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- Eslami, Maryam, et al.
(författare)
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Deposition and Characterization of Cerium-Based Conversion Coating on HPDC Low Si Content Aluminum Alloy
- 2017
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Ingår i: Journal of the Electrochemical Society. - : Electrochemical Society. - 0013-4651 .- 1945-7111. ; 164:9, s. C581-C590
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Tidskriftsartikel (refereegranskat)abstract
- Cerium-based conversion coatings were deposited on high pressure die cast (HPDC) Al-Si alloys using an immersion method. Hydrogen peroxide and sodium chloride were added to the conversion solution to accelerate the coating formation and to understand its formation mechanism. These studies showed that the deposition of cerium hydroxide/ oxide conversion layer starts from iron-rich intermetallic particles, which are located inside the eutectic region and then the coating growth continues to cover the entire alloy surface. This phenomenon passivates the active interfaces between iron-rich intermetallic particles and/ or the eutectic silicon phase and the aluminum matrix, which are prone to localized corrosion in chloride ions containing environments. Accordingly, values of the total impedance in EIS measurements significantly increased for the treated substrates. Morphologies of the conversion coatings and the oxidation state of cerium compounds were found to be dependent on the composition of the solution and the presence of chloride ions and/ or hydrogen peroxide. Aluminum alloy with higher silicon content showed a more active surface during immersion in the conversion solution. This makes it more difficult to be treated using aggressive conversion solutions.
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| 2. |
- Eslami, Maryam, et al.
(författare)
-
Study of selective deposition mechanism of cerium-based conversion coating on Rheo-HPDC aluminium-silicon alloys
- 2017
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Ingår i: Electrochimica Acta. - : Pergamon Press. - 0013-4686 .- 1873-3859. ; 255, s. 449-462
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Tidskriftsartikel (refereegranskat)abstract
- Cerium-based conversion coatings were deposited on Rheo-High Pressure Die Cast (HPDC) Al-Si alloys by immersion in cerium nitrate aqueous solutions. Rheocast Al-Si alloys have a heterogeneous microstructure and present a challenge for the conversion treatment. Different parameters were studied to optimize the conversion coating, and NaCl or H2O2 were also added to the solution to modify or accelerate the deposition process. The mechanism of the coating formation was studied by means of focused ion beam milling (FIB) assisted SEM. The results show that applying cerium-based conversion coating to Al-Si alloys, is possible and a preferential deposition is obtained due to the presence of iron-rich intermetallic particles inside the eutectic region. The formation mechanism of selectively deposited cerium-based conversion coating includes dissolution of aluminium matrix, selective dissolution of aluminium from the noble intermetallic particles, oxidation of iron from the intermetallic particles, and the deposition of cerium hydroxide/oxide layer. The results reveal that the improvement in corrosion resistance in the presence of selectively deposited cerium-based conversion coating is more significant compared to the homogenous coating deposited from the conversion solution containing H2O2.
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