High performance alloys are often the materials used for various  components exposed to high temperature environments. In many cases,  protective coatings are applied in these alloys, providing higher  corrosion and oxidation resistance, compared to the base material. This  study investigates the feasibility to apply boro-aluminising treatment  on P91 steel by pack cementation process, to increase the steel high  temperature properties in oxidising and corrosive environments. Packs  activated by AlCl₃, NH₄Cl and KBF₄ were  used to carry out the coating deposition at a temperature of 715°C for 6  h. The coating formed was analysed by means of SEM and XRD, and the  compounds formed were identified. Cyclic steam oxidation for a total of  1008 h at 650°C revealed an oxide scale of 50 mum for the uncoated P91  steel, while the coated steel shows practically no oxidation effect.Publicad

Maragoudakis, N

Michailidis, N

Omar, H.

Tsipas, Sophia Alexandra

Omar, H

Name not available

Boro-aluminising of P91 steel by pack cementation for protection against steam oxidation

Universidad Carlos III de Madrid e-Archivo

This document is published in:Corrosion Engineering, Science and Technology (2011). 46(6), 697-700.DOI: http://dx.doi.org/10.1179/147842209X12579401586564Ins t i tu t ional  Repos i tory  © 2011  Institute of Materials, Minerals and Mining. Published by Maney on behalf of the InstituteBoro-aluminising of P91 steel by packcementation for protection against steamoxidationH. Omar1, S. A. Tsipas2, N. Maragoudakis1 and N. Michailidis*14Abstract: High performance alloys are often the materials used for various components exposed to high temperature environments. In many cases, protective coatings are applied in these alloys, providing higher corrosion and oxidation resistance, compared to the base material. This study investigates the feasibility to apply boro-aluminising treatment on P91 steel by pack cementation process, to increase the steel high temperature properties in oxidising and corrosive environments. Packs activated by AIC3, N H CI and KBF4 were used to carry out the coating deposition at a temperature of 715ºC for 6 h. The coating formed was analysed by means of SEM and XRD, and the compounds formed were identified. Cyclic steam oxidation for a total of 1008 h at 650ºC revealed an oxide scale of 50 µm for the uncoated P91 steel, while the coated steel shows practically keywords: Boro-aluminising, Pack cementation, Heat resistance, Corrosion and oxidationIntroductionAmong the various techniques to apply surface coatings(laser plasma spraying, chemical vapour deposition,pack cementation, etc.),1 the pack cementation processis an efficient and cost effective method to improve thesurface properties of materials. Pack aluminising hasbeen widely applied to steels and superalloys to improvetheir high temperature oxidation and sulphidationresistance.2,3 Boronising is a thermochemical diffusionsurface treatment, where boron atoms diffuse into thesurface of workpiece, thus forming hard borides with thebase material.4–7 Aluminising is a similar process whereAl is diffused into the surface of materials. Aluminium,alumina and halide salts such as ammonium chloride areusually used as pack cementation compounds.8 Thephase composition and properties of the coating dependon the process parameters (e.g. composition of packcementation powder, treatment temperature, time, etc.).Many papers have been published on the properties ofthe boride and aluminide layers on alloy steels.9–11,13–15In this paper, the codeposition of boron andaluminium on P91 heat resistant steel, applying a one-step pack cementation process, was investigated. Themorphology of the layers formed within the coating, aswell as the distribution of the diffused B and Al in thecoating and the interface, was analysed using scanningelectron microscopy (SEM) and X-ray diffraction(XRD). A cyclic steam oxidation process for both theuncoated P91 steel and the coated one, in combinationwith an elemental analysis on corresponding cross-ExperimentalBoro-aluminising processThe boro-aluminising process parameters (i.e. tempera-ture, time and pack cementation powder composition)are given in Table 1. The P91 steel chemical compositionwas (wt-%): C: 0?08–0?12, Si: 0?2–0?5, Mn: 0?3–0?6, P:(0?02, S: (0?01, Cr: 8?0–9?5, Mo: 0?85–1?05 and Fe:balance). Test pieces to be boro-aluminised were placedin a sealed retort in contact with the powders and thenheated in an electrical resistance furnace at 715uC for6 h. A combination of NH4Cl, AlCl3 and KBF4 wereused as activators. All the powders were pharmaceuticalmixed for 30 min, using a ball milling apparatus (Cole-Palmer) and Al2O3 ceramic spheres.Coating characterisationFor the examination of the microstructure, cross-sections from each specimen were cut and mountedand prepared metallographically. The observation of thecoatings was performed using a scanning electronmicroscope (SEM) associated with an energy dispersiveX-ray spectroscopy (EDS) analyser. The specimens wereexamined with a 20 kV Jeol 840A SEM equipped withan Oxford ISIS 300 EDS analyser and the necessarysoftware in order to perform point microanalysis. Thenature of the phases was determined using XRD. TheXRD experiments were carried out in a conventional1Physical Metallurgy Laboratory, Department of Mechanical Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece2Department of Materials Science and Engineering, Universidad Carlos III de Madrid, Avda de la Universidad 30, 28911, LeganesMadrid, Spain* Corresponding author, email nmichail@eng.auth.grno oxidation effect.sectioned specimens, will reveal the efficiency of theproduced coating.´1powder diffractometer (Seifert 3003 TT with Cu Karadiation) in Bragg–Brentano geometry. The XRDpatterns originate from the whole coating volume.Cyclic steam oxidationA cyclic steam oxidation process was selected to test theefficiency of the oxidation resistance of the producedcoating. Flowing Ar2 and steam in a 1 : 1 volume ratiowere supplied at 650uC for 24 h, followed by a coolingperiod for 1 h at room temperature, and again the samecycle was repeated for total 1008 h (42 cycles). The exactheating and cooling cycles presented in Fig. 1 weremeasured by placing a thermocouple close to thespecimen.Results and discussionA SEM cross-sectional image of the produced coating isshown in Fig. 2, revealing a coating thickness ofy15 mm. Three distinct regions were identified: (I) anouter surface layer consisting mainly of Al and Fe; (II) atransition region containing Fe, Cr, B and Al; (III) aninner region containing Fe, Cr, B, and Al. The weightand atomic percentages for each element in these threeregions are also presented in the same figure.The Al concentration in the three different regions inthe coating varies from approximately 42 at-% in theouter region to 4 at-% in the inner region and the Bcontent varies from 38 at-% in region II to 24 at-% inregion III. Therefore, it can be concluded that simulta-neous diffusion of both elements, Al and B, has beenachieved. The XRD spectrum of the coating obtainedafter boro-aluminising on P91 with pure aluminium andboron as source elements and NH4Cl, AlCl3 and KBF4as activators is shown in Fig. 3. The XRD resultsshowed the existence of FeAl, Fe2AlB2 and Fe2Bcompounds.Energy dispersive X-ray spectroscopy analysis in thethree different regions indicated in the micrographshows a uniform multi-layer coating of y15 mm. Theoutermost layer appears to correspond to the FeAlphase (59 at-%Fe–41 at-%Al). Moving towards thesubstrate, EDS analysis indicates a layer containingFe, B, Al and some Cr (37, 39, 14 and 10 at-%respectively). It is possible that this layer correspondsto the Fe2AlB2 phase. Furthermore, a boride layer,which contains very little Al, possibly corresponding tothe Fe2B phase, appears to be present (54 at-%Fe–24Table 1 Chemical composition (wt-%) and conditions for boro-aluminising powderAgent B–Al, % Filler Al2O3, % Activators, % Holding time, h Temperature, uC18 76 6 6 7151 Cyclic steam oxidation process2 Cross-sectional SEM image of produced coating, show-ing three distinct regions with different compositions (I,II and III)3 X-ray diffraction spectrum of coating obtained afterboro-aluminising on P91 with pure aluminum andboron as source elements and NH4Cl, AlCl3 and KBF4as activators: SEM image of coating is shown in Fig. 22at-%B16 at-%Cr–6 at-%Al). B appears to diffuse to adepth of at least 15 mm and reacts with Fe and Cr toform borides. Al diffusion seems to be limited to theouter region of the coating and up to about 10–15 mm.It is also clear from both the EDS analysis and theunidentified peaks from the XRD spectrum that otheraluminide (such as FeAl3 and Fe2Al5) or boride phases(FeB) may also be present in the coating in smallerquantities. The coating mainly seems to consist of FeAlintermetallic in the outer region, as expected from the Fe–Al phase diagram.12 The predominant iron boride phaseappears to be Fe2B (region III), with an intermediatemiddle zone (region II) consisting of Fe2AlB2.Fe2B has been found to dissolve to 11 at-%Alaccording to the ternary Fe–B–Al isothermal (800uC)phase diagram,16 and the compound Fe2AlB2 has beenalso identified during the boronising of aluminisedsteel.17FeAl phase, which seems to be the predominant phaseformed in the outer region of the coating, is the desirableone, since iron aluminides with higher aluminiumcontent, such as FeAl3 and Fe2Al5, tend to embrittlethe produced layer and to reduce its oxidation resis-tance. The coating layer formation is a temperature andtime dependant process, which means that it is adiffusion controlled reaction. The intermetallic layer isfollowed by an interdiffusion zone, containing mainly Aland B, both elements diffusing into the substrate.In order to evaluate the oxidation resistance of theproduced coating, a cyclic steam oxidation process wasapplied. A schematic representation of the selected cycleis presented in Fig. 1. Flowing Ar2 and steam in a 1 : 1volume ratio were supplied at 650uC for 24 h, followedby a cooling period for 1 h at room temperature, andagain the same cycle was repeated for total 1008 h (42cycles).The cyclic steam oxidation process in Fig. 1 wasperformed for both the coated and uncoated P91 steel.Figure 4a presents the SEM images and elementalanalysis of the cross-sectioned oxidised P91 steel, showingthat there is a 50 mm external oxide scale formed.In this scale, there are two distinct zones (A and B)observed, separated by a crack, running through thescale, parallel to the oxide/substrate interface. In theexternal zone A of the scale, the EDS analysis showedthe presence of oxygen and Fe (64 at-%O2–36 at-%Fe),suggesting that the compound formed is possibly Fe2O3.In the inner zone B, the EDS analysis indicated theincorporation of Cr (y9 at-%) together with minoramounts of Mo and Si, into the formed oxide, suggestingan Fe(Cr)2O3 spinel type compound. The differenceobserved in the chemical composition of the two adjacent4 Micrographs (SEM) and elemental analyses of cross-sectioned specimens a for uncoated P91 steel and b for coatedone, after 1008 h cyclic steam oxidation3oxide compounds formed in zones A and B will lead todifferent values of the coefficient of thermal expansion forthese compounds, thus leading to crack formation at theinterface between the two zones and eventually spalling,during repeated thermal cycling. Also for 9–12 wt-%Crsteels operating in steam containing environments, attemperatures between 550 and 650uC, a detailed mechan-ism has been proposed for the formation of vacancies,18micropores and eventual breakaway at the interface ofsuch oxide layers. The corresponding SEM image andelemental analysis of the cross-sectioned B–Al coatedsteel shows a 30 mm affected zone (Fig. 4b). This zoneconsists in the outer part mostly of Fe and Al (72 and25 at-% respectively, indicating the phase FeAl3),together with a small amount of Cr (2 at-%) and Fe,Cr, Al, B and O (11 at-%Al–36 at-%B–13 at-%O). Theexternal FeAl layer formed on the initial coating seems tooffer sufficient protection from high temperature steamover the tested period of 1008 h under cycling conditions.Both Al and B were also found in significant amounts inthe zone. The affected zone shows good adherence to thesubstrate with no cracks, voids or pores, limited oxygendiffusion and an overall improvement in the steamoxidation resistance compared to the uncoated steel.ConclusionIn this paper, the pack cementation method was used asa coating technique to produce B–Al layers by diffusionin P91 steel. The B–Al coating was achieved in a singlestep process, and both Al and B diffused into thesubstrate forming a homogeneous layer of 15 mm inthickness. Three distinct regions were identified in thecoating. Close to substrate, the Fe2B boride is formed,followed by an intermediate zone rich in Al and B andprobably Fe2AlB2. The outer region is rich in Fe and Al,indicating that it is FeAl. The existence in the coating ofother aluminide or borides compounds such as FeAl2,Fe2Al5 or FeB cannot be rejected. The coating wasuniform and showed good bonding with the substrate.Finally, it showed an increased oxidation resistance, astested by cyclic steam oxidation for a total of 1008 h at650uC.References1. G. W. Goward: Surf. Coat. Technol., 1998, 108–109, 73–79.2. N. E. Maragoudakis, G. Stergioudis, H. Omar, H. Paulidou and D. N. Tsipas: Mater. Lett., 2002, 53, 406–410.3. A. M. Hodge and D. C. Dunand: Intermetallics, 2001, 9, 581–589.4. V. A. Ravi, P. Choquet and R. A. Rapp: Proc. MRS Int. Meet. Adv. Mater., 1989, 4, 483–500.5. A. K. Sinha: ‘Boriding (boronizing)’, in ‘ASM handbook’, Vol. 4, 437–447; 1991, Materials Park, OH, ASM International.6. R. H. Biddulph: Thin Solid Films, 1977, 45, 341–347.7. M. Carbucicchio, L. Bardani and G. P. Palombarini: J. Mater. Sci., 1980, 15, 711–719.8. E. Basuki, A. Crosky and B. Gleeson: Mater. Sci. Eng. A, 1997, A224, 27–32.9. D. N. Tsipas and J. Rus: J. Mater. Sci. Lett., 1987, 6, 118–120.10. D. N. Tsipas, J. Rus and H. Noguera: Met. 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Boro-aluminising of P91 steel by pack cementation for protection against steam oxidation

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