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High Temp. Mater. Proc. 2015; 34(2): 115 –121

Z. Grzesik*, M. Migdalska and S. Mrowec The Influence of Yttrium on High Temperature Oxidation of Valve Steels

Abstract: The influence of small amounts of yttrium, elec- [4–6]. It is then obvious that under thermal cycle condi- trochemically deposited on the surface of four steels uti- tions, the scale adherence to the substrate constitutes the lized in the production of valves in car engines, on the most important factor, determining the corrosion resis- protective properties of the scale and its adherence tance of valve material. to the surface of the oxidized materials has been studied The combustion gases of petrol and fuel oil are highly under isothermal and thermal cycle conditions. Oxidation aggressive and during last few years this corrosion danger measurements have been carried out at 1173 K. It has considerably increased due to the addition to these fuels been found that yttrium addition improves considerably various bio-components [6–14]. In this situation, there is the scale adherence to the substrate surface, increasing then an urgent need to improve the corrosion resistance thereby corrosion resistance of the studied materials. of engine valves. In order to make a step forward on this way, it is necessary to explain in more details the mecha- Keywords: steel, thermal cycling, oxidation nism of high temperature corrosion of valve materials. This problem is a subject of investigation in our laboratory PACS® (2010). 81.65.Mq since several years [6, 15–18]. As the chemistry of combus- tion gases is rather complex, the aggressiveness of partic- DOI 10.1515/htmp-2014-0023 ular components is being studied separately one after Received January 29, 2014; accepted March 8, 2014; another. The only conclusion at present is that the syner- published online: May 14, 2014 gistic effect seems to be most important. On the other hand, since more than 50 years it is well known that ­reactive elements (RE) like yttrium, and lantha- 1 Introduction num may considerably improve the corrosion resistance of chromia forming alloys [1–3, 19–22]. It has been shown Four austenitic -nickel steels (X33CrNiMn23-8, for instance, that small addition of yttrium improves the X50CrMnNiNbN21-9, X53CrMnNiN20-8 and X55CrMnNiN20- chromia scale adherence and simultaneously its protec- 8) are being generally utilized nowadays in the production tive properties [1–3, 23]. Thus, the aim of the present paper of valves in automobile engines. These materials are is an attempt to prove if yttrium addition may improve the working in very sever conditions due to rather high tem- chromia scale adherence in the case of valve steels under peratures (873–1173 K) and in particular, due to sudden thermal cycle conditions. temperature changes, described in the literature as ther­ mal cycles [1–3]. Under these conditions high mechanical stresses are developed in the scale-substrate system due to different coefficients of both materi- 2 Materials and experimental als [4, 5]. Thus, during heating and cooling of engine, procedure cracking and spalling of the scale are observed, consider- ably lowering the corrosion resistance of valve materials Chemical compositions of steels under investigation are summarized in Table 1. The steel samples in the form of flat discs (18 mm in *Corresponding author: Z. Grzesik: Department of Physical diameter and 1 mm thickness) have been cut from steel Chemistry and Modelling, Faculty of Materials Science and rods, abraded with emery papers (up to 800 SiC) and , AGH University of Science and Technology, al. A. before the covering their surface by yttrium, polished Mickiewicza 30, 30-059 Krakow, Poland. E-mail: [email protected] using pastes in order to get mirror-like surfaces. M. Migdalska and S. Mrowec: Department of Physical Chemistry and Modelling, Faculty of Materials Science and Ceramics, AGH There are several possibilities to incorporate reactive ele- University of Science and Technology, al. A. Mickiewicza 30, 30-059 ments into the growing scale. The oldest method consists Krakow, Poland in the incorporation of reaction elements into the bulk of 116 Z. Grzesik et al., The Influence of Yttrium on High Temperature Oxidation

Table 1: The chemical compositions (wt.%) of X33CrNiMn23-8, X50CrMnNiNbN21-9, X53CrMnNiN20-8 and X55CrMnNiN20-8 valve steels.

Type of steel C Mn Si Cr Ni N W Nb S P Mo Fe

X33CrNiMn23-8 0.35 3.3 0.63 23.4 7.8 0.28 0.02 – <0.005 0.014 0.11 bal. X50CrMnNiNbN21-9 0.54 7.61 0.30 19.88 3.64 0.44 0.86 2.05 0.001 0.031 – bal. X53CrMnNiN20-8 0.53 10.3 0.30 20.5 4.1 0.41 – – <0.005 0.04 0.12 bal. X55CrMnNiN20-8 0.55 8.18 0.17 20.0 2.3 0.38 – – <0.005 0.03 0.11 bal.

Fig. 1: The scheme of setup used in oxidation tests under thermal shock conditions.

the material using metallurgical procedures [1–3]. Nowa- matically in Fig. 1. These experiments consisting in rapid days these elements are being applied rather by surface heating of a given sample from room temperature up to treatment using implantation technique or/else elec- 1173 K and after treating it at this temperature during two trochemical deposition methods [21]. In our experiments hours subsequently quenched to room temperature. The we used the last of these methods, consisting in electro- duration of heating time was approximately equal to 1 chemical deposition of yttrium on the surface of studied minute. After two hours heating the furnace was moved steels from alcohol solution of yttrium . Before out from reaction chamber and the sample was quenched yttrium deposition steel samples were degreased in ace­ to room temperature. The cooling time in air flux (5 × 10−5 tone using ultrasonic washer and finally treated at 343 K m3/min) proceeded about 2 minutes. After that, heating in acetone and trichloroethylene vapors during 30 min­ was repeated by moving the furnace again in the first utes. The experimental procedure consisted in electro- working position. The corrosion tests consisted in deter- chemical deposition of yttrium from 0.01 M solution of mining the mass changes of oxidized samples as a func-

Y(NO3)3 in ethyl alcohol. The electrodeposition has been tion of a number of thermal cycles. The maximum number carried out by voltage of 10 V and current intensity of 10−2 of shocks, reaching 500, has been chosen in order to sim- A during 45 seconds. During this time the whole amount ulate the time of standard procedure utilized in testing of deposited yttrium equals 2 × 10−6 kg/m2. This is equiva- properties of car engines. lent of about 1023 Y+3 /m2. The final treatment of covered material consisted in drying the sample at 323 K during 10 minutes and annealing after that at 673 K during 3 Results and discussion 30 minutes. The oxidation kinetics under isothermal conditions Figure 2 illustrates the kinetics of the isothermal oxidation have been studied gravimetrically in air at 1173 K in the of the sample of the X33CrNiMn23-8 steel at 1173 K covered microthermogravimetric apparatus [24]. The weight gains electrochemically with yttrium, on the background of of oxidized samples as a function of time have been regis- analogous results obtained using the same steel sample tered automatically with the accuracy of 10−9 kg. The ap- without yttrium addition [25]. As can be seen, the influ- paratus and the details of experimental procedure have ence of yttrium on the rate of oxidation of the discussed been described elsewhere [24]. X33CrNiMn23-8 steel is rather small. In fact, the difference The experiments under thermal cycle conditions of mass gains of the oxidized samples related to their unit have been carried out in the experimental set shown sche- surface area is of the order of 20% after 50 hours. On the Z. Grzesik et al., The Influence of Yttrium on High Temperature Oxidation 117

Fig. 2: The kinetics of the isothermal oxidation of the Fig. 4: The kinetics of the isothermal oxidation of the X33CrNiMn23-8 steel at 1173 K covered electrochemically with X53CrMnNiN20-8 steel at 1173 K covered electrochemically with yttrium on the background of analogous results obtained with the yttrium on the background of analogous results obtained with the same steel without yttrium addition (Δm/S – weight changes of the same steel without yttrium addition (Δm/S – weight changes of the oxidized sample per unit surface area). oxidized sample per unit surface area).

Fig. 3: The kinetics of the isothermal oxidation of the Fig. 5: The kinetics of the isothermal oxidation of the X50CrMnNiNbN21-9 steel at 1173 K covered electrochemically with X55CrMnNiN20-8 steel at 1173 K covered electrochemically with yttrium on the background of analogous results obtained with the yttrium on the background of analogous results obtained with the same steel without yttrium addition (Δm/S – weight changes of the same steel without yttrium addition (Δm/S – weight changes of the oxidized sample per unit surface area). oxidized sample per unit surface area). other hand, in the case of three remaining steels with quently, the oxidation resistance of the X33CrNiMn23-8 lower chromium concentration, the positive influence of steel sample covered by yttrium is only insignificantly yttrium is more clearly visible (Figs. 3–5). This different better than that of uncovered material. In the case of re- behavior of investigated materials can be explained in maining steels, the chromium concentration being on the terms of different chemical composition of valve steels. level of 20 mass%, is not high enough for selective oxida-

It has been found, namely, that during oxidation of the tion of chromium and formation of Cr2O3 oxide [1–3, 15]. X33CrNiMn23-8 steel containing the highest chromium Thus, heterogeneous scales are formed, which protec- content, the oxidation product forming the scale is build tive properties are much worse than that assured by chro- mainly from highly protective Cr2O3 oxide [15]. In this mium oxide. Consequently, yttrium electrochemically de- ­situation, yttrium addition can improve only slightly very posited on the surface of steel samples with low chromium good oxidation properties of Cr2O3 oxide [1–3] and conse- content, promoting chromia oxide formation instead of 118 Z. Grzesik et al., The Influence of Yttrium on High Temperature Oxidation

Fig. 8: The results of thermal cycles of the X50CrMnNiNbN21-9 Fig. 6: The collective plot, summarizing the results of the isothermal steel, covered electrochemically with yttrium on the background oxidation of four studied steels covered electrochemically with of analogous results obtained with the same steel without yttrium yttrium. addition.

Fig. 7: The results of thermal cycles of the X33CrNiMn23-8 steel, Fig. 9: The results of thermal cycles of the X53CrMnNiN20-8 steel, covered electrochemically with yttrium on the background of covered electrochemically with yttrium on the background of analogous results obtained with the same steel without yttrium analogous results obtained with the same steel without yttrium addition. addition. heterogeneous spinel scales, considerably increases the of the X33CrNiMn23-8 steel without yttrium above about oxidation resistance of investigated steels. The positive 150 cycles gradually starts to lose its mass indicating that effect of yttrium addition in the case of all investigated in this situation the scale adherence is worse [26]. From steels is illustrated once more in collective plot presented X-ray diffraction patterns presented in Fig. 11 it follows in Fig. 6. that in the case of discussed steel with yttrium, the scale

Analogous situation is also observed in the case of is build mainly of highly protective Cr2O3 oxide, as a result ­oxidation of steels under thermal cycle conditions. From of selective oxidation of chromium. On the surface of Figures 7–10 it follows, namely, that under these condi- this layer have been found. Chromia scale is tions considerable influence of yttrium on the scale adher- growing by the outward diffusion of cations and this is ence is clearly visible. From Fig. 7 it follows that in the the reason why the adherence of the scale to the substrate case of the X33CrNiMn23-8 steel with the highest chro- is insufficiently good [23]. On the other hand, the same mium concentration, the mass of the oxidized sample scale on the surface of chromia formers containing reac- containing yttrium does not virtually change with a tive element additions is growing by different mechanism, number of thermal cycles. On the other hand, the sample i.e. by the inward diffusion of oxygen [23]. Yttrium is Z. Grzesik et al., The Influence of Yttrium on High Temperature Oxidation 119

Fig. 10: The results of thermal cycles of the X55CrMnNiN20-8 Fig. 12: The collective plot, summarizing the results of thermal steel, covered electrochemically with yttrium on the background cycles of four studied steels covered electrochemically with yttrium. of analogous results obtained with the same steel without yttrium addition. the positive influence of yttrium on the adherence of scales to these steels is much higher than that observed for the X33CrNiMn23-8 steel. For example, it can be seen in Fig. 8 that for the X50CrMnNiNbN21-9 steel doped with yttrium, the scale starts to crack and spell off from the surface after about 450 cycles, while in the case of the same steel without yttrium the spallation is observed after about 100 cycles. In the case of two other steels with lower chromium content covered by yttrium, the situation is analogous but the scale spallation starts to be observed after 400 cycles for the X53CrMnNiN20-8 steel and after about 300 cycles for the X55CrMnNiN20-8 steel. These dif- ferences are summarized in Fig. 12. The lower oxidation resistance of discussed uncov- ered three steels with comparison to that observed in the case of the X33CrNiMn23-8 steel is a result of lower chro- Fig. 11: X-ray diffraction patterns of the X33CrNiMn23-8 steel surface without yttrium addition after 500 thermal cycles. mium concentration in these steels, which is not high enough for selective oxidation of chromium. Consequently, heterogeneous scales are formed, composed mainly of

­segregated along the grain boundaries blocking, as a con- spinel phases with the Fe3O4 and Fe2O3 oxides on the sequence the outward diffusion of cations. Segregating surface (Fig. 13), which adherence to the substrate is along the grain boundaries yttrium atoms form together much worse than that of Cr2O3 oxide [1–3]. As already men- with Cr2O3, the yttrium-chromium , YCrO3. Pre- tioned, yttrium addition promotes chromia oxide forma- dominant defects in this compound are located in anion tion instead of heterogeneous spinel scale, considerably sublattice, forming thus the ways for inward diffusion of increasing the scale adherence. Nevertheless, it can be oxygen. This process proceeds slower than the outward noted that addition of yttrium is not enough high to stop diffusion of cations in the case of pure Cr2O3, which results completely the growth of iron oxides and other spinel in better protective properties of scales doped with yttrium phases. Thus, after initial of oxidation, low adher- as compared to pure chromia scale [23]. Much better scale ent oxide phases are formed and spallation process is adherence of yttrium doped chromia scale results, in turn, ­initiated. To explain differences in degradation rate of from the compensation of consumption zone by steels with low chromium content, in addition to chro-

Cr2O3 molecules, forming at the metal-scale interface. mium concentration in the steels also the nickel and In the case of three remaining steels with lower chro- ­ contents should be considered. From litera- mium concentration (Figs. 7–10), it has been found that ture data it follows, namely, that the presence of nickel in 120 Z. Grzesik et al., The Influence of Yttrium on High Temperature Oxidation

gated materials. This positive influence of yttrium is par- ticularly high in the case of steels with low chromium content. However, in spite of this effect, the highest resis- tance against oxidation during long period of reaction was observed only for the X33CrNiMn23-8 steel with the highest chromium concentration, covered by yttrium (see Fig. 12). This situation is a result of the formation of highly protective and well adherent to the substrate chromium oxide. In the case of three remaining steels with lower chromium concentration, after long period of oxidation heterogeneous scales start to be formed, build mainly

from iron Fe3O4 and Fe2O3 oxides, which protective proper- ties and adherence to the surface of steels is much worse Fig. 13: X-ray diffraction patterns of the X50CrMnNiNbN21-9 steel than that observed in the case of chromium oxide. The surface without yttrium addition after 500 thermal cycles. final conclusion is that engine valves should be produced in the future only from X33CrNiMn23-8 steel, containing chromium steels favors oxidation resistance of chromium small amount of yttrium. steels [27], but that of manganese shows worsening effect [1]. Con­sidering these facts and taking into account Funding: This work was supported by The National Centre ­chemical composition of studied steels (see Table 1), it can for Research and Development in Poland no. N R15 0013 be stated that slightly faster degradation rate of the 06/2009. ­X55CrMnNiN20-8 steel from those of X50CrMnNiNbN21-9 and X53CrMnNiN20-8 steels can be a result of compromise between manganese and nickel contents in these materi- References als. This effect is clearly visible not only during oxidation of valve steels under thermal cycle conditions, where [1] S. Mrowec and T. Werber, Scaling-resistant Iron- Alloys in mass losses for the X55CrMnNiN20-8 steel are the highest Modern Scaling-Resistant Materials, National Bureau of (see Figs. 7–10) but also during oxidation under isother- Standards and National Science Foundation, Washington D.C., 1982. mal conditions, where mass gains for the same steel are [2] P. Kofstad, Development of Stresses and Strains, the highest (see Figs. 2–5). Because yttrium is presented Non-protective Scales, Boundary Reactions in High only in the surface layer of investigated steel samples, Temperature Corrosion, Elsevier Applied Science, London and the scale spallation denotes gradual decrease of yttrium New York, 1988. amount in the oxidized material and consequently, the [3] N. Birks, G.H. Meier and F.S. Pettit, Introduction to the High dramatic reduction of adherence of the scale to the sub- Temperature Oxidation of , Cambridge University Press, 2009. strate. As a consequence, the oxidation kinetic of yttrium [4] D. Naumienko, L. Singheiser and W.J. Quadakkers, Proceedings containing sample starts to be similar to that observed for of the EFC Workshop, Frankfurt/Main, (1999), pp. 287–306. samples without yttrium addition. It may be then stated [5] M. Beukenberg, Proceedings of the Turbine Forum 2006, in the conclusion that in spite of high positive influence Advances Coatings for High Temperatures, Nice, April 26–28, of yttrium on scale adherence observed in the case of (2006). [6] Z. Grzesik, G. Smola, K. Adamaszek, Z. Jurasz and S. Mrowec, three steels with the lower chromium content, still the Oxid. Metals, 80 (2013) 147–159. X33CrNiMn23-8 steel has the best scale adherence during [7] A.S.M.A. Haseeb, M.A. Fazal, M.I. Jahirul and H.H. Masjuki, long period of oxidation. Fuel, 90 (2011) 922–931. [8] Z.W. Yu and X.L. Xu, Engineering Failure Analysis, 13 (2006) 673–682. [9] C.G. Scott, A.T. Riga and H. Hong, Wear, 181–183 (1995) 4 Conclusions 485–494. [10] D. Schlager, C. Theiler and H. Kohn, Mater. Corros., 53 (2002) From the results of oxidation rate measurements, ob- 103–110. tained under isothermal and thermal cycle conditions, it [11] M. Velliangiri and A.S. Krishnan, J. Energy Technol. Policy, 2 follows that the presence of small amounts of yttrium in (2012) 42–53. all studied steels improves protective properties of scales [12] P. Lawrence, P.K. Mathews and B. Deepanraj, J. Sci. Ind. Res., 70 (2011) 789–794. as well as the scale adherence to the surface of investi- Z. Grzesik et al., The Influence of Yttrium on High Temperature Oxidation 121

[13] T. Hejwowski, Vacuum, 80 (2006) 1386–1390. [21] B.A. Pint, in Developments in High-temperature Corrosion and [14] Z. Grzesik, G. Smola, K. Adamaszek, Z. Jurasz and S. Mrowec, Protection of Materials, Woodhead Publishing in Materials, Corr. Sci., 77 (2013) 369–374. Cambridge, England (2008), pp. 398–432. [15] K. Adamaszek, Z. Jurasz, L. Swadzba, Z. Grzesik and S. Mrowec, [22] J. Jedliński, Z. Żurek, M. Homa, G. Smoła and J. Camra, Def. Diff. High Temp. Mater. Processes (London), 26 (2007) 115–122. Forum, 289–292 (2009) 541–555. [16] Z. Jurasz, K. Adamaszek, R. Janik, Z. Grzesik and S. Mrowec, [23] K. Przybylski, Mater. Sci. Eng., A121 (1989) 509–517. J. State Electrochem., 13 (2009) 1709–1714. [24] S. Mrowec and Z. Grzesik, J. Phys. Chem. , 65 (2004) [17] Z. Grzesik, M. Migdalska and S. Mrowec, High Temp. Mater. 1651–1657. Processes (London), 29 (2010) 203–214. [25] Z. Grzesik, Z. Jurasz, K. Adamaszek and S. Mrowec, High Temp. [18] Z. Grzesik, Z. Jurasz, K. Adamaszek and S. Mrowec, High Mater. Processes (London), 31 (2012) 775–779. Temp. Mater. Processes (London), 31 (2012) 775–779. [26] Z. Grzesik, E. Leyko, Z. Jurasz, K. Adamaszek and S. Mrowec, [19] A. Galerie, in Shreir’s Corrosion, 4th Edition, Elsevier Ltd., Proceedings of the 2nd International Conference Corrosion Amsterdam, The Netherland, (2010), vol. 1, pp. 583–645. and Material Protection, 19–22 April, Prague, Czech Republic [20] P.F. Tortorelli and M.P. Brady, in Shreir’s Corrosion, 4th Edition, (2010). Elsevier Ltd., Amsterdam, The Netherland, (2010), vol. 1, [27] G. Wood, M. Hobby and B. Vaszko, J. Iron Steel Inst. 202 (1964) pp. 541–582. 685–691.