Atsushi Nishikata

We studied the passivation behavior of hot-dip galvanized steel (HDG) rebar and ordinary steel (black) rebar in fresh concrete using electrochemical techniques. Although the passivation behavior was considerably different, both types of rebar were fully passivated. The black rebar was immediately passivated after exposure to fresh concrete; however, the HDG rebar was passivated after being kept active for tens of hours. The corrosion rates of both types of rebars after passivation seemed comparable. To further investigate the difference, we monitored the passivation processes by electrochemical impedance spectroscopy using carbon steel and zinc electrodes in fresh mortar and saturated Ca(OH)2 solution, which simulated the water in the concrete pores; furthermore, we observed similar trends for fresh concrete. Initially, the Zn surface was partially covered with calcium hydroxy zincate (CHZ) whose coverage increased with exposure time. Finally, the surface was fully covered with a CHZ film after passivation.

To investigate hydrogen absorption behavior into carbon steel during corrosion in an aqueous sodium chloride (NaCl) droplet, a simultaneous measurement system of the corrosion potential, Ecorr and hydrogen permeation current, iper was developed using the Kelvin probe (KP) technique and the Devanathan–Stachurski (DS) method, respectively. This system outputs the interrelation between corrosion and hydrogen absorption into steel throughout the drying process of an NaCl droplet. Our results showed that hydrogen absorption into the steel occurred when the Ecorr shifted in less noble direction under wet conditions, and ceased at a higher potential of Ecorr when the steel surface dried up. Based on the results of the transients of the iper, the amount of hydrogen absorbed during the drying of the NaCl droplet increased with NaCl concentration, which was attributed to the negative shift of the Ecorr. Furthermore, the amount of hydrogen absorbed within one wet-dry cycle changed with the number of cycles, due to the expansion of the corroded area and the formation of iron rust.

　Carbon steel is widely used as structural materials in soil, but it suffers from corrosion. There are lots of environmental factors that may influence the corrosion of the steel structures in soil. For instance, water content, soil covering depth, soil particle size, pH, temperature, anions (Cl⁻ and SO₄ ²⁻) and microorganism¹ may be considered as the environmental factors. In order to understand the corrosion mechanism in soil, the influences of these factors must be systematically investigated. In this study, the corrosion of carbon steel has been monitored in silica sands of different water contents and covering depths by electrochemical impedance spectroscopy (EIS). A pair of identical carbon steel (SM490A) sheets was embedded 2 mm apart in parallel in epoxy resin. The dimension of the carbon steel sheet was 5 mm × 10 mm. The silica sand of 100 μm particle size was used as artificial soil. The mixture of the silica sand and 3% NaCl solution was placed on the carbon sheet electrode using an acrylic pipe (inner diameter 30 mm). The soil covering depth was defined by the height of the silica sand. It was changed to 10, 30 and 50 mm. The pore water content W in soil was changed to 100, 90, 80, 70%. It was calculated by W=(V _Water/ V _Pore)×100 [%] where V _Pore is the pore volume in soil and V _Water is volume of the added NaCl solution. The V _Pore was determined from the volume of the solution when all the pores were completely filled with the solution. EIS was measured at the open circuit potential with amplitude of 10 mV and frequency range of 100 kHz to 10 mHz. The charge transfer resistance R _ct was determined by curve-fitting. The reciprocal R _ct ^-1 was used as an index of the corrosion rate of carbon steel. At all the pore water contents, the corrosion rate of carbon steel decreased with time at the early stage because oxygen in soil was consumed by the corrosion reaction, and then reached a steady value within several days. The corrosion rate under the steady state condition was strongly dependent of the pore water content. The maximum corrosion rate was observed at 90 % water content, which was about 19 times higher than 100%. This is attributed to the enhancement of the cathodic reaction (diffusion of oxygen) due to the increase of the gas phase. On the other hand, the corrosion rate decreased with decreasing the water content below 90%, probably due to the suppression of the anodic reaction by decrease of the liquid phase. At 100% water content, the corrosion rate under the steady state condition was independent of the covering depth because the cathodic reaction of the corrosion changed from oxygen reduction to water reduction. At 90% water content, however, the corrosion rate decreased with the increase of the covering depth. The corrosion rate was inversely proportional to the covering depth. It means that oxygen diffusion through the silica sand controls the corrosion of carbon steel at 90%. On the other hand, at 80 and 70%, the corrosion rate was independent of the covering depth. This is because that the corrosion is controlled by the anodic reaction. 1) I.S. Cole and D. Marney, Corros. Sci., 56, 5 (2012).

In this study, we investigated the hydrogen absorption behavior of pre-rusted steel in an NaCl droplet by measuring the hydrogen permeation current and corrosion potential. To prepare the pre-rusted steel, a 30-μL droplet of a 0.1-mol/dm³ NaCl solution was placed on the surface of steel and exposed to wet-dry cyclic conditions by changing the relative humidity (RH) repeatedly from 40% to 95% at a constant rate of 27.5%/h and 298 K. The hydrogen permeation current and corrosion potential of the pre-rusted steel were measured during the drying of an electrolyte droplet using an electrochemical hydrogen permeation test and the Kelvin probe (KP) technique, respectively. The corrosion potential of the pre-rusted steel shifted in the negative direction simultaneous with the evolution of the hydrogen permeation current during the drying of the droplet. The hydrogen permeation current increased with the number of wet-dry cycles in the early stages of the rusting process, which related to the negative shift in corrosion potential. As the whole surface of the steel was covered with iron rust in the later stages of the rusting process, the corrosion potential shifted in the positive direction and the hydrogen absorption current decreased gradually.

The dissolution behavior of Pt−Cu nanoparticles prepared by pulse-electrodeposition under potential cycling was investigated. Electrodeposited Pt−Cu consisted of numerous Pt−Cu nuclei (∼3 nm in diameter) that agglomerated to form larger secondary particles (30–50 nm), and its composition were determined to be Pt−35 at% Cu (Pt−35Cu). A Pt−enriched shell formed on the Pt−35Cu surface due to Cu-selective dissolution from Pt−35Cu during the initial immersion in 0.5 M H₂SO₄. Cu dissolution from Pt−35Cu was then strongly suppressed under potential cycling between 0.05−0.6 and 1.0 V, owing to the Pt−enriched shell. On the contrary, when Pt−35Cu was potential-cycled between 0.05 and 1.4 V, Cu-selective dissolution was not suppressed because Pt dissolved from Pt−35Cu even after the Pt−enriched shell formation. Surface morphology changes during potential cycling were strongly related to both upper (E_L) and lower (E_U) potential limits. Surfaces of Pt−35Cu were smoothened when E_U = 1.0 and 1.4 V, than when E_U = 0.6 V. When E_L was fixed at 0.6 V to prevent the dissolved Pt ions from re-depositing on the surface, surface smoothening of Pt−35Cu did not occur during potential cycling between 0.6−1.0 V. Based on these results, the dominant factor for surface reconstruction of Pt−35Cu was Pt dissolution and re-deposition.

In this study, pH measurement was performed in a thin electrolyte droplet with a thickness <1000 µm by the measurement of the equilibrium electrode potential of an Sb/SbxOy electrode used as a pH sensor. The equilibrium potential of the Sb/SbxOy electrode was evaluated by using the Kelvin probe (KP) technique. To investigate the potential response of the Sb electrode in a thin electrolyte droplet, the dependency of the Volta potential difference between the Sb and a gold wire as a KP on electrolyte droplet thickness was measured. The Volta potential difference had a linear response with respect to the buffer solution pH, independent of the droplet thickness. This result indicates that the KP technique, combined with an Sb electrode, is sensitive to the pH of a thin electrolyte droplet of thickness ≥50 µm. This pH measurement technique was also applied to measure pH in a corrosion model of steel. The corrosion model consisted of two steel plates in the same plane as the anode and cathode, with a constant current between them. During the corrosion process, the pH value decreased from 6 to 5 near the anode and increased from 6 to 12 at the cathode. The changes in pH measured in the thin electrolyte droplet were in good agreement with the color changes of the solution containing pH indicators.

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