Yutaka Shinata

NaCl-induced hot corrosion of stainless steels☆

Abstract The hot corrosion behaviour of NaCl-coated SUS-430, SUS-304, SUS-316 and SUS-329J1 stainless steels was examined in a temperature range from 923 to 1173 K by thermogravimetry, measurement of corrosion loss, X-ray diffraction of the scale and electron probe microanalysis. Corrosion of these stainless steels is markedly accelerated by the presence of NaCl at every temperature, but the rate of acceleration is considerably higher above the melting point of NaCl. In hot corrosion, chromium is oxidized selectively, forming a non-protective Cr2O3 scale. Therefore the corrosion loss of these four types of steel increases with increasing chromium content. The nickel contained in the steels restricts the detrimental effect of chromium. In the SUS-329J1 two-phase stainless steel, the ferritic phase is attacked more than the austenitic phase because the ferrite has a high chromium content and a low nickel content.

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.

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

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.

The effect of a small amount of HCl gas on high temperature oxidation of pure iron and chromium

The effect of a small amount of HCl on high temperature oxidation of pure Fe and Cr was examined by thermogravimetry and analysis of corrosion products. Oxidation of Fe at 873K was accelerated slightly by a small amount of HCl whereas the oxidation at temperatures above 973K was restrained by a small amount of HCl. Mass gain of Fe due to oxidation turned negative after an oxidation period of 20ks at temperatures higher than 1073K. Under the condition for which the mass gain turned negative, the formation of volatile FeCl2 was supposed. On the other hand, the oxidation of Cr at temperatures lower than 1100K was extremely accelerated by a small amount of HCl. At temperatures higher than 1123K, however, the oxidation rate in O2 containing HCl was the same as that in pure O2. It was found that the Cr2O3 scale whose formation was accelerated by HCl contained Cl. Such oxidation behaviors for Fe and Cr under O2 containing a small amount of HCl were discussed thermodynamically.

Photostimulated Exoelectron Emission (PSEE) during Tensile Deformation of Aluminum Single Crystal

Photostimulated exoelectron emission (PSEE) from aluminum single crystals during tensile deformation was measured with a picoammeter in air and in a vacuum. The PSEE intensity is discussed in relation to surface consitioning and crystal orientation. The integrated surface areas of slip steps formed by tensile deformations were calculated and the values were compared with the experimental data. The PSEE intensity during tensile deformation increased with a progression of the elongation. However the rates of the increases differed depending on the surface conditioning and crystal orientation. The intensity in a vacuum was larger than that in air, but the relative change during deformation was nearly equal in both conditions. The PSEE intensity was proportional to the integrated surface area of slip steps for every orientation.