Electrochemical corrosion behaviour of nanostructured bainitic steel
Kazum, Oluwole (2014) Electrochemical corrosion behaviour of nanostructured bainitic steel. Masters (Research) thesis, James Cook University.
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Abstract
Nanostructured bainitic steels are gaining high interest due to their excellent mechanical properties. However, high carbon content in nanostructured bainitic steels can influence their general and localized corrosion susceptibility. In this study, the corrosion behaviour of nanostructured bainitic steel was compared with the well-known martensitic steel in chloride-containing solution using electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation. EIS results showed that the polarisation resistance (R(p)) for nanostructured bainitic steel (3400 Ω cm²) was higher than that of martensitic steel (2000 Ω cm²). Potentiodynamic polarisation results showed an 85% lower corrosion current density (i(corr)) for nanostructured bainitic steel as compared to martensitic steel. Interestingly, galvanostatic polarisation of the steels showed different corrosion morphology, i.e., martensitic steel revealed intergranular corrosion (IGC) and the nanostructured bainitic steel exhibited lamellar structure suggesting selective dissolution.
In order to understand the corrosion mechanism of the nanostructured bainitic steel, two different isothermal temperatures were used to produce nanostructured baintic steel with different percentages of retained austenite (RA) and bainitic ferrite (BF). Nanostructured bainite formed at 200 °C (RA: 21%) exhibited marginally higher corrosion resistance compared with that at 350 °C (RA: 53%). Post-corrosion analysis of the galvanostatically polarised samples revealed localised corrosion for both the steels, but the degree of attack was higher in the 350 °C steel than in the 200 °C steel. The localised corrosion attack was due to the selective dissolution of the RA phase. The higher volume fraction and larger size of RA in the 350 °C steel as compared to that of the 200 °C steel contributed to the pronounced corrosion attack in the 350 °C steel.
To enhance the corrosion resistance of the nanostructured bainitic steel, a conducting polymer, polyaniline (PANI), was coated on the steel using galvanostatic method. Samples coated for 10 mins with lower current density (5 mA cm⁻²) exhibited higher R(P) (3.2 × 10⁴ Ω cm²) as compared to the samples coated with 20 mA cm⁻² (9.82 × 10³ Ω cm²). Although the R(P) of the coating increased with increase in the coating thickness (i.e., by increasing the coating time), under long-term exposure the R(P) of the coated samples dropped drastically. This can be attributed to the large pores in the coatings. To reduce the porosity in the coating, the coating process was performed under stirred-condition. The stirred-condition coating (20 mA cm⁻² for 20 mins) exhibited only a few fine defects and the R(P) was almost two orders of magnitude higher than that of the unstirred-condition coated sample. Long-term EIS results for the stirred-condition coated sample showed an initial increase in R(P) (4.3 × 10⁶ Ω cm²) after 78 h exposure, and then gradually decreased to 7.0 × 10⁵ Ω cm² after 168 h exposure. However, the R(P) was significantly higher than that of the bare metal. Potentiodyanamic polarisation results confirmed the higher corrosion resistance of the stirred-condition coated sample as compared to the unstirred-condition coated sample.
The study suggests that nanostructured bainitic steel exhibited higher corrosion resistance than martensitic steel in chloride-containing solution, but the RA in the nanostructured bainitic steel dissolved selectively. PANI coating using galvanostatic method under stirred-condition, however, improved the general and localized corrosion resistance of nanostructured bainitic steel significantly.
| Item ID: | 41344 |
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| Item Type: | Thesis (Masters (Research)) |
| Keywords: | bainitic steel; carbon content; corrosion; electrochemical corrosion; electrolytic corrosion; galvanostatic polymerisation; microstructure; nanostructure; polyaniline, steel |
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| Additional Information: | Publications arising from this thesis are available from the Related URLs field. The publications are: Kazum, O., Mathan, M. Bobby, Beladi, H., Timokhina, I.B., Hodgson, P.D., and Khoddam, S. (2014) Aqueous corrosion performance of nanostructured bainitic steel. Materials & Design, 54. pp. 67-71. Kazum, Oluwole, Kannan, Mathan Bobby, Beladi, Hossein, Timokhina, Ilana, Hodgson, Peter, and Khoddam, Shahin (2014) Selective dissolution of retained austenite in nanostructured bainitic steels. Advanced Engineering Materials, 16 (4). pp. 442-444. Kazum, O., and Mathan, M. Bobby (2013) Galvanostatic polymerisation of aniline on steel: improving the coating performance in chloride-containing environment. Synthetic Metals, 180. pp. 54-58. Kazum, O., and Kannan, M. Bobby (2013) Galvanostatic coating of polyaniline on steel. In: Proceedings of the Australasian Corrosion Association Annual Conference, pp. 1-6. From: ACA 2013: Australasian Corrosion Association Annual Conference, 10-13 November 2013, Brisbane, QLD, Australia. |
| Date Deposited: | 17 Dec 2015 03:39 |
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