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The oxidation rates of WC and WC-Ni hard alloys containing 2,4 and 6% Ni as a binder were determined by means of a magnetic balance apparatus. The relationships between the characteristics of the oxide films and the oxidation rates were also studied with the help of SEM and X-ray diffraction technique. WC and WC-Ni hard alloys weve oxidized at 560-830℃ under two different oxygen pressures (2.6×10^(-2) and 1.3×10^(-1) Pa). The reactions were parabolic during initial stages and linear after 30 min. The oxidation rates increased with temperature and reached a maximum at about 740℃ in 2.6×10^(-2) Pa O₂ and at about 650℃ in 1.3 ×10^(-1) Pa O₂, after which the rate decreased with temperature. It waa identified by X-ray diffraction method that WO₃ was formed in the low temperature and W_(18)O_(49) in the high temperature. The decreased oxidation rate was associated with more protective characteristics of W_(18)O_(49) than WO₃. Adding Ni in WC transformed the paralinear rate of WC only to parabolic rate. The oxidation rate decreased with increasing Ni concentration. Under above conditions the oxide scale was consisted of mainly double oxide NiWO₄, which was dense and protective film.
Electrochemical polarization techniques were utilized to study the effect of the alternating current on the galvanic corrosion of zinc coupled to iron with AV modulation and ZRA methods. The microstructures of specimens and their corrosion products were identified by using optical microscope, scanning electron microscope and X-ray analysis. The passive films of the Zn-Fe alloys were densificated with increasing the ζ phase precipitated to reduce the effect of alternating current on the galvanic corrosion of alloys. The passivation time in the alloys was increased with increasing the ζ phase precipitated in the absence of superimposed alternating current, while it was decreased under the superimposed alternating current.
As a study on the corrosion property change at each part near weldment in AISI type 316 Stainless Steel, canes for the decrease of the corrosion resistance, change of the passive tendency and process of pitting were investigated by metallographic and electrochemical techniques. The equilibrium corrosion potentials, the pitting potentials and the current densities in the passive region in 3% NaCl solution at 19-20℃ were studied with the potentiostatic anodic polarization curves. And the shapes of the pits were investigated as soon as the experiment for anodic polarization curve had been finished(i.e. in the transpassive region). The followings are the main conclusions drawn from the above experiments. The decrease of corrosion resistance is caused by the residual stress as well as the thermal effect. The passivity decreased at HAZ, and a certain correlation between the passive potential range and the current density in the passive region was revealed. Pits were most frequently generated at HAZ, and pitting initiated near the grain boundaries.
연료유에 의한 금속재질의 부식을 억제하기 위하여 Fe-Cr-Ni강 (ASTM A447) 재질의 V₂O_5-Na₂SO₄系 인조재 속에서의 시간, 온도, 조성에 따른 부식율 변화와 금상학적 변화를 조사하였으며, X-선법에 의해 질석이 인조재에 미치는 영향도 조사하였다. 이상의 결과에 따르면 질석이 인조재의 희석제로의 작용에 의해 부식율 감소가 얻어진 것이 아니라 인조재와 첨가된 질석이 어떤 복화합물을 형성하므로서 인조재의 부식성을 감소시킨다고 믿어진다. 인조재에 질석을 40%이상 첨가하였을 때 뚜렷한 방식효과가 나타났으며, 이것으로 방식 첨가제로 연료유에 첨가할 질석의 적합량을 환산할 수 있었다. The corrosion rates of Fe-Cr-Ni alloy(ASTM A447) in V₂O_5-Na₂SO₄ synthetic ash were measured in the range of 650-850℃ with time and various ash compositions. For the reduction of corrosion rate of the alloy in the presence of fuel oil ash, a Korean vermiculite was used as an inhibition additive. The corrosion effects of the vermiculite were studied by using metallographic and X-ray methods. The inhibition effect might be obtained by the formation of a complex compound with synthetic ash. Considering corrosion prevention and economic view, the adequate amount of vermiculite addition to the ash was measured as 40%, and this amount might be applied to fuel oil comparing its ash content.
The oxidation rate of chromium carbide has been measured continuously using thermogravimettic analysis at different oxygen pressures ranging from 1.33×10^(-2) to 2.67×10^(-1)t Pa O₂ at 1000-1300℃. The oxidation of pure chromium has also been studied between 1000-1300℃ under 6.67×10^(-2) Pa O₂ and compared with that of chromium carbide. The oxidation of chromium carbide showed a linear behavior which was different from that of chromium. The oxidation rate of chromium carbide increased with increasing temperature and oxygen pressure was lower than of pure chormium. Above 1200℃, the volatile oxide was formed and evaporated causing a weight loss. The compositions and morphorjogy of the oxide were studied with X-ray diffractometer and scanning electron microscope, respectively. The morphorlogy of oxide changed with varying temperature and pressure. The oxide scale was consisted of mainly two different layers of Cr₂O₃ and CrO, and the properties of oxide scale were correlated with oxidation behavior. The oxide film formed in the above test condition has been detached from the carbide surface. The crack and pore were thought to be from CO gas evolving at the interface of chromium carbide and its oxide and the major factor of the linear behavior of chromium carbide.