Coatings are widely used to mitigate corrosion of structural steel in aggressive humid environments. However, the service life is often diminished in aggressive environments. Repair of coatings can be costly due to materials, labor and environmental controls. So, in search for novel coatings, Chemically Bonded Phosphate Ceramic (CBPC) coating was investigated for marine bridge application. The research on CBPC coating considered various exposure environments such as inland, beach, salt-fog, wet and alternate wet and dry exposure to identify the degradation mechanism. To assess the corrosion damage, the coating was evaluated by visual inspection, thickness, adhesion, microscopy and X-ray diffraction. The CBPC coating degraded initially due to alternate wet and dry exposure. The unreacted coating constituent reacted further in moist environment to form magnesium phosphate hydrate and enhanced bulk coating porosity. That facilitated moisture to the coating substrate and formed apparent protective iron phosphate hydrate by interaction with steel substrate to the coating constituents. Passive-like conditions were observed in wet test of chloride-free solutions for the formation of hydration product of magnesium but that type of hydrate was not identified in chloride solution for the apparent high solubility. The resolved coating impedance parameters were introduced to characterize the bulk ceramic degradation. The solution resistance did show a decrease for all samples due to leaching of minerals from the bulk material. The resolved pore resistance did not show any distinct change, though there was an indication of bulk coating degradation by MIP. Water saturation level during exposure was also calculated from the resolved capacitance. An approach was proposed to transfer the pre-exponential term, Yo to coating capacitance, CC for ceramic coating. The estimated value of the coating capacitance from the developed technique indicated early saturation with water during exposure due to the porosity. So, the extent of CBPC coating permeability and degradation could not be resolved only by conventional analysis approaches. However, the formation of iron hydrogen phosphate hydrate and iron phosphate hydrate from reaction of unreacted coating constituents was thought to provide apparent enhanced corrosion protection but there is a probability of steel substrate corrosion in extended exposure in humid environment.

The thesis is focused first on the research of literature on the topic of phosphate ceramics and then on the search for a suitable synthesis of phosphate binder based on Y2O3-P2O5-H2O and characterization of the resulting products. Various types of syntheses were prepared with subsequent characterization of the phase and chemical composition of the final products. The samples were measured on an electron microscope to determine their surface morphology and also element maps were obtained. The samples were also analyzed by heat microscopy to determine the temperature behavior of the samples. After measurement on a heat microscope, the samples were measured again on an electron microscope, in order to detect changes caused by the action of high temperatures, in particular to assess the presence of binder or sintered phases.

The doctoral thesis is focused on the study, research and development of refractories with phosphate bonding. Primary motivation is preparation of phosphate-based binders that can be applied for non-shaped insulating material fabricated by “in situ” foaming. The phosphate binders based on Al2O3-H3PO4 system are one of the most common in refractory technology. The amphoteric nature of the aluminium and its oxides require the setting of Al2O3-H3PO4 binders under the higher temperatures. Application of higher temperatures is necessary to obtain the minimal manipulation strength of as-prepared green bodies. The main goal of the work is preparation of ytrium phosphate-based binders via different preparation techniques. Yttrium based binders would allow phosphate refractory preparation by standard way, i.e via reaction between binder and reactive aggregate, without structural disintegration of foamed material even before getting manipulation strength or the final firing procedure. Yttrium is more basic than aluminium and therefore more reactive in Y2O3-H3PO4 system.. Yttrium oxide is highly refractory material and sintered yttrium monophosphate has excellent properties from the thermal and corrosive stability point of view. This fact should compensate higher price of this material. Experimental part of this work studies the possibilities of binders preparation in Y2O3-H3PO4 system. The mixtures were designed to prepare binding phase Y(H2PO4)3. Products of these reactions were characterized in phase and chemical point of view. In order to study and describe setting and hardening processes of phosphate binders, the products were “in situ” analyzed by the thermal analyses and high-temperature diffraction analysis up to the 1300 °C. High temperature products were then characterized by means of phase and chemical composition and morphology.