Haruhiko Kajimura

Corrosion resistance of zirconium and zirconium-titanium alloy in hot nitric acid

Abstract Since zirconium (Zr) has superior corrosion resistance to concentrated nitric acid (HNO 3 ) at elevated temperature, it has been successfully used as a material for chemical plants to produce HNO 3 or spent nuclear fuel reprocessing plants. However, the occurrence of stress corrosion cracking (SCC) has recently been reported in hot nitric acid conditions. The necessary conditions for SCC in Zr, and the preventive method against SCC by alloying were investigated. The slow strain rate technique test, electrochemical measurements, strain electrode tests, and analysis of corrosion products were carried out in this work. SCC does not occur in Zr in boiling HNO 3 at concentrations less than 70%, whereas SCC is initiated in 6–94% HNO 3 when a potential is applied above the critical potential for SCC. A new invention of Zr-15%Ti alloy is immune to SCC under applied potential conditions in hot HNO 3 ; this alloy forms a stable passive film composed of ZrO 2 and TiO 2 .

Erratum: Change of Microstructure during Thermal Fatigue at Maximum Temperature 1073 K in Nb-Added Ferritic Stainless Steels

In this study, thermal fatigue tests at maximum temperature 1073 K were performed using 13% Cr–Nb–Si and 18% Cr–Nb–Mo steels as representative heat–resistant ferritic stainless steels for automotive exhaust systems. The changes in the microstructure, the crystal orientation and the hysteresis loop during thermal fatigue in the temperature range from 473 K to 1073 K were investigated. As a result of comparing thermal fatigue life under these conditions, 18% Cr–Nb–Mo steel with high temperature strength was found to have a longer thermal fatigue life than 13% Cr–Nb–Si steel. During the thermal fatigue process, the material was softened by reducing of the amount of solute Nb, and the coarsening of Nb precipitation. By this softening, the form of the hysteresis loops changed with the increase in cycles. By considering the softening of the material, the change in the hysteresis loops could be predicted to some extent. Furthermore, by EBSD analysis, it was recognized that the dynamic recovery and recrystallization accompanied by the uniaxial and fine grain formation occurred during the thermal fatigue process. From the viewpoint of change of the microstructure, the thermal fatigue damage was quantified by the ratio of the low–angle grain boundary, and the change of this index with the progress of the cycle in 18% Cr–Nb–Mo steel had a smaller than 13% Cr–Nb–Si steel. It was thought that this point was caused by the retardation of recrystallization by solute Mo. [doi:10.2320/jinstmet.JC201701]