United States Department of Transportation
i
Corrosion evaluation of novel coatings for steel components of highway bridges.
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2015-03-01
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Abstract:The Florida Department of Transportation (FDOT) had expressed interest in gauging the available coating
technologies that may have suitable applications for steel components in highway bridges. The motivation was to
possibly identify coating systems that would provide corrosion durability of steel components in highway bridges
and reduce costs associated with regular inspection and maintenance of the coating systems. Chemically
bonded phosphate ceramics (CBPC) and the thermal diffusion galvanizing (TDG) process have been identified
for further testing due to growing interest in the systems for possible corrosion mitigation, the lack of sufficient
data to determine their effectiveness for corrosion protection of steel structures, and their commercial availability.
An issue for further evaluation of long term durability and corrosion protection by CBPC coatings is the degree
of deterioration of the ceramic coating in aggressive environments. Initial testing showed that the material was
not durable in highly alkaline solutions (pH 13), and in exposure conditions that cycle between frequent high
moisture contents and drying conditions. The rather short-term outdoor exposures investigated so far have
produced some promising results, but the significant extent of undercoating surface oxidation that was observed
may compromise long-term durability. The intermediate alloy layer was not consistently identified and its role in
corrosion mitigation has not been elucidated.
Although no severe steel corrosion was observed for TDG in outdoor exposure, degradation of the topcoat,
when present, and subsequent consumption of the TDG would result in a shorter service life of the coating for
corrosion mitigation. Variations in quality of the topcoats resulted in variations in coating performance. The
findings suggest that sufficient application of the TDG and robust topcoats are required for long-term durability.
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