Interfacial bonding mechanisms of carboxylic coatings on pretreated zinc surfaces

The aim of this Ph.D. work is to investigate the adhesion and de-adhesion of organic coatings on galvanized steel substrate. Chapter 1 introduces the project aim and research outline. Chapter 2 investigates Zn oxide semiconductor properties as the major compound formed on galvanized steel. The results showed a clear effect of surface treatment on Zn oxide doping concentration and flatband potential. Moreover, an obvious relation between Zn oxide composition and flatband potential was detected. The deposition mechanism of Zr-based conversion coatings, as potential replacement of the existing conversion coatings, on differently pretreated Zn surfaces has been studied in chapter 3. It was found that Zr deposition rate and the composition of the obtained conversion layer strongly depend on the initial hydroxyl fraction and surface roughness. Studying the metal-polymer interfacial bonding is extremely difficult due to the relatively high polymer thickness masking the interface region. Consequently, the metal-polymer interfaces are modeled in chapters 4-7 by adsorption of the representative molecules. Chapter 4 compares the adsorption properties of carboxylic based molecules on Zn and Fe substrates. It was shown that the obtained interfacial configurations are different on Zn and Fe substrates. In this case, one end adsorption of succinic acid molecules was promoted on Fe, while adsorption of Zn was conducted mainly with both ends. Consequently, Zn was selected for continuation of the research to explore more details of the interfacial properties. Chapter 5 examines the effects of an external potential on adsorption properties of the carboxylic model compounds. The results showed that the applied potential slightly increases the amount of adsorbates, which may contribute to a better adhesion. Moreover, the applied potential increases one-end adsorption of succinic acid molecules due to a C-C bonding cleavage. Desorption of the formed carboxylates was examined by means of cyclic voltammetry in chapter 6. It was shown that the molecular type and surface pretreatment affect the desorption process and consequently the interfacial bonding strength. It was found the metal-polymer interaction is accompanied by an electron exchange in the interfacial region. Consequently, scanning Kevin probe (SKP) measurements were performed to evaluate the Volta potential changes due to the adsorption. The results showed that the amount of electron transfer is correlated to the interfacial bonding density and molecular chain length. Additionally, an effect of adsorption configuration was detected on the obtained Volta potential shift. In the next step, the interfacial bonding properties of the real carboxylic coatings and Zn substrate were evaluated in chapters 8-10. Chapter 8 assesses the interfacial bonding of the residue polymer layers obtained after a dedicated polymer removal procedure. A comparable interfacial bonding mechanism was detected for the real coatings and model compounds applied on Zn surfaces. Chapter 9 investigates in-situ the interfacial bonding formation upon application of the polymers on Zn surfaces in a Kretschmann geometry. An equivalent interfacial bonding mechanism was detected compared to those obtained through the adsorption of the model compounds and residue layers. Additionally, it was shown that the metal-polymer interaction takes place in a relatively short time, while polymerization proceeds afterwards. The interfacial bonding degradation was examined by means of an integrated ATR-FTIR and EIS set-up in a Kretschmann geometry and described in chapter 10. The results showed a beneficial role of the polymerization process in the formation of lateral bondings blocking water penetration through the polymer structure. Moreover, it was found that the duration of the disbonding process depended on the Zn surface hydroxyl fraction.