The microwave irradiation has been applied to the chemical analysis, the materials synthesis, sintering and joining of ceramics, and heating for food processing etc. In this review, the hydrothermal process combined with the microwave irradiation is introduced at the viewpoint of the material synthesis. The results prepared by the microwave-hydrothermal are also presented. The microwave-hydrothermal method is an energy-saving process because of rapid heating and increase of the kinetics of reaction compared to the conventional hydrothermal method.
The purposes of this study are to clarify fundamental aspect of the corrosion behavior of reinforcing steel in concrete and to develop an accurate monitoring method. In order to simulate the corrosion of concrete structures in a marine environment, carbon steel rods embedded in mortar were exposed to alternate conditions of 8h-immersion in 3mass%NaCl solution and 16h-drying in room temperatures. The carbon steel rods were embedded at the different positions in the thickness of the mortar (mortar thickness φ: 5, 10, 20, 40mm). The wet-dry cycles were carried out for the period of 80 days. The impedance was measured in the frequency range from 10kHz to 1mHz when the mortar was submerged in 3mass%NaCl solution. The carbon steel and mortar interface is explained by an equivalent circuit consisting of a solution resistance, a charge transfer resistance and a CPE (Constant Phase Element). As the corrosion progresses, the diffusion impedance appears in the low frequency region. At the initial stage of the wet-dry cycles, the corrosion rate is accelerated due to the enhancement of the anodic dissolution. This enhancement is attributed to introduction of chloride ions into the mortar by the wet-dry cycles. In the middle and final stages, the carbon steel shows higher corrosion rate and less noble corrosion potential as the mortar thickness decreases, indicating the acceleration of anodic process due to neutralization of solution and concentration of chloride ions in the mortar by the wet-dry cycles.
We observed the corrosion process that occurs between copper and ammonium sulfate (NH4)2SO4 or sodium sulfate Na2SO4 particles above a critical relative humidity (CRH) to study copper corrosion caused by corrosive particles. We also analyzed the corrosion products that formed on the copper by using X-ray diffraction (XRD) and Raman spectroscopy. Our results can be summarized as follows. 1) The droplet formed by the deliquescence of (NH4)2SO4 above CRH was colorless at first and became pale blue after 3 hours exposure. Further exposure turned the droplet dark blue. Patina precipitation was observed after 24 hours exposure. We attributed the color change to the formation of cupric species in the droplet. 2) With Na2SO4, the droplet color remained unchanged after 24 hours exposure. However, copper corrosion started as soon as a droplet formed on the copper surface. Further exposure resulted in an enlargement of the corroded area. 3) We analyzed the corrosion products using XRD and Raman spectroscopy and found that antlerite Cu3SO4 (OH)4 formed on copper that had reacted with (NH4)2SO4. By contrast, cuprite Cu2O was formed with Na2SO4. The results show that different corrosion products were formed when different sulfate salts reacted with copper above CRH.
Corrosion behavior of various cast alloys has been studied in a high temperature air heater environment for a waste gasification and ash melting system. This air heater was placed in a severe corrosion condition; the temperatures of both the exhaust gas and the heat exchanger tube surface exceed 1000°C. As heat exchanger tube materials, ceramics and metals may be applicable, cast alloys are preferable due to economical reason. However, the corrosion behavior is not completely known. The cast alloys have been exposed to such environment of a demonstration plant in order to examine the corrosion behavior. The test results suggested the presence of a corrosion peak likely in the temperature range from 800 to 1000°C, and internal corrosion was also found in all the cast alloys. The materials with small carbon content particularly showed from extreme intergranular corrosion. 33Cr-50Ni-15W alloy showed the best corrsion resistance among the alloys.
Ni-Cr-W cast alloy is considered one of the promising materials for high temperature air heaters in gasification and ash melting systems due to their favorable properties for both corrosion resistance and high temperature strength, as well as economical concerns over costs. The exposure test on a demonstration plant indicates that these cast alloys showed their specific temperature dependence while corroded in a high temperature exhaust gas environment. The corrosion rate rapidly increased in a temperature range from 800°C to 900°C, and slightly decreased when the temperature went beyond that range. Chloride was formed inside the alloys and oxidized to produce chlorine. This chlorine was then involved in another chlorination. Thus, consecutive reactions of the chlorinated corrosion occurred. This chlorine recycling inside the alloys can be a cause of severe corrosion. The rapid corrosion increase in the temperature range from 800°C to 900°C was assumed due to the progress of the chlorine recycling caused by the melting of the formed CrCl2. Meanwhile, the decline of the corrosion rate at higher temperatures could have been due to the suppression of the recycling by the formed chloride becoming volatilized and dispersed for fugacity from the alloys.