발간논문

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High velocity oxy-fuel thermally sprayed coating of the WC-Co cermet material is a well-established process for modifying the surface properties of the structural components exposed to the corrosive and wear attacks. The hard WC phase in the coating resists to the wear while the soft metallic Co increases the adhesive and cohesive bonding properties. The coating properties deposited by the HVOF process are greatly dependent on the feedstock materials and processing parameters. The effects of the feedstock material and process parameters including the inflight particle parameters and resultant coating microstructures were observed in this study. Both the NW10Co powder containing nano-sized WC particle and the micro-sized WC embedded MW12Co powder were thermally sprayed by the table of orthogonal array of the processing parameters. The in-flight particle parameters such as the particle temperature and velocity were directly measured along the hot gas downstream using the DPV-2000 system. In-flight particle parameters were affected by the properties of the feedstock materials. Especially, the particle temperature of the NW10Co was higher than that of the MW12Co from the barrel exit point to the deposition point, irrespective of the condition of the processing parameters. Porosity in the as-sprayed HVOF coating was measured using an image analyzer. And the Vickers microhardness was measured under applied load of 300 g for 15 seconds. Phase compositions of the as-sprayed HVOF WC-Co coatings were measured and analyzed using the X-ray diffraction. The effects of the feedstock materials and process parameters on the coating properties of the as-sprayed HVOF coatings were discussed in terms of the in-flight particle properties during processing.

keyword : Nanostructured WC-Co, Feed Stock, In-situ analysis, Coating properties, DPV-2000

In order to develop a new corrosion sensor for detecting and monitoring the corrosion of buried pipeline, sensor element design and the correlation of its output to corrosion rate of steel pipe itself were evaluated by laboratory test methods in synthetic groundwater. It uses well-known principles of galvanic corrosion and consists of two dissimilar metals (anode and cathode) installed on the buried pipeline. In this paper, two types of electrochemical probes were used: galvanic cells containing pipeline steel (CS)-copper (Cu) and pipeline steel (CS)-type 304 stainless steel (SS) couples. The corrosion behavior in synthetic groundwater for the different electrodes was investigated by potentiodynamic test. The galvanic corrosion test was accomplished by a zero resistance ammeter technique. Weight loss measurements were conducted to obtain the corrosion rates of pipeline steel in synthetic groundwater. The correlation between galvanic current and corrosion rate was obtained by galvanic current measurement and corrosion rate measurements. The results of the potentiodynamic test indicated that copper exhibited an active corrosion behavior, while stainless steel demonstrated spontaneous passivation. In galvanic corrosion tests, the galvanic current of copper-steel couple was higher than that of other couple. The comparison of the sensor output and corrosion rate revealed that a linear relationship was found between the probe current and the corrosion rate. Especially, a better linear quantitative relationship was found between the Cu-CS probe current and the weight loss data of the pipeline steel coupons. In addition, the Cu-CS probe is more suitable for high resistance soil than SS-CS probe, due to the high current output.

keyword : Galvanic couples, Corrosion sensor, Pipeline, Galvanic current, Stainless steel, Copper

The effect of heat treatment on corrosion behavior and mechanical properties of Zr-1.5 wt.% Nb alloy was investigated. The alloys of Zr-1.5 wt.%Nb containing different oxygen content of 700 and 2000 ppm were annealed for 1, 10 100, 500 hours at 470, 520 and 570oC. The microstructural characteristics of annealed specimens were investigated by using OM and TEM. The corrosion tests were carried out at 360oC water. The oxide characteristics were investigated by using low angle XRD and EIS. And mechanical properties were analyzed by using micro hardness, tensile and creep tester. The recrystallization behavior of Zr-1.5Nb alloy was affected by more annealing temperature than annealing time. The corrosion resistance was improved with increasing annealing time at each temperature, but was not affected by oxygen content. The hardness, tensile and creep properties of annealed alloy were decreased with increasing annealing time and temperature, but elongation was increased. The hardness of them were increased with increasing oxygen content.

keyword : Zr-1.5Nb, Corrosion, Oxygen, Annealing, Recrystallization

We have investigated the magnetic properties of Co(P) nanowire arrays fabricated by ac electroforming into self-ordered nanosized pores in anodic aluminum oxide (AAO), prepared by anodizing the pure aluminum foil. The nanopores were found to have an average diameter of 20 nm and aspect ratios of more than several hundreds. Magnetic properties of Co(P) nanowires were found to have the coercivity of more than 2.6 kOe due to the shape anisotropy and squareness(M_(r)/M_(s)) of ~0.8. It was particularly interesting to note that Co(P) nanowires showed different crystallographic structures depending on the applied potential. Co(P) nanowire prepared at ac-10 V_(rms) exhibited better magnetic properties than one prepared at ac-11 V_(rms) due to the crystallographic structural variation.

keyword : Co(P), Nanowire, Ac electroforming, Applied voltage, Magnetic properties

A kinetic study on Bi removal during copper matte oxidation was designed under the presumptions of low FeO activity and of no fayalite slag formation. Copper matte with 59 wt% Cu was oxidized by bubbling Ar+O₂mixture gas through a submerged nozzle. The effects of melt temperature, gas flow rate and oxygen partial pressure on the volatilization rate of Bi in copper matte are discussed. Bi was significantly removed during Ar gas blowing, and the volatilization rate of Bi was markedly increasing with the melt temperature. However, the dependence of the volatilization rates on the oxygen partial pressure was affected in a complicated way by the effects of the melt temperature and the reduced amount of exhausted gas. The volatilization of Bi in copper matte was primarily controlled by the mass transfer rate through the gas film boundary layer around rising gas bubbles.

keyword : Copper, Matte, Mass transfer, Bubbling, Volatilization, Bismuth