A study on the microstructure and cyclic oxidation behavior of the pack aluminized Hastelloy X at 1100 °C
Surface and Coatings Technology, Volume 201, Issue 7, 20 December 2006, Pages 3867-3871
Jyh-Wei Lee, Yu-Chu Kuo
Abstract : A pack aluminizing process at 950 °C for 9 h has been employed on the nickel-base superalloy Hastelloy X to deposit a 75 μm thick β-NiAl aluminide layer on the surface. A nanoscale dendritic structure is observed on the surface of the aluminide coating. A finger-like interdiffusion zone is found between the aluminide layer and the substrate. Fine precipitates with complex phases are distributed in the NiAl layer. The cyclic oxidation tests of aluminized alloys and untreated substrates were conducted at 1100 °C for 196 h. It was observed that the aluminizing process greatly enhances the cyclic oxidation resistance of Hastelloy X at 1100 °C due to a dense and protective alumina layer formed on the surface. Complex phase transformation reactions occurred in the aluminide layer. Owing to the oxidation and interdiffusion reactions at high temperature, the Al content of the NiAl layer was depleted to form some low Al containing γ-substrate grains on the surface and a continuous γ layer between the aluminide layer and substrate. Thermal stress induced, transverse cracks in the interdiffusion zone, were observed possibly due to the difference of thermal expansion coefficients among the substrate, aluminide layer and interdiffusion zone.SD
Formation of aluminide coatings on low alloy steels at 650DGC by pack cementation process
Xiang, Z D; Datta, P K
Materials Science and Technology. Vol. 20, no. 10, pp. 1297-1302. Oct. 2004
Abstract: The pack aluminising process is normally conducted on alloy steels at temperatures higher than 900DGC at which mechanical properties of steels would degrade. This study aims to investigate the feasibility of pack aluminising a commercial 9Cr-1Mo alloy steel at 650DGC in an attempt to increase its high temperature oxidation and corrosion resistance without adversely affecting its mechanical properties and consequently to increase its long-term structural operating temperatures to up to 700DGC. It was demonstrated that this could be achieved using packs containing AlCl3 as an activator and elemental Al as a depositing source. The coatings formed under these conditions consisted of an outer Fe14A186 layer and an inner FeAl3 layer with an abrupt interface between the coating and substrate, suggesting that the coating is formed via a mechanism of the inward Al reaction -diffusion. The pack Al content was varied from 1 to 6 wt-% to investigate its effects on the coating formation process. It was found that the pack Al content in this range affected only the coating thickness and therefore the growth rate of the coating, but not the surface Al concentration. A post-aluminising heat treatment study was also undertaken for an aluminised specimen at 650DGC under an argon atmosphere to investigate the kinetics of converting the brittle Fe14Al86 and FeAl3 phase layers to a more ductile FeAl phase layer. It was observed that this was a slow process requiring 1132 h for an initial coating layer thickness of 33 mm. The coating after the conversion consisted of a uniform top FeAl layer with all other alloying elements in the solid solution and a diffusion zone underneath. (Application: steam power plants).CSA
Pack aluminisation of low alloy steels at temperatures below 700 deg C
Xiang, Z D; Datta, P K
Surface and Coatings Technology. Vol. 184, no. 1, pp. 108-115. 1 June 2004
Abstract: This study aims to investigate the feasibility of aluminising low alloy steels at temperatures below 700 deg C by pack cementation process to increase their high temperature durability in oxidative and corrosive environments without adversely affecting their mechanical strength and creep resistance at elevated temperatures. Packs activated by AlCl3, AlF3, NaCl and NaF were used to carry out coating deposition experiments at 650 deg C in an attempt to identify the most suitable activator for the intended pack aluminising process. Once this was achieved, a series of further experiments were undertaken to investigate the effects of pack composition, deposition temperature and time on the kinetics of coating growth process. The pack Al content was varied from 1 to 6 wt.%, deposition temperature from 600 to 750 deg C and deposition time from 1 to 16 h. Within the ranges of these parameters, it was observed that these parameters only affected the coating thickness, but not the surface Al concentration and with the packs activated by AlCl3, the coatings formed via an inward reactive Al diffusion mechanism, which led to the formation of a surface Fe14Al86 layer and an inner FeAl3 layer. Thermochemical calculations were also undertaken to analyse the equilibrium vapour pressures of depositing halide species (AlF or AlCl) generated at deposition temperatures in packs activated by different halide salts. The results obtained were discussed in relation to the observed deposition tendencies of these packs and their influence on the kinetics of coating growth.CSA
Surface orientation dependent oxidation behavior of aluminized DS CM 247 LC nickel-base superalloy
Yuan, E H; Yoo, Y S; Jo, C Y; Choi, B G; Hu, Z Q
Surface and Coatings Technology. Vol. 183, no. 1, pp. 106-110. 1 May 2004
Abstract: A study has been carried out into the oxidation behavior of a directionally solidified superalloy. Three types of specimens were cut transversely (T), longitudinally (L) and at 45 deg angle (45 deg ) in relation to the solidifying direction of the DS CM 247 LC rods. These specimens were aluminized by the pack cementation method and then were oxidized at 1000 deg C for 200 h. The coating grows epitaxially on (001) surfaces of the directionally solidified superalloy. Epitaxy of the coating on the L specimens is also evident. All the coatings on the above specimens consist of a precipitation-rich layer, a precipitation-free layer and an interdiffusion layer. Lamellar alpha-Cr precipitates directly in the substrate underneath the interdiffusion layer during the aluminization process, and in the subsequent interdiffusion process during the oxidation tests. The formation of the *c-Al2O3 leads to a fast oxidation rate for the T, L and 45 deg specimens in the transient stage. The 45 deg specimens oxidized at the fastest rate, while the T specimens gave the smallest weight gain. After 20 h, further weight gains of all these specimens are very small, due to more stable *a-Al2O3 growth and transformation from *c-Al2O3 to *a-Al2O3. The slower oxidation rate in the case of the T specimens might be a result of epitaxial growth of *c-Al2O3 on the *b-NiAl coating.CSA
A new two-step process of pack aluminizing.
Huang, Z-R; Xu, H; Li, P-N
Heat Treatment of Metals (
Abstract: The structure and property of HK40 steel aluminized layer after a new two-step pack aluminizing was studied by use of optical microscopy, scanning electron microscopy, X-ray diffraction and microhardness tester. The results showed that the phases of the aluminizing layer consist of NiAl and Ni3Al, and the process has advantages over the traditional methods, such as high infiltration rate, flatter microhardness profile and finer surface quality. The causes of the new process's accelerating the pack cementation process were also discussed.CSA
A study of aluminide coatings on TiAl alloys by the pack cementation method
Zhou, C; Xu, H; Gong, S; Kim, K Y
Materials Science and Engineering A (Switzerland). Vol. 341, no. 1-2, pp. 169-173. 20 Jan. 2003
Abstract: The lower case halide-activated pack cementation method was utilized to deposit aluminide coatings on TiAl alloys. Emphasis was placed on the effect of alloying elements on the aluminizing behavior of TiAl alloy. The addition of a small amount of Nb or Cr in the TiAl improved significantly the aluminizing kinetics of TiAl alloys by increasing the solid-state diffusion of Al through the formation of stable TiAl sub 3 layer. The TiAl sub 3 layer formed on the TiAl alloyed with Nb or Cr had better toughness than the TiAl sub 3 formed on the non-alloyed TiAl. The reason for better toughness of the coating formed on TiAl was that partial TiAl sub 3 with tetragonal structure was changed to high symmetry cubic L1 sub 2 structure since Nb or Cr was dissolved into TiAl sub 3 . The TiAl sub 3 layer formed on the TiAl alloyed with Nb or Cr had much better oxidation resistance than the TiAl sub 3 layer formed on the non-alloyed TiAl. It was attributed to change in the crystal structure of TiAl sub 3 from the brittle tetragonal DO sub 22 to the ductile cubic L1 sub 2 by addition of small amount of Nb or Cr.CSA
7.
Microstructure of first-stage aluminized coating on a Ni-Cr alloy
Huang, H.L.; Chen, Y.Z.; Gan, D.
Materials Science and Engineering A (
Abstract: First-stage aluminized coating on pure Ni-Cr alloy are prepared by pack cementation aluminizing method at 750 deg C. The microstructures were analyzed with scanning electron microscope, transmission electron microscopy and electron probe micro-analyzer. The coating consists of several layers which, in sequence from substrate to coating surface, are substrate gamma , polygonized gamma , gamma '-Ni sub 3 Al, beta -NiAl + alpha -Cr, delta -Ni sub 2 Al sub 3 , and delta -Ni sub 2 Al sub 3 + Cr sub 5 Al sub 8 layers. A high compositional gradient is present across these interfaces together with a high density of dislocations. No crystallographic relationship was found at the gamma / gamma ', gamma '/ beta and beta / delta interfaces. The gamma ' and beta + alpha layers consist of very fine grains with some scattered amorphous regions.CSA
Pack cementation coatings on Ti sub 3 Al-Nb allys to modify the high-temperature oxidation properties
Koo, C H; Yu, T H
Surface and Coatings Technology (
Abstract: Ti sub 3 Al-based alloys have excellent high-temperature strength, but their poor oxidation resistance restricts the applications of these alloys at high temperature. In this study the microstructure, oxidation resistance, and mechanical properties of pack cementation Al, Si, and Cr coatings on Ti sub 3 Al-based alloys, i.e. Ti-25Al-10Nb and Ti-25Al-13Nb, were investigated and discussed. The morphology of aluminized coating is a single layer of the TiAl sub 3 phase. The siliconized coating shows a multilayer structure with the composition of TiSi sub 2 , TiSi, TiAl sub 2 and TiAl from the outer surface to the substrate. The chromized coating is a Cr-rich beta phase. The high-temperature oxidation resistance of Al and Si coatings is quite superior, and their temperature limit of oxidation resistance is up to 1100 deg C. In contrast to Al and Si coatings, the oxidation resistance of the chromized coating is poor, since it is unable to improve the oxidation resistance of the Ti-25Al-10Nb alloy. The high-temperature tensile test of Ti-25Al-13Nb with Al or Si coatings shows that the hard coating layer may reduce the ductility of the base alloy. The alloy with the Al or Si coating is suitable for applications which operate at high temperatures but low stress levels.CSA
Pack cementation coatings on Ti sub 3 Al-Nb allys to modify the high-temperature oxidation properties
Koo, C H; Yu, T H
Surface and Coatings Technology (
Abstract: Ti sub 3 Al-based alloys have excellent high-temperature strength, but their poor oxidation resistance restricts the applications of these alloys at high temperature. In this study the microstructure, oxidation resistance, and mechanical properties of pack cementation Al, Si, and Cr coatings on Ti sub 3 Al-based alloys, i.e. Ti-25Al-10Nb and Ti-25Al-13Nb, were investigated and discussed. The morphology of aluminized coating is a single layer of the TiAl sub 3 phase. The siliconized coating shows a multilayer structure with the composition of TiSi sub 2 , TiSi, TiAl sub 2 and TiAl from the outer surface to the substrate. The chromized coating is a Cr-rich beta phase. The high-temperature oxidation resistance of Al and Si coatings is quite superior, and their temperature limit of oxidation resistance is up to 1100 deg C. In contrast to Al and Si coatings, the oxidation resistance of the chromized coating is poor, since it is unable to improve the oxidation resistance of the Ti-25Al-10Nb alloy. The high-temperature tensile test of Ti-25Al-13Nb with Al or Si coatings shows that the hard coating layer may reduce the ductility of the base alloy. The alloy with the Al or Si coating is suitable for applications which operate at high temperatures but low stress levels.CSA
Effect of aluminising on high temperature oxidation resistance of TiAl compounds
Kim, S; Paik, D; Kim, I; Kim, H; Park, K
Materials Science and Technology (UK). Vol. 14, no. 8, pp. 822-825. Aug. 1998
Abstract: Intermetallic compounds of TiAl were aluminised by pack cementation in the temperature range 700-900 deg C with a powder mixture of aluminium, NH sub 4 Cl, and Al sub 2 O sub 3 under a flow of argon gas. The coating products and oxidised products formed on the TiAl substrate were investigated by optical microscopy, x-ray diffraction, and scanning electron microscopy. Single layers of TiAl sub 3 were formed on the substrate of all the aluminised specimens. Through thickness cracks and pores were often observed inside the coating layers. High quality coating layers (approx30 mu m) containing a very small amount of microcracks and pores were obtained by a treatment of 800 deg C for 3 h. The average surface hardness of the aluminised specimens (approx1010 HV (*25 mg)) was much higher than that of the TiAl (approx396 HV (25 mg)), thus improving the wear resistance. In particular, the aluminising significantly improved the high temperature oxidation resistance. After the high temperature oxidation tests, four sublayers, i.e. alpha -Al sub 2 O sub 3 , TiAl sub 2 , TiAl with a high Al content, and TiAl with a low Al content, from the surface, were formed on the substrate. These four sublayers contributed to a significant improvement in the high temperature oxidation resistance.CSA
Thermodynamics of pack aluminization
Medina, F; Wettinck, E; Moors, M
Deformacion Metalica (
Abstract: The thermodynamics of the Pack Aluminization has been studied for several activators, temperatures and master alloy compositions. Nodular cast iron (matrix ferrite-pearlite) was the substrate selected to carry out this study. The better Al depositions were attained at 950 deg C for activators such as: AlF sub 3 , NH sub 4 Cl and NaCl which generate the higher partial pressures of the Al halides in equilibrium with the pack. It was proved that activators are ranked as: Fapprox =Cl > Br > l. The iron additions reduced the undesirable surface growth. The surface composition always reached a steady state after a short time of treatment.CSA
Pack aluminizing of copper
El-Azim, M E A; Soliman, H M
Journal of Materials Science & Technology (
Abstract: Aluminizing of Cu by a pack cementation process was performed to improve its surface properties. The effect of variation of pack aluminizing temperature from 800 to 900 deg C and aluminizing time from 1 to 6h on the microstructure and the thickness of the aluminide coating of Cu was investigated. Pack aluminizing of Cu significantly improved the microhardness and the oxidation resistance. The microhardness was increased about seven times and the oxidation resistance, after 96h exposure in air at 900 deg C, was extremely increased ten times by aluminizing Cu at 900 deg C for 3h.CSA
OXIDATION CHARACTERISTICS OF Ti3Al-Nb ALLOYS AND IMPROVEMENT IN THE OXIDATION RESISTANCE BY PACK ALUMINIZING
Jha, S K; Khanna, A S
Oxidation of Metals. Vol. 47, pp. 465-493. 1997
Abstract: The oxidation behavior of three Ti3-Al-Nb alloys: Ti-25Al-11Nb, Ti-24Al-20Nb, and Ti- 22Al-20Nb was investigated in the temperature range of 700-900 C in air. The uncoated alloy Ti-25Al-11Nb showed the lowest weight gain with nearly parabolic oxidation rate; while the other two alloys had much higher weight gain, accompanied by excessive oxide scale spalling. The scale analysis, using XRD, SEM/EDAX, and AES revealed that the scale was a mixture of TiO2, Al2O3, and Nb2O5 with the outer layer rich in TiO2. The effect of variation in Al and Nb content on the oxidation behavior is discussed. A decrease in Al content of the alloy adversely affects the oxidation resistance; and it seems that a Nb content as high as 20 at.% is also not beneficial. Hence these alloys, especially Ti-24Al- 20Nb and Ti-22Al-20Nb, should not be used in the as-received condition above 750 C. An attempt was made to improve the oxidation resistance of these alloys by pack aluminizing which led to the formation of an Al rich TiAl3 surface layer doped with
Simultaneous Chromizing--Aluminizing Coating of Low-Alloy Steels by a Halide-Activated, Pack-Cementation Process
Geib, F D; Rapp, R A
Oxid. Met. Vol. 40, no. 3-4, pp. 213-228. Oct. 1993
Abstract: The simultaneous chromizing--aluminizing of low-alloy Cr--Mo steels has achieved Kanthal-like surface compositions of 16-21 Cr and 5-8 wt.% Al by the use of cementation packs with a Cr--Al materalloy and an NH sub 4 Cl activator salt. An initial preferential deposition of Al into the alloy induces the phase transformation from austenite to ferrite at the 1150 deg C process temperature. The low solubility of carbon in ferrite results in the rejection of solute carbon into the austenitic core, thereby preventing the formation of an external chromium carbide layer, which would otherwise block aluminizing and chromizing. The deposition and rapid diffusion of Cr and Al into the external bcc ferrite layer follows. Parabolic, cyclic-oxidation kinetics for alumina growth on the coated steels in air were observed over a wide range of realtively low temperatures (637-923 deg C).CSA
Improvement of Cyclic High-Temperature Corrosion Resistance of Pack-Aluminized Heat-Resistant Stainless Steels
Kim, K Y; Jung, H G; Seong, B G; Hwang, S Y
Oxid. Met. Vol. 41, no. 1-2, pp. 11-35. Feb. 1994
Abstract: Modification of pack aluminizing for heat-resistant stainless steels was studied to improve corrosion resistance by controlling the microstructure of the coating layer. The major process parameters examined include the pack powder composition, coating time, and temperature. Depending on the combination of these parameters, the microstructure of the coating layer can be controlled to form either a continuous layer of internal-diffusion barrier (IDB) or an inter-diffusion zone (IZ). At the coating-process temperatures, the IDB forms as a mixture of sigma- and Beta-aluminide, whereas the IZ forms as a mixture of alpha-ferrite and Beta-aluminide. But the sigma phase shown in the IZ at room temperature is formed by transformation from the alpha phase during cooling. Even though the hardness of the IDB is higher than the other phases present in the coating layer, the aluminide coating layer with the IDB shows outstanding cyclic high-temperature corrosion resistance. As long as the stable IDB forms, the corrosion resistance increases with the thickness of the aluminide-coating layer.CSA
Pack aluminization of nickel anode for molten carbonate fuel cells
Chun, H S; Park, G P; Lim, J H; Kim, K; Lee, J K; Moon, K H; Youn, J H
Elsevier Science SA, Journal of Power Sources (Switzerland). Vol. 49, no. 1-3, pp. 245-255. Apr. 1994
Abstract: The aluminum pack cementation (pack aluminization) process on a porous nickel anode for molten carbonate fuel cells has been studied to improve anode creep resistance. The porous Ni substrates used in this study were fabricated by doctor blade equipment followed by sintering (850 deg C). Packs surrounding the Ni anode were made by mixing Al sub 2 O sub 3 powder, Al powder, and NaCl as activator. The pack aluminization was performed at 700 to 850 deg C for 0.5-5.0 h. After pack aluminization, the principal Ni-Al intermetallic compounds detected were Ni sub 3 Al at 700 deg C, NiAl at 750 deg C and Ni sub 3 Al sub 2 at 800 deg C. The aluminum content in the aluminized Ni anode was proportional to the square root of pack aluminizing time. With increasing the Al content in the anode, the creep of the anode decreased. It was nearly constant (2.0%) when the Al content was above 5.0%. Although the exchange current density (24 mA/cm exp 2 ) for the aluminized (2.5 wt.%) Ni anode was somewhat lower than that of the pure Ni anode (40 mA/cm exp 2 ), the performance of a single cell using an aluminized Ni anode was similar to that of the one with pure Ni anode.CSA
High-Temperature Coating for Titanium Aluminides Using the Pack-Cementation Technique
Kung, S-C
Oxidation of Metals. Vol. 34, no. 3-4, pp. 217-228. Oct. 1990
Abstract: The practical application of titanium aluminide metal-matrix composites (MMCs) at high temperatures requires suitable surface coatings to provide the needed oxidation resistance. Without a coating, the titanium aluminide alloys suffered from rapid oxidation attack at elevated temperatures, particularly under thermal cyclic conditions. The pack-cementation coating process was utilized to aluminize the surface region of a Ti sub 3 Al-base alloy to TiAl sub 3 , the most oxidation-resistant phase. With the existence of an adherent conversion coating, a thin protective alumina scale formed on the outer surface, and a significant improvement in the corrosion resistance was observed. Excellent coating efficiency and geometric flexibility were demonstrated by the pack-cementation technique. Further development of the cementation process will focus on the elimination of surface cracking in the coating. Graphs, Photomicrographs. 8 ref.—AA.CSA
Protection of Low Alloy Steel Against Thermal Oxidation or Sulphidation by Superficial Pack or Laser Aluminisation
Skalli, A; Galerie, A; Caillet, M
Mater. Sci. Technol. Vol. 5, no. 8, pp. 853-857. Aug. 1989
Abstract: It is shown that superficial aluminisation largely improves the behaviour of Fe--2.25Cr--1Mo steel at elevated temperatures in oxygen, SO sub 2 , and a typical coal gasification atmosphere. The results are discussed with the aid of the stability diagrams of the system involving Fe, Cr, Mo, O and sulphur with or without addition of Al. 7 ref.—AA.CSA
Fundamental Kinetic Study of Aluminization of Iron by Pack Cementation at 900 deg C
Kung, S C; Rapp, R A
Surf. Coat. Technol. Vol. 32, no. 1-4, pp. 41-56. Nov. 1987
Abstract: The kinetics of pack aluminization of Fe at 900 deg C, using NaCl as the activator under both Ar and forming gas (5% hydrogen + Ar) atmospheres, has been studied. The coating system was simplified by using a low Al activity masteralloy of 42 wt.% Al--58 wt.% Fe to avoid the formation of intermetallic compounds at the substrate surface. The chemically depleted zone in the pack was eliminated by a continuous tumbling of the system during coating. The substrate was wrapped with a ceramic sheath to allow a particle-free surface and to introduce a diffusion barrier for the transporting gaseous species. The rate-limiting step for the aluminizing process was shown to be series transport of the gaseous species from the pack to the substrate surface and solid state diffusion in the substrate. A kinetic model is proposed and compared with the experimentally obtained data, and results are in good agreement. 21 ref.—AA.CSA
Simultaneous Chromizing--Aluminizing of Iron and Iron-Base Alloys by Pack Cementation
Rapp, R A; Wang, D; Weisert, T
High Temperature Coatings; Orlando, Florida; USA; 7-9 Oct. 1986. pp. 131-141. 1987
Abstract: Iron and a low-alloy steel (1.25Cr--1Mo) were simultaneously chromized and aluminized by means of a pack cementation process. A masteralloy of high Cr/Al activity ratio is essential for deposition of both Cr and Al. Therefore, a 95Cr--5Al (by weight) alloy was used as the masteralloy for coating a pure Fe substrate, and a 90Cr--10Al alloy was suitable for a 2.25Cr--1Mo steel. A mixed activator salt of 1NaCl:2AlCl sub 3 provided the proper ratio of volatile chromium to aluminum halides to achieve the desired surface composition. A rotation (tumbling) of the pack enhanced metal deposition by eliminating the depletion zone around the substrate. The resulting diffusion coatings had a Kanthal-like surface composition, Fe--20Cr--4Al, which provides superior high-temperature oxidation resistance. Thermogravimetric oxidation testing (TGA) in air at 1000 deg C confirmed the excellent oxidation resistance of this coating. Preliminary studies showed that yttrium could also be introduced into the coated surface by direct mixing of fine yttria powder into the cementation pack. Very limited testing showed a significant increase in the oxide adhesion for cyclic oxidation. 12 ref.—AA.CSA
Pack-Aluminization of Titanium
Galis, M F; Guille, J L; Clauss, A
Titanium--Science and Technology. Vol. 2;
Abstract: An adherent coating can be obtained by pack-aluminization of titanium in easily reached technological conditions. Analysis of the composite coatings yields to the characterization of a new TiAl sub 2 phase. The mechanical behaviour reamins almost unaltered for thin ( approx 2 mu m) coated wires. Such a coating could be a very efficient barrier against gaseous hydrogen penetration and could find industrial application in preventing hydrogen embrittlement of titanium. 8 ref.—AA.CSA
Thermodynamics and Kinetics of Pack Cementation Processes
Seigle, L L
Surface Engineering: Surface Modification of Materials; Les Arcs;
Abstract: Pack cementation processes (aluminizing, chromizing, siliconizing) have received widespread application in the deposition of coatings to improve the surface oxidation and corrosion resistance of heat resistant alloys at elevated temp. A review of the basic thermodynamics and kinetics of such processes is presented, with emphasis on those of interest for coating gas turbine components. 49 ref.--AA 
The Kinetics of Gas Transport in Halide-Activated Aluminizing Packs
Kandasamy, N; Seigle, L L; Pennisi, F J
Thin Solid Films. Vol. 84, no. 1, pp. 17-27. 2 Oct. 1981
Abstract: Int. Conf. on Metallurgical Coatings,
24.
Basic Principles of Diffusion Coating
Wachtell, R L
Science and Technology of Surface Coatings, Academic Press,
Boundary Conditions for Diffusion in the Pack-Aluminizing of Ni
Sivakumar, R; Menon, N B; Seigle, L L
Metall. Trans. Vol. 4, no. 1, pp. 396-398. Jan. 1973
26.
Pack diffusion coating of metals
Baldi, Alfonso L; Damiano, Victor V
Abstract: In the diffusion coating of aluminum onto brazed metal, the penetration of the aluminum into the brazing is reduced by conducting the diffusion coating with hydrated aluminum chloride, bromide or iodide as energizer, with the energizer preferably kept out of contact with the work being coated until the energizer volatilizes. This is particularly suited for aluminizing chromium-containing surfaces. Chromium diffusion coatings are less apt to form undesirable oxide inclusions when the diffusion coating is from a pack containing at least about 3% Ni.sub.3 Al. Also the formation of undesirable alpha-chromium is reduced when the pack diffusion is carried out with a retort effectively not over 5 inches in height. Pack aluminizing where the aluminizing is inhibited by the presence of chromium in the pack makes a very effective top coating over platinum plated or platinum coated nickel-base superalloys. Aluminized nickel can also have its aluminum attacked and at least partially removed with aqueous caustic to leave a very highly active catalytic surface. Pack diffusion can also be arranged to simultaneously provide different coatings in different locations by using different pack compositions in those locations. An aluminizing pack containing a large amount of chromium provides a thinner aluminized case than an aluminizing pack containing less chromium and some silicon. Also a cobalt-chromium pack deposits essentially a chromized case when energized with a chloride, but deposits large amounts of cobalt along with chromium when energized with an iodide.
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