NaCl-induced hot corrosion of a titanium aluminide alloy

Materials Science and Engineering

NaCl-induced

A192/193

(1995) 994-1000

hot corrosion of a titanium aluminide alloy Z. Yao, M. Marek

School of Materials Science and Engineering,

Georgia Institute of Technology, Atlanta, GA 30332, USA

Abstract NaCl-induced hot corrosion of a TiAl-based intermetallic, Ti-47at.%Al-2at.%Nb-2at.%Mn + 7 vol.% TiB,, was studied. Sodium chloride was vapor deposited on the surface of the specimens. The reaction of the metal, oxygen and deposited sodium chloride caused the formation of a non-protective scale and accelerated the oxidation of the alloy. Pits and possible grain boundary attack were observed on the corroded surfaces. Oxygen-free exposure showed no corrosion of the TiAl alloy due to NaCl at temperatures below and above the melting temperature of NaCl. Thermodynamic calculations were carried out to predict the possible corrosion products. The corrosion mechanisms are discussed. Keywords:

Sodium; Chlorine;

Corrosion;

Titanium; Aluminium;

Alloys

1. Introduction

Intermetallic compounds, such as TiAl, are considered to be potential replacements for some superalloys for parts of turbine engines, jet engines, diesel engines, etc. In addition to their low density, intermetallic compounds possess a number of excellent high temperature properties, including creep resistance, fatigue resistance and static strength retention. TiAlbased alloys also exhibit good oxidation resistance due to their high aluminum content. However, hot corrosion occurs when salt deposits accumulate on the surface of the alloy. In the past, the study of TiAl intermetallic compounds has concentrated mainly on the mechanical properties. Very little work has been carried out on the environmental attack of these types of materials at elevated temperatures. The hot corrosion of various superalloys [ 1,2] and other types of alloy [ 31 has been reviewed by several workers, and the mechanisms of hot corrosion have been explained by Rapp [4]. The hot corrosion of intermetallic compounds has been studied by a few researchers [5-71, but no well-developed corrosion mechanisms have been published. NaCl is one of the major components of salt deposits that accumulate on the alloy surface in the field. It has been found condensed on blades of turbine 092 l-5093/95/$9.50

0 1995 - Elsevier Science S.A. All rights reserved

SSlIf OY21-5093(95)03345-5

engines by the researchers at Rolls-Royce. The objective of this research is to determine the mechanisms of the NaCl-induced hot corrosion of a TiAl alloy.

2. Experimental

procedure

2.1. Materials

The material studied was a y-TiAl-based intermetallic: Ti-47at.%Al-2at.%Nb-2at.%Mn + 7 vol.% TiB,. The material had been hot-isostatic-pressed (HIP) at 2300 “F at a pressure of 25 ksi for 4 h, and heat treated at 1850 “F for 50 h. The material has a fine lamellar microstructure consisting of y-T% and a*Ti,Al formed by a eutectoid reaction. 2,2. Specimen preparation The specimens were cut and sliced with a diamond saw, and were then ground using silicon carbide abrasive papers through 600 grit. The size of the specimens was about 0.5 mm X 5 mm X 11 mm. The dimensions of the specimens were measured with a micrometer with an accuracy of 0.01 mm. Following grinding, the specimens were degreased in isopropyl alcohol using an ultrasonic cleaner for 5 min, and dried

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in air. The weight of each specimen was determined using an electronic balance with a resolution of 0.01 mg.

meter with a curved position incident beam angle was 5”.

2.3. NaCl vapor deposition

3. Results

NaCl can be coated on the specimen surface by immersion in saturated salt water and drying [8,9]. When using this technique it is difficult to provide a uniform layer of salt on the specimen surface. In this research, NaCl was vapor deposited onto the specimen surface. NaCl powder was placed in an alumina crucible and melted in a custom designed furnace at about 850 “C. The melting point of NaCl is 801 “C. When all the salt was melted and the vapor stream was stable, the specimen was placed in the center of the stream at a constant distance above the crucible for 30 s, and then turned over and held for another 30 s. A thin layer of very fine salt crystals condensed on the surface of the specimen. The amount of NaCl deposited on the specimen surface was approximately 2.3 x 10e4 mg mmm2.

3.1. Weight change

sensitive detector.

The

Kinetic curves of oxidation of the alloy with and without deposited NaCl were generated. Fig. 1 shows the weight change of specimens exposed at 650, 750 and 850 “C. The weight gain of specimens with NaCl was higher than for those without NaCl throughout the exposure. The rate of weight gain of specimens coated with NaCl was higher during the first 5 h. Corrosion of the TiAl alloy due to the presence of NaCl was observed at temperatures as low as 400 “C. Kinetic oxidation curves for temperatures below 650 “C are not shown here, because the weight gains were very small and it was difficult to obtain accurate data. Results of the weight gain vs. exposure temperature are plotted in Fig. 2.

2.4. Oxidation in air with and without salt deposit 7

A Lindberg tube furnace was used to oxidize the alloy in air. The specimens were placed on a ceramic boat which was then inserted into the hot zone of the furnace. Air was flowed through the quartz tube throughout the exposure. The exposure temperatures were 350, 450, 550, 650, 750 and 850°C. After exposure, the specimens were air cooled. The pieces of spalled scale were carefully collected and weighed together with the remaining specimen. Fresh specimens were used for each exposure.

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2.5. Oxygen-free exposure NaCl-coated specimens were heated to 550, 650, 750 and 850 “C for 2 h in a vacuum furnace at a pressure lower than 10 ~’ Torr. 2.4. Characterization of the degradation Weights of the specimens before and after exposure were measured using a Mettler analytical electronic balance with a resolution of 0.01 mg. The surface of the alloy before and after exposure and the corrosion products were analyzed using an automatic powder diffractometer (PW 1800) at a time constant of 10 s and a step size of 0.025”. The surface microstructure and phases after oxidation, and the oxidation products, were examined using a Hitachi S-800 scanning electron microscope and an Inel XRG 3000 X-ray diffracto-

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Time, hr Fig. 1. Weight gain vs. exposure time at different temperatures: with NaCl deposits; 0, without NaCl deposits.

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In order to determine whether sodium chloride reacts with metal in the absence of oxygen, NaCIcoated specimens were heated to 550, 650, 7.50 and 850 “C in a vacuum furnace at a vacuum of 10~ i Torr and held for 2 h. No corrosion was observed on any of the samples, except for some residual NaCl deposits on the surface. 3.2. Morphology 3.2.1. initial oxidation stuge A specimen deposited with NaCl was exposed to air at 750 “C for 6 min, examined by scanning electron microscopy (SEM), and then placed back into the furnace and held for another 6 min. The 6 min exposure time included the heating time of the specimen from room temperature to 750°C which was believed to be very short since the specimen was very small and thin. Fig. 3 shows the change in appearance of an NaCl crystal on the surface of the specimen. Before exposure, the NaCl crystal showed flat, smooth surfaces. After the first 6 min of exposure, as shown in Fig. 3(a), many small nodules could be seen on the surface of the crystal. Some small nodules could also be seen on the surface of the specimen adjacent to the NaCl crystal. After another 6 min (total 12 min) of exposure, as shown in Fig. 3(b), the surface of the original NaCl crystal was covered with small rectangular crystal-like particles. The area around the crystal was also covered with small, rectangular-shaped crystals. These are believed to be oxides of titanium and aluminum. The nodule formation is believed to

Fig. 3. Change in appearance of an NaCl crystal on the surface of the TiAl alloy after exposure at 750 “C: (a) exposed for 6 min; (b) exposed for 12 min.

have caused the observed accelerated early weight gain for the specimen with the NaCl deposit. The elemental changes of an NaCl crystal after oxidation were determined using SEM energy dispersive spectroscopy (EDS) analysis. During oxidation,

the percentage of titanium and aluminum increased with time, while sodium and chlorine decreased with time. The appearance and composition change of the deposited NaCl crystal with oxidation time suggest that, during oxidation, the metal ions diffuse outwards through the NaCl crystal and react with oxygen on the surface of the NaCI crystal. The accelerated oxidation in the vicinity of the NaCl crystals is probably due to surface diffusion of NaCl, since both sodium and chlorine were detected in this area. The areas distant from the NaCl crystals did not show accelerated oxidation. The nodule formation resulted in a faster weight gain during the early stages of oxidation. Examination of the morphologies after short time exposure at 650 and X50 “C was also performed. The morphologies of the specimen surface were similar, i.e. the accelerated oxidation occurred at and near the deposited NaCl crystals, and the oxides eventually covered the entire surface. The difference was in the rate of oxidation, which increased with increasing exposure temperature.

Fig. 3 shows a fracture surface of the oxide scale formed on the TiAl alloy exposed at 750 “C for 20 h without an NaCl deposit. The oxide scale was dense and lacked pores. The scale was adherent and did not crack or blister on cooling. In order to examine the cross-section, the oxidized specimen was notched with

a diamond saw. The specimen was then fractured by bending. Some separation of the scale from the metal substrate was caused by bending fracture. Fig. 5 shows the fracture surface of the oxide scale of specimens deposited with NaCl and oxidized at 750 “C for 20 h. The oxide scale was porous and nonadherent, and cracked and blistered on cooling. EDS maps showed that both with and without NaCl deposits the scale had two sublayers of different composition. The inner layer contained mainly Al?O, (corundum), while the outer layer contained mainly TiOz (rutile). The fracture surface of the metal substrate after oxidation exposure is shown in Fig. 6. There is a welldefined layer showing internal attack. The surface of the specimen after descaling showed pitting. After ion milling the surface exhibited evidence of possible grain boundary attack as shown in Fig. 7. Oxygen was detected in this layer, but no oxides were found using an X-ray diffractometer with a position sensitive detector and an incident beam angle of 5”. Some chlorine was detected in this layer, but very little chlorine was detected after about 2 pm of material was removed from the surface by ion milling. It was unclear whether or not there was sodium on the surface. EDS results also showed that aluminum was depleted at the surface next to the oxide scale. This resulted in the formation of a layer in the alloy next to the interface containing a larger amount of T&Al than that in the original alloy. Fig. 8 shows X-ray diffraction results for

Fig. 4. Fracture surface of oxide MAC for a specimen air without NaCl deposits at 750 “C for 20 h.

Fig. 5. Fracture surface of oxide scale for a specimen air with NaCI deposits at 750 “C for 20 h.

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Internal corrosion

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the surface of a specimen before oxidation and after oxidation and removal of the oxide scale. The ratio of the T&Al phase to the TiAl phase was higher for the oxidized specimen. All major Ti,Al diffraction peaks were observed. Only a few are presented in Fig. 8 to show the change in the relative quantity of T&Al with respect to that of the original alloy. Fig. 6. Fracture surface of the substrate oxidation at 750 “C for 72 h.

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Fig. 7. Surface after removal of oxide scale followed by 3 h of ion milling. The specimen with NaCl deposits was oxidized at 750 “C for 12h.

4. Discussion The faster weight gains of the salt-coated specimens in the first few hours of exposure are believed to be caused by reactions involving metal, sodium chloride and oxygen, which prevent the formation of a protective oxide scale. After longer exposures, the scales formed in the presence of NaCl were thicker than the scales formed without NaCl, but porous. Therefore the difference in the rate of weight gain was smaller than in the early stages of oxidation. It is unclear why the weight gain is higher for specimens without an NaCl coating after 70 h of exposure at 850 “C. For the same exposure time, the weight gain of the TiAl alloy coated with NaCl increases with increasing temperature, as shown in Fig. 2. The slope of the curve is highest between 650 and 750 “C. Below 350 “C, no corrosion was observed. Above 800 “C, which is the melting temperature of NaCl, the residence time of NaCl on the surface is short compared with that at lower temperatures. This may be the reason for the lower slope of the curves in Fig. 2 above 800 “C. The results of the oxygen-free exposures are consistent with thermodynamic calculations, which predict that no reactions should occur between titanium or aluminum and sodium chloride in the absence of oxygen at temperatures between 550 and 850 “C. The results of the thermodynamic calculations (Fig. 9) show

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Fig. 9. Results of thermodynamic calculation of equilibrium phases formed when aluminum is exposed to air and NaCl at different temperatures. Reactants Al :NaCl : O2 = 1 : 1: 5 (moles).

the amount of equilibrium phases formed for the oxidation conditions used in this study. There is no significant change in the amount of products with changing temperature from below to above the melting temperature of sodium chloride, i.e. 800 “C. Thermodynamic calculations also show that at temperatures between 320 and 950 “C, aluminum and titanium react with sodium chloride and oxygen to form NaAlO,, Na,TiO, and other products. It is believed that, below 800 “C, the following reactions may occur 2NaCl+ Al + 40, -+ 2NaAl0, 2NaCl+ Ti + 20, - Na,TiO,

+ Cl, + Cl,

(1)

(1995) 994-1000

999

As mentioned earlier, the oxide scale contained mainly TiO, rutile in the outside layer and A&O, in the inner layer. According to Taniguchi et al. [ll], the diffusivity of titanium in rutile is faster than that of oxygen, whereas aluminum diffuses at a very low rate in alumina. This results in an outward growth of rutile and an inward growth of alumina. The formation of A&O, next to the matrix alloy resulted in an aluminumdepleted layer. This in turn caused the formation of the next lower aluminide, i.e. Ti,Al. The longer the oxidation exposure, the larger the amount of Ti,Al formed underneath the Al,O, layer. The oxide scales formed in the presence of NaCl deposits were porous (Fig. 4). When specimens were coated with NaCl, the oxides showed an acicular whisker shape and grew outwards at different angles to the surface of the specimen. They increased in diameter until the individual whiskers touched each other and could grow no further. This may be the cause of the observed porosity of the scale. Another possible reason for the porosity of the scale is the formation of volatile phases, such as Cl,, Na,Cl, and NaCl vapor. As shown in Fig. 6, internal corrosion was observed. The corroded layer was not uniform, making it difficult to quantify the depth of internal corrosion. In general, for the same oxidation temperature, the depth of the attack increased with the exposure time. Oxygen and chlorine were detected on this corroded layer. Chlorine had penetrated at least 2 pm, and could be responsible for the attack. According to Hiramatsu et al. [9], chlorine released from NaCl causes accelerated corrosion.

(2)

Below its melting temperature, NaCl does not react with alumina. This was demonstrated by coating a piece of bulk alumina and pre-oxidized TiAl alloy with NaCl. After exposure to air (alumina for 48 h and preoxidized TiAl for 10 min) at temperatures between 550 and 750°C no corrosion of alumina or preoxidized TiAl alloy was observed. At temperatures above the melting temperature of NaCl(80 1 “C), A&O, may dissolve in molten NaCl [9,10]. However, in the case of deposited NaCl crystals, at temperatures above 800 “C, the residence time of NaCl on the specimen is very short. Therefore it was difficult to verify whether either Al,O, or TiO, dissolved in NaCl before the latter evaporated completely. In this study, X-ray powder diffraction analysis was performed on the oxidation products. TiO,, A&O,, TiN and TiB, were the only phases found. On the basis of thermodynamic calculations, NaAlO, is stable at temperatures between 320 and 950 “C. However, only about 1 ppm of the oxides formed can be attributed to this compound, which is too small to be detected by X-ray diffraction.

5. Conclusions

(1) The

presence of NaCl deposits accelerated the oxidation of TiAl at temperatures between 400 and 850 “C. NaCl reacted with oxygen, titanium and aluminum and formed non-protective oxide scales. The porosity of the scales formed at high (2) temperatures when NaCl deposits were present may have resulted from the growth of oxides in the form of whiskers. Alternatively, volatile phases formed during oxidation, such as Cl,, Na,Cl, and NaCl(v), may be responsible for the porosity of the scale. The large thickness of scale may be due to the fast diffusion of oxygen and metal ions through the porous oxide. (3) The degradation of the surface of the alloy in the presence of NaCl was non-uniform. Pitting was observed. The internal corrosion that occurred at high temperatures was probably related to the diffusion of chlorine into the alloy.

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(4) In an oxygen-free environment, no reactions between sodium chloride deposits and titanium or aluminum occurred in the temperature range 550-850 “C. Acknowledgment

The authors thank the Howmet Corporation supply of the alloy. References [l] J. Stringer, Mater. Sci. Technol., 3( 1987) 483-493. [2] P. Hancock, Mater. Sci. Technol., 3 (1987) 540.

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G.H. Meier, Muter. Sci. Eng., A120(1989)

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;:; R.A. Rapp, Corrosion, 42 (10) (1986) 568-577. [51 P.F. Tortorelli and P.S. Bishop, in R.H. Jones and R.E.

Environmental Effects on Advanced Ricker (eds.), Materials,TMS, 1991,pp.91-105. [fd G.Y. Lai, J.J. Barnes and J.E. Barnes, Am. Sot. Mech. Eng., Pap., ASME, New York, June, 1991, pp. l-10. [71 C.L. Zeng, Y. Niu, W.T. Wu, F. Gesmundo and J.T. Guo, Solid State lonics, September ( 1993) 672-677. [81 H. Fujikawa and N. Maruyama, Muter. Sci. Eng., A 120 (1989) 301-306. [91 N. Hiramatsu, Y. Uematsu, T. Tanaka and M. Kinugasa, Muter. Sci. Eng., A 220 (1989) 3 19-328. [lOI Y. Sinata and Y. Nishi, Oxid. A4et., 26 (3-4) (1990) 313-322. 1111 S. Taniguchi, T. Shibata and S. Itoh, Mater. Trans. JIM, 32 (1991) 151-156.