Quantitative analysis during surface pretreatment by coupling in situ elementally resolved electrochemistry and electrogravimetry – QUEENE

The degradation of the materials may cause significant problem in our society by limiting the performance and sustainability of a system using those materials. Controlling the degradation of the materials is critical to minimize the financial burden and also to economize the scarce resources that we consume in the world. This often involves the surface pretreatment to protect the materials from the degradation, and to enhance the adhesion property between the substrate and coating which may provide a longer lifetime of a system. The highly efficient Cr(VI)-based formulation has been used for a several decades, however, these have been phase out since 2006 by European Union members due to their genotoxicity causing irreversible health damages to live organisms. To the end, alternative surface pretreatment process without Cr(VI) have been suggested by the researchers. Understanding reaction mechanisms of the newer generation of environmentally friendly surface pretreatment is significant to effectively replace the toxic Cr(VI)-based conventional treatments. The layered double hydroxides (LDHs) have been considered to be efficient and eco-friendly surface pretreatment materials due to their unique characteristics of anion/cation exchange ability, and physical/chemical barrier of the corrosion including self-healing properties. The objective of this project is to quantitatively understand each reaction step of the surface layer formation (e.g., LDHs and conversion coatings) and its reactivity using a novel coupling between atomic emission spectroelectrochemistry (AESEC) and electrochemical quartz crystal microbalance (EQCM). The AESEC gives real-time elemental dissolution rates for a material/electrolyte combination during all possible electrochemical measurements. The EQCM directly measures the mass and the mechanical change of the films often coupled with the AC-electrogravimetry (AC-EG), an advanced EQCM method, providing the in situ interfacial changes with high-precision. In this way, coupling EQCM with AESEC can give a real-time elementally resolved complementary information of the mass change, mechanical evolution, and dissolved species during electrochemical reactions of the surface pretreatment which have not been clearly elucidated to date. The formation and reactivity of the MgAl-, ZnAl-LDHs on the Mg-, Al, and Zn-based alloy coatings will be monitored by the AESEC-EQCM method with in situ/ex situ surface characterization to identify chemical species formed in each step of reaction. The AESEC-EQCM will also be coupled with the electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) analysis, which are the specialties of the host laboratory LISE (Laboratoire Interfaces et Systèmes Electrochimiques, Sorbonne Université), to give an in-depth understanding of faradaic and non-faradaic reaction mechanisms during surface layer formation. The multi-scale novel electrochemical/spectroscopic approach developed in this project will be a milestone in our understanding of the surface pretreatment mechanisms. We aim to suggest an optimized surface pretreatment procedure by the novel AESEC-EQCM coupling in this project which will provide the quantitative real-time information of the surface change. In this way, the total cost of the surface pretreatment can be reduced by decreasing the consumption of the materials and by improving the corrosion resistance of the system. More significantly, this project will contribute to replace the toxic Cr(VI)-based formulations to more eco-friendly materials.