2(g), CrOOH(g), CrO2(OH)2(g), Cr(OH)6(g), FeOOH(g), Fe0.947O(g), Fe2O3
(g), CO, CO2, Mo(g), and HNi(g) were identified by means ofmass spectrometry during the oxidation of the P91 steel withoutcoating. The presence of the volatile species CrO2(OH)2(g)during 100 h of steam oxidation indicated that the P91 steelloses its protective chromia (Cr2O3) scale due to the strongdeterioration of steam. This result verifies results by otherauthors of the oxidation of these steels and results of bytheoretical calculations under the same conditions [7–15]. Inaddition, when we found the fragmentation of the volatilespecies that belong to oxides and oxyhydroxides of iron, theypredict the formation of the majority layer of wüstite andtherefore it indicates that the sample is in state of “catastrophiccorrosion”.
The Al(g), AlO(g), and FeOOH(g) volatile species have beendetected for FeAl-slurry/P91 sample oxidized at the sameconditions for 110 and 160 h. The difference between thesetimes of the test consisted, that the species of Al(g) and AlO(g)had diminished at the greater time of oxidation, while theFeOOH(g) species remained constant. The reduction of Al inthe sample during the oxidation would be related later to the lossof alumina (Al2O3) and for that reason it would not be protectedto the oxidation. Also, this loss of Al during the oxidation wouldbe related thermally to its diffusion towards the interior of thesubstrate (P91 steel) and after together with the iron would formFe2Al5 intermetallic. In the oxidation up to 200 h of FeAlcoating sample we did not found another species volatile of
chromium that could determine the sample in state ofcatastrophic corrosion.
Thermogravimetric measurements indicated that the FeAl-slurry coating increased the oxidation resistance of the P91 steelup to 40 times. SEM images of the cross section of the thicknessof oxides in both samples confirmed also the advantages of theFeAl coating. This sample with coating after the test presentedsmall “oxidation nodes”.
The mappings of composition, EDS analysis of the crosssection of small area and X-Ray Diffraction of FeAl-slurry/P91oxidized confirmed the presence of layers as, alumina (Al2O3),hematite (Fe2O3), magnetite (Fe3O4), other majority interme-tallic Fe2Al5, and finally a protective internal layer Cr2FeO4 ofmixed spinel type. These results help us to understand theimportance that the aluminium enhances in the formation ofprotective coatings on ferritic steels.
Acknowledgment
The authors acknowledge the financial support of theEuropean Community under project No ENK5-CT-2002–00608-SUPERCOAT.
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