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Title:	Mechanical properties and corrosion resistance characteristics of plasma sprayed titania coatings deposited on mild steel for marine structures
Authors:	Seyed Mohsen Forghani (P46821)
Supervisor:	Mariyam Jameelah Ghazali, Associate Prof.
Keywords:	Corrosion Marine structures Corrosion and anti-corrosives
Issue Date:	20-Jun-2014
Description:	Salutan seramik diguna secara meluas untuk mengatasi masalah haus dalam banyak industri. Dalam aplikasi marin, komponen sistem pendorongan seperti aci dan komponen dalaman dalam kapal dan kapal selam menunjukkan keupayaan salutan seramik dalam mengurangkan haus dan menyokong kekuatan kepada struktur yang dilindungi. Titania (TiO2) adalah satu contoh yang baik bagi salutan seramik. Tujuan kajian ini adalah untuk mengoptimumkan sifat mekanik dan rintangan kakisan salutan titania pada keluli lembut untuk aplikasi marin. Untuk mencapai matlamat ini, dua jenis endapan titania telah dipilih iaitu bahan suap konvensional bersaiz mikro (berpadu dan hancur, bersaiz 10-45 μm) dan berstruktur nano (tergumpal dan tersinter bersaiz 5-32μm). Bagi kerja pengoptimuman, suatu reka bentuk eksperimen (DOE) dengan menggunakan perisian Design Expert berdasarkan kaedah Analisis Varian (ANOVA) telah digunakan. Mikrokekerasan salutan berstruktur nano optimum didapati adalah hampir sama dengan salutan mikrostruktur mikro. Dalam kajian haus, salutan berstruktur nano telah menunjukkan kadar haus terendah berbanding salutan berstruktur mikro. Keputusan menunjukkan bahawa rintangan haus salutan berstruktur nano adalah 20% lebih tinggi berbanding dengan salutan konvensional bersaiz mikro. Analisis pada permukaan haus dan subpermukaan endapan menunjukkan mekanisma haus lelapisan yang dominan telah berlaku. Punca utama bagi penambahbaikan kerintangan haus struktur bersaiz nano disebabkan oleh penambahbaikan kelasakan patah dan kehadiran struktur dwi-modal dalam salutan, yang menghalang penyebaran retak di dalam salutan. Keupayaan litar terbuka (OCP) telah diukur menggunakan elektrod kalomel tepu (SCE) di dalam larutan 3.5% NaCl. Semua keluli lembut yang bersalut didapati lebih bersifat elektropositif dengan julat antara -0.40 V hingga -0.50 V, berbanding dengan substrat tanpa salutan dengan keupayaan lebih kurang -0.7 V. Bagi kesan saiz partikel, keupayaan salutan mikropartikel adalah lebih stabil berbanding dengan yang berstruktur nano. Kadar hakisan yang diperolehi daripada plot ekstrapolasi Tafel menunjukkan yang salutan berstruktur mikro adalah lebih tahan kakisan berbanding dengan yang berstruktur nano. Bagi salutan berstruktur nano kewujudan struktur dwi-modal menggalakkan larutan menembusi permukaan logam, manakala salutan bersaiz mikro pula menghalang penembusan larutan pada permukaan salutan daripada berlaku. , Salutan berstruktur nano mempunyai kadar kakisan 0.072 mm/tahun manakala lapisan berstruktur mikro menunjukkan kadar kakisan 0.023 mm/tahun. Di samping itu, analisis spektroskopi impedans elektrokimia (EIS) juga menunjukkan tren yang serupa di mana salutan nanopartikel tergumpal mempunyai rintangan kakisan yang lebih rendah berbanding salutan bersaiz mikro. Secara ringkasnya, salutan konvensional bersaiz mikro adalah pilihan terbaik dalam mengawal masalah kakisan keluli lembut. Walau bagaimanapun, salutan berstruktur nano masih boleh menjadi pilihan yang baik untuk perlindungan kakisan yang stabil dengan sifat rintangan haus yang baik.,Ceramic coating is widely used to overcome wear problems in many industries. In marine applications, components of the propulsion systems such as shafts and interior components in ships and submarines indicate the ability of the ceramic coatings in reducing wear and supporting strength to the protected structures. Titania (TiO2) is one good example of these ceramic coatings. The aim of this study is to optimise the mechanical and corrosion resistant properties of titania coatings on mild steels for marine applications. To achieve this goal, two different types of titania deposition were selected; namely a conventional micro-sized (fused and crushed, particle size 10-45 μm) and a nano-structured (agglomerated and sintered, particle size 5-32 μm) feedstocks. For optimisation works, a design of experiment (DOE) by using a Design Expert software based on Analysis of Variance (ANOVA) method was utilised. It was found that the microhardness of the optimised nanostructured coatings was similar to the micro-structured coatings. In wear test, the nanostructured coatings exhibited the lowest wear rate than those in the microstructured coatings. The results showed that the wear resistance of the nanostructured coatings was about 20% higher than the conventional micro-sized coatings. Analyses on the worn surfaces and the subsurface of the depositions indicated that a dominant wear mechanism of delamination had occurred. The main cause for the improved wear resistant of the nano-sized structure was due to the improved fracture toughness and the presence of the bi-modal structures in its coatings, resisting crack propagation within the deposition. The open circuit potential (OCP) was measured by using a Saturated Calomel Electrode (SCE) in a 3.5% aqueous NaCl solution. It was found that all coated mild steels were more electropositive; ranging between -0.40 V to -0.50 V, than the bare substrates with a potential about -0.7 V. As for the particle size effects, the potential of the micro-particle deposition was more stable compared to the nanostructured ones. The corrosion rates obtained from the Tafel extrapolation plots showed that the microstructured coatings were better corrosion resistant than the nanostructured ones. In the nanostructured coatings, the existence of a bi-modal structure encouraged the solution penetration on the metal surface, whereby the micro-sized deposited coating resisted against the penetration of the corrosive medium. Relatively, the nanostructured coating possessed a corrosion rate of 0.072 mm/year whereas the microstructured coating showed a corrosion rate of 0.023 mm/year. The micro-porosities resulted from the deposition of a bi-modal microstructure in the nanostructured coating reduced the corrosion resistance. The electrolyte could diffuse into the substrate and cause more damage than it could do in the denser conventional coating. Moreover, the Electrochemical Impedance Spectroscopy (EIS) analysis also indicated a similar trend in which the agglomerated nanoparticle coatings had lower corrosion resistance than the micro-sized coatings. In short, the conventional micro-sized coatings were most preferable in controlling corrosion problems for mild steels. However, the nano-structured coatings may still be a good option for a relatively stable corrosion protection with good wear resistant characteristics.,PhD
Pages:	200
Call Number:	TA467.F646 2014 3 tesis
Publisher:	UKM, Bangi
Appears in Collections:	Faculty of Engineering and Built Environment / Fakulti Kejuruteraan dan Alam Bina

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