Sains Malaysiana 39(2)(2010): 249–259

 

Kesan Wap Air Terhadap Pengoksidaan Berkitar pada Aloi Fe-Cr

(Water Vapour Effects of Cyclic Oxidation on Fe-Cr Alloys)

 

N.K. Othman*

Fakulti Sains dan Teknologi, Universiti Kebangsaan Malaysia

43600 Bangi, Selangor D.E., Malaysia

 

N.Othman

Bahagian Teknologi Industri, Agensi Nuklear Malaysia Bangi

43000 Kajang, Selangor D.E., Malaysia

 

J. Zhang & D.J. Young

School of Materials Science & Engineering

University of New South Wales, 2052, Sydney Australia

 

Diserahkan: 28 Mei 2009 / Diterima: 25 Ogos 2009

 

ABSTRAK

 

Aloi Fe-Cr (Cr: 9-30 brt%) telah dikenakan pengoksidaan berkitar dalam Ar-20%O2, Ar-20%O2-5%H2O dan Ar-10%H2-5%H2O (tekanan separa oksigen, pO2 = 1.1 ¥ 10-16atm), pada suhu 950oC. Pengukuran perubahan berat pengoksidaan menunjukkan secara umunnya bahawa peningkatan kandungan Cr dalam aloi akan mengurangkan kadar pengoksidaan. Untuk aloi Cr≤17%, ‘pengoksidaan pemecahan’ telah dapat diperhatikan. Untuk aloi Cr≥25%, penambahan berat adalah sangat rendah disebabkan oleh pembentukan lapisan Cr2O3. Penambahan 5%H2O kepada Ar-20%O2 dapat mempercepatkan lagi kadar pengoksidaan pada aloi yang mempunyai Cr≤20%. Walau bagaimanapun, kesan pencepatan ini tidak berlaku pada aloi yang mempunyai kandungan Cr yang tinggi. Analisis XRD menunjukkan aloi yang mempunyai kandungan Cr≤20%, wustit dan spinel terbentuk pada pO2 yang rendah, manakala pada pO2 yang tinggi, hematit, oksida besi dan spinel didapati terbentuk. Untuk aloi yang mempunyai kandungan Cr yang tinggi, hanya Cr2O3 dapat dikesan. Analisis keratan rentas menunjukkan bahawa spinel terbentuk melalui pengoksidaan dalaman. Ketebalan dan saiz zon oksida dalaman bertambah dengan bertambahnya kandungan Cr. Lapisan kromium oksida tunggal yang tumpat dapat diperhatikan apabila kandungan Cr melebihi 25%. Kerak oksida yang terbentuk di dalam wap air adalah lebih poros dan kurang padat berbanding kerak yang terbentuk dalam keadaan oksigen kering. Kesan wap air ke atas peningkatan pengoksidaan aloi Fe-Cr telah dibincangkan daripada aspek interaksi gas-pepejal.

 

Kata kunci: Aloi Fe-Cr; Cr2O3; pengoksidaan berkitar; wap air

 

ABSTRACT

 

Fe-Cr alloys (Cr: 9-30 wt%) were subjected to cyclic oxidation in Ar-20%O2, Ar-20%O2-5%H2O and Ar-10%H2-5%H2O (partial pressure of oxygen, pO2 = 1.1 ¥ 10-16atm). Oxidation weight change measurement showed that in general, increasing Cr content reduced the oxidation rate. At Cr < 17%Cr, breakaway oxidation was observed. However, for Cr > 25%, the weight gain became very low due to formation of Cr2O3 layer. Adding 5%H2O to Ar-20%O2 accelerated the oxidation rate of alloys with Cr < 20%. However, this accelerating effect did not appear in high chromium content alloys. XRD analysis showed that for alloys with Cr < 20%, wustite and spinel were formed at low pO2, while at high pO2, hematite, iron oxide and spinel were formed. For high Cr content alloy only Cr2O3 was detected. Cross-section analysis showed that the spinel was formed by internal oxidation. The thickness and size of this internal oxide zone increased with increasing Cr content. A dense chromia layer was observed when the Cr content was above 25%. The oxides scales formed in water vapour were more porous and less compact than the scale formed in dry oxygen. The accelerating effect of water vapour on Fe-Cr oxidation is discussed in terms of gas-solid interactions.

 

Keywords: Cr2O3; cyclic oxidation; Fe-Cr alloy; water vapour

 

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*Pengarang untuk surat-menyurat; email: insan@ukm.my