Layered double hydroxide based smart protective coating systems
Creators: name: Iqbal, Muhammad Ahsan
Publisher: name: Università degli studi di Trento
Year: 2021

description: Corrosion protection of aluminum and its alloys is a challenging subject to widen the service life of material and its industrial usage. Numerous protective measures have been employed for the corrosion protection of aluminum alloys. Chromate based coatings system was in use for a long time to protect the light metals alloys, but after the ban on Chromates based coating (2017) in European Union due to strict health and safety rules and regulations, there is a need to develop environmentally friendly, greener coating systems to functionalize aluminum surface and to provide sufficient chemical and electrochemical stability against the aggressive environment and can possess multifunctional properties. The present study investigates the designing of the "Layered double hydroxide (LDHs)" based system for the protection of AA6082 alloy, which until now has given not much attention. LDHs demonstrate unique characteristic which helps to obtain special microstructure to attain multi-functional properties along with improved corrosion resistance properties. The novel environmentally friendly “Layered double hydroxides” (LDHs) thin films were synthesized, through chemical conversion approach, directly on the aluminum AA6082 substrates, and precisely characterized by various physical and electrochemical approaches. The defining of proper synthetic conditions for the synthesis of LDH is problematic due to the influence of certain parameters which made an impact not only on the structural properties of the LDHs but also on the anticorrosion behavior of LDH. This study focused on the thorough investigation of the optimization of synthetic conditions for LDHs and their effect on LDH assembly, surface morphology, thickness, interaction with corrosion inhibitors to control the corrosion resistance properties. The first step of the work is to control the synthetic parameters to obtain unique LDHs structural geometries for the development of anti-corrosion thin films. The addition of a complexation agent i.e. urea, ammonium hydroxide was also introduced to obtain distinct surface morphologies. Various LDHs classes were developed on the aluminum substrate in that scenario, for example, MgAl-LDH, ZnAl-LDH, NiAl-LDH, CaAl-LDH, and a detailed comparative study is reported about the dependency of structural- electrochemical relationships. The aim was to find the ideal films' properties, which leads to widening the potential window of corrosion resistance films in the defined electrolytes. To analyses high-temperature applications, calcination of developed MgAl-LDHs is investigated to understand the effect of thermal treatments on the LDHs structure, basal spacing, intercalated anions, and its effects on corrosion resistance properties. The second part of the thesis is focused on the modification of the LDHs. Initially, graphene is introduced inside the LDHs network due to graphene chemical inertness and electrochemical characteristics. A thorough investigation is reported about the interaction of graphene with LDHs, and the ability of graphene to tune the properties of LDH for the designing of improved protective LDHs films. Specifically, the research focused on the use of graphene to seal the micropores of LDHs films to promote barrier properties and explore the graphene interaction with the LDHs. Overall, the impedance modulus of the films was compared with the results of virgin LDHs and the efficacy of the graphene-based LDH system is described. In the third part of the thesis, cerium modified LDHs are developed on the anodic AA6082 substrate to understand the self-healing characteristic of the modified LDHs based system and their long-term protective ability. Rare earth elements found to have a significant inhibiting effect and are the object of considerable scientific interest, exhibiting nontoxic nature. Conceptually, the cerium modified LDH grown on the anodized surface can seal the micropores of the anodized surface (improved barrier properties), while LDHs themselves provide active protection via entrapment of aggressive species and through self-healing properties. The final part of the thesis comprises the introduction of a double-doped effect, where cerium was introduced inside LDHs galleries and further doped with superhydrophobic species to obtain compact LDH multifunctional films with enhanced LDHs corrosion resistance properties. In the case of the double doped cerium-based LDHs films, the improvement in the corrosion resistance properties were observed during the long-term EIS measurements, while superhydrophobic, self-cleaning characteristics, and UV radiation impact on coating was thoroughly reported. Double doped LDHs lead to an influential increment in corrosion resistance properties, durability, and long-term stability. Furthermore, the role of LDHs as adsorbents for the heavy metals present in drinking water is also reported. Different adsorption models are studied, and adsorption kinetics is reported to understand the adsorption behavior of LDHs against arsenic impurities. The results depicted the successful removal of arsenic from drinking water with high efficiency than traditionally used materials.