| Summary: | 博士 === 國立臺灣海洋大學 === 材料工程研究所 === 95 === The objective of this investigation is to evaluate the effects and mechanisms of different surface treatments on reinforcing steel and concrete of cement-based composites. The microstructure of coating layer and electrochemical behavior of rebars exposed to various environments were studied. The galvanized metals such as zinc and zinc-aluminum alloy acting as sacrificial anodes to protect the substrate metal. The corrosion behavior of epoxy coating specimens, zinc-aluminum electrical arc spraying coating specimens and stainless steel specimens where subjected to cyclic salt spray condition and were evaluated by using weight loss method. The surface treatment methods for cement-based composite consisted of methyl methacrylate polymer impregnation and penetrated silicate sealer application.
Scanning electronic microscopy (SEM), backscattered electron imaging (BEI), energy dispersive X-ray spectrometry (EDS), and X-ray diffractometer (XRD) were employed to observe the interface between the coating layer and substrate and to evaluate the microstructure of the specimens. To assess the effect of coating materials in an alkaline chloride solution with various pH and chloride ion concentrations, the direct current polarization measurement was performed. The resulting pitting corrosion was evaluated by a microscopic analysis. The absorptions and total charge passed from the rapid chloride ion penetration test (RCPT) indicated the effect of the various surface treatments on the cement-based specimens. Pore structures and pore-size distribution for the treated specimens were observed using SEM micrographs and mercury intrusion porosimetry (MIP) spectrum, which were corresponding to the macro-observation.
Test results of specimens with 30 μm zinc-aluminum and 100 μm zinc coatings indicated the corrosion potential of ZA specimen was lower than those of ZR specimens and untreated specimens when immersed in a chloride ion solution. However, ZA specimens showed a lower corrosion current density and a lower corrosion rate than in ZR specimens. The coating material would rapidly deteriorate if conventional ZR rebars were exposed to pH 7 and pH 11 solutions with a chloride ion concentration exceeding 1.0 wt%. Under a chloride-ion attack, the corrosion rate of zinc-aluminum alloy coating was 10-57% lower than those specimens with conventional zinc coatings in a pH 7, 9 and 11 solutions. Thin and dense structure of ZA coating minimizes the crack initiation and propagation resulting from cold working and additionally improving the corrosion protection. The hot-dipped process produces an alloy layer between the substrate and coating metals which provides a better bonding affinity for coating materials.
The product which filling the voids (above 10 μm), micro-cracks, or large capillary pores (50 nm - 10 μm) was generated from the reaction of solid silicate sealer (SA) and calcium hydroxide. The flaky material, which was observed in SB specimens, covered the hydration product, densified the internal structure and reduced the size of large and medium capillary pores (10 nm - 10 μm). The polymer treated specimens show the most promising testing results because the polymer particles are probably finer than the medium capillary pores (10-50 nm). The mechanism of polymer impregnation was to narrow down the size of large capillary pores (50-10000 nm) and fill the medium capillary pores. The polymer impregnation would densify the pore structure of the interfacial transition zone and enhance the bonding strength between aggregate and paste.
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