Haruo Baba

Role of nitrogen on the corrosion behavior of austenitic stainless steels

Abstract The role of nitrogen in the mechanisms of localized corrosion resistance and repassivation were electrochemically investigated using a nitrogen-bearing austenitic stainless (SUS316L) steel in 0.1 and 0.5 M Na 2 SO 4 solutions and a 3.5% NaCl solution. Almost 100% of the nitrogen compounds dissolved into the bulk solution after crevice corrosion were transformed into NH 3 . That is, the mole amount of ammonia in the solution was approximately equivalent to the mole amount of nitrogen dissolved in the steel. This suggests that NH 4 + consuming H + in the pit controlled the local decrease of pH and promoted the repassivation. NO 3 –N and NO 2 –N were not detected by chemical analyses in the high potential and thermodynamically stable zone as NO 3 − in the potential–pH diagram. The repassivation in nitrogen-bearing SUS316L steel in a 0.1 M Na 2 SO 4 solution was studied using the scratching electrode technique, which measured the partially destroyed passivation films on the steel. This technique showed that nitrogen dissolved in the steel has a strong repassivation capacity.

The effect of water quality on pitting corrosion of copper tube in hot soft water

Abstract Laboratory reproduction of pitting on copper was attempted in a hot, circulating water loop for the purpose of clarifying the mechanism and establishing countermeasures to the pitting. Type 2 pitting was observed to occur on the internal surface of copper tubes after long-time exposure to Tokyo water, when it was heated to 60°C and residual chlorine of 2∼3 mg/l was added. After prolonged loop testing, the electrode potential of copper exceeded the critical value, +150 mV(SCE). The pits were deep and acute, and were covered with greenish blue products, which were identified as basic copper sulfate. Pitting did not occur in water free of residual chlorine even under air-saturated conditions, in which copper retained a low potential value.

The effect of oxidizing ions on the passivity of the valve metals in boiling nitric acid solutions

Abstract In the reprocessing plant of spent nuclear fuel, excellent corrosion resistant materials are required for the construction of the equipment, since concentrated boiling nitric acid solutions are used in the Purex process and powerful oxidizers from the fission products also take part in the corrosion process. In such a highly oxidizing and acidic environment, valve metals are basically corrosion resistant. For this reason, the passivating behavior of the valve metals such as Ti, Nb, Ta, Zr, and Hf was investigated in boiling nitric acid solutions in the presence and absence of Cr 6+ ion as an oxidizing agent. Anodic polarization curves, the frequency response of AC impedance, and galvanic coupling with stainless steel were applied to evaluate the stability of the passive films on the valve metals.

POTASSIUM-SODIUM-RUBIDIUM NIOBATE SINGLE CRYSTALS AND ELECTRIC PROPERTIES

Potassium-sodium-rubidium niobate single crystals are grown using an original pulling down method, to improve their composition change during a crystal growth, by means of co-doping of small ionic size sodium and large ionic size rubidium into potassium niobate. Even by the co-doping, single crystals can be grown with orthorhombic single-phase at room temperature, as well as pure potassium niobate. Their electric properties, such as the dielectric constant and the impedance, are changed depending on the doping ions.