Analysis of localized corrosion failures shows that troubles caused by materials, design and fabrication are a few, while those caused by operation occurr freqently over 50% of the total failures. Some of them probably develop social anxiety and may cause serious disaster such as explosion, fires and death. In this report, a yearly change of number of failures in complex up to 1988 is analyzed and problems associated with the troubles caused by localized corrosion are discussed from the viewpoint of corrosion control.
In this investigation, the structure of a rust layer on iron and steel materials exposed to atmospheric and soil environments for a long peroid, up to approximately 1600 years, is discussed. It is found that the rust constituents change with exposure time. The rust layer transforms and stabilizes to α-FeOOH. It can be pointed out that the structure of a rust layer suggests the environmental condition in which the materials have been preserved.
In order to improve the corrosion resistance of MCFC separator, a new coating was made by combination of ion plating and heat treatment. The corrosion resistance of coating was investigated by electrochemical corrosion tests in a molten carbonate and MCFC single cell tests. The composition of coating was analyzed by means of EPMA and XRD. The characteristics required for the wet seal area of separator such as the flatness of surface were discussed. The results obtained are as follows: (1) Through the plating of Al layer on stainless steel and the heat treatment under a particular condition, the multi-layer coating having aluminum oxide phase at the outer side and Al-Cr-Ni diffusion alloy phase at the inner side is obtained. (2) The morphology of the coating film is affected by the thickness of Al layer plated on stainless steel. (3) Corrosion resistance in a molten carbonate environment is dependent on the composition of the oxide layer.
The anodic polarization behavior and corrosion products of Cr carbide-Ni-Cr alloy in soduim sulfate solution containing sodium chloride were studied using electrochemical techniques, scanning electron microscopy and energy dispersive spectroscopy. The investigation was carried out on the thermal spray films processed by the high velocity oxygen-fuel spraying method. An increase in NaCl concentration in the deaerated Na2SO4 solution resulted in the increase of the degree of passive current and caused the production of uplifting corrosion film on the Ni-Cr alloy matrix. The Ni element in the Cr3C2-Ni-Cr alloy preferentially dissolved at less noble potential in the passive region and not only Ni-Cr alloy matrix but Cr carbide dissolved at more noble potential than 0.7V vs. Ag/AgCl.
Corrosion behavior of thermal sprayed carbide cermet coating in an alkaline aqueous solution that was exposed to air and aerated was examined by electrochemical processes. In this study, the galvanic corrosion due to contact with under-coating or substrate was considered. The results obtained are following: (1) Anodic polarization of thermal sprayed Cr3C2/NiCr coating indicates active dissolution. The NiCr matrix may have major influence on it. (2) The corrosion rate of thermal sprayed NiP under-coating tends to be faster than that of carbon steel. (3) Chloride ions in 1M·NaOH solution may have minor influence on the corrosion behavior of thermal sprayed Cr3C2/NiCr and NiP coatings and carbon steel.
Tantalum is used in a chemical plant as an excellent corrosion resistant material. But occasionally it suddenly causes embrittlement fracture by absorbed hydrogen. In this study, the effect of hydrogen absorption on its electrical properties has been studied. Tantalum showed the embrittlement at 250ppm of hydrogen, which appeared as the decrease of elongation and reduction of area in the tensile test. On the other hand, the eddy current test showed a distinct dependence on absorbed hydrogen even at 150ppm, which scarcely appeared on the mechanical properties and electric resistivity. Therefore, the eddy current test is a more sensitive method for the evaluaion of hydrogen embrittlement of tantalum.
Nickel titanium films were fabricated by RF magnetron sputtering method with dual targets, NiTi and titanium. The titanium content of NiTi films was controlled by changing the input power into titanium target. The fabricated films were solution-treated at 900°C for 30 minutes and aging-treated at 400°C for 6 hours. The results are obtained as follows. (1) It was possible to controll the titanium content of NiTi film by changing the input power into titanium target. (2) Two types of films were obtained. One was nickel rich film that had the mixed structure of NiTi, Ti2Ni and Ni3Ti, and the other was titanium rich film that had the mixed structure of NiTi and Ti2Ni. (3) NiTi films were transformed into martensite phase _??_ rhombohedral phase _??_ austenite phase. These transformation temperatures were determined by a differential scanning calorimeter. The transformation temperature of nickel rich layer was lower than that of titanium rich film. The temperature hysteresis between rhombohedral and martensite phases was small. (4) The shape memory characteristics were observed at the transformation between austenite and rhombohedral phases. It was expected to use NiTi film for microactuators and microsensors.
A low-pressure-plasma spray (LPPS) process is used to overlay coatings of MCrAlY alloy for protection against high temperature corrosion and oxidation. This coating process has been found to be very effective for gas turbine components. On the other hand, diffusion coating processes have been applied for many years to improve similarly the environmental resistance by enriching the surface of a substrate with Cr, Al, or Si. Recently, aluminizing on the MCrAlY coatings is used for improving further the high temperature oxidation resistance. However, the mechanical properties of aluminized MCrAlY coating, which have an important effect on coating life, have not always been clarified. In this study, five kinds of free-standing MCrAlY specimens (CoCrAlY, CoNiCrAlY, CoNiCrAlY+Ta, NiCrAlY, NiCoCrAlY) were machined from the thick LPPS coatings. And, the heat treated MCrAlY specimens (1393K, 2h, Ar atmosphere) and the aluminized specimens (Al-Cr-Al2O3-NH4Cl pack, 1173K-1273K, 10h) after heat treatment were used in the experiments. The Vicker's hardness, Young's modulus, Poisson's ratio and four point bending strength of the aluminized MCrAlY coatings were measured at room temperature in comparison with the MCrAlY substrates. The experimental results suggest that the volume percentage of precipitated aluminum compound in the MCrAlY coatings and the residual stress induced by the aluminizing have an important effect on the bending strength. Namely, there is a tendency that the bending strength decreases by the aluminizing and also with increasing the volume percentage of aluminum compound. The Vickers hardness and Young's modulus of the aluminized layers show higher values in comparison with the MCrAlY coatings.
Recently, polymer matrix composites have become a primary structural material for aircraft and spacecraft in addition to their secondary structural material. Therefore, high reliability over a long period has become required in regards to the deformation of these composite structures. A thermosetting resin used as the matrix for composites shows a considerable change of mechanical properties with time and temperature not only above the glass transition temperature Tg but also below Tg. Therefore, it is expected that the composites, generally used below Tg, show viscoelastic behavior in this temperature range. It is well known that a resin is generally not in a thermodynamically equilibrium state below Tg. This non-equilibrium state is unstable and gradually moves to the equilibrium state. This behavior called physical aging remarkably affects the mechanical properties, especially the viscoelastic behavior of resin. Therefore, it is important to investigate the influence of physical aging on the viscoelastic behavior of thermosetting resin below its Tg in order to evaluate the reliability over a long period for composite structures. The purpose of this study is to evaluate accurately the effect of physical aging on the creep compliance of epoxy resin. First, the creep compliance of epoxy resin subjected to various aging times and temperatures are measured at various test temperatures, and the master curve of creep compliance of epoxy resin with various aging conditions is obtained based on the modified reciprocation law of time and temperature. Second, the degree of aging progress as the amount of suppression of creep compliance by physical aging is evaluated by using the master curve of creep compliance. Finally, the aging time and temperature dependences on this degree of aging progress are discussed.
The effect of microstructure in Ti-6Al-4V alloy on low-cycle fatigue behavior at elevated temperature was studied. Three kinds of microstructure in Ti-6Al-4V alloy were prepared under different heat treatment conditions; that is, equiaxed α structure, lenticular α structure and bimodal (mixed of equiaxed α and lenticular α) structure. Total strain controlled low-cycle fatigue test was performed at 773K in air. Crack initiation was investigated by D.C. potential method. The following conclusions were obtained. (1) The highest low-cycle fatigue strength was obtained for the bimodal structure, followed by the equiaxed α structure and the lowest was the lenticular α structure. (2) The crack initiation life of the lenticular α structure was smaller than those of other structures. (3) The effect of microstructure on crack propagation life was small as compared with that on crack initiation life. (4) Cracks initiated inside of grains of the equiaxed α structure and the bimodal structure. Cracks of the lenticular α structure initiated at the interface between α layer precipitated at grain boundary and lenticular α structure.
In this report, air-entraining, high-range water reducing admixture A, water reducing agent for extremely stiff consistency concrete B, admixture B with viscosity increasing admixture C and normal water reducing agent D were used and each one was added to the concrete for roller compacted pavement having the water cement ratio of 0.37. In order to compare the effect of each admixture on the mixture proportion and workability of concrete, the unit water content and sand percentage were varied and vibrating compaction value measured by a Swedish-type Vebe apparatus, compaction ratio, surface vibrating compaction time to get a fully compacted state and segregation degree taken place by falling were determined. The conclusions were as follows; (1) In the case of a constant unit water content, the minimum vibrating compaction value, minimum surface vibrating compaction time and maximum compaction ratio were obtained at a sand percentage for each admixture but the segregation degree became maximum. This phenomenon was closely related to the fluidity and viscosity of mortar component and mixture proportion such as the ratio of mortar content to coarse aggregate content. (2) In the case of a constant vibrating compaction value and maximum compaction ratio, the sand percentage was least by adding admixture B and the unit water content was least by adding A or C, and the surface vibrating compaction time was least by adding admixture D and the segregation degree was least by adding A or C, respectively. (3) It can be thought that admixture B is effective to improve the abrasion resistance and A or C is of use to reduce the unit cement content and segregation degree. An admixture with viscosity increasing admixture is more effective to reduce the segregation degree.
The chloride permeability of normal weight and lightweight concretes subjected to freeze-thaw damage at the age of 3 days was evaluated by using the AASHTO T277 chloride permeability test method and the freeze-thaw test method similar to that recommended by JSCE-1986. The relative dynamic modulus of elasticity was obtained from the results of ultrasonic pulse velocity measurements for each specimen before and after the freezing and thawing test. The results showed that the chloride permeablity of the normal weight concretes having an air content of at least 7.6% changed little with the repeated cycles of freezing and thawing ranging from 18 to 44 cycles, irrespective of the presence and the type of mineral admixtures. It was found that the application of freeze-thaw cycles obviously decreased the values of the relative dynamic modulus of elasticity in the normal weight concretes, but that the reduction in the relative dynamic modulus of elasticity did not always result in an increase in the chloride permeablity of the concretes. The test results also indicated that most of lightweight concretes made with fully-saturated expanded shale aggregates exhibited an extremely high chloride permeability, when they were subjected to a single freeze-thaw cycle. Furthermore, the type of microcracks developing in the normal weight concretes under the repeated freeze-thaw cycles was discussed.