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Development of anticorrosive Zn-based alloy coatings for cold formed steel body parts

Abstract : Numerous coating systems are used by the automotive manufacturers for the corrosion protection of cold formed steel body parts. According to the literature, various approaches and solutions were suggested over the past decades. However, only a few systems were able to go into serial production. The objective of the present doctorate thesis is to develop and characterize alternative anticorrosive coatings based on Zn-alloys for the corrosion protection of cold formed steel components used for automotive body parts. Based on a thorough literature review, two interesting approaches were considered, consisting in the pulse electrodeposition of Zn-Mn and ZnMn-Al2O3 coatings. The deposits were characterized in order to determine their composition, microstructure as well as their crystallographic properties. The suitability of these new deposits was then assessed based on their anticorrosive behaviour, cold formability and paintability. Using pulse electrodeposition permitted to deposit monophasic, pore-free and compact Zn-Mn alloys containing about 15 wt.% Mn at high current efficiencies. Electrodeposition on various substrates exposed a stable and reproducible deposition process. Electrochemical experiments and accelerated corrosion tests revealed a promising anticorrosive performance of the Zn-Mn alloys. A substantial improvement in the time before red rust appearance and a significant decrease of the corrosion rate were observed in comparison to the traditional Zn deposits. While a slight increase of the hardness was observed in comparison to the Zn coatings, a lower crack density was observed on the Zn-Mn deposits after tensile experiments. Preliminary phosphating and painting tests revealed the good suitability of the Zn-Mn alloys for those processes with the formation of a dense and homogeneous phosphate crystals network and the successful deposition of a homogeneous and pore-free electrophoretic paint layer. However, some challenges were identified with Zn-Mn deposits presenting some unsatisfactory properties when immersed in chloride-containing environments. With the aim to solve these issues, the deposition of ZnMn-Al2O3 composite coatings was considered. The impact of the particles and electric parameters were investigated with a particular attention given to the distribution of the particles in the metallic matrix. Using optimized deposition parameters, homogeneous, compact and monophasic composite coatings containing about 15 wt.% Mn were obtained at high efficiencies. Further analyses revealed the presence of particles on the surface as well as at the substrate – deposit interface. First mechanical tests revealed interesting properties of the composite coatings with a substantial increase of the coating hardness. Electrochemical experiments also revealed some promising insights on the corrosion behaviour of the ZnMn-Al2O3 deposits.
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Submitted on : Tuesday, December 14, 2021 - 2:48:01 PM
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Florian Claudel. Development of anticorrosive Zn-based alloy coatings for cold formed steel body parts. Materials. Université de Lorraine, 2019. English. ⟨NNT : 2019LORR0264⟩. ⟨tel-02558306⟩



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