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Investigation of the effect of tantalum to rhenium ratio on the high-temperature corrosion resistance of ZhS-32 creep-resisting nickel alloy.

One of the most unfavourable types of damage in the working blades, resulting in a decrease of the reliability and economic parameters of gas turbines, is associated with the corrosive effect of sol and gas products of combustion of fuel arising in the flow section of the turbine. The stability of the external surface is the non-variable condition of the surface reliability of the components of gas turbine engines (GTE). Although various coatings are deposited on the blades of the gas turbine engines, the resistance of the alloy-base to high temperature gas corrosion is of considerable importance [1, 2].

The resistance of gas corrosion depends primarily on the chemical composition of the alloy, temperature and structure. Therefore, every alloy has individual characteristics of high temperature corrosion resistance (HTCR) [2-7]. It should be mentioned that the oxidation rate is an order of magnitude lower than the rate of high temperature corrosion at the same temperature. This indicates that the main process, controlling the service life of components of the cross-section of the gas turbine engine at a constant temperature-force conditions is the high-temperature corrosion resistance, resulting in the changes of the geometry of the blade system and premature failure of the latter [4].

In the surface systems of the D 336-1T and D336-2T type, developed on the basis of the D36 and D436T engines, the main components are produced from conventional aviation materials. On the one hand, this approach results in the high working parameters of the systems whose efficiency is higher than the parameters of the most economical turbines, constructed for gas-pumping stations. On the other hand, special features of the operation of gas turbine systems results in a number of problems associated with the effect of the products of fuel combustion on the components of the hot section, especially the working blades of the turbine.

Recently, industrial creep-resisting cast nickel alloy ZhS-32, containing 4.5-5.5% chromium with no corrosion resistance, has been used for producing the working blades of high-pressure turbines. The creep resistance of the surface of the blades is increased by depositing heat-resisting coatings, ensuring the required service life.

The ZhS-32VI alloy, designed for producing the components of gas turbine systems by the method of directional (mono) crystallisation, is characterised by relatively low high-temperature corrosion resistance. However, as regards the resistance to high temperature gas corrosion resistance, this alloy does not satisfy the requirements on the nickel creep-resisting alloys designed for operation in corrosive media.

The main task of the investigations was the evaluation of the effect of the ratio of the concentration of tantalum and rhenium in the ZhS-32 alloy on the rate of high temperature gas corrosion in order to optimise this ratio for improving the parameters of high-temperature corrosion resistance.

For this purpose, the standard charge of the ZhS-32 alloy was used for casting single crystal specimens of the simulation compositions 15 by the method of directional (mono) crystallisation in equipment UVNK-8P (in accordance with standard technology) with the solidification rate of 10 mm/min. The simulation compositions were characterised by different ratios of the concentrations of tantalum and rhenium in the investigated range (from 1 to 6). The specimens were cast in special ceramic moulds with the starting seed crystals, made of the Ni-W binary alloy, placed in the mould.

The cast single crystal blanks of the experimental compositions No. 1-5 and of the laser alloy were subjected to 100% inspection of the macrostructure by etching in a solution containing ferric chloride (700-800 g), hydrochloric acid (120-150 [cm.sup.3]) and water (up to 1 d [m.sup.3]), and also to inspection of the crystallographic orientation in DRON-3M equipment.

The chemical analysis of the experimental melts was carried out by the standard methods, in accordance with the requirements of TU 14-1689-73 and OST 1.90127-85. The spectral chemical analysis was carried out using ARL-4460 optical emission equipment (the quantometer for simultaneous multichannel analysis). The chemical composition of the melts of the investigated alloys is presented in Table 1.

Table 2 gives the ratio of tantalum and rhenium in the investigated alloys, and also the heat treatment conditions. The first stage of heat treatment [t.sup.1.sub.hom]--high temperature homogenising--for each experimental composition No. 1-5 was studied individually on the basis of the results obtained by the method of differential thermal analysis in the course of the investigation of the temperature ranges of phase transformations.

The second stage of thermal treatment (low temperature homogenising) of all the experimental compositions was carried out at tIIhom = 1050[degrees]C, taking into account the technological temperature of the deposition of the protective coating. The heat treatment of ZhS-32 alloy was carried out in accordance with the standard conditions. The tests of high-temperature corrosion resistance of the specimens of the experimental compositions No. 1-5 in comparison with the ZhS-32 alloy, were carried out after heat treatment.

The experimental investigations of the high-temperature corrosion resistance of the specimens of the experimental compositions No. 1-5 with the directional (mono) [001] structure, in comparison with the ZhS-32 alloy, was carried out in synthetic sol at temperatures of 800 and 850[degrees]C using the procedure, developed at the I.I. Polzunov TsKTI Institute [1, 5-7].

The corrosion tests were carried out using the standard cylindrical specimens with the diameter of 10 mm and 12 mm long, which were deposited, after preliminary degreasing, measurements and weighing with the accuracy of [+or -] 0.0005g, with 15 mg/[cm.sup.2] of synthetic sol, simulating the products of combustion of the gas turbine fuel with the following composition, wt.%: [Na.sub.2]S[O.sub.4]--66.2; [Fe.sub.2][O.sub.3] 20.4; NiO 8.3; CaO 3.3; [V.sub.2][O.sub.5] 1.8.

The specimens of the investigated compositions were held in a furnace on a platform made of a heat-resisting materials in air. The tests at both temperatures lasted 600 h.

The corrosion products were removed by the method of hydrogen reduction of scale [5]. After the corrosion tests, the specimens were investigated by the methods of gravimetric, metallographic and x-ray diffraction phase analysis. The high-temperature corrosion resistance of the experimental compositions was estimated on the basis of the mean corrosion rate [v.sub.q].

To evaluate the effect of the tantalum and rhenium ratio on the parameters of the high-temperature corrosion resistance of the ZhS-32 alloy, part of rhenium in the alloy was replaced with a step of 0.5 wt.%, in the concentration range 1.5-4.0 wt.%, by tantalum with a step of 1 wt.% in the concentration range 4-9%. Thus, in the experimental compositions, we completely cover the range of the tantalum to rhenium ratio of 1-6, and the concentration of the remaining alloying elements did not change (Table 1).

Figure 1 shows the dependence of the mean corrosion rate [v.sub.q] on the ratio of the concentration of tantalum and rhenium in the investigated range, in comparison with ZhS-32 alloy.

The analysis of the experimental results shows that the corrosion rate of the specimens of the experimental alloys No. 1-5 decreases in the entire range of the investigated ratios of the tantalum and rhenium concentrations from 1 to 6 at both test temperatures. This is caused by the changes of the chemical and phase composition of the products of high-temperature corrosion in comparison with ZhS-32 alloy of the standard chemical composition.

The optimum ratio of tantalum to rhenium in the investigated range, resulting in the lowest rate of high-temperature corrosion in synthetic ash at both values of the test temperature, corresponding to the experimental compositions No. 5 in which it was equal to 6 (Table 2, Fig. 1). In comparison with the ZhS-32 alloy, the rate of high-temperature corrosion of the experimental compositions No. 5 decreased 2.8 times at a temperature of 800[degrees]C and by a factor of 2.5 at 850[degrees]C.

The investigation of the phase composition of the corrosion products by x-ray diffraction analysis shows that the composition of the products of the relatively complicated and the products contain a relatively large number of different spinels, oxide and sulphide phases since the investigated alloys are alloyed with a large number of elements (Table 3). In comparison with the initial ZhS-32 alloy and experimental alloys No. 1 and 2, the alloys No. 3-5 characterised by a relatively dense and resistant film of corrosion products. The phase composition of the corrosion products contained a relatively large amount of the spinel, based on the chromium oxide, in comparison with the spinel based on aluminium oxide.

[FIGURE 1 OMITTED]

The x-ray diffraction analysis on the composition of the surface layer of the corrosion products, formed on the specimens of the ZhS-32 alloy after testing in synthetic sol at both temperatures show that the main phase components are the chromium spinel NiO x [Cr.sub.2][O.sub.3] and the nickel oxide NiO and also (in small quantities), the nickel oxides Ni[Al.sub.2][O.sub.4], Ni[Cr.sub.2][O.sub.4], chromium oxide [Cr.sub.2][O.sub.3], tantalum oxide [Ta.sub.2][O.sub.5] and nickel sulphide [Ni.sub.3][S.sub.2].

Comparative analysis of the results shows that with the increase of the tantalum and rhenium ratio from 1 to 6, the phase composition of the products of high-temperature corrosion of the experimental alloys No. 1-5 is characterised by a gradual decrease of the concentration of the nickel oxides NiO, Ni[Al.sub.2][O.sub.4] and the nickel sulphide [Ni.sub.3][S.sub.2]. This is accompanied by the increase of the concentration of the chromium spinel NiO x [Cr.sub.2][O.sub.3], the chromium oxides [Cr.sub.2][O.sub.3] and tantalum oxide [Ta.sub.2][O.sub.5], resulting in a decrease of the rate of high-temperature corrosion in the conditions of sulphide-oxide corrosion, in comparison with ZhS-32 alloy.

In the experimental alloy No. 5 with the tantalum and rhenium ratio of 6, which showed the lowest corrosion rate, in contrast to the phase composition of the progress of corrosion of ZhS-32 alloy, the amount of the phase components Ni x [Cr.sub.2][O.sub.3] increase of 1.8-2.0 times; [Cr.sub.2][O.sub.3] increases 2.2-3.0 times; [Ta.sub.2][O.sub.5] 2-3 times. This is accompanied by a decrease of the amount of the phase components, NiO up to 4 times, Ni[Al.sub.2][O.sub.4] 3-5 times, [Ni.sub.3][S.sub.2] 2-3 times.

In the investigation of the phase composition of the corrosion products, formed on the surface of the specimens with a different ratio of tantalum to rhenium, the following phases were additionally identified: in ZhS-32 alloy and experimental compositions No. 1, 2--[Cr.sub.2][S.sub.3], Ni[Cr.sub.2][O.sub.4], Co[S.sub.2], in the experimental compositions 3-5 these phase constituents were not detected. Table 3 shows that in the phase composition of the products of high-temperature corrosion of the ZhS-32 alloy, in which the ratio of tantalum to titanium is 1 (Table 2), there is a large amount of the phase components which do not differ in the protective properties (NiO, Ni[Al.sub.2][O.sub.4], Ni[Cr.sub.2]O, [Ni.sub.3][S.sub.2]) and a small amount of the compounds with the protective properties (chromium spinel NiO x [Cr.sub.2][O.sub.3], chromium oxides [Cr.sub.2][O.sub.3] and tantalum oxide [Ta.sub.2][O.sub.5]).

At the same time, the layer of the corrosion products, formed on the surface of the specimens of all experimental compositions in which the ratio of tantalum to rhenium is higher than in the ZhS-32 alloy, the phase composition of the products of high-temperature corrosion contains a large amount of the compounds, differing in the protective properties--chromium spinel Ni x [Cr.sub.2][O.sub.3], chromium oxide [Cr.sub.2][O.sub.3] and tantalum oxide [Ta.sub.2][O.sub.5], tantalum sulphides Ta[S.sub.2], and at considerably smaller amount of the compounds (or no compounds the tour) which do not differ in the protective properties--oxides NiO, Ni[Al.sub.2][O.sub.4], and sulphides [Cr.sub.2][S.sub.3], Ni[Cr.sub.2][S.sub.4], Co[S.sub.2], in comparison with the ZhS-32 alloy.

Thus, the increase of the Ta/Re ratio increases the concentration of the thermodynamically more stable compound [Cr.sub.2][O.sub.3], [Ta.sub.2][O.sub.4], Ta[S.sub.2] in the phase composition of the corrosion products of the experimental alloys No. 1-5, indicating a decrease of the rate of the diffusion processes with the increase of the tantalum content of the ZhS-32 alloy. This results in a decrease of the rate of high-temperature corrosion in all experimental alloys No. 1-5, in comparison with the ZhS-32 alloy of the standard chemical composition (Fig. 1).

Comparative analysis of the data shows that the increase of the tantalum content to 9 wt.% (composition No. 5) increases the concentration of the chromium oxide [Cr.sub.2][O.sub.3] and tantalum oxide [Ta.sub.2][O.sub.5] on the oxide-metal interface, reducing the rate of formation of the compounds which do not differ in the protective properties (Ni[Cr.sub.2][O.sub.4], Ni[Al.sub.2][O.sub.4], NiO). In all likelihood, the [Ta.sub.2][O.sub.5] oxide bonds NiO preventing the formation of the compounds of the type NiMo[O.sub.4] and Ni[WO.sub.4].

The comparison of the results shows that the stabilisation of the complex oxides takes place as a result of the implantation of tantalum or the dissolution of tantalum oxide [Ta.sub.2][O.sub.5] leading to the formation of a more complicated form of structural formulae [(Cr,Ta).sub.2][O.sub.3] and [Ni(Cr,Ta).sub.2][O.sub.4]. These mixed oxides are thermodynamically more stable than form as a result of their capacity to produce solid solutions with the chromium oxides [Cr.sub.2][O.sub.3] and tantalum oxide [Ta.sub.2][O.sub.5].

Analysis of the phase composition of the products of high-temperature corrosion shows that after testing it the two temperatures, the surface of the specimens of the experimental compositions No. 3-5 is characterised by the formation of a scale layer, consisting of a relatively large amount of the oxides [Cr.sub.2][O.sub.3] and [Ta.sub.2][O.sub.5] at a considerably higher concentration of the chromium spinel NiO * [Cr.sub.2][O.sub.3] and a lower concentration of the oxides NiO and nickel sulphide [Ni.sub.3][S.sub.2]. It should be mentioned that for all investigated alloys, the phase composition of the corrosion products contain low concentrations of the sulphides with the mixed base [(Cr, Ta).sub.3][S.sub.4].

Thus, the increase of the high-temperature corrosion resistance of ZhS-32 alloy is achieved by reducing the rate of diffusion processes and also changing the chemical and phase composition of the surface layer of the products of corrosion is a result of the capacity of tantalum for the formation of thermodynamically stable oxides and sulphides with sulphur and oxygen.

Conclusions

The highest parameters of high-temperature corrosion resistance in synthetic sol of the gas turbine fuel at the temperatures of 800 and 850[degrees]C for 600 h are recorded in the ZhS32 alloy at a mass ratio of the tantalum and rhenium Ta/Re = 6 (experimental compositions No. 5), when the tantalum content is 9 wt.% and the rhenium content is 1.5 wt.%.

The increase of the tantalum/rhenium ratio in the ZhS-32 in the range from 1 to 6 increases the concentration in the surface layer of the scale of the chromium spinel NiO * [Cr.sub.2][O.sub.3], chromium oxides [Cr.sub.2][O.sub.3] and tantalum oxide [Ta.sub.2][O.sub.5], which complicates the interaction of nickel with sulphur and oxygen, suppressing the mechanism of formation in the zone of the oxide NiO and nickel sulphides [Ni.sub.3][S.sub.2].

References

[1.] Nikitin V.I., Corrosion and protection of gas turbine blades, Mashinostroenie, Leningrad, 1987.

[2.] Shalin R.E., et al., Single crystals of nickel creep resisting alloys, Mashinostroenie, Moscow, 1997.

[3.] Paton B.E., et al., Creep strength of cast nickel alloys and oxidation protection, Naukova Dumka, Kiev, 1987.

[4.] Koval' A.D., et al., Scientific fundamentals of the alloying of creep resisting nickel alloys, resistant to high-temperature corrosion, preprint KIEV UMK VO, ZMI, 1990.

[5.] Nikitin V.I., et al., effect of the composition of nickel alloys on their corrosion resistance in the sol of gas turbine fuel, Trudy TsKTI im. I.I. Polzunova, No. 158m Leningrad, 1978, 71-75.

[6.] Andrienko A.G., et al., Novi Mateiali i Tekhnologii v Metallurgii ta Mashinobuduvanni, 2005, No. 1, 61-64.

[7.] Gaiduk S.V., et al., Vestn. Dvigatelestroeniya, 2007, No. 1, 150-154.

Submitted 7.10.2009

A.G. Andrienko, S.V. Gaiduk, V.V. Kononov and I.S. Malashenko

Zaporozh'e National Technical University

E.O. Paton Electric Welding Institute, Kiev
Table 1. Chemical composition of the investigated alloys

            Mass fraction of alloying elements, %
 Alloy
variant    C     Cr    Co     w    Mo    Al    Nb

ZhS-32    0.15   4.9   9.3   8.2   1.1   5.8   1.6
   1      0.14   5.1   9.1   8.0   1.0   5.9   1.4
   2      0.15   4.9   9.0   8.2   0.9   6.1   1.5
   3      0.14   4.8   8.9   8.1   1.0   6.0   1.6
   4      0.15   5.0   9.0   8.0   0.9   6.1   1.5
   5      0.16   5.2   9.1   7.9   1.1   6.2   1.6

          Mass fraction of alloying elements, %
 Alloy
variant   Ta    Re     Zr      B       Ni

ZhS-32    4    4.0    0.05   0.015   Balance
   1      5    3.5    0.05   0.015
   2      6    3.0.   0.05   0.015
   3      7    2.5    0.05   0.015
   4      8    2.0    0.05   0.015
   5      9    1.5    0.05   0.015

Table 2. Heat treatment conditions for the investigated alloys

Alloy variants   Ta/Re   [[tau].sup.1.sub.hom]

    ZhS-32       1.00            1280
      1          1.43            1275
      2          2.00            1270
      3          2.80            1265
      4          4.00            1260
      5          6.00            1255

1. Holding time in all cases was up to 4 h. 2. Cooling between
stages was conducted in air.

Table 3. The phase composition of the corrosion products of the
investigated alloys, wt.%

 Alloy    NiO-[Cr.sub.2]   NiO   [Cr.sub.2]   Ni[Al.sub.2]
variant     [O.sub.3]            [O.sub.3]     [O.sub.4]

                  Test temperature 800 [degrees]C

ZhS-32          15         25         7           10
   1            19         21        12            8
   2            20         18        17            6
   3            24         13        22            6
   4            26         10        24            4
   5            27          6        25            3

                 Test temperature 850 [degrees]C

ZhS-32          16         28         9           11
   1            21         23        13            6
   2            26         18        15            5
   3            29         15        18            3
   4            31         10        19            3
   5            32          7        20            2

 Alloy    [Cr.sub.2]   Ni[Cr.sub.2]   [Ta.sub.2]   Ta[S.sub.2]
variant   [S.sub.3]     [O.sub.4]     [O.sub.5]

                   Test temperature 800 [degrees]C

ZhS-32        8             9              9            3
   1          6             5             14            4
   2          4             2             19            6
   3          --            --            24            6
   4          --            --            26            6
   5          --            --            29            7

                  Test temperature 850 [degrees]C

ZhS-32        9             --            11            4
   1          6             --            15            6
   2          6             --            17            6
   3          --            --            20            9
   4          --            --            22           10
   5          --            --            23           11

 Alloy    Co[S.sub.2]   [Ni.sub.3][S.sub.2]
variant

          Test temperature 800 [degrees]C

ZhS-32         4                10
   1           3                 8
   2           1                 7
   3          --                 5
   4          --                 4
   5          --                 3

          Test temperature 850 [degrees]C

ZhS-32        --                12
   1          --                10
   2          --                 7
   3          --                 6
   4          --                 5
   5          --                 5
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Title Annotation:GENERAL PROBLEMS OF METALLURGY
Author:Andrienko, A.G.; Gaiduk, S.V.; Kononov, V.V.; Malashenko, I.S.
Publication:Advances in Electrometallurgy
Date:Oct 1, 2009
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