Document Type

Dissertation

Degree

Doctor of Philosophy (PhD)

Department

Civil Engineering

First Advisor's Name

Kingsley Lau

First Advisor's Committee Title

Committee chair

Second Advisor's Name

Norman D. H. Munroe

Second Advisor's Committee Title

Committee member

Third Advisor's Name

Ton-Lo Wang

Third Advisor's Committee Title

Committee member

Fourth Advisor's Name

Hesham Ali

Fourth Advisor's Committee Title

Committee member

Fifth Advisor's Name

Zhonghong Tang

Fifth Advisor's Committee Title

Committee member

Sixth Advisor's Name

Dale DeFord

Sixth Advisor's Committee Title

Committee member

Keywords

CBPC, Coating, Steel bridge, Corrosion, EIS

Date of Defense

3-22-2017

Abstract

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.

Identifier

FIDC001797

Available for download on Tuesday, October 24, 2017

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