PRELIMINARY STUDY ON METALLIC COATING OF STEEL SUBSTRATES THROUGH HOT DIP ALUMINIZING BY USING ENERGY DISPERSIVE X-RAY SPECTROSCOPY (EDX) TECHNIQUE

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1 PRELIMINARY STUDY ON METALLIC COATING OF STEEL SUBSTRATES THROUGH HOT DIP ALUMINIZING BY USING ENERGY DISPERSIVE X-RAY SPECTROSCOPY (EDX) TECHNIQUE 1 Hishamuddin Hj. Husain, 2 Muhamad Daud, 1 Anasyida Abu Seman and 1 Abdul Razak Daud 1 Universiti Kebangsaan Malaysia, Bangi, Kajang, Selangor D.E., Malaysia. 2 Malaysian Nuclear Agency, Bangi, Kajang, Selangor D.E., Malaysia. hishamuddin@nuclearmalaysia.gov.my ABSTRACT Even though a lot of new advanced materials have been developed nowadays, steel remains a major material in construction, automobiles, appliances, industrial machinery as well as in the nuclear industry. Due to steel easily corroded, a proper surface protection is required to avoid any failures and extended the life cycle of the components. Surface coating is an efficient and economical method to obtain desirable material surfaces properties. Hot dip aluminizing technique was utilized in this study. Experiments have been conducted on the mild steel substrates with 12mm diameter. Prior to hot dipping process, observation on grain growth at three different temperatures had also been conducted to understand the behaviour of steel under application of heat. The substrates were heated at 700ºC, 800ºC and 900ºC for 1 hour and the microstructure was analyzed. The temperature of 800C was chosen for hot dipping. The substrates were dipped into the molten aluminum maintained at temperature 800ºC for 2,4,6,8,10,15 and 20 minutes. Optical microscopy and energy dispersive X-ray spectroscopy were used in this investigation. From the microstructure observation, it showed the appearance of intermetallic layer covered by the top layer of Al on the mild steel substrate increased with the increase in dipping time ranging from 36 to 282μm. The result of EDX analysis revealed the existence of Fe and Al in form of Fe 2 Al 5 phase for all the dipping time. ABSTRAK Walaupun bahan-bahan termaju yang baru telahpun dibangunkan pada masa sekarang, keluli masih menjadi bahan utama di dalam pembinaan, automobile, perkakasan industry dan juga di dalam industry nuclear. Oleh kerana keluli mudah terkakis, perlindungan permukaan yang sempurna adalah diperlukan bagi mencegah sebarang kegagalan dan meningkatkan kitaran hayat bagi komponen-komponen. Penyalutan permukaan adalah kaedah yang berkesan dan ekonomik di dalam menghasilkan sifat-sifat permukaanbahan yang dikehendaki. Teknik salutan panas aluminium telah dikaji. Ujikaji telah dijalankan ke atas substrat keluli lembutberdiameter 12mm. Sebelum proses pencelupan panas dijalankan, pemerhatian ke atas pertumbuhan butiran pada suhu-suhu yang berbeza telah dijalankan bagi memahami kelakuan keluli di bawah pengaruh suhu. Substrat di[panaskan pada suhu 700ºC, 800ºC dan 900ºC untuk tempoh 1 jam dan mikrostrukturnya substrat telah di analisis. Suhu 800C telah dipilih untuk proses pencelupan panas. Substrat telah dicelup ke dalam leburan aluminum yang di kekalkan pada suhu 800ºC untuk tempoh celupan 2,4,6,8,10,15 dan 20 minit. Pemerhatian menggunakan mikroskop optic dan spektroskopi serakan sinar-x telah dijalankan. Daripada pemerhatian mikrostruktur, didapati terbentuknya lapisan antara logam yang dilitupi oleh lapisan atas 63

2 aluminium di atas substrat keluli. Ketebalan lapisan ini meningkat dengan peningkatan tempoh pencelupan dalam julat 36 hingga 282μm. Keputusan analisis EDX menunjukkan kewujudan elemen Fe dan Al dalam bentuk fasa Fe 2 Al 5 untuk semua tempoh celupan. Keywords: surface coating, hot dip aluminizing and intermetallic layer. INTRODUCTION Much of loss caused by the corrosion comes from the corrosion of iron and steel. This is because among many new materials have been developed, steel remains the principal construction material for automobiles, appliances, industrial machinery as well as in nuclear industries. In nuclear industries, there are many items that are made of steel such as vessels, containments, piping systems, pumps, valves, core support structures and storage tanks, including their respective supports (ASTM standards, 2000). Since the nuclear plants are designed for decades of operation, scheduled inspections and maintenance are crucial to be conducted. Beside the initial cost of building the nuclear reactors, maintenance and nuclear components replacement are very costly. Therefore, a proper protection of the nuclear components especially steel components is important to avoid any failures and to extend the service life of the nuclear reactors. Because of steel s vulnerability to attack by aggressive chemical environments or even from simple atmospheric oxidation, it is necessary to provide various degree of protection against the environmental attacks. Cathodic protection, alloy selection, metallic coating and organic coating are commonly used methods for providing corrosion protection to various structures and components. Metallic coating methods are ranging from hot-dipped and electroplated process to tough polymer and flame-sprayed ceramics. Cost-effective coatings can save money by increasing the lifetime of industrial components and by allowing for the substitution of expensive materials by less-expensive substrate and coating combinations. Surface coating is an efficient and economical way to obtain desirable material properties by altering physical, chemical, or electrical characteristics of a material surface. The hot dip aluminizing, which is one of the surface coating processes, is the most widely used method of coating a mild steel sheet with aluminum. The hot dip aluminizing is carried out to increase corrosion and oxidation resistance as well as hardness of the sheet. When steel is in contact with a molten aluminum maintained at affixed temperature for a certain period, a reaction occurs to form a brittle interlayer of intermetallic compounds (Li et al., 2002, Awan & Hasan, F., 2008, Han et al., 2009). The intermetallic layer develops between the steel substrate and the coated aluminum. The intermetallic layer grows and dissolves concurrently into the molten aluminum, which is directly associated with the loss of the steel substrate. The growth and dissolution rates of the intermetallic layer determine the thickness of the layer. The rates are closely related with the temperature of the molten aluminum and dipping time of the steel. The thickness of the layer also varies depending on chemical composition of the molten. Aluminum is very successful as a protective coating for steel because in most environments to which steel will be subjected, aluminum will act as the anode; that is it will dissolve in preference to the steel. 64

3 METHODOLOGY For substrates preparation, mild steel rods with 12mm diameter were used in the experiment. Machining process was performed to remove the oxidized surface and to acquire a smooth surface of the rods. Traces of machining were removed by grinding with SiC paper prior to cleaning by ultrasonic. Observation on the grain growth of the substrates was conducted by heating at 700ºC, 800ºC and 900ºC for 1 hour. The samples were polished and etched using Nital solution. Microstructures of steel substrates were analyzed using the optical microscope. For hot dipping process, pure aluminum (99.9% from Aldrich Chemical Company Inc) were melted in a graphite crucible at the temperature maintained to 800ºC. Then, the substrates were dipped into the molten aluminum for 2, 4, 6, 8, 10, 15, and 20 minutes. Then, the cross section of the coated samples were analyzed by using optical microscope and Energy dispersive X-ray (EDX). RESULTS Figure 1 shows clearly the increment of grain size as temperature increase. The grain size before heating is about 11 µm in average. The grain size increase with temperature at 14 µm, 25 µm and 38 µm in average for heating at 700ºC, 800 ºC and 900 ºC respectively. The increment of grain size will allow higher penetration of aluminum atoms into the substrate to form the intermetallic layer. The cementite phase was observed in the microstructures at all temperatures. This is due to the reaction of carbon in steel substrate. As the concentration of carbon increase, the fraction of cementites in the microstructure will increase and this will lowering down the diffusion rate of Fe and Al across the layer (Hwang et al., 2005). At 800ºC microstructure shows moderate grain size which result in a moderate diffusion to take place. Figure 2 shows the cross section with coating layers formed at the surface of mild steel contains two different layers i.e the pure aluminum layer and the intermetallic layer. The intermetallic layers are in tounge-like shape (Wang et al., 2003). The tounge-like shape intermetallic layer projected into the substrate with irregular projection length. Figure 3 shows the effect of dipping time on the thickness of the intermetallic layer. The thickness of the coating increase with increasing in dipping time. As the dipping time increase, diffusion of aluminum atoms increase. Aluminum atoms continuously diffused in the coating forming the intermetallic layer of Fe-Al. Sufficient thickness of the intermetallic layer would promote a good adhesion and protection against corrosion. The EDX analysis (Figure 4) shows the existence of the intermetallic phase, Fe 2 Al 5, dominating the samples at all different dipping times. Determination of phase was calculated based on the atomic percentage of Al and Fe elements. Although the growth of FeAl 3 columnar grains was simultaneous with the growth of the Fe 2 Al 5 layer, the coating layer developed into a single phase of Fe 2 Al 5 due to the preferential growth of the Fe 2 Al 5 layer (Kobayashi & Yakou, 2002). 65

4 a. Before heating (as-received) b. Heated at 700ºC (1 hour) c. Heated at 800ºC (1 hour) d. Heated at 900ºC (1 hour) Figure 1. Microstructure of substrate heated at different temperatures for 1 hour 66

5 JOURNAL of NUCLEAR And Related TECHNOLOGIES, Vol. 6, No. 2, December, minutes 4 minutes 6 minutes 8 minutes 10 minutes 15 minutes 20 minutes Figure 2. Cross section of coated sample dipped for 2, 4, 6, 8, 10, 15, and 20 minutes viewed by the optical microscope at different magnifications. 67

6 300 Intermetallic thickness (um) Dipping time (minutes) Figure 3. Intermetallic layer thickness at different dipping times. Figure 4. EDX analysis on the intermetallic layer. 68

7 CONCLUSIONS The grain size increases with the temperature and it exhibited a moderate grain size at 800ºC. The increment of grain size will allow higher penetration of aluminum atoms into the substrate to form the intermetallic layer. As the dipping time increase, the intermetallic layer thickness would increase. The EDX analysis shows the existence of the intermetallic phase, Fe 2 Al 5, dominating the samples at all different dipping times. ACKNOWLEDGMENTS The authors acknowledge the management of Malaysian Nuclear Agency especially MTEC group, Industrial Technology Division for the kind support in carrying out this work. REFERENCES Annual book of ASTM standards (2000), Section 1 Iron & Steel products Vol Coated steel products. Li, Y., Wang, J., Zhang, Y. & Holly, X. (2002), Fine structures in Fe3Al alloy layer of a new hot dip aluminized steel. Bulletin Material Science 25, pp: Awan, G.H.& Hasan, F. (2008), The morphology of coating/substrate interface in hot-dipaluminized steels. Materials Science and Engineering A 472, pp: Han, S., Li, H., Wang, S., Jiang, L. & Liu, X. (2009), Influence of Silicon on hot-dip aluminizing process & subsequent oxidation for preparing hyrogen/tritium permeation barrier. International Journal of Hydrogen Energy XXX, pp: 1-5. Hwang, S.H., Song, J.H. & Kim, Y.K. (2005), Effects of carbon content of carbon steel on its dissolution into a molten aluminum alloy. Materials Science and Engineering A 390, pp: Wang Deqing, Shi Ziyuan, Zou Longjiang (2003), A liquid aluminum corrosion resistance surface on steel substrate, Applied Surface Science 214, pp: Kobayashi, S & Yakou, T. (2002), Control of intermetallic compound layers at interface between steel and aluminum by diffusion-treatment. Materials Science and Engineering A, pp:338 :