Tokiko Nakagawa

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.