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Impact of the microstructure of ZnAlMg coatings for steel on the formation mechanisms and performance of Cr VI -free conversion coatings

Abstract : To replace Zn coatings for steel by ZnAlMg in painted applications, it is necessary to optimize conversion coatings (CC) ensuring paint adherence and corrosion resistance. While on Zn-coated steels, phosphate CC (PCC) and Ti CC (TiCC) were comparable, accelerated tests demonstrated a lower blistering resistance of ZnAlMg with TiCC. The PhD aimed at understanding this difference, especially the role of the ZnAlMg microstructure and to propose a solution improving TiCC performances. Using SEM-EDS, XPS and nano-AES, it was demonstrated, at different steps of CC procedure, that PCC is homogeneously distributed on ZnAlMg and pure phases, while TiCC better covers eutectic phases than Zn dendrites. CC distribution correlated with the reactivity order expected from galvanic coupling and corresponding variation of local pH, necessary for CC precipitation. The reactivity difference was leveled by the crystal size of PCC, unimpacted by ZnAlMg heterogeneity. For TiCC, a Cu cementation pretreatment was formulated, ensuring a complete coverage of ZnAlMg surface and a higher blistering resistance after painting.
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Submitted on : Sunday, April 21, 2019 - 6:51:00 PM
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Perrine Tanguy. Impact of the microstructure of ZnAlMg coatings for steel on the formation mechanisms and performance of Cr VI -free conversion coatings. Inorganic chemistry. PSL research University, 2019. English. ⟨tel-02105785⟩

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