The correlation between the microhardness and microstructure features of anodic films on 2024 aluminum alloy formed in the mixed sulfuric acid/oxalic acid electrolyte was studied using micro-hardness tester and scanning electron microscope(SEM). The results show that the microhardness of the anodic fi lm is infl uenced by the microstructure of the anodic fi lm such as the fi lm porosity, and the order and continuity of the hexagon columnar cells. The fi lm microhardness increases as the porosity of the anodic fi lm decreases and the order and continuity of the fi lm cells increase. With the same current density, as the anodic fi lm thickens with anodizing time, the fi lm microhardness increases because the fi lm porosity decreases and the order and continuity of the cells are also improved. Under the condition of the same anodizing time, as the current density increases, the fi lm microhardness decreases due to the higher fi lm porosity and the poorer order and continuity of the fi lm cells. The fi lm porosity increases because the increased current density can accelerate the oxidation reaction, strengthen the fi led-assisted dissolution and the heating effect in the anodic fi lm, resulting in decreased fi lm order and continuity.
Two kinds of simulated concrete pore solutions(SPSs) were treated with different amounts of synthetic calcium silicate hydrate(C-S-H). The variation of the [Cl^-]/[OH^-] ratio in SPS was measured and the corrosion susceptibility of carbon steel in the SPS was investigated with potentiodynamic polarization, EIS and weight lose tests. The experimental results showed that for the SPS at p H 12.5, as the amount of C-S-H increases, the [Cl^-]/[OH^-] ratio increases thereby causing an increase in the corrosion susceptibility of the steel. While for the SPS at p H 9.7, with increasing C-S-H amount, the drop amplitudes of both [Cl^-]/[OH^-] ratio and steel corrosion rate first decrease and then increase, and a 3% C-S-H addition shows the best inhibition effect. XPS results demonstrate that after C-S-H treating in p H 12.5 SPS the [Fe^(3+)]/[Fe^(2+)] ratio in the film on steel surface is reduced while in p H 9.7 SPS the [Fe^(3+)]/[Fe^(2+)] ratio is increased. The different effects of the C-S-H amount on the two SPSs and the steel corrosion behavior result from the influences of C-S-H on the SPS p H, which is related to the composition of the SPS.
The adsorbing effect of calcined layered double hydroxide(CLDH) for chloride ions in simulated concrete pore(SCP) solutions was investigated with the potentiodynamic polarization method, impedance measurement, ion selective electrode analysis and XRD. CLDH could effectively adsorb Cl- and increase pH value in SCP solutions containing NaCl. The chloride to hydroxyl ions ratio([Cl-]/[OH-]) of the solution greatly decreased by CLDH treatment. In CLDH treated SCP solution with Cl-, the pitting potential of carbon steel notably increased, and the surface impedance was much higher, indicating strengthened passivation. The process of CLDH adsorbing chloride ions from SCP solutions was accompanied with the reconstruction of the layered structure.
The tensile strength of a corroded rebar in a 53-year-old concrete structure was studied. The microstructure of the metallic substrate, the fracture surface, and the corrosion product layers were investigated. Metallographic observation results showed that the carbon steel was constituted of ferrite and some pearlite. The tensile test results indicated that the corroded rebar presented low strength and elongation. In addition, the fracture surface of the rebar in the tensile test displayed dimple fracture behavior. The Raman spectroscopy results indicated that corrosion products at the general corrosion zone were obviously different from those at the localized corrosion zone. The rust layer at the general corrosion zone was composed of goethite(α-FeOOH), magnetite(Fe3O4), and hematite(α-Fe2O3), while that of the pitting zone was made of feroxyhyte(δ-FeOOH), goethite(α-FeOOH), and hematite(α-Fe2O3). However, the general tendencies that the corrosion products were constituted of a mix of oxides and hydroxides, the oxides mainly existed in the internal part and the hydroxides more presented in the external layer were observed.
