Yoichi Kojima

In situ measurement system for deformation and solidification phenomena of yttria-stabilized zirconia droplets impinging on quartz glass substrate under plasma-spraying conditions

The authors have developed an in situ measurement system for precise one-to-one correlation between splat morphology and thermal history during particle impingement on a temperature-controlled substrate inside an airtight chamber under plasma-spraying conditions. The system has made it possible to collect about 10 single particles successively within a 10 s time frame, and to correlate exactly the relationship between the size, the temperature, and the impacting velocity of each droplet, and the morphology of the splats. The most striking finding is that a part of the yttria-stabilized zirconia (YSZ) droplets may be actually in supercooled condition before impinging, although a marked difference was not found in the splat morphology. In addition, as secondary results, we could evaluate the viscosity, μ, of YSZ, and the thermal contact resistance between YSZ splat and the quartz glass substrate as μ [Pa · s]=0.0037 exp (6110/T) and 3 × 10−6-4×10−5 m2 K/W, respectively.

Repassivation Method to Predict Long Term Integrity of Low Alloy Titanium for Nuclear Waste Package

Long term corrosion resistance is evaluated for G-2 and G-12 titanium as a candidate of the high-level nuclear waste packages. The repassivation potential for crevice corrosion, E R , in comparison with the well known spontaneous potential, E SP , of a metal passivated in given environment, allows conclusions to be drawn whether it performs “permanent” or does not. This repassivation method is extended to determine critical conditions in terms of NaCl concentration and temperature for specimens kept at an electrode potential which is more noble than E R and is included in E SP ranges. Thus obtained NaCl concentration - temperature - crevice corrosion map could predict critical conditions for the titanium used in geologic formations.

Simulation of The State of Carbon Steel n Years After Disposal with n Years of Corrosion Product on its Surface in a Bentonite Environment

The use of bentonite as buffer and carbon steel as overpack material for the geological disposal of nuclear waste is under investigation. To better assess the long term integrity of the carbon steel overpack, a quantitative analysis of the corrosion behavior on the steel surface for time frames beyond that of feasible empirical determination is required. The state n years after disposal, consisting of Carbon Steel/Corrosion products + Bentonite/Water, was simulated and the corrosion behavior of the carbon steel in this state investigated. The following facts became apparent. Both the corrosion rate and the non-uniformity of it increased with increase in the corrosion product content in the compacted bentonite. When the corrosion product layer was formed between the carbon steel and the bentonite, it enabled the corrosion potential and increased the corrosion rate.

Crevice corrosion -- NaCl concentration map for Alloy 625 at elevated temperature

The repassivation potentials, Er, for metal/metal-crevice of Alloy 625 were determined in 0.3--10% NaCl solutions for temperatures up to 250 C. The ER were found to be the least noble at temperatures around 100 and 125 C. The Er became more noble as temperature increased; this tendency was particularly strong in diluted solutions. Based on the experimental data, a crevice corrosion map showing the critical condition in terms of temperature and NaCl concentration was presented. As for the map, a wide repassivation region was found in elevated temperatures, similar to that of commercially pure titanium, C.P.Ti.

Crevice corrosion - NaCl concentration map for grade-2 titanium at elevated temperature

The repassivation potential, ER, for metal/metal-crevice of Commercially Pure Titanium, C.P.Ti, was determined in NaCl solutions at temperatures up to 250C. The ER has its least noble value near 100C and becomes more noble as the temperature increases. As shown in previous research, the shrinkage of the repassivation region should continue with increasing temperatures. However, in conducting this same experiment at temperatures higher than 100C, an examination of the NaCl concentration - temperature - crevice corrosion map verifies that the repassivation region began to expand again when the temperature exceeded 140C. This expansion continued as the temperature continued to increase.