cold-rolling and annealing. Rectangular specimens were cut to the dimensions of 15×12×2 mm by a water-cooled cutting machine. After machining the samples were degreased in acetone bath and finally cleaned ultrasonically in ethanol bath, and dried in air before hot dip aluminizing. The Al-10wt.Si% aluminum alloy was melted in an alumina crucible and maintained at 700°C. Specimens for further hot-dip aluminizing treatment were hung by stainless steel wires and coated with a uniform welding flux. The up/down speed of the specimen elevator was 15 cm/min-1. After 16 s of exposure they were pulled out and air-cooled to room temperature. Platinum wire (25μm diameter) was used as a marker by spot-welding on the surface of some specimens. The marker specimens were carried out by hot-dipping process under the same conditions. The hot-dipped specimens were cleaned by a mixed aqueous solution of nitric acid, phosphoric acid, and water in a 1:1:1 volume ratio at 25
°C. After hot dip aluminizing, metallographical examination was carried out to study the developing mechanism and the thickness of the intermetallic layers. The aluminized samples were placed in a furnace heated in static air at temperatures of 750, 850 and 950°C for 10 min to 56 hr. The oxidation kinetics were evaluated by thermogravimetrix analysis (TGA). After oxidation tests, the phases constitutions in the intermetallic layers of all samples were investigated by means of metallographical examination, scanning electron microscopy (SEM) with energy dispersive X-ray facility analysis (EDX) and X-ray diffraction (XRD) using Cu-Kα radiation.
3. Results and discussion
3.1 Microstructure and phase constitution
A typical cross-section morphology of an as-coated steel is shown in Fig. 1(a) where three layers are presented, topcoat aluminum (dissolved Si), intermetallics and the steel substrate. XRD analysis showed that the coating layers consisted of three phases, where Al,
4
FeAl3, Fe2Al5 were detected from external topcoat to the aluminide/steel substrate. Typically, the thickness of the coating layers was about 25 μm and the coating layers showed good adhesion to the steel substrate. The surface morphology of the as-coated specimen is showed in Fig. 2(a), where eutectic Si particle formed on the coating surface. For determining the formation mechanism of the aluminide layer during the hot-dip process, some specimens with platinum markers were performed. An optical micrograph of the coated specimen with platinum marker is shown in Fig. 1(b). The platinum marker was located at the interface between the aluminum topcoat and aluminide layer. This suggested that the formation of the aluminide layer was mainly due to outward diffusion of iron, while the inward diffusion of Al contributed to the growth of the aluminide layer.
In order to understand the oxidation behavior and the phase formation in the aluminide layer, oxidation tests were carried out at 750, 850 and 950°C in air for various durations of time. For 10 min oxidation at 750 and 850°C as shown in Fig. 1, the aluminum topcoat could still be observed in the coating layers. After oxidation for 20 min at both temperatures, the aluminum topcoat disappeared and an aluminide layer formed owing to the outward diffusion of iron from the matrix and inward diffusion of aluminum. The aluminide layers being approximately 75 μm thickness consisted of a thinα-Al2O3 layer on the surface, followed by a thicker layer of Fe2Al5+FeAl2 on the steel substrate side. As the oxidation time increased to 1hr at 850°C, some cracks generated in the aluminide layer due to the thermal expansion mismatch at the interface between the brittle Fe2Al5+FeAl2 layer and the steel substrate [11].
The thickness of aluminide layer kept almost the same for all testing duration of time.
However, the dominating Fe2Al5+FeAl2 in the aluminide layer transformed to FeAl gradually with increasing time owing to the aluminum dilution. The Al consumption in the aluminide layer was mainly by two ways: one is the outward diffusion of Al to form Al2O3 scale, and the other is Al inward diffusion toward the steel substrate [12]. Thus, FeAl formed beneath the oxide scale and at the interface between Fe2A5+FeAl2 phases and the steel substrate as shown
5
in Fig. 3.
For oxidation at 850°C for 2 hr, some voids formed at the interface between the aluminide layer and the steel substrate as a result of Kirkendall effect [13]. Vacancies generated at the interface and might condense into voids due to the rapid inter-diffusion of ferrite and aluminum during oxidation or heat treatment processes [14] as indicated in the black area between the steel and the aluminide layer as shown in Fig. 1. The accumulation of voids might generate cracks and further lead to the degradation of the aluminide layer. The black area in the aluminide layer was likely to be oxides formed from the specimen surface by high temperature oxidation. For specimens oxidized at 750°C for 8 and 56 hr, the phases present in the aluminide layer and cross-sectional morphologies were similar to that of specimen oxidized at 850°C for 3 hr. A cross-sectional micrograph of coated specimen oxidized at 750°C for 56 hr and its corresponding EDX line profiles of elements Fe, Al and Si distributions across the coating layer on the steel substrate were depicted in Fig. 3. Three distinct phases presented in the aluminide layer which consisted of FeAl, Fe2Al5+FeAl2 and FeAl starting from outer aluminide layer/steel substrate. Silicon was enriched in FeAl area due to the fact that the solubility of Si ranged from 15 to 17 at.% when FeAl was in equilibrium with the Fe2Al5 and FeAl2 [15]. The SEM micrograph as shown in Fig. 2(b) revealed the oxide morphology of the aluminized steel specimen after oxidation at 750°C for 56 hr; the surface morphology of the oxide scale revealed dense character, which consisted of Al2O3. More voids and pores were observed in the aluminide layers and at the interface between the aluminide layer and the steel substrate after oxidation at 850°C for 4 hr.
As mentioned above, it can be observed that crack generated across the interface between the aluminide layer and the steel substrate after oxidation at 850°C for 8 hr. Crack at the interface between the aluminide layer and the steel substrate might promote exfoliation of the aluminide layer. Consequently, the microcracks became short-circuit diffusion paths for
6
oxygen penetration when mechanical failure of the aluminide layer took place, leading to the nucleation and growth of oxide nodules on the coating surface. During high-temperature oxidation, a sign of failure as the formation of spore-like oxidation nodules of dark color formed on the surface of the aluminide layer. Protrusion of iron oxide on the surface of coated specimen occurred after oxidation at 850°C for 24hr as shown in Fig. 1(j). Meanwhile, through the top view investigation of scanning electron micrograph, it was clear to observe that the oxide nodules formed on the coated specimen as shown in Figs. 2(c) and (d). The degradation might cause the voids formation between the aluminide layer and the steel substrate. The phenomenon would be more pronounced when the aluminide layer lost its protective ability. The cross-sectional micrograph of the aluminide layer with protrusion of iron oxide and its corresponding EDX quantitative analysis were depicted in Fig. 4. It can be seen clearly that the steel substrate was oxidized directly by oxygen penetration through the cracks. The iron oxides formed at the surface of the aluminde layer and in the internal-oxidation layer. The brittle Fe2Al5+FeAl2 phases formed during the hot-dip aluminizing process transformed completely to FeAl phase after oxidation at 850°C for 56 hr.
It also can be observed that voids formed in the aluminide layer after oxidation at 950°C for 8 hr due to fast inter-diffusion rates of Fe/Al in the coating layer. Thus, the Fe-Al intermetallic in the coating layer totally transformed to α-Fe(Al), and oxidation behavior would be similar to that of the steel substrate.
The mechanism of the internal voids formation was attributed to the Kirkendall effect.
The different diffusion rates between Fe and Al caused a net flux of vacancies to form voids at the interface between the aluminide layer and the steel substrate. Fig. 3 shows surface morphology and fractured section of aluminide layer after oxidation at 750°C for 24 hr. It can be seen that some voids formed at the interface between the aluminide layer and the steel substrate.
7
3.2 oxidation resistance
The oxidation kinetics of the coated specimens were studied at 750, 850 and 950°C for 72 hr. Fig. 6 shows changes in weight gain per unit surface area versus time, revealing that the kinetics curves approximately obeyed the parabolic rate law, regardless of temperature. In addition, it can be seen that the weight gain of specimens increase with increasing temperature. It was due to the fact that voids condensed to form cracks extensively at the interface between the aluminide layer and steel substrate and provide more substrate surface area for oxidation, leading to faster weight-gain kinetics at 850°C. On the other hand, the Fe-Al intermetallic transformed completely to α-Fe(Al) after oxidation at 950°C for a short period of time. The coating layer fully lost its efficacy and afterward oxidation would almost match the direct oxidation of the substrate steel. As mentioned above, severe cracking and internal oxidation would occur in the coated specimen after high-temperature oxidation. Thus these limitations degrade the use of aluminized coatings in high temperature oxidation environments.
4. Conclusions
The high-temperature oxidation behavior of low carbon steel with hot-dip aluminizing coating was investigated in the temperature range from 750 to 950°C in air for various duration of time. The results may be summarized as follows:
1. The thickness of aluminide layer kept almost the same for all testing durations of time.
However, the Fe2Al5+FeAl2 dominated aluminide layer transformed to FeAl gradually with increasing time owing to the aluminum dilution.
2. The accumulation of voids at the interface between the aluminide layer and the steel substrate might produce cracks and result in the degradation of the aluminide layer.
8
Therefore, protrusion of iron oxide nodules on the surface of coated specimen occurred after oxidation at 850°C for 24hr.
3. The oxidation kinetics of the coated specimens followed the parabolic rate law at all three temperatures. Furthermore, severe cracking and internal oxidation occur in the coated specimen after high-temperature oxidation. Thus, these limitations degraded the use of aluminized coatings in high temperature oxidation environments.
Acknowledgements
The authors are very grateful to the National Science Council of Republic of China for funding support under Grant No. NSC 93-2216-E-011-023.
9
References
1. Y. N. Chang, F. I. Wei, J. Mater. Sci. 24 (1989) 14.
2. B. Schmid, N. Aas, Q. Grong, R. Qdegard, Oxid. Met. 57 (2002) 116.
3. S. Taniguchi, H. Juso, T. Shibata, Oxi. Met. 49 (1998) 325.
4. W. Retallick, M. Brady, D. Humphrey, Intermetallics 6 (1998) 335.
5. C. J. Wang, C. C. Li, Surf. Coat. Technol. 177-178 (2003) 37.
6. C. J. Wang, J. W. Lee, T. H. Twu, Surf. Coat. Technol. 163-614 (2003) 37.
7. H. Glasbrenner, O. Wedemeyer, J. Nucl. Mater. 257 (1998) 274.
8. R. W. Richards, R. D. Jones, P. D. Clements, H. Clarke, Int. Mater. Rev. 39 (1994) 191.
9. G. Eggeler, W. Auer, H. Kaesche, J. Mater. Sci. 21 (1986) 3348.
10. G. M. Bedford, J. Boustead, Met. Technol. 1 (5) (1974) 233.
11. V. N. Yeremenko, Y. V. Natanzon, V. I. Dybkov, J. Mater. Sci. 16 (1981) 1748.
12. K. Y. Kim, H. G. Jung, B. G. Seong, S. Y. Hwang, Oxid. Met. 41 (1994) 12 13. A. D. Smigelskas, E. O. Kirkendall, Trans. AIME, 171 (1947) 130.
14. S. Taniguchi, Mater. Corro. 48 (1997) 1.
15. T. Maitra, S. P. Gupta, Mater. Character. 49 (2003) 293.
10
Table 1. Chemical composition of alloys employed (wt.%).
Alloy Si Mn Cr Ni C Fe Al
Low carbon steel 0.28 0.97 0.02 0.11 0.20 Bal. - Al-10%Si 10.00 0.50 0.40 - - 0.70 Bal.
Figure 1. Optical cross-sectional micrographs of coated specimens oxidized for various time at 750, 850 and 950°C in air.
Figure 2. Surface morphologies of (a) As-coated specimen; (b) coatied specimen after oxidized at 750°C for 56hr; (c) coated specimen after oxidized at 850°C for 24 hr; (d) enlarged view of (c).
Figure 3. (a) Cross-sectional micrograph of coated specimen oxidized at 750°C for 56 hr; and (b) its corresponding EDX line profiles of elements Fe, Al and Si distributions across the coating layer on the steel substrate.
Figure 4. (a) Cross-sectional micrograph of coated specimen oxidized at 850°C for 24 hr; and (b) its corresponding EDX quantitative analysis results showing the concentration variations of Al, Fe, Si, O and Mn elements.
Figure 5. SEI fractograph of aluminide layer formed on the coated specimen after oxidation at 750°C for 24 hr.
Figure 6. Plot of oxidation kinetics of coated specimens after oxidation at 750, 850 and 950°C for 72 hr.
11
Fig. 1. Optical cross-sectional micrographs of coated specimens oxidized for various time at 750, 850 and 950°C in air.
12
Fig. 2. Surface morphologies of (a) As-coated specimen; (b) coated specimen after oxidized at 750°C for 56hr; (c) coated specimen after oxidized at 850°C for 24 hr; (d) enlarged view of
(c).
13
(a)
(b)
Fig. 3. (a) Cross-sectional micrograph of coated specimen oxidized at 750°C for 56 hr; and (b) its corresponding EDX line profiles of elements Fe, Al and Si distributions across the coating
layer on the steel substrate.
14
(a)
(b)
Fig. 4. (a) Cross-sectional micrograph of coated specimen oxidized at 850°C for 24 hr; and (b) its corresponding EDX quantitative analysis results showing the concentration variations
of Al, Fe, Si, O and Mn elements.
15
Fig. 5. SEI fractograph of aluminide layer formed on the coated specimen after oxidation at 750°C for 24 hr.
16
Fig. 6. Plot of oxidation kinetics of coated specimens after oxidation at 750, 850 and 950°C for 72 hr.