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Harvard Thesis Template XXX cycleXXX Doctoral School in Materials, Mechatronics and Systems Engineering Surface Treatments to Protect Conventional and Rheo-High Pressure Die Cast Al-Si Alloys from Corrosion Maryam Eslami January 2019 THESIS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Surface Treatments to Protect Conventional and Rheo- High Pressure Die Cast Al-Si Alloys from Corrosion Maryam Eslami Approved by PhD commission Prof. Flavio Deflorian Prof. Jinshan Pan Department of Industrial School of Chemical Science and Engineering Engineering University of Trento, Italy KTH Royal Institute of Technology, Sweden Dr. Caterina Zanella Dr. Cecilia Monticelli Department of Materials and Department of Engineering Manufacturing University of Ferrara, Italy Jönköping University, Sweden Dr. Massimo Pellizzari Department of Industrial Engineering University of Trento, Italy University of Trento Department of Industrial Engineering Trento, Italy 2019 Surface Treatments to Protect Conventional and Rheo-High Pressure Die Cast Al-Si Alloys from Corrosion Cover Photo: From the top left: the microstructure of Rheo-HPDC Al-Si alloy, localized corrosion (trenching) around the eutectic silicon, a cerium- based nucleus, a cross-sectional view of the same nucleus showing the selective deposition site (an intermetallic particle), the surface morphology of polypyrrole coating and the cross-sectional view of oxidation occurring at the polypyrrole/Al interface. Doctoral Thesis © Maryam Eslami, 2019 Published in Trento (Italy) – by University of Trento ISBN: - - - - - - - - - Will is character in action. William McDougall Abstract Surface Treatments to Protect Conventional and Rheo-High Pressure Die Cast Al-Si Alloys from Corrosion Rheocasting process integrated with the high pressure die casting method (Rheo-HPDC) has established itself as a new promising technology to produce high-quality components. However, different types of microstructural segregation induced by the semi-solid process influence the properties of the final component. The semi-solid microstructural features and the new compositions require detailed corrosion studies and verification. The first part of this thesis deals with microstructural and corrosion studies of the conventional and Rheo-HPDC Al-Si alloys. In this part, corrosion properties of two Al–Si alloys containing 2.5 and 4.5 wt % silicon cast by Rheo-HPDC method were examined in the diluted Harrison solution using polarization and electrochemical impedance spectroscopy (EIS) techniques on as-cast and ground surfaces. The microstructural studies revealed that samples taken from different positions (with respect to the feeding gate) contain different fractions of solid and liquid parts of the initial slurry. It was shown that the Rheo-HPDC Al-Si alloys are prone to the localized form of corrosion inside the eutectic region at the interface of aluminum with silicon phase and intermetallic particles. Electrochemical behavior of as-cast, ground surface, and bulk material was shown to be different due to the presence of a segregated skin layer and the surface quality. Corrosion properties of the two Al-Si alloys cast by the conventional and Rheo-HPDC process were also evaluated and compared in 0.01, 0.05, 0.1 and 0.6 M NaCl solutions. The conventional HPDC and semi-solid alloys presented similar EIS responses. However, the semi-solid samples with a lower fraction of the eutectic phase showed slightly higher impedance values in the more diluted sodium chloride solutions. Corrosion morphological I features, including localized corrosion, trenching and co-operative corrosion rings were comparable for both types of alloys. However, in the anodic polarization test, the semi-solid alloys presented a higher resistance to pitting corrosion. To protect aluminum alloys from corrosion, chromium-based conversion coating has been successfully used for decades, due to its extensive protection. However, rising concerns and new restrictions on the environmental hazards of Cr (VI) compounds have led to intensive efforts to develop alternative coatings. The second and third parts of this thesis address the effort to investigate two alternatives in this field. Cerium-based conversion coatings were deposited on the conventional and Rheo-HPDC Al-Si alloys by immersion in cerium nitrate aqueous solutions. 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 results revealed that applying cerium-based conversion coating on Al-Si alloys, is possible and a selective deposition is obtained due to the presence of iron-rich intermetallic particles inside the eutectic region. Under the accelerated conditions, the deposition mechanism includes dissolution of the aluminum matrix, selective dissolution of aluminum from the noble intermetallic particles, oxidation of iron from these particles, and the deposition of cerium hydroxide/oxide layer. The results revealed that the improvement in corrosion resistance in the presence of selectively deposited cerium-based conversion coating is more significant compared to the homogenous coating obtained from the conversion solution containing H2O2. The aluminum alloy with a higher amount of silicon showed more active surface during the conversion process which reduces the required concentration of Ce(NO3)3 but also makes it difficult to work with more aggressive solutions. II In the third part of this thesis, the possible protective effect of polypyrrole coating on pure aluminum and Rheo-HPDC Al-Si alloys was investigated. Different electropolymerization solutions containing the Py monomer, SDS, DHBDS (Tiron), C6H8O7 and NaNO3 were used. The presence of nitrate anions led to the passivation of the aluminum electrode (both pure and alloy) during the electropolymerization and to the deposition of a thicker/more conductive coating. These facts resulted in longer and more efficient corrosion protection in NaCl solutions. This polypyrrole coating was able to keep the alloys’ surface potential noble for at least 168 hours. Which can be attributed to the anodic protection provided by the reduction of the polymer. It was shown that the presence of silicon phase or intermetallic particles has a positive effect on the electropolymerization of polypyrrole film. Therefore, the coatings deposited on the alloys possess higher thicknesses compared to those deposited on the pure aluminum. In the presence of chloride ions, all coatings suffered from the formation of blisters as a result of severe (localized) galvanic interaction of polypyrrole with aluminum. This may question the application of polypyrrole coating in concentrated NaCl solutions. However, it is shown that the protection efficiency can be improved by altering the solution chemistry which affects the polymer/metal interface and the conductivity and the barrier properties of the coating. Therefore, the application of polypyrrole in corrosion protection is not totally ruled out but needs specific considerations. III Acknowledgments I would like to express my sincere gratitude to my supervisors Prof. Flavio Deflorian and Dr. Caterina Zanella (Jönköping University, Sweden) for their scientific guidance and support during my PhD. They truly supported me through many tough times and never gave up on me. I would also like to acknowledge Dr. Michele Fedel for many fruitful discussions and his helpful comments and suggestions. Many sincere thanks go to Mr. Luca Benedetti for his unconditional and kind assistance to my experiments. I gratefully acknowledge Dr. Giorgio Speranza (FBK-CMM) for his great help with the XPS measurements and analysis. I would also like to thank Prof. Sasha Omanovic (McGill University, Canada) for his hospitality, guidance and support during my internship in Montreal. In addition, I would like to express my gratitude to the colleagues and staff at Electrochemistry/Corrosion lab and Chemical Engineering department at McGill University. I am also grateful to the members of the Material and Manufacturing group at Jönköping University for their collaboration and hospitality during my stay in Jönköping. I would like to especially thank Dr. Anders E. W. Jarfors and Dr. Mostafa Payandeh for their prestigious help with the first part of my research. I would also like to acknowledge Dr. Nils-Eric Andersson and Dr. Ehsan Ghassemali for their assistance with the SEM-FIB analysis. I would like to thank COMPtech AB (Sweden) for the production of alloy components and the technical support. I would also like to appreciate the European Cooperation in Science and Technology for partially funding this project. Last but not least, I would like to acknowledge the unconditional love and support from my family and friends, especially my mom and my cousin Roya and her lovely family. I would also like to thank my friend Hamid Hassani for being a kind brother to me in Trento. A special appreciation goes to my dearest friend Rajeev Piyare who has been always there for me. IV List of Publications This thesis is primarily based on the work presented in the following papers. In the text they are referred by roman numerals. Journal publications: Paper I M. Eslami, M. Fedel, G. Speranza, F. Deflorian, N-E. Andersson, C. Zanella, Study of selective deposition mechanism of cerium-based conversion coating on Rheo-HPDC aluminum-silicon alloys, Electrochimica Acta 255 (2017) 449-462. Paper II M. Eslami, M. Fedel, G. Speranza, F. Deflorian, C. Zanella, Deposition and characterization of cerium-based
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