In the previous paper, the critical impact velocity in the erosion of metallic materials by solid particles impact was experimentally determined. In this paper, the critical impact velocity was theoretically derived through analyzing the behavior of the material surface impacted by a spherical solid particle: at the moment of oblique impact, an indentation is formed; at the same time, the material is strained tangentially to the surface in the direction of particle movement. As the extent of strain goes over the elastic limit, the impacted solid particle does skid on the surface, which brings about wear to the surface by cutting. The threshold strain was derived as a function of the impact velocity, the rotating velocities as well as the duration of particle surface contact. Those parameters which characterized the impact behavior were derived from the coefficient of friction and the rebounding coefficient, all of which were obtained from the mechanical properties of the target material and particle, and other factors concerning the particle. Consequently, the theoretical value of the critical impact velocity was given solely as a function of the mechanical properties of the target and the particle. The calculated critical impact velocity attained the lowest value at a low impact angle, and a good correlation was found between the critical velocity determined by experiment and that predicted by the theoretical equation.

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Dry Low-Level Radioactive Wastes (LLW) which mean incombustible solid LLW generated from nuclear power stations are scheduled to be packed in steel drums followed by solidification with mortar. The solidified dry LLW is then to be disposed to shallow underground at Rokkasho LLW Disposal Center. Dry LLW includes some amphoteric metals among which aluminum is the most corrosive with gas evolution in high alkaline media such as mortar. The evolved gas may accelerate the leaching of solidified dry LLW with mortar. Despite the planned removal of aluminum from dry LLW, small inclusion of aluminum is unavoidable. The present study focuses on the effect of environmental factors such as pH and temperature on gas evolution behavior caused by aluminum corrosion. Large effects of pH and temperature on corrosion rate of aluminum and gas evolution were recognized. Principal corrosion product of aluminum was calcium aluminate compound when it was immersed in simulated mortar environments. It is demonstrated that 1.5mol hydrogen gas evolves with the corrosion of 1mol aluminum in environments of 12<pH<13 at temperatures below 60°C.

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Creep strength of 2 1/4 Cr-1Mo Steel used for steam generator tube of a fast breeder reactor is decreased by decarburization in high temperature sodium utilized for coolant of the reactor. However, proposed evaluation methods for carbon decrease in the steel were not sufficient in the predictability. Then, an evaluation method taking account of the behavior of carbide in the steel was studied in this report. The following results were obtained.
(1) Model of the evaluation method for decarburization of 2 1/4 Cr-1Mo steel was set from decarburization test results in 500 to 700°C of sodium temperature, and the method was developed.
(2) Decomposition rate of carbide in the steel to fit test results was calcu lated with the method.
(3) It was clarified that carbon concentration calculated with the method was decreased rapidly at initial stage of decarburization for diffusion of solution carbon, and at next stage the carbon concentration was decreased slowly under influence of decomposition rate for carbide to be dominant rate determining step of decarburization.
(4) This method presented the most smallest decarburization rate compared with proposed evaluation methods. And the method presented slight decrease of concentration for steam generator tube for 32 years at 500°C whose thickness was 0.3cm.

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An atomospheric exposure test has been conducted since Sept. 26, 1994 at a five member family steel framed prefabricated house in Nogi-chou, Shimotuga-gun, Tochigi prefecture. The ACM (Atmospheric Corrosion Monitor) corrosion sensors and zinc-coated steel coupons are exposed in nine indoor sites-ceilings, floors and wall cavities-and one outdoor site sheltered from rain. Three year of measured results are reported here. The ACM sensor outputs galvanic current, I, between carbon steel and silver, integral of which over a day -daily average electricity, Q- is related to corrosion rates of carbon steel and zinc to be used as a measure of corrosivity for each site. The sensor output, I, is found to depend not only on relative humidity, RH, but also on amount of deposits on the sensor, which, expressed as equivalent amount of deposited sea salt, Ws, is determined by referencing to I vs. RH relationship observed under various predetermined amounts of deposited sea salt in laboratory test. Corrosion rates of carbon steel and zinc at their indoor sites are around one fifth of the outdoor one. Such lower corrosion rates at the indoor sites are attributed mainly to lower Ws values which are around one tenth of the outdoor one. Moreover, amount of sulfur trioxide collected for a year is 0.001mg SO₃/100cm²/day at an indoors site, one twenty sixth of the outdoor one.

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A heat exchanger for vaporizing liquefied natural gas (LNG) is made of aluminum alloy tubes, and the way of protecting the tubes from corrosion in sea-water is to form a sacrificial anode layer by thermal-spray coating of Al-Zn alloy on aluminum alloy tubes. However, corrosion damage has been sometimes found in the lower part of heat exchangers exposed to low temperature of freezing sea-water. The purpose of this study is to investigate the corrosion behavior of A3003 aluminum alloy in freezing sodium chloride solution and to examine a possibility of electrochemical impedance spectroscopy for monitoring of the corrosion process of heat exchanger. The aluminum alloy corrodes at 253K in freezing sodium chloride solution. The characteristic of polarization was a cathodic control at this temperature. The corrosion rate obtained by the measurement of electrochemical impedance spectroscopy didn't change with immersion time, corresponding with the corrosion rate obtained by the weight loss method. It was found that this technique is a convenient tool for monitoring of the corrosion processes of heat exchanger in freezing sea-water.

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