This study provides new perspectives on why the oxidation rates of silicon carbide and silicon nitride are lower than those of silicon and on the conditions under which gas bubbles can form on them. The effects on oxidation of various rate-limiting steps are evaluated by considering the partial pressure gradients of various species, such as O,, CO, and N1. Also calculated are the parabolic rate constants for the situations when the rates are controlled by oxygen andlor carbon monoxide (or nitrogen) diffusion. These considerations indicate that the oxidation of silicon carbide and silicon nitride should be mixed controlled, influenced both by an interface reaction and diffusion. [

The kinetics of Na2SO4-induced corrosion were measured by accelerated oxidation tests on Co-30Cr and Ni-30Cr as a function of temperature from 600~ "C, SO3 in the environment and deposit composition. The alloys were rapidly attacked at temperatures between 650 ~ and 750~C when a liquid sulfate phase was obtained from an initially pure solid Na2SO4 deposit. The rapid rate of attack resulted ~rom sulfation of the transient surface nickel or cobalt oxides and the dissolution of these transition metal sulfates into Na2SO4 to yield a liquid phase. This retarded the formation of a protective Cr203 scale. The exposure conditions under which liquids could form from Na2SO4-CoSO4 mixtures were calculated from thermodynamic considerations.Many fossil-fueled devices for energy conversion are susceptible to rapid degradation of high temperature alloy components when subjected to molten salt deposits. Because of the nearly universal presence of sulfur and alkali metals in fossil fuels, the corrosive deposits usually contain alkali sulfates (Na2SO4, K2SO4). The mechanism of Na2SO4-induced corrosion at high temperatures, where Na2SO4 (mp _--884~ is liquid, has been extensively studied. This form of attack, commonly known as hot corrosion, results in typical sulfidation with an aluminum and/or chromium depletion zone in the alloy.Recently, a different, but related, form of hot corrosion has been identified in marine propulsion gas turbines under service conditions where the surface temperatures of the first stage blades and vanes were in the range of about 650~176(1). The attack on Co-base coatings resulted in an unusual morphology: pitting, little or no depletion of A1 or Cr in the alloy, and corrosion products which contained water soluble Co and Ni. Burner rig tests and other studies have been able to reproduce these characteristic features of intermediate temperature attack only when Na2SO4containing deposits are used in conjunction with sufficient SO3 in an oxidizing environment (2, 3).Only a few investigators have studied the mechanism of alkali sulfate induced corrosion at intermediate temperatures (600~176 (4-6). Umland and Voigt (4) investigated the formation of soluble Ni, Co, and chromate ions in crucible-type corrosion tests. Several nickel-, cobalt-, and iron-base alloys were exposed to melts containing Na2SO4, K2SO4, or mixtures thereof in air or in air containing SQ over the temperature range 650~176They found a maximum in corrosion rate at 750~ which coincided with the highest recovery of Ni and Co sulfates from the melt. This study was subsequently extended by Balajka and Danek (5), who observed enhanced corrosion of Ni in alkali sulfate melts containing CoSO4 at intermediate temperatures. They interpreted their results in terms of Umland and Voigt's suggestion that cobalt * Electrochemical Society Active Member.

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