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Factors governing slurry aluminization of steels
Surface & Coatings Technology 309 (2017) 179–186
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Factors governing slurry aluminization of steels X. Montero ⁎, I. Demler, V. Kuznetsov, M.C. Galetz a r t i c l e
i n f o
Article history: Received 5 July 2016 Revised 9 November 2016 Accepted in revised form 17 November 2016 Available online 21 November 2016 Keywords: Coating Aluminide Slurry High temperature Steel
a b s t r a c t Slurry aluminization has been used for a long time. This work presents the study of several important parameters affecting the formation of aluminum diffusion coatings onto iron base alloys via slurry application. The study concentrates on the formation mechanism of aluminides using the coating surface coverage and the coating thickness of different steels as evaluation method. Steel composition, heat treatment atmosphere, and steel surface finishes were the studied parameters. These three parameters have to be in accordance in order to obtain aluminum in the melted phase, avoid excessive substrate oxidation, promote fast diffusion of elements forming aluminides, and avoid diffusion of elements blocking the exothermic reaction that forms aluminides during the heat treatment. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Aluminization is a very common technique used to mitigate high temperature corrosion [1–4]. Among the different aluminization techniques the slurry route is one of the most interesting ones applied to industrially coat large parts [5–8]. An aluminum-containing slurry can be applied onto the component to be coated, and during heat treatment the slurry will react with the substrate forming an aluminide. The main benefit of the slurry compared to chemical vapour deposition (CVD) such as pack cementation is the easier manufacturing and the lower cost especially for large parts [9–11]. For CVD or derivate methods the parts have to be introduced entirely into furnaces with inert gas or vacuum, where aluminide precursors can be transferred to the gas phase to react with the substrate. All sections not to be coated have to be laboriously masked. The necessary equipment is very expensive due to the required controlled atmosphere; precursors can also be expensive and environmentally critical, thus all waste has to be treated. Even if the advantages seem clear, the use of slurry coatings has not been extended for various reasons. Standard slurries contain several toxic and harmful constituents [12–14]; however, this problem has been successfully addressed with the development of chromate- and phosphate-free water-based slurries [15,16], and the aluminization mechanism of nickel based alloys via the slurry route is well understood [18,19]. It was observed that on these alloys aluminization even in air is reliable due to the instantaneous reaction propagating via the combustion synthesis of nickel aluminide formation. Contrarily, under the same conditions aluminization of iron-based alloys in air is often not sufficient as shown in Fig. 1, when Ni-rich Alloy 800 and almost unalloyed Fe-based 13CrMo44 are compared. To determine the best condition and parameter field for the production of aluminide coatings on iron-based alloys is the ⁎ Corresponding author. E-mail address: [email protected] (X. Montero).
http://dx.doi.org/10.1016/j.surfcoat.2016.11.067 0257-8972/© 2016 Elsevier B.V. All rights reserved.
motivation of this study. All the reactions that promote or block the aluminide formation have to be identified. As Ni was addressed in a previous work [17], the study focuses on substrates of Fe and Cr, which are the main elements participating in surface reactions of iron-based alloys and several steels. Besides the chemical composition, the influence of different parameters such as surface treatment and atmospheres was also studied. The formation of aluminides was quantified by means of successful surface coverage using statistical analysis. The presented study was designed to identify most adequate treatments or limiting parameters for aluminizing steels. Consequently we expect that the results could trigger a wider use of slurries for products out of the state of the art application range. 2. Experimental procedure Differential thermal analysis (DTA) of pure aluminum and powder mixtures was carried out in order to investigate the coating formation processes such as melting, solidification, combustion reaction and oxidation as well as their corresponding temperatures. Mixtures of iron and chromium (Goodfellow) with 50 wt.% aluminum (MMG) were measured in air and argon using a Netzsch STA 449 F3 (Netzsch, Germany) thermo balance. Samples were heated at a heating rate of 10 °C/min to 750 °C, cooled to 200 °C, heated again to 750 °C, and finally cooled to room temperature. Pure iron, chromium (Goodfellow), and iron base steels 10CrMo910, P91, K41, and AISI 347 were selected to apply aluminum based slurries (see composition in Table 1). Square coupon samples (10 × 10 × 2 mm3) with different surface finishes (see Table 2) were rinsed in water followed by acetone. Afterwards the water-based slurry with polyvinyl alcohol (10:1) as binder was mixed with the aluminum (1.3:1) and deposited by air spraying. The deposited slurry quantity was around 12 mg·cm−2 which gave an approximate coating thickness of 50 μm before heat treatment. The coated specimens were heat treated in argon (99.99%) or air in a furnace. The samples were heated at a rate between 1 and 10 °C·min−1 to 680 °C with holding times of 2 h
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a
b Al slurry Diffusion spot
Diffusion layer
oxide
substrate 20 µm
substrate 20 µm
Fig. 1. Alloy 800 (a) and 13CrMo44 (b) after Al slurry coating heat treated in air at 690 °C.
in order to evaluate the formation of coating formation and reveal the microstructure in the subsurface zone. The microstructure and elemental distribution of diffusion zones were characterized using a SEM (JEOL 5410LV at 20 kV) in combination with EDX (Oxford). The surface roughness and average grain size of samples was measured before coating application (see Table 2). For surface roughness measurements in situ AFM investigations were carried out on a Bruker Multimode 8 microscope with a Nanoscope V controller, image sizes 102, 202 μm2. Electrolytic etching with 10% oxalic acid was applied to cross sections of uncoated samples in order to reveal the grain structure of the different tested surface finishes. Quantification of the grain sizes was performed by using optical light microscopy following the linear intercept method. Fig. 2 shows the etched cross-sections of several analyzed samples. The coating thickness of the samples was measured every 300 μm (ca. 110 measurements per sample) by optical light microscopy. The thickness was determined including the interdiffusion zone and possible pores. With these data cumulative probability plots were created in order to quantify the coating coverage of the surface. A cumulative distribution was obtained by determining the number of observations greater or smaller than the mean coating thickness and determining the range of
coating thickness. The cumulative distribution of the corrosion data is displayed in normalized probability plots. 3. Results 3.1. Heat treatment atmosphere The results obtained from coating of iron samples with Al slurry show a successful formation of a diffusion layer after heating to 680 °C in Ar. At this temperature aluminum was fully melted, which enabled the reaction between slurry and substrate [17]. In argon no oxide layers were formed at the iron surface and thus the so-called combustion synthesis could proceed to form the aluminide. The coating shows a continuous and consistent diffusion layer without interruptions. EDX measurements revealed that the composition of the aluminide changed from Fe2Al5 to FeAl closer to the substrate (see Fig. 3a). Fe2Al5 contains horizontal cracks formed during the metallographic preparation of the sample because of the brittleness of this Al-rich intermetallic phase. The epitaxial orientation of the Fe2Al5 phase demonstrates the fast growth of this phase along the c-axis [18]. When the heat treatment
Table 1 Chemical composition of the used substrates (wt.%). Substrate
Fe
Fe Cr 10CrMo9-10 P91 AISI347 K41
Bal. Bal. Bal. Bal. Bal.
Ni
0.19 9.75
Cr Bal. 2.25 8.35 17.72 17.96
Ti
Mo
0.88 0.87 0.14
Nb
≤0.40 0.47
C
0.08–0.14 0.08–0.12 ≤0.08 ≤0.03
Mn
0.48 0.41 1.33 0.25
Si
0.19 0.31 0.34 0.52
V
0.21
CTE/x 10−6 K−1 [from materials datasheet] 11.8 4.9 14.1 12.7 18.6 11
Table 2 Tested steel finishes [29,30]. Finishing
Definition
Roughness [Rq/nm]
Average grain size/μm
2B 2R RB P6 120 grit 2400 grit Sand blasted
Cold-rolled, annealed, pickled and skin passed Cold-rolled, bright-annealed and skin passed Cold rolled in non oxidant atmosphere No. 6 polished/Dry polished with 240 - grit abrasive belts Water polished with 120 grit abrasive paper Water polished with 2400 grit abrasive paper Glass pearls with 90° of incidence
56.2 21.4 28.5 24.8 129 15.1 357
22.4 14.4 16.2 28.2 31.2 33.0 28.9
X. Montero et al. / Surface & Coatings Technology 309 (2017) 179–186
a
b
c
d
e
f
181
Fig. 2. Differential interference contrasted Light Microscopy of electrolytic polished 120 grit finished P91(a) P6 industrially finished K41(b), 120 grit finished K41 (c), 2400 grit finished K41 (d) sand blasted K41 (e) and lower magnification of sand blasted K41 (f).
was conducted in air instead, pure iron produced a thick oxide layer between the Al particles of the slurry and the substrate. This oxide layer prevented the reaction between aluminum melt and substrate and, thus, the formation of a diffusion layer (see Fig. 3b). Chromium substrates formed a diffusion layer consisting of up to five different chromium aluminides (Fig. 3c). The higher number of phases can be explained by the chromium/aluminum phase diagram [19]. Similar as in the case of iron substrate, all the Al-rich intermetallic phases are brittle line compounds and show horizontal cracks produced during cooling. In contrast to the observations on iron, slurry coating of chromium in air produced a thick and uneven diffusion layer, but with similar composition as the one formed under argon (compare Fig. 3c and d). This demonstrates that the oxide scale formed on Cr at this temperature did not prevent the formation of a diffusion barrier as it was the case with pure iron. DTA analysis of the Al from the slurry demonstrated that Al melts in air and Ar (see Fig. 4). The measurement of slurry powder mixtures in argon showed the expected signals for melting and solidification without any signs of oxidation. Close to the solidification peaks a noise signal was observed, which occurred in all of the measurements. DTA signals in synthetic air showed, besides the expected signals at melting and solidification temperatures, an exothermic signal before the melting which is interpreted as oxidation. At these temperatures Al oxidation is slow and forms a thin shell on the Al particles during heat treatment
in air. This shell does not pose a problem for the formation of the diffusion layer, as proven by the formation of a homogeneous diffusion layer on the pure chromium substrate (Fig. 3). On iron the situation is different due to the much higher oxide growth rates of this metal. Such oxides form a barrier against the aluminum supply, when they reach a certain thickness before the aluminum becomes available upon melting. This makes the coating of many steels much more challenging in air than in argon, also when slow heating rates have to be used for the heat treatment, giving the oxides more time to grow [20]. As shown in Fig. 3, the oxidation behavior of the substrate led to the formation of a thick iron oxide scale barrier at the surface which hindered the formation of the desired diffusion layers. Substrates with an unfavorable kinetic, such as iron, oxidize faster and thereby pose a higher difficulty to form the desired diffusion layers than substrates with a lower oxide growing rate. As discussed in detail in [17], the formation of protective aluminum diffusion layers is subject to the requirement of bringing the slurry into a liquid phase in order to be able to wet the substrate surface and interact with it. Now a second requirement can be added: Thick oxide scale formation on the substrate must be avoided, since the oxide layer can form a barrier for the reaction between molten aluminum and substrate. In order to evaluate the reaction mechanism, differential thermal analysis (DTA) measurements were carried out, using mixtures of chromium and iron element powders with aluminum. Results of these
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a
b Al Fe2Al5 FeAl2
Fe2O3
FeAl
substrate substrate
10 µm
10 µm
c
d Al7Cr Al7Cr
Al11Cr2
Al11Cr4
Al11Cr2
Al4Cr
Al9Cr4 substrate
Al4Cr 10 µm
Al11Cr4 substrate
Al9Cr4
10 µm
Fig. 3. SEM images of Al slurry diffusion coatings after heat treatment of Fe in Ar (a) and air (b) as well as Cr in Ar (c) and air (d) at 680 °C.
investigations could help to understand the reaction taking place during the heat treatment of slurry in contact with such metals, since eutectics or intermetallics that formed during the first heat cycle should significantly change the signal from the second heating cycle. The mixture of iron and aluminum heated in argon showed a strongly pronounced exothermic signal during the first heating cycle (Fig. 5a) in accordance with literature [21,22]. This signal cannot be assigned to an oxidation since the measurement was conducted in nearly oxygenfree atmosphere. The reasonable explanation is the occurrence of combustion synthesis which is a highly exothermic process [17]. Another point confirming this theory is the absence of an aluminum solidification peak in the first run and a melting peak of aluminum in the second heating cycle. This indicates the consumption of aluminum in a reaction to form a higher melting temperature intermetallic phase. When heated in air (Fig. 5b), the oxidation of iron appeared in the form of a round bodied signal after the aluminum melting signal. Contrarily to the case of exposure in Ar, Al melts and oxidizes in the second cycle because it has not been consumed by the formation of iron aluminide intermetallics. The mixture of Cr and Al in Ar shows an exothermic signal with a lower intensity compared to iron (compare Fig. 5a and c) or nickel [17]. This signal is assigned to combustion synthesis with a lower intensity and thus a lower driving force for intermetallic formation. In addition, the relative position of the combustion synthesis peak in Ar varies depending on the sample. Fe shows instantaneous combustion synthesis whereas Cr also shows a delay between the aluminum melt and the combustion synthesis (intermetallic formation). Similar to the results of measurements in argon atmosphere, Cr and Al powder mixtures show a small aluminum melting peak during the second heating cycle in air, indicating that aluminum did not react with chromium (Fig. 5d). However, the corresponding solidification peaks were very small and partly not visible, indicating either oxidation of the remaining Al or further intermetallic formation.
3.2. Steel composition In the following several industrially manufactured alloys with chromium contents between 2% and 18% were investigated. All samples were prepared with 120 grid sand paper, coated with Al slurry, and heat treated at 680 °C (10 °C/min) in argon atmosphere. In all cases diffusion layers were formed successfully. These layers were, however, irregular and showed some major disruptions. The diffusion layer consisted of Fe2Al5 on the ferritic steels (10CrMo9-10, P91 and K41). The austenite AISI 347 presented an outer FeAl3 layer, and the Fe2Al5 which contained nickel (up to 6 at.%) was found between the FeAl3 and the substrate (Fig. 6). Interestingly the austenitic steel was the only one which did not form any vertical cracks in the diffusion layer. These cracks are presumed to be formed due to the thermal expansion coefficient mismatch between aluminum-rich iron aluminides (Fe2Al5 ≈ 17.4 × 10−6 K−1 [23]) and the substrates (see Table 1). Fig. 7 shows the results of measurements of diffusion layer thickness. Coating of 10CrMo9–10 specimens resulted in a similar coating thickness, but around 2% of the surface was not coated. P91 substrate showed the thickest diffusion layer with 3% of the sample surface uncoated. The increase in Cr seems to decrease the coverage of the coating, which confirms the negative effect of Cr to form aluminide coatings revealed in the previous section. The ferritic steels coverage decreases with increasing Cr content. K41 formed a rather thin diffusion layer and had the largest uncoated fraction with 10% of the sample surface. Additionally, the steeper slope of the K41 curve indicates that the coating thickness is more irregular than in the previous cases, ranging between 52 and 5 μm. Instead AISI 347 showed a complete covering of the substrate with a coating of around 40 μm thickness. AISI347 and K41 have the same Cr content, but they vary in Ni, and thus AISI347 has an austenitic lattice, while K41 (see Table 1) remains ferritic even at the high temperatures used for coating manufacturing. Interestingly the formed phases
X. Montero et al. / Surface & Coatings Technology 309 (2017) 179–186
183
a
Melting
Solidification
b
Melting
Solidification
Oxidation
Fig. 4. DTA analysis of Al powder in Ar (a) and air (b).
also differed, AISI347 formed Al-rich FeAl3 whereas K41 only produced Fe2Al5. Both the Ni content and austenitic lattice in steels seem to improve the quality of the diffusion layer and promote Al diffusion. By the higher Ni content the higher energy of formation compared to iron can help to form the coating and compensate for the lower heat of formation in Cr aluminides, as discussed for the results shown in Fig. 5. In K41 the Cr content lowers the heat of formation, also lowering the reactivity. This effect had not yet been observed at Cr contents of 9% or lower. In addition, the heat of formation may also depend on the “parent” lattice present in the steel before aluminization, which should be further investigated in the future. 3.3. Surface finishing Various surface treatments were also tested in order to coat Fe\\Cr alloys with Al slurry. Descriptions of the surface finishes are summarized in Table 2. Micrograph images of coated samples (not shown to avoid repetition of figures) showed a nearly continuous diffusion
layer. However the quality of the formed diffusion layer showed considerable differences (Fig. 8). Ferritic-martensitic structured P91 treated with sand paper 120 grit or sand blasted showed the highest and almost identical coating thickness, whereas 2400 grit sand paper treated samples showed a lower coverage (around 85%) and only almost half of the coating median thickness (see Fig. 8a). In this case high surface roughness and dislocations seems to promote homogeneous and thicker aluminide formation. Fig. 2 shows how dislocations are present in 120 grit and sand blasted surfaces whereas 2400 girt treated surface is dislocations free. The depth of the dislocations is higher for the sand blasted surface (with almost 10 μm depth) than for the 120 grit treated surface (around 3 μm depth). Industrial surface finished samples were also tested in order to verify the influence of the surface roughness on the coating thickness (see Fig. 8b). In this case the experiment was carried out only with the ferritic structured K41 steel. The median coating thickness was similar with all of the tested finishings, but the coverage differed. However, the smoother the surface, the higher was the coating coverage on K41
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a
Combustion synthesis
b
Al melt
Al solidification
Al melt/ solidification
Fe oxidation
c
having been introduced during the grit blasting promoted recrystallization of the substrate in the vicinity of the substrate/coating interface when the aluminizing process proceeded. The recrystallization accelerated the interdiffusion between Al and Ni via grain boundaries. Generally it is considered that in oxidation cases diffusion of metallic or oxidizing elements is faster along grain boundaries, dislocations, pores, and cracks than through the lattice and tends to dominate the oxidation process at low and intermediate temperatures [25]. This principle could also be considered for the case of aluminum and metallic elements of the substrate to form aluminide diffusion coatings. The large grains in the fine polished alloys provide a slower diffusion of Al to form the diffusion layer. Trindade et al. [26] showed that a reduced grain size on Fe\\Cr alloy enhances the diffusion of Cr, Fe, or O to form the oxide scale as the small grain size increases the number of sites for diffusion. Finally, rougher surfaces oxidize faster than polished ones, which may induce more stress into the oxide. This is mostly due to the presence of many more dislocations on the rougher surface due to the mechanical abrasion with large SiC grains, as dislocations induce faster diffusion [28]. In a similar manner this kind of surface will improve the formation of aluminides. These explanations are in accordance with the observations on ferritic-martensitic P91 substrate in which high roughness, small grain sizes and dislocations seem to improve the aluminide formation. In the case of the K41 ferritic substrate these principles cannot be directly applied. Cold-worked materials (industrial finishes) seem to form more homogeneous coatings because they usually have finer grains and therefore oxidize faster [27]. However, the higher roughness and dislocations has a negative influence on this kind of substrate. Diffusion of negatively affecting elements is also promoted by increasing roughness and, as explained in the previous section, Cr contained in this substrate is detrimental for the homogeneous formation of the coating. 4. Conclusions
Al melt
Combustion synthesis
d
Fig. 5. DTA analysis of Al powder mixed with powders of Fe in Ar (a) and air (b); Cr in Ar (c) and air (d); at proportions of 50 at.%.
steel. The average grain size does not seem to have a real influence as P6 and 2B, having the larger grain sizes, show the highest and lowest surface coverage rates respectively. To the best of the authors' knowledge, there are very few reports showing the influence of surface preparation on aluminide coating formation [24]. For Ni-based alloys the authors claimed that surface strain
In this work the influences of exposure atmosphere as well as the influence of alloy composition and surface treatment were investigated in order to achieve an aluminide diffusion layer manufactured by slurry application. DTA was applied as an effective method in order to identify detrimental substrate materials. The coating formation mechanism involved three major steps: melting of the slurry, wetting of the substrate, and combustion synthesis. In order to induce these reaction steps, a heat treatment is required, which can be conducted in argon or air. By using argon atmosphere protection against oxidation was provided. Experiments in this protective atmosphere showed positive results for the coating of iron and chromium with Al heat treatment at 680 °C. When the heat treatment was carried out in air, coating formation was controlled by a constant competition between oxidation of the metal substrate and transformation of slurry into the liquid state. This makes the coating of substrates in air much more challenging than in argon. The kind of substrate determines thereby the degree of influence by oxidation. Substrates with an unfavorable kinetic, such as iron, oxidize faster and thereby pose a higher difficulty to form the desired diffusion layers than substrates with a lower oxide growing rate. Coating of various steels with an Al slurry and heat treatment at 680 °C in argon atmosphere showed an increasing probability of uncoated surface with increasing chromium content. However the presence of nickel in the alloy seemed to weaken this effect and contribute to a continuous and regular diffusion layer. As shown by the experiments regarding surface treatment, high roughness and small grain size providing treatments promote formation of thicker and homogeneous aluminides. The experiments have also shown that the optimization of one of the analyzed parameters can affect another negatively and finally form lower quality aluminide coatings. This balance was observed in the case of Cr addition to steel. On the one hand this addition decreases
X. Montero et al. / Surface & Coatings Technology 309 (2017) 179–186
185
a
Fe2Al5
Al
10 µm
10 µm
Fe
10 µm
low
high
Fe
10 µm
Cr
10 µm
Cr
10 µm
Cr
20 µm
b
Al
Fe2Al5
10 µm
low
10 µm
high
c
Al
Fe2Al5
20 µm
Fe
20 µm
low
high
10 µm
d
5 µm FeAl3
Al
20 µm
Fe
20 µm
Cr
20 µm
Ni
20 µm
Ni inclusions
Fe2Al5
low
high
10 µm
Fig. 6. SEM images and element maps of a) 10CrMo9-10 b) P91 c) K41 and d) AISI 347 after coating with Al slurry and heat treatment at 680 °C in argon atmosphere.
the oxide blocking effect at the reaction interface, which permits the formation of aluminides in non-protective atmosphere, but an inadequate surface treatment that promotes excessive diffusion of Cr could decrease the quality of the formed aluminide layer. Acknowledgements The authors gratefully acknowledge P. Girardon from APERAM (France) for providing K41 substrate material with different surface finishes.
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X. Montero et al. / Surface & Coatings Technology 309 (2017) 179–186 90 80 10CrMo9-10 P91 AISI347 K41
diffusion layer thickness/µm
70 60 50 40 30 20 10 0 0%
10%
20%
30%
40% 50% 60% cumulative probability
70%
80%
90%
100%
Fig. 7. Cumulative probability plot of the diffusion layer thickness of various substrates coated with Al after heat treated at 680 °C in argon.
100
coating thickness/µm
a
90
120grid grit 120
80
2400grid grit 2400
70
sand blasted Sand blasted
60 50 40 30 20 10 0 0%
b
70
40% 60% cumulative probability
80%
100%
40% 60% cumulative probability
80%
100%
K41;Al;2B;Ar;fast 2B
60 coating thickness/µm
20%
K41;Al;2R;Ar;fast 2R K41;Al;RB;Ar;fast RB
50
K41;Al;P6;Ar;fast P6
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K41;Al;Sic120;Ar;fast 120 grit
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Fig. 8. Comparison of P91 (a) and K41 (b) specimens after different surface treatment, coating with Al-slurry and heat treatment at 680 °C (10 °C/min) in argon atmosphere.
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Factors governing slurry aluminization of steels X. Montero ⁎, I. Demler, V. Kuznetsov, M.C. Galetz a r t i c l e
i n f o
Article history: Received 5 July 2016 Revised 9 November 2016 Accepted in revised form 17 November 2016 Available online 21 November 2016 Keywords: Coating Aluminide Slurry High temperature Steel
a b s t r a c t Slurry aluminization has been used for a long time. This work presents the study of several important parameters affecting the formation of aluminum diffusion coatings onto iron base alloys via slurry application. The study concentrates on the formation mechanism of aluminides using the coating surface coverage and the coating thickness of different steels as evaluation method. Steel composition, heat treatment atmosphere, and steel surface finishes were the studied parameters. These three parameters have to be in accordance in order to obtain aluminum in the melted phase, avoid excessive substrate oxidation, promote fast diffusion of elements forming aluminides, and avoid diffusion of elements blocking the exothermic reaction that forms aluminides during the heat treatment. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Aluminization is a very common technique used to mitigate high temperature corrosion [1–4]. Among the different aluminization techniques the slurry route is one of the most interesting ones applied to industrially coat large parts [5–8]. An aluminum-containing slurry can be applied onto the component to be coated, and during heat treatment the slurry will react with the substrate forming an aluminide. The main benefit of the slurry compared to chemical vapour deposition (CVD) such as pack cementation is the easier manufacturing and the lower cost especially for large parts [9–11]. For CVD or derivate methods the parts have to be introduced entirely into furnaces with inert gas or vacuum, where aluminide precursors can be transferred to the gas phase to react with the substrate. All sections not to be coated have to be laboriously masked. The necessary equipment is very expensive due to the required controlled atmosphere; precursors can also be expensive and environmentally critical, thus all waste has to be treated. Even if the advantages seem clear, the use of slurry coatings has not been extended for various reasons. Standard slurries contain several toxic and harmful constituents [12–14]; however, this problem has been successfully addressed with the development of chromate- and phosphate-free water-based slurries [15,16], and the aluminization mechanism of nickel based alloys via the slurry route is well understood [18,19]. It was observed that on these alloys aluminization even in air is reliable due to the instantaneous reaction propagating via the combustion synthesis of nickel aluminide formation. Contrarily, under the same conditions aluminization of iron-based alloys in air is often not sufficient as shown in Fig. 1, when Ni-rich Alloy 800 and almost unalloyed Fe-based 13CrMo44 are compared. To determine the best condition and parameter field for the production of aluminide coatings on iron-based alloys is the ⁎ Corresponding author. E-mail address: [email protected] (X. Montero).
http://dx.doi.org/10.1016/j.surfcoat.2016.11.067 0257-8972/© 2016 Elsevier B.V. All rights reserved.
motivation of this study. All the reactions that promote or block the aluminide formation have to be identified. As Ni was addressed in a previous work [17], the study focuses on substrates of Fe and Cr, which are the main elements participating in surface reactions of iron-based alloys and several steels. Besides the chemical composition, the influence of different parameters such as surface treatment and atmospheres was also studied. The formation of aluminides was quantified by means of successful surface coverage using statistical analysis. The presented study was designed to identify most adequate treatments or limiting parameters for aluminizing steels. Consequently we expect that the results could trigger a wider use of slurries for products out of the state of the art application range. 2. Experimental procedure Differential thermal analysis (DTA) of pure aluminum and powder mixtures was carried out in order to investigate the coating formation processes such as melting, solidification, combustion reaction and oxidation as well as their corresponding temperatures. Mixtures of iron and chromium (Goodfellow) with 50 wt.% aluminum (MMG) were measured in air and argon using a Netzsch STA 449 F3 (Netzsch, Germany) thermo balance. Samples were heated at a heating rate of 10 °C/min to 750 °C, cooled to 200 °C, heated again to 750 °C, and finally cooled to room temperature. Pure iron, chromium (Goodfellow), and iron base steels 10CrMo910, P91, K41, and AISI 347 were selected to apply aluminum based slurries (see composition in Table 1). Square coupon samples (10 × 10 × 2 mm3) with different surface finishes (see Table 2) were rinsed in water followed by acetone. Afterwards the water-based slurry with polyvinyl alcohol (10:1) as binder was mixed with the aluminum (1.3:1) and deposited by air spraying. The deposited slurry quantity was around 12 mg·cm−2 which gave an approximate coating thickness of 50 μm before heat treatment. The coated specimens were heat treated in argon (99.99%) or air in a furnace. The samples were heated at a rate between 1 and 10 °C·min−1 to 680 °C with holding times of 2 h
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a
b Al slurry Diffusion spot
Diffusion layer
oxide
substrate 20 µm
substrate 20 µm
Fig. 1. Alloy 800 (a) and 13CrMo44 (b) after Al slurry coating heat treated in air at 690 °C.
in order to evaluate the formation of coating formation and reveal the microstructure in the subsurface zone. The microstructure and elemental distribution of diffusion zones were characterized using a SEM (JEOL 5410LV at 20 kV) in combination with EDX (Oxford). The surface roughness and average grain size of samples was measured before coating application (see Table 2). For surface roughness measurements in situ AFM investigations were carried out on a Bruker Multimode 8 microscope with a Nanoscope V controller, image sizes 102, 202 μm2. Electrolytic etching with 10% oxalic acid was applied to cross sections of uncoated samples in order to reveal the grain structure of the different tested surface finishes. Quantification of the grain sizes was performed by using optical light microscopy following the linear intercept method. Fig. 2 shows the etched cross-sections of several analyzed samples. The coating thickness of the samples was measured every 300 μm (ca. 110 measurements per sample) by optical light microscopy. The thickness was determined including the interdiffusion zone and possible pores. With these data cumulative probability plots were created in order to quantify the coating coverage of the surface. A cumulative distribution was obtained by determining the number of observations greater or smaller than the mean coating thickness and determining the range of
coating thickness. The cumulative distribution of the corrosion data is displayed in normalized probability plots. 3. Results 3.1. Heat treatment atmosphere The results obtained from coating of iron samples with Al slurry show a successful formation of a diffusion layer after heating to 680 °C in Ar. At this temperature aluminum was fully melted, which enabled the reaction between slurry and substrate [17]. In argon no oxide layers were formed at the iron surface and thus the so-called combustion synthesis could proceed to form the aluminide. The coating shows a continuous and consistent diffusion layer without interruptions. EDX measurements revealed that the composition of the aluminide changed from Fe2Al5 to FeAl closer to the substrate (see Fig. 3a). Fe2Al5 contains horizontal cracks formed during the metallographic preparation of the sample because of the brittleness of this Al-rich intermetallic phase. The epitaxial orientation of the Fe2Al5 phase demonstrates the fast growth of this phase along the c-axis [18]. When the heat treatment
Table 1 Chemical composition of the used substrates (wt.%). Substrate
Fe
Fe Cr 10CrMo9-10 P91 AISI347 K41
Bal. Bal. Bal. Bal. Bal.
Ni
0.19 9.75
Cr Bal. 2.25 8.35 17.72 17.96
Ti
Mo
0.88 0.87 0.14
Nb
≤0.40 0.47
C
0.08–0.14 0.08–0.12 ≤0.08 ≤0.03
Mn
0.48 0.41 1.33 0.25
Si
0.19 0.31 0.34 0.52
V
0.21
CTE/x 10−6 K−1 [from materials datasheet] 11.8 4.9 14.1 12.7 18.6 11
Table 2 Tested steel finishes [29,30]. Finishing
Definition
Roughness [Rq/nm]
Average grain size/μm
2B 2R RB P6 120 grit 2400 grit Sand blasted
Cold-rolled, annealed, pickled and skin passed Cold-rolled, bright-annealed and skin passed Cold rolled in non oxidant atmosphere No. 6 polished/Dry polished with 240 - grit abrasive belts Water polished with 120 grit abrasive paper Water polished with 2400 grit abrasive paper Glass pearls with 90° of incidence
56.2 21.4 28.5 24.8 129 15.1 357
22.4 14.4 16.2 28.2 31.2 33.0 28.9
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a
b
c
d
e
f
181
Fig. 2. Differential interference contrasted Light Microscopy of electrolytic polished 120 grit finished P91(a) P6 industrially finished K41(b), 120 grit finished K41 (c), 2400 grit finished K41 (d) sand blasted K41 (e) and lower magnification of sand blasted K41 (f).
was conducted in air instead, pure iron produced a thick oxide layer between the Al particles of the slurry and the substrate. This oxide layer prevented the reaction between aluminum melt and substrate and, thus, the formation of a diffusion layer (see Fig. 3b). Chromium substrates formed a diffusion layer consisting of up to five different chromium aluminides (Fig. 3c). The higher number of phases can be explained by the chromium/aluminum phase diagram [19]. Similar as in the case of iron substrate, all the Al-rich intermetallic phases are brittle line compounds and show horizontal cracks produced during cooling. In contrast to the observations on iron, slurry coating of chromium in air produced a thick and uneven diffusion layer, but with similar composition as the one formed under argon (compare Fig. 3c and d). This demonstrates that the oxide scale formed on Cr at this temperature did not prevent the formation of a diffusion barrier as it was the case with pure iron. DTA analysis of the Al from the slurry demonstrated that Al melts in air and Ar (see Fig. 4). The measurement of slurry powder mixtures in argon showed the expected signals for melting and solidification without any signs of oxidation. Close to the solidification peaks a noise signal was observed, which occurred in all of the measurements. DTA signals in synthetic air showed, besides the expected signals at melting and solidification temperatures, an exothermic signal before the melting which is interpreted as oxidation. At these temperatures Al oxidation is slow and forms a thin shell on the Al particles during heat treatment
in air. This shell does not pose a problem for the formation of the diffusion layer, as proven by the formation of a homogeneous diffusion layer on the pure chromium substrate (Fig. 3). On iron the situation is different due to the much higher oxide growth rates of this metal. Such oxides form a barrier against the aluminum supply, when they reach a certain thickness before the aluminum becomes available upon melting. This makes the coating of many steels much more challenging in air than in argon, also when slow heating rates have to be used for the heat treatment, giving the oxides more time to grow [20]. As shown in Fig. 3, the oxidation behavior of the substrate led to the formation of a thick iron oxide scale barrier at the surface which hindered the formation of the desired diffusion layers. Substrates with an unfavorable kinetic, such as iron, oxidize faster and thereby pose a higher difficulty to form the desired diffusion layers than substrates with a lower oxide growing rate. As discussed in detail in [17], the formation of protective aluminum diffusion layers is subject to the requirement of bringing the slurry into a liquid phase in order to be able to wet the substrate surface and interact with it. Now a second requirement can be added: Thick oxide scale formation on the substrate must be avoided, since the oxide layer can form a barrier for the reaction between molten aluminum and substrate. In order to evaluate the reaction mechanism, differential thermal analysis (DTA) measurements were carried out, using mixtures of chromium and iron element powders with aluminum. Results of these
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a
b Al Fe2Al5 FeAl2
Fe2O3
FeAl
substrate substrate
10 µm
10 µm
c
d Al7Cr Al7Cr
Al11Cr2
Al11Cr4
Al11Cr2
Al4Cr
Al9Cr4 substrate
Al4Cr 10 µm
Al11Cr4 substrate
Al9Cr4
10 µm
Fig. 3. SEM images of Al slurry diffusion coatings after heat treatment of Fe in Ar (a) and air (b) as well as Cr in Ar (c) and air (d) at 680 °C.
investigations could help to understand the reaction taking place during the heat treatment of slurry in contact with such metals, since eutectics or intermetallics that formed during the first heat cycle should significantly change the signal from the second heating cycle. The mixture of iron and aluminum heated in argon showed a strongly pronounced exothermic signal during the first heating cycle (Fig. 5a) in accordance with literature [21,22]. This signal cannot be assigned to an oxidation since the measurement was conducted in nearly oxygenfree atmosphere. The reasonable explanation is the occurrence of combustion synthesis which is a highly exothermic process [17]. Another point confirming this theory is the absence of an aluminum solidification peak in the first run and a melting peak of aluminum in the second heating cycle. This indicates the consumption of aluminum in a reaction to form a higher melting temperature intermetallic phase. When heated in air (Fig. 5b), the oxidation of iron appeared in the form of a round bodied signal after the aluminum melting signal. Contrarily to the case of exposure in Ar, Al melts and oxidizes in the second cycle because it has not been consumed by the formation of iron aluminide intermetallics. The mixture of Cr and Al in Ar shows an exothermic signal with a lower intensity compared to iron (compare Fig. 5a and c) or nickel [17]. This signal is assigned to combustion synthesis with a lower intensity and thus a lower driving force for intermetallic formation. In addition, the relative position of the combustion synthesis peak in Ar varies depending on the sample. Fe shows instantaneous combustion synthesis whereas Cr also shows a delay between the aluminum melt and the combustion synthesis (intermetallic formation). Similar to the results of measurements in argon atmosphere, Cr and Al powder mixtures show a small aluminum melting peak during the second heating cycle in air, indicating that aluminum did not react with chromium (Fig. 5d). However, the corresponding solidification peaks were very small and partly not visible, indicating either oxidation of the remaining Al or further intermetallic formation.
3.2. Steel composition In the following several industrially manufactured alloys with chromium contents between 2% and 18% were investigated. All samples were prepared with 120 grid sand paper, coated with Al slurry, and heat treated at 680 °C (10 °C/min) in argon atmosphere. In all cases diffusion layers were formed successfully. These layers were, however, irregular and showed some major disruptions. The diffusion layer consisted of Fe2Al5 on the ferritic steels (10CrMo9-10, P91 and K41). The austenite AISI 347 presented an outer FeAl3 layer, and the Fe2Al5 which contained nickel (up to 6 at.%) was found between the FeAl3 and the substrate (Fig. 6). Interestingly the austenitic steel was the only one which did not form any vertical cracks in the diffusion layer. These cracks are presumed to be formed due to the thermal expansion coefficient mismatch between aluminum-rich iron aluminides (Fe2Al5 ≈ 17.4 × 10−6 K−1 [23]) and the substrates (see Table 1). Fig. 7 shows the results of measurements of diffusion layer thickness. Coating of 10CrMo9–10 specimens resulted in a similar coating thickness, but around 2% of the surface was not coated. P91 substrate showed the thickest diffusion layer with 3% of the sample surface uncoated. The increase in Cr seems to decrease the coverage of the coating, which confirms the negative effect of Cr to form aluminide coatings revealed in the previous section. The ferritic steels coverage decreases with increasing Cr content. K41 formed a rather thin diffusion layer and had the largest uncoated fraction with 10% of the sample surface. Additionally, the steeper slope of the K41 curve indicates that the coating thickness is more irregular than in the previous cases, ranging between 52 and 5 μm. Instead AISI 347 showed a complete covering of the substrate with a coating of around 40 μm thickness. AISI347 and K41 have the same Cr content, but they vary in Ni, and thus AISI347 has an austenitic lattice, while K41 (see Table 1) remains ferritic even at the high temperatures used for coating manufacturing. Interestingly the formed phases
X. Montero et al. / Surface & Coatings Technology 309 (2017) 179–186
183
a
Melting
Solidification
b
Melting
Solidification
Oxidation
Fig. 4. DTA analysis of Al powder in Ar (a) and air (b).
also differed, AISI347 formed Al-rich FeAl3 whereas K41 only produced Fe2Al5. Both the Ni content and austenitic lattice in steels seem to improve the quality of the diffusion layer and promote Al diffusion. By the higher Ni content the higher energy of formation compared to iron can help to form the coating and compensate for the lower heat of formation in Cr aluminides, as discussed for the results shown in Fig. 5. In K41 the Cr content lowers the heat of formation, also lowering the reactivity. This effect had not yet been observed at Cr contents of 9% or lower. In addition, the heat of formation may also depend on the “parent” lattice present in the steel before aluminization, which should be further investigated in the future. 3.3. Surface finishing Various surface treatments were also tested in order to coat Fe\\Cr alloys with Al slurry. Descriptions of the surface finishes are summarized in Table 2. Micrograph images of coated samples (not shown to avoid repetition of figures) showed a nearly continuous diffusion
layer. However the quality of the formed diffusion layer showed considerable differences (Fig. 8). Ferritic-martensitic structured P91 treated with sand paper 120 grit or sand blasted showed the highest and almost identical coating thickness, whereas 2400 grit sand paper treated samples showed a lower coverage (around 85%) and only almost half of the coating median thickness (see Fig. 8a). In this case high surface roughness and dislocations seems to promote homogeneous and thicker aluminide formation. Fig. 2 shows how dislocations are present in 120 grit and sand blasted surfaces whereas 2400 girt treated surface is dislocations free. The depth of the dislocations is higher for the sand blasted surface (with almost 10 μm depth) than for the 120 grit treated surface (around 3 μm depth). Industrial surface finished samples were also tested in order to verify the influence of the surface roughness on the coating thickness (see Fig. 8b). In this case the experiment was carried out only with the ferritic structured K41 steel. The median coating thickness was similar with all of the tested finishings, but the coverage differed. However, the smoother the surface, the higher was the coating coverage on K41
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a
Combustion synthesis
b
Al melt
Al solidification
Al melt/ solidification
Fe oxidation
c
having been introduced during the grit blasting promoted recrystallization of the substrate in the vicinity of the substrate/coating interface when the aluminizing process proceeded. The recrystallization accelerated the interdiffusion between Al and Ni via grain boundaries. Generally it is considered that in oxidation cases diffusion of metallic or oxidizing elements is faster along grain boundaries, dislocations, pores, and cracks than through the lattice and tends to dominate the oxidation process at low and intermediate temperatures [25]. This principle could also be considered for the case of aluminum and metallic elements of the substrate to form aluminide diffusion coatings. The large grains in the fine polished alloys provide a slower diffusion of Al to form the diffusion layer. Trindade et al. [26] showed that a reduced grain size on Fe\\Cr alloy enhances the diffusion of Cr, Fe, or O to form the oxide scale as the small grain size increases the number of sites for diffusion. Finally, rougher surfaces oxidize faster than polished ones, which may induce more stress into the oxide. This is mostly due to the presence of many more dislocations on the rougher surface due to the mechanical abrasion with large SiC grains, as dislocations induce faster diffusion [28]. In a similar manner this kind of surface will improve the formation of aluminides. These explanations are in accordance with the observations on ferritic-martensitic P91 substrate in which high roughness, small grain sizes and dislocations seem to improve the aluminide formation. In the case of the K41 ferritic substrate these principles cannot be directly applied. Cold-worked materials (industrial finishes) seem to form more homogeneous coatings because they usually have finer grains and therefore oxidize faster [27]. However, the higher roughness and dislocations has a negative influence on this kind of substrate. Diffusion of negatively affecting elements is also promoted by increasing roughness and, as explained in the previous section, Cr contained in this substrate is detrimental for the homogeneous formation of the coating. 4. Conclusions
Al melt
Combustion synthesis
d
Fig. 5. DTA analysis of Al powder mixed with powders of Fe in Ar (a) and air (b); Cr in Ar (c) and air (d); at proportions of 50 at.%.
steel. The average grain size does not seem to have a real influence as P6 and 2B, having the larger grain sizes, show the highest and lowest surface coverage rates respectively. To the best of the authors' knowledge, there are very few reports showing the influence of surface preparation on aluminide coating formation [24]. For Ni-based alloys the authors claimed that surface strain
In this work the influences of exposure atmosphere as well as the influence of alloy composition and surface treatment were investigated in order to achieve an aluminide diffusion layer manufactured by slurry application. DTA was applied as an effective method in order to identify detrimental substrate materials. The coating formation mechanism involved three major steps: melting of the slurry, wetting of the substrate, and combustion synthesis. In order to induce these reaction steps, a heat treatment is required, which can be conducted in argon or air. By using argon atmosphere protection against oxidation was provided. Experiments in this protective atmosphere showed positive results for the coating of iron and chromium with Al heat treatment at 680 °C. When the heat treatment was carried out in air, coating formation was controlled by a constant competition between oxidation of the metal substrate and transformation of slurry into the liquid state. This makes the coating of substrates in air much more challenging than in argon. The kind of substrate determines thereby the degree of influence by oxidation. Substrates with an unfavorable kinetic, such as iron, oxidize faster and thereby pose a higher difficulty to form the desired diffusion layers than substrates with a lower oxide growing rate. Coating of various steels with an Al slurry and heat treatment at 680 °C in argon atmosphere showed an increasing probability of uncoated surface with increasing chromium content. However the presence of nickel in the alloy seemed to weaken this effect and contribute to a continuous and regular diffusion layer. As shown by the experiments regarding surface treatment, high roughness and small grain size providing treatments promote formation of thicker and homogeneous aluminides. The experiments have also shown that the optimization of one of the analyzed parameters can affect another negatively and finally form lower quality aluminide coatings. This balance was observed in the case of Cr addition to steel. On the one hand this addition decreases
X. Montero et al. / Surface & Coatings Technology 309 (2017) 179–186
185
a
Fe2Al5
Al
10 µm
10 µm
Fe
10 µm
low
high
Fe
10 µm
Cr
10 µm
Cr
10 µm
Cr
20 µm
b
Al
Fe2Al5
10 µm
low
10 µm
high
c
Al
Fe2Al5
20 µm
Fe
20 µm
low
high
10 µm
d
5 µm FeAl3
Al
20 µm
Fe
20 µm
Cr
20 µm
Ni
20 µm
Ni inclusions
Fe2Al5
low
high
10 µm
Fig. 6. SEM images and element maps of a) 10CrMo9-10 b) P91 c) K41 and d) AISI 347 after coating with Al slurry and heat treatment at 680 °C in argon atmosphere.
the oxide blocking effect at the reaction interface, which permits the formation of aluminides in non-protective atmosphere, but an inadequate surface treatment that promotes excessive diffusion of Cr could decrease the quality of the formed aluminide layer. Acknowledgements The authors gratefully acknowledge P. Girardon from APERAM (France) for providing K41 substrate material with different surface finishes.
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diffusion layer thickness/µm
70 60 50 40 30 20 10 0 0%
10%
20%
30%
40% 50% 60% cumulative probability
70%
80%
90%
100%
Fig. 7. Cumulative probability plot of the diffusion layer thickness of various substrates coated with Al after heat treated at 680 °C in argon.
100
coating thickness/µm
a
90
120grid grit 120
80
2400grid grit 2400
70
sand blasted Sand blasted
60 50 40 30 20 10 0 0%
b
70
40% 60% cumulative probability
80%
100%
40% 60% cumulative probability
80%
100%
K41;Al;2B;Ar;fast 2B
60 coating thickness/µm
20%
K41;Al;2R;Ar;fast 2R K41;Al;RB;Ar;fast RB
50
K41;Al;P6;Ar;fast P6
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K41;Al;Sic120;Ar;fast 120 grit
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