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 oxide 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, cerium 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 chromium-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 thermal expansion 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 Ceramics, 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 diamond 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 ion 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 nitrate. 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 ions/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].