- Email: [email protected]
Gas phase aluminizing of nickel alloys with hydrogen chloride
Available online at www.sciencedirect.com
Surface & Coatings Technology 202 (2007) 613 – 616 www.elsevier.com/locate/surfcoat
Gas phase aluminizing of nickel alloys with hydrogen chloride J. Kohlscheen ⁎, H.-R. Stock Foundation Institute of Materials Science, Badgasteiner Str. 3, 28359 Bremen, Germany Available online 10 July 2007
Abstract Aluminizing of novel nickel alloy Haynes 282 and alloy 201 was performed by chemical vapor deposition. Aluminum chloride served as precursor and was generated in-situ by feeding gaseous hydrogen chloride (HCl) into the retort reacting with a solid high-melting aluminum alloy at about 1350 K. In comparison to over-pack aluminizing this process offers the advantage to control the coating growth by simply adjusting the HCl gas flow. Either continuous HCl flow or discontinuous addition was applied. Hydrogen (H2) to HCl gas flow ratios from 0:1 up to 9:1 (total flow of 50 l/h in every case) were tested at atmospheric pressure. Coating thickness was determined from glow discharge spectroscopy and metallographic cross sections. It is shown that formation of the desired β-NiAl phase is relatively insensitive to the gas flow ratio, yet forms best at hydrogen excess with a H2 to HCl flow ratio of approx. 4:1 (40:10 l/h). Discontinuous addition of HCl does not lower coating growth rate significantly in either case because the chloride introduced into the chamber is not consumed in the aluminum deposition process. The results are explained by thermo-chemical calculations showing that an excess of hydrogen chloride reduces formation of NiAl on the part to be coated. Isothermal oxidation tests revealed an improved oxidation resistance of the aluminized surfaces. © 2007 Elsevier B.V. All rights reserved. Keywords: Nickel alloy; Diffusion coating; Oxidation resistance
1. Introduction Aluminizing is a well-established technique to enhance the oxidation resistance of certain metallic materials like nickel and iron based alloys. This is necessary for applications where parts like turbine blades are operated in a high temperature environment, e.g. in stationary power generation units. The high aluminum content at the surface leads to formation of a dense protecting oxide layer in-service. Pack cementation or out-ofpack processes are often used for aluminizing treatments [1,2]. The parts are placed into a vacuum furnace together either with pure aluminium (high activity process) or with an aluminium donating alloy (low activity process) and activating halide powders (e.g. NH4F) [3]. The arrangement is heated typically to about 1300 K where a considerable aluminum halide pressure becomes available. The process runs for several hours in order to allow sufficient aluminum and nickel to diffuse and to form the desired β-NiAl phase. Typical layer thicknesses are in the range of 10 to 100 μm. In low activity processes the topmost layer consists of a stoichiometric intermetallic aluminide (e.g. β-NiAl) formed ⁎ Corresponding author. Tel.: +49 421 218 5470; fax: +49 421 218 5333. E-mail address: [email protected] (J. Kohlscheen). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.07.003
by outward diffusion of nickel and reaction with the aluminum which is continuously supplied by the gas phase [4]. Below the initial surface of the substrate, a columnar structured zone forms due to the loss of nickel and subsequent enrichment of alloying metals often accompanied by precipitation of metal carbides in a NiAl matrix. When aluminizing with high activity aluminum (at lower temperature) top layers richer in aluminum are formed (e.g. Ni2Al3) which may require an additional heat treatment. However, pack-based processes have the drawback that they are not controllable once the retort is heated to process temperature because all depositing species are formed within. Therefore, arrangements with a controllable generation of aluminum halides are desirable. Hydrogen chloride (HCl) is a good candidate for the use as activator gas. Warnes et al. have studied activation of Al donating alloys by hydrogen chloride leading to generation of metal chlorides either outside or inside of the deposition chamber [5,6]. Aluminizing of stainless steel using different hydrogen/HCl gas mixtures has also been reported [7,8]. A high amount of hydrogen added to the HCl gas flow has been found to increase the coating growth rate (ratio of approx. 10:1). In this paper, gas phase aluminization activated by HCl is investigated for almost pure nickel (alloy 201) and a novel alloy, which is especially designed for heat resistant parts
614
J. Kohlscheen, H.-R. Stock / Surface & Coatings Technology 202 (2007) 613–616
in power generation units (alloy Haynes 282). For these applications a high resistance to steam oxidation is often needed which can be provided by aluminized surfaces. 2. Experimental Samples of nickel alloy 201 and alloy Haynes 282 were used for the experiments. Table 1 shows their elemental composition determined by optical emission spectroscopy. For alloy 201 samples of 50 mm diameter and 5 mm thickness were cut from an extruded rod. Alloy Haynes 282 was chosen as a newly developed alloy with potential to be used in hot environments (e.g. in power plants). Specimens with dimensions 25 × 25 mm were cut from sheet material with 1.5 mm thickness. Coating deposition was performed in a slightly modified commercial CVD apparatus with heating and cooling hood [9]. The inside of the steel retort was equipped with a stainless steel coating box approx. 800 mm high (150 mm in diameter) covered by a lid. The samples were suspended inside the box at half height. The retort could be fed with argon and hydrogen, whereas the box itself had a separate gas duct for supply with HCl and hydrogen gas. Aluminum-chromium alloy granules with an average diameter of 10 mm served as donating material because a low activity process was intended. The donator was placed at the bottom of the coating box. After evacuation the retort was purged with argon for 30 min. The heating hood was placed on the retort and a hydrogen gas flow was adjusted to yield an atmospheric pressure inside the retort. Once a temperature of 1350 K was reached, the diffusion process was carried out for 2 h. Afterwards, the retort was allowed to cool down assisted by a flow of argon. The high reactivity of HCl led to corrosion of the steel ducts and contamination of the first aluminized samples. This could be solved by using a nickel alloy as duct material. The structure of the diffusion zone was visualized by metallographic cross-section images without etching. The chemical composition of the near surface layers was quantified by glow discharge optical emission spectroscopy (GDOES) with a LECO type GDS 750A apparatus. Calibrated NiAl and Ni3Al samples of GFE Materials, Nuremberg, served as standards. The evaluation of the oxidation resistance of selected samples was done by cyclic oxidation. The test temperature was 1173 K. The samples were taken out of the furnace after certain time intervals (typically 24 h) and cooled down to room temperature with compressed air. Their weight change due to oxidation was determined with a high precision balance.
Table 2 Overview of different flow ratios of hydrogen to hydrogen chloride used for gas phase aluminizing of nickel alloys 201 and 282 H2 flow [l/h]
45
40
35
25
20
10
0
HCl flow [l/h] Flow ratio H2:HCl
5 9:1
10 4:1
15 7:3
25 1:1
30 2:3
40 1:4
50 0:1
from hydrogen excess with little addition of HCl (ratio 9:1) up to pure HCl were chosen (0:1). Fig. 1 shows the metallographic cross-section images together with associated elemental depth profiles obtained by GDOES for aluminized alloy 201 (flow ratio 4:1). In this case the cross section reveals a simple layer sequence consisting of βNiAl (dark or blue) phase (“A”) and a brighter zone composed presumably of the Ni5Al3 phase (“B”). A diffusion zone (“C”) with aluminum solved in the nickel matrix is also present which passes into the nickel base material (“D”). The total coating thickness is defined as the zone where the aluminum content is above 35 at.% (i.e. almost 35 μm in Fig. 1, sum of zone A + B). As one can see, there is a good correspondance between GDOES profile and cross-section layer sequence. Fig. 2 shows the same analyses for alloy 282. An aluminide top layer is clearly visible (“A”). The main difference to Fig. 1 is a zone where an enrichment of alloying metals (chromium, cobalt, molybdenium) can be identified at approx. 20 μm below the sample surface (“B”). This is a typical phenomenon occurring for aluminized superalloys [4]. It is explained by the fact that nickel diffuses outwards during the low activity process considered here. The concentration of the alloying metals (mainly chromium) builds up in the nickel depleted zone and precipitates of metal carbides occur. This zone acts like a diffusion barrier for aluminum, which is proven by rapid decrease of the aluminum concentration when compared to Fig. 1 (therefore no zone “C”) and transition to the base alloy (“D”). The aluminum content falls below 35 at.% already at a depth of about 15 μm (i.e. thickness of zone “A”). As was reported earlier hydrogen addition to HCl activating gas has a significant effect on the diffusion coating growth rate [7,8]. The effect of different H2 to HCl gas flow ratios on the resulting coating thickness on alloy 201 after 2 h of aluminizing is shown in Fig. 3. The total coating thickness is shown together with the thickness of the NiAl phase layer which forms part of the total thickness (see Fig. 1). As can be seen, only a little
3. Results and discussion Table 2 shows the different H2 to HCl flow ratios tested for aluminizing. Increasing HCl contents of the process atmosphere Table 1 Elemental composition of alloys used for aluminizing tests (in wt.%) Alloy
Ni
Cr
Co
Mo
Ti
Al
Fe
Mn
Si
C
201 Haynes 282
98.5 57
– 19.5
1.0 10
– 8.5
– 2.1
– 1.5
0.25 1.5
0.35 0.3
0.25 0.15
0.02 0.06
Fig. 1. GDOES elemental depth profile and metallographic cross-section image of aluminized alloy 201 surface (flow ratio H2 to HCl of 4:1).
J. Kohlscheen, H.-R. Stock / Surface & Coatings Technology 202 (2007) 613–616
Fig. 2. GDOES elemental depth profile and metallographic cross-section image of aluminized alloy 282 surface (flow ratio H2 to HCl of 4:1).
amount of HCl is necessary to achieve a maximum amount of aluminide for these process conditions. Increasing the HCl flow does not increase the coating thickness further and a saturation is reached at 15 l/h HCl flow (flow ratio 7:3). The drop in coating thickness between 20 and 30 l/h HCl flow may be within error limits but should be elucidated further. Activating with pure HCl lowers the coating thickness again. The observed trend in coating thickness depending on HCl flow may be explained by the fact that formation of aluminum chloride is limited by the available reacting surface of the Al containing donator granulate. Therefore, adding more HCl does not necessarily produce more aluminum chloride. However, the ocurring reactions are also strongly dependent on thermodynamic properties of the reacting substances [10]. To better understand the effect of different gas mixtures, the occurring chemical reactions and the coating results, thermochemical calculations were performed with the software package FactSage [11]. The following input for modelling the chemical reaction during the aluminizing process was considered: HCl þ H2 þ Al9 Cr4 þ Ni
615
Fig. 4. Thermochemical calculation of amounts of resulting NiAl and AlCl depending on HCl concentration in the process atmosphere.
achieved justified by the high process temperature. In the real experiment not all the material present takes part in the reaction due to kinetic limitations. However, it is expected that the calculation results reflect the general trend observed in real experiments. A presence of sufficient reacting nickel and AlCr donator (i.e. 100 g each) was taken as input at a pressure of 1 atmosphere and a temperature of 1350 K. First, an addition of 1 mol H2 with a minor addition of 0.1 mol HCl was considered. In a second calculation, equal inputs of 1 mol H2 and HCl were chosen. A third calculation was performed with pure HCl addition. The resulting equilibrium contents of all possible reaction products were calculated. Fig. 4 shows the calculated amounts of the reaction products NiAl and AlCl. The aluminum rich precursor molecule AlCl is considered paramount for the ability of the process gas to deposit aluminum [10] (in contrast to the trichloride, AlCl3, which by stoichiometry is poorer in aluminium) in the following disproportionation reaction: 3AlCl þ 2Ni↔2NiAl þ AlCl3
The compound Al9Cr4 is very similar in composition when compared to the donator alloy used in the experiments. For simplification it was assumed that thermodynamic equilibrium is
According to the calculation an excess of hydrogen yields the highest amount of AlCl in the gas phase (approx. 0.008 mol) and the highest amount of NiAl (almost 150 g) as shown in Fig. 4. If only HCl is offered (right data points), the aluminum chloride
Fig. 3. Coating thickness depending on activator gas flows (HCl) in case of aluminizing alloy 201 for 2 h.
Fig. 5. Weight change of untreated and aluminized nickel alloy Haynes 282 (flow ratio 4:1) during oxidation testing at 900 °C (air).
616
J. Kohlscheen, H.-R. Stock / Surface & Coatings Technology 202 (2007) 613–616
compounds richer in chlorine, e.g. AlCl3 favorably form [7] and the resulting AlCl content of the gas phase is significantly lowered to 0.003 mol according to the calculation. This hinders deposition of aluminum and lowers the amount of calculated NiAl to about 145 g. This is a similar trend when compared to the real NiAl layer thickness in Fig. 3. When aluminizing in a pure HCl atmosphere (right side of Fig. 3) a 10% decrease of NiAl layer thickness is measured. Fig. 5 shows the plot of sample weight change per area during oxidation testing at 900 °C for untreated and aluminized alloy 282, respectively. During the first hours of exposition, the sample weight increases rapidly in both cases due to formation of oxides. Afterwards, the sample weight does not increase much due to a protective oxide layer either consisting of chromium oxide (untreated alloy) or aluminum oxide (aluminized surface). The aluminized surface, however, shows a better protection behavior with respect to these test conditions because the weight increases at a significantly lower rate. A more detailed study of the oxidation behavior will be published in a forthcoming paper. 4. Conclusion Nickel alloy 201 and novel nickel based alloy Haynes 282 were aluminized by using gaseous HCl as activator. Compared to conventional out-of-pack processes this offers the advantage of better process control by adjusting the HCl gas flow. In packbased processes the only way to influence the diffusion coating growth is to lower the temperature. An Al–Cr alloy was used as in-situ aluminum donating material for a low activity diffusion process. The effect of different H2 to HCl process gas mixtures on the formation of a nickel aluminide diffusion coating was studied. It was found that little HCl with an excess of hydrogen is sufficient for releasing a considerable amount of aluminum mono-chloride which serves as precursor for deposition of aluminum and
subsequent formation of nickel aluminide. Increasing the HCl content of the process atmosphere does not increase the coating growth rate because the available reacting surface of the granulated donator is limited and the aluminum poor molecule AlCl3 abounds which is unable to release aluminum near the nickel surface. A high temperature oxidation test at normal atmosphere shows the increased protection of the aluminide layer on alloy 282 when compared with untreated material. Therefore, gas phase aluminization offers potential if this nickel alloy is intended for use in high temperature environments. Acknowledgements The authors would like to thank Robert Zapp Werkstofftechnik GmbH, Ratingen, for supplying us with HAYNES® 282™ alloy sheet. We are also grateful to Dr Klaus Hack of GTT Technologies, Herzogenrath, for performing thermochemical calculations. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
G.W. Goward, Surf. Coat. Technol. 108–109 (1998) 73. M.J. Pomeroy, Mater. Des. 26 (2005) 223. D.J. Levine, M.A. Levinstein, U.S. patent no. 3,667,985, 1972. R. Bianco, R.A. Rapp, in: K.H. Stern (Ed.), Metallurgical and Ceramic Protective Coatings, Chapman Hall, London, 1996, p. 236. B.M. Warnes, D.C. Punola, Surf. Coat. Technol. 94–95 (1997) 1. B.M. Warnes, Surf. Coat. Technol. 146–147 (2001) 7. F. Pedraza, C. Gomez, M.C. Carpintero, M.P. Hierro, F.J. Perez, Surf. Coat. Technol. 190 (2005) 223. F.J. Perez, M.P. Hierro, F. Pedraza, C. Gomez, M.C. Carpintero, Surf. Coat. Technol. 120–121 (1999) 151. J. Kohlscheen, H.R. Stock, H.W. Zoch, H. Pillhoefer, HTM Z. Werkst. Wärmebeh. Fert. 62 (2007) 3, 103. A. Squillace, R. Bonetti, N.J. Archer, J.A. Yeatman, Surf. Coat. Technol. 120–121 (1999) 118. G. Eriksson, K. Hack, Metall. Trans., B, Process Metall. 21B (1990) 1013.
Surface & Coatings Technology 202 (2007) 613 – 616 www.elsevier.com/locate/surfcoat
Gas phase aluminizing of nickel alloys with hydrogen chloride J. Kohlscheen ⁎, H.-R. Stock Foundation Institute of Materials Science, Badgasteiner Str. 3, 28359 Bremen, Germany Available online 10 July 2007
Abstract Aluminizing of novel nickel alloy Haynes 282 and alloy 201 was performed by chemical vapor deposition. Aluminum chloride served as precursor and was generated in-situ by feeding gaseous hydrogen chloride (HCl) into the retort reacting with a solid high-melting aluminum alloy at about 1350 K. In comparison to over-pack aluminizing this process offers the advantage to control the coating growth by simply adjusting the HCl gas flow. Either continuous HCl flow or discontinuous addition was applied. Hydrogen (H2) to HCl gas flow ratios from 0:1 up to 9:1 (total flow of 50 l/h in every case) were tested at atmospheric pressure. Coating thickness was determined from glow discharge spectroscopy and metallographic cross sections. It is shown that formation of the desired β-NiAl phase is relatively insensitive to the gas flow ratio, yet forms best at hydrogen excess with a H2 to HCl flow ratio of approx. 4:1 (40:10 l/h). Discontinuous addition of HCl does not lower coating growth rate significantly in either case because the chloride introduced into the chamber is not consumed in the aluminum deposition process. The results are explained by thermo-chemical calculations showing that an excess of hydrogen chloride reduces formation of NiAl on the part to be coated. Isothermal oxidation tests revealed an improved oxidation resistance of the aluminized surfaces. © 2007 Elsevier B.V. All rights reserved. Keywords: Nickel alloy; Diffusion coating; Oxidation resistance
1. Introduction Aluminizing is a well-established technique to enhance the oxidation resistance of certain metallic materials like nickel and iron based alloys. This is necessary for applications where parts like turbine blades are operated in a high temperature environment, e.g. in stationary power generation units. The high aluminum content at the surface leads to formation of a dense protecting oxide layer in-service. Pack cementation or out-ofpack processes are often used for aluminizing treatments [1,2]. The parts are placed into a vacuum furnace together either with pure aluminium (high activity process) or with an aluminium donating alloy (low activity process) and activating halide powders (e.g. NH4F) [3]. The arrangement is heated typically to about 1300 K where a considerable aluminum halide pressure becomes available. The process runs for several hours in order to allow sufficient aluminum and nickel to diffuse and to form the desired β-NiAl phase. Typical layer thicknesses are in the range of 10 to 100 μm. In low activity processes the topmost layer consists of a stoichiometric intermetallic aluminide (e.g. β-NiAl) formed ⁎ Corresponding author. Tel.: +49 421 218 5470; fax: +49 421 218 5333. E-mail address: [email protected] (J. Kohlscheen). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.07.003
by outward diffusion of nickel and reaction with the aluminum which is continuously supplied by the gas phase [4]. Below the initial surface of the substrate, a columnar structured zone forms due to the loss of nickel and subsequent enrichment of alloying metals often accompanied by precipitation of metal carbides in a NiAl matrix. When aluminizing with high activity aluminum (at lower temperature) top layers richer in aluminum are formed (e.g. Ni2Al3) which may require an additional heat treatment. However, pack-based processes have the drawback that they are not controllable once the retort is heated to process temperature because all depositing species are formed within. Therefore, arrangements with a controllable generation of aluminum halides are desirable. Hydrogen chloride (HCl) is a good candidate for the use as activator gas. Warnes et al. have studied activation of Al donating alloys by hydrogen chloride leading to generation of metal chlorides either outside or inside of the deposition chamber [5,6]. Aluminizing of stainless steel using different hydrogen/HCl gas mixtures has also been reported [7,8]. A high amount of hydrogen added to the HCl gas flow has been found to increase the coating growth rate (ratio of approx. 10:1). In this paper, gas phase aluminization activated by HCl is investigated for almost pure nickel (alloy 201) and a novel alloy, which is especially designed for heat resistant parts
614
J. Kohlscheen, H.-R. Stock / Surface & Coatings Technology 202 (2007) 613–616
in power generation units (alloy Haynes 282). For these applications a high resistance to steam oxidation is often needed which can be provided by aluminized surfaces. 2. Experimental Samples of nickel alloy 201 and alloy Haynes 282 were used for the experiments. Table 1 shows their elemental composition determined by optical emission spectroscopy. For alloy 201 samples of 50 mm diameter and 5 mm thickness were cut from an extruded rod. Alloy Haynes 282 was chosen as a newly developed alloy with potential to be used in hot environments (e.g. in power plants). Specimens with dimensions 25 × 25 mm were cut from sheet material with 1.5 mm thickness. Coating deposition was performed in a slightly modified commercial CVD apparatus with heating and cooling hood [9]. The inside of the steel retort was equipped with a stainless steel coating box approx. 800 mm high (150 mm in diameter) covered by a lid. The samples were suspended inside the box at half height. The retort could be fed with argon and hydrogen, whereas the box itself had a separate gas duct for supply with HCl and hydrogen gas. Aluminum-chromium alloy granules with an average diameter of 10 mm served as donating material because a low activity process was intended. The donator was placed at the bottom of the coating box. After evacuation the retort was purged with argon for 30 min. The heating hood was placed on the retort and a hydrogen gas flow was adjusted to yield an atmospheric pressure inside the retort. Once a temperature of 1350 K was reached, the diffusion process was carried out for 2 h. Afterwards, the retort was allowed to cool down assisted by a flow of argon. The high reactivity of HCl led to corrosion of the steel ducts and contamination of the first aluminized samples. This could be solved by using a nickel alloy as duct material. The structure of the diffusion zone was visualized by metallographic cross-section images without etching. The chemical composition of the near surface layers was quantified by glow discharge optical emission spectroscopy (GDOES) with a LECO type GDS 750A apparatus. Calibrated NiAl and Ni3Al samples of GFE Materials, Nuremberg, served as standards. The evaluation of the oxidation resistance of selected samples was done by cyclic oxidation. The test temperature was 1173 K. The samples were taken out of the furnace after certain time intervals (typically 24 h) and cooled down to room temperature with compressed air. Their weight change due to oxidation was determined with a high precision balance.
Table 2 Overview of different flow ratios of hydrogen to hydrogen chloride used for gas phase aluminizing of nickel alloys 201 and 282 H2 flow [l/h]
45
40
35
25
20
10
0
HCl flow [l/h] Flow ratio H2:HCl
5 9:1
10 4:1
15 7:3
25 1:1
30 2:3
40 1:4
50 0:1
from hydrogen excess with little addition of HCl (ratio 9:1) up to pure HCl were chosen (0:1). Fig. 1 shows the metallographic cross-section images together with associated elemental depth profiles obtained by GDOES for aluminized alloy 201 (flow ratio 4:1). In this case the cross section reveals a simple layer sequence consisting of βNiAl (dark or blue) phase (“A”) and a brighter zone composed presumably of the Ni5Al3 phase (“B”). A diffusion zone (“C”) with aluminum solved in the nickel matrix is also present which passes into the nickel base material (“D”). The total coating thickness is defined as the zone where the aluminum content is above 35 at.% (i.e. almost 35 μm in Fig. 1, sum of zone A + B). As one can see, there is a good correspondance between GDOES profile and cross-section layer sequence. Fig. 2 shows the same analyses for alloy 282. An aluminide top layer is clearly visible (“A”). The main difference to Fig. 1 is a zone where an enrichment of alloying metals (chromium, cobalt, molybdenium) can be identified at approx. 20 μm below the sample surface (“B”). This is a typical phenomenon occurring for aluminized superalloys [4]. It is explained by the fact that nickel diffuses outwards during the low activity process considered here. The concentration of the alloying metals (mainly chromium) builds up in the nickel depleted zone and precipitates of metal carbides occur. This zone acts like a diffusion barrier for aluminum, which is proven by rapid decrease of the aluminum concentration when compared to Fig. 1 (therefore no zone “C”) and transition to the base alloy (“D”). The aluminum content falls below 35 at.% already at a depth of about 15 μm (i.e. thickness of zone “A”). As was reported earlier hydrogen addition to HCl activating gas has a significant effect on the diffusion coating growth rate [7,8]. The effect of different H2 to HCl gas flow ratios on the resulting coating thickness on alloy 201 after 2 h of aluminizing is shown in Fig. 3. The total coating thickness is shown together with the thickness of the NiAl phase layer which forms part of the total thickness (see Fig. 1). As can be seen, only a little
3. Results and discussion Table 2 shows the different H2 to HCl flow ratios tested for aluminizing. Increasing HCl contents of the process atmosphere Table 1 Elemental composition of alloys used for aluminizing tests (in wt.%) Alloy
Ni
Cr
Co
Mo
Ti
Al
Fe
Mn
Si
C
201 Haynes 282
98.5 57
– 19.5
1.0 10
– 8.5
– 2.1
– 1.5
0.25 1.5
0.35 0.3
0.25 0.15
0.02 0.06
Fig. 1. GDOES elemental depth profile and metallographic cross-section image of aluminized alloy 201 surface (flow ratio H2 to HCl of 4:1).
J. Kohlscheen, H.-R. Stock / Surface & Coatings Technology 202 (2007) 613–616
Fig. 2. GDOES elemental depth profile and metallographic cross-section image of aluminized alloy 282 surface (flow ratio H2 to HCl of 4:1).
amount of HCl is necessary to achieve a maximum amount of aluminide for these process conditions. Increasing the HCl flow does not increase the coating thickness further and a saturation is reached at 15 l/h HCl flow (flow ratio 7:3). The drop in coating thickness between 20 and 30 l/h HCl flow may be within error limits but should be elucidated further. Activating with pure HCl lowers the coating thickness again. The observed trend in coating thickness depending on HCl flow may be explained by the fact that formation of aluminum chloride is limited by the available reacting surface of the Al containing donator granulate. Therefore, adding more HCl does not necessarily produce more aluminum chloride. However, the ocurring reactions are also strongly dependent on thermodynamic properties of the reacting substances [10]. To better understand the effect of different gas mixtures, the occurring chemical reactions and the coating results, thermochemical calculations were performed with the software package FactSage [11]. The following input for modelling the chemical reaction during the aluminizing process was considered: HCl þ H2 þ Al9 Cr4 þ Ni
615
Fig. 4. Thermochemical calculation of amounts of resulting NiAl and AlCl depending on HCl concentration in the process atmosphere.
achieved justified by the high process temperature. In the real experiment not all the material present takes part in the reaction due to kinetic limitations. However, it is expected that the calculation results reflect the general trend observed in real experiments. A presence of sufficient reacting nickel and AlCr donator (i.e. 100 g each) was taken as input at a pressure of 1 atmosphere and a temperature of 1350 K. First, an addition of 1 mol H2 with a minor addition of 0.1 mol HCl was considered. In a second calculation, equal inputs of 1 mol H2 and HCl were chosen. A third calculation was performed with pure HCl addition. The resulting equilibrium contents of all possible reaction products were calculated. Fig. 4 shows the calculated amounts of the reaction products NiAl and AlCl. The aluminum rich precursor molecule AlCl is considered paramount for the ability of the process gas to deposit aluminum [10] (in contrast to the trichloride, AlCl3, which by stoichiometry is poorer in aluminium) in the following disproportionation reaction: 3AlCl þ 2Ni↔2NiAl þ AlCl3
The compound Al9Cr4 is very similar in composition when compared to the donator alloy used in the experiments. For simplification it was assumed that thermodynamic equilibrium is
According to the calculation an excess of hydrogen yields the highest amount of AlCl in the gas phase (approx. 0.008 mol) and the highest amount of NiAl (almost 150 g) as shown in Fig. 4. If only HCl is offered (right data points), the aluminum chloride
Fig. 3. Coating thickness depending on activator gas flows (HCl) in case of aluminizing alloy 201 for 2 h.
Fig. 5. Weight change of untreated and aluminized nickel alloy Haynes 282 (flow ratio 4:1) during oxidation testing at 900 °C (air).
616
J. Kohlscheen, H.-R. Stock / Surface & Coatings Technology 202 (2007) 613–616
compounds richer in chlorine, e.g. AlCl3 favorably form [7] and the resulting AlCl content of the gas phase is significantly lowered to 0.003 mol according to the calculation. This hinders deposition of aluminum and lowers the amount of calculated NiAl to about 145 g. This is a similar trend when compared to the real NiAl layer thickness in Fig. 3. When aluminizing in a pure HCl atmosphere (right side of Fig. 3) a 10% decrease of NiAl layer thickness is measured. Fig. 5 shows the plot of sample weight change per area during oxidation testing at 900 °C for untreated and aluminized alloy 282, respectively. During the first hours of exposition, the sample weight increases rapidly in both cases due to formation of oxides. Afterwards, the sample weight does not increase much due to a protective oxide layer either consisting of chromium oxide (untreated alloy) or aluminum oxide (aluminized surface). The aluminized surface, however, shows a better protection behavior with respect to these test conditions because the weight increases at a significantly lower rate. A more detailed study of the oxidation behavior will be published in a forthcoming paper. 4. Conclusion Nickel alloy 201 and novel nickel based alloy Haynes 282 were aluminized by using gaseous HCl as activator. Compared to conventional out-of-pack processes this offers the advantage of better process control by adjusting the HCl gas flow. In packbased processes the only way to influence the diffusion coating growth is to lower the temperature. An Al–Cr alloy was used as in-situ aluminum donating material for a low activity diffusion process. The effect of different H2 to HCl process gas mixtures on the formation of a nickel aluminide diffusion coating was studied. It was found that little HCl with an excess of hydrogen is sufficient for releasing a considerable amount of aluminum mono-chloride which serves as precursor for deposition of aluminum and
subsequent formation of nickel aluminide. Increasing the HCl content of the process atmosphere does not increase the coating growth rate because the available reacting surface of the granulated donator is limited and the aluminum poor molecule AlCl3 abounds which is unable to release aluminum near the nickel surface. A high temperature oxidation test at normal atmosphere shows the increased protection of the aluminide layer on alloy 282 when compared with untreated material. Therefore, gas phase aluminization offers potential if this nickel alloy is intended for use in high temperature environments. Acknowledgements The authors would like to thank Robert Zapp Werkstofftechnik GmbH, Ratingen, for supplying us with HAYNES® 282™ alloy sheet. We are also grateful to Dr Klaus Hack of GTT Technologies, Herzogenrath, for performing thermochemical calculations. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
G.W. Goward, Surf. Coat. Technol. 108–109 (1998) 73. M.J. Pomeroy, Mater. Des. 26 (2005) 223. D.J. Levine, M.A. Levinstein, U.S. patent no. 3,667,985, 1972. R. Bianco, R.A. Rapp, in: K.H. Stern (Ed.), Metallurgical and Ceramic Protective Coatings, Chapman Hall, London, 1996, p. 236. B.M. Warnes, D.C. Punola, Surf. Coat. Technol. 94–95 (1997) 1. B.M. Warnes, Surf. Coat. Technol. 146–147 (2001) 7. F. Pedraza, C. Gomez, M.C. Carpintero, M.P. Hierro, F.J. Perez, Surf. Coat. Technol. 190 (2005) 223. F.J. Perez, M.P. Hierro, F. Pedraza, C. Gomez, M.C. Carpintero, Surf. Coat. Technol. 120–121 (1999) 151. J. Kohlscheen, H.R. Stock, H.W. Zoch, H. Pillhoefer, HTM Z. Werkst. Wärmebeh. Fert. 62 (2007) 3, 103. A. Squillace, R. Bonetti, N.J. Archer, J.A. Yeatman, Surf. Coat. Technol. 120–121 (1999) 118. G. Eriksson, K. Hack, Metall. Trans., B, Process Metall. 21B (1990) 1013.