Motoi Hara

The galvanic corrosion behavior of coupled nickel-platinum in molten Na2CO3

Abstract The galvanic corrosion behavior of a platinum and nickel couple in molten Na2CO3 at 1173K was investigated. Particular attention was given to the influence of O2 gas on the corrosion behavior. The galvanic current passed from platinum to nickel under atmospheres of both O2 and Ar. Nickel and platinum became the anode and cathode, respectively. The galvanic current passed under an O2 atmosphere was larger than that under the Ar atmosphere, indicating that O2 gas dissolved in the molten salt and hastened the galvanic corrosion. During the galvanic corrosion test under an O2 atmosphere, the masses of platinum and nickel, respectively, decreased and increased greatly. After the corrosion test, extensive attack was observed on the platinum surface, whereas the formation of NiO and metallic platinum crystals was observed on the nickel specimen. On the basis of these results, the cathodic reaction on platinum and the anodic reaction on nickel during the galvanic corrosion test were proposed.

The corrosion of nickel under cathodic polarization in molten NaNO3

The loss of nickel under cathodic polarization in molten NaNO3 increased with an increase in the quantity of electricity for the cathodic reaction. The formation of NiO was observed on the surface of the nickel after cathodic polarization. A large amount of powdered corrosion product consisting of NiO was detected in the NaNO3 salt after cathodic polarization. The cathodic polarization reaction and the products by the cathodic polarization reaction were examined in order to determine a mechanism for the cathodic corrosion of nickel in molten NaNO3. On the basis of the results it was proposed that the corrosion of nickel under cathodic polarization in molten NaNO3 occurred by a chemical reaction of nickel with NO2 and O2− which appeared to be cathodic products.

Preparation of Ni Aluminide/Ni Bilayer Coating on Nb–W Alloys by Molten Salt Electrodeposition and Oxidation Resistance

Coatings consisting of Ni aluminide/Ni bilayer on Nb–W and Nb–W–Mo alloys were prepared by the electrodeposition of Al using a molten salt electrolyte after the Ni electrodeposition. The coating specimen with a thin Ni layer showed a vigorous oxidation during the initial period. On the contrary, the coating specimen with a thick Ni layer showed a high oxidation resistance. For the coating specimen with a thick Ni layer, a surface layer consisting of NiAl was formed after the oxidation test. For this coating specimen, further, intermediate layers consisting of Ni–Nb alloys were formed by the diffusion of the Ni layer and the alloy substrate during the oxidation test. These intermediate layers contributed to the maintenance of a high concentration of Al in the surface layer and the suppression of spallation of the surface layers, leading to the improvement in the oxidation resistance of the coating.

Formation of Ni Aluminide Layer Containing La by Molten-Salt Electrodeposition and Cyclic-Oxidation Resistance

In order to improve the cyclic-oxidation resistance of Ni and SUS304 steel, the coating of a Ni aluminide containing La on both samples were carried out by the electrodeposition of Al and La in a molten salt. The electrodepositions of Al and La were conducted using a potentiostatic polarization method at 1023 K in an equimolar NaCl-KCl melt containing 3.5 mol%AlF3, and that containing 4 mol%La2O3-24 mol%NH4Cl, or 3.5 mol%LaF3. Observation and analysis of the cross-section of the specimen after polarization showed that a deposited layer consisting of a thick inner Ni aluminide layer and a thin outer layer containing a large amount of La was formed on both samples. The cyclic-oxidation resistance of the Ni specimen covered with the deposit layer having the La layer as the outer layer was higher than those of the untreated Ni specimen and the Ni specimen coated with the deposit layer without La. The stainless steel covered with the Ni aluminide showed a high cyclic-oxidation resistance in the atmosphere containing water vapor at 1273 K in comparison with untreated stainless steel, without depending on the existence of La in the deposited layer.

High temperature oxidation of laser surface treated Fe-Cr alloys in HCl-containing atmosphere

Abstract Laser surface treatment of Fe-1, 2, 5, 10, 14 and 19%Cr alloys was accomplished by CO2 gas laser. High temperature oxidation behavior of laser treated alloys was investigated in HCl-containing atmosphere by thermogravimetric technique. Mass change of Fe-Cr alloys in 1%HC1-50%O2-N2 atmosphere at the temperature range from 1073–1273 K was effectively reduced by laser treatment. The effectivity of this treatment was most significant in case of Fe-14%Cr alloy. From analyses of the scales formed in the initial stage and in the later stage, the mechanism of improvement in oxidation resistance in 1%HCl atmosphere was discussed.

Cathodic corrosion of nickel in molten sodium sulphate and sodium carbonate at 900°C

The corrosion behaviour of nickel under cathodic polarization in both molten Na 2 SO 4 and Na 2 CO 3 at 900 o C has been investigated. Corrosion losses of nickel under cathodic polarization in both molten salts increased with an increasing quantity of electricity for the cathodic reaction. Large amounts of powdered corrosion products, rich in nickel, were detected in both salts after cathodic polarization. It is thought that such cathodic corrosion of nickel proceeded by a chemical reaction of nickel with the cathodic products

The effects of polarization potential and concentration of hydrochloric acid on the photopotential of passive films formed on titanium in hydrochloric acid solution

Abstract The effects of polarization potential and concentration of hydrochloric acid on the photopotential of the passive film formed on titanium in hydrochloric acid solutions were investigated. The photopotentials measured for titanium at open circuit condition after potentiostatic polarization at potentials in the passive region showed negative values, and increased in the negative direction with an increase in polarization potential. The increase in concentration of hydrochloric acid for the solution led to an increase in passive current density and a lowering of the photopotential. The results were interpreted on the basis of Oshes model, and discussed from the viewpoint of semiconductor properties of passive film.

Preparation of Highly Oxidation-Resistant Surface by Molten Salt Electrodeposition

In order to prepare a highly oxidation-resistant surface for TiAl and SUS 304 stainless steel, the molten salt electrodeposition of Al or Y on these metals was carried out. The electrodeposition was conduced using a potentiostatic polarization method at constant potentials in an equimolar NaCl-KCl melt containing AlF3 or YF3 at 1023 K. After the Al electrodeposition, homogenous deposit layers were formed on the TiAl and the stainless steel. The deposited layer formed on the TiAl consisted of TiAl3. The deposited layer formed on the stainless steel consisted of some Fe aluminides. The TiAl and the stainless steel covered by the electrodeposited layers were far more resistant than the bare TiAl and stainless steel to high temperature oxidation. The Y electrodeposition on the stainless steel induced the deposition of Y particles on the stainless steel. The cyclic-oxidation resistance of the electrodeposited stainless steel was remarkably improved as compared to the untreated stainless steel.

HOT CORROSION OF REACTION-SINTERED Si 3 N 4 IN MOLTEN Na 2 SO 4

The isothermal hot corrosion behavior of reaction -sintered Si 3 N 4 was evaluated at 1173 to 1293 K for 1.8 to 36 ks. Measurement of mass change, X-ray diffraction, surface observation and EPMA analysis were conducted during and after corrosion experiment. At temperatures above 1223 K mass of specimen slightly increased initially and then decreased rapidly with time. This decrease in mass was due to dissolution of Si 3 N 4 in molten Na 2 SO 4 evolving acidic and reductive gas. Below 1173 K mass-loss was not observed. Corrosion rate increased with increasing temperature. And the rate in argon was faster than that in oxygen. A glassy phase was formed on corroding specimen, and many small holes were observed on the phase. No corrosion product was identified by X-ray diffraction. Based in the above-mentioned experimental results, reaction mechanism was discussed.

High Temperature Corrosion Resistance of Siliconized Stainless Steel under Continuous Deposition of Salt

The surface alloying of Si into SUS304 austenitic stainless steel was carried out by a halide-activated pack-cementation method. By this treatment, the silicon diffusion layer containing about 13 at.% Si was formed. The high temperature corrosion resistance of this specimen was evaluated under the continuous deposition of salt. The result of the corrosion test showed that the oxidation mass gain of the siliconized stainless steel was lower than that of non-treated stainless steel. It was found from the observation of the cross-section of the specimen after the corrosion test that a thin scale was formed on the silicon diffusion layer and silicon oxide was formed as an inner layer of the scale. A mechanism of the oxidation suppression for the siliconized steel under the continuous deposition of salt was investigated by the oxidation test of pure silicon, iron, chromium or nickel powder mixed with equimolar NaCl-KCl. As a result, it was found that the high corrosion resistance of the siliconized steel was attributable to the fact that the silicon oxide formed on the silicon diffusion layer was inert to the chemical reaction with the NaCl-KCl salt.