Oxidation Studies

High Temperature Oxidation of Ceramics

Silicon nitride and silicon carbide ceramics oxidise slowly at temperatures above 900°C. With standard commercial sintered silicon nitride, for example, an oxide film some 10 µm thick can be formed after exposure to air at 1375°C for 24 hours. This corresponds to a mass gain of around 3 mg for the standard cubes used of 7 mm side. Mass gains are monitored and recorded using a CI Microbalance and control unit, which is linked to a PC. This system is programmed to correct automatically for the mass losses due to volatilisation of the platinum suspension system at these high temperatures. Controlled curve smoothing is required to eliminate the effects of sample vibration in the hot gas environment. Corrected data can be conveniently and easily presented in a variety of formats.

Continuous Weight-Gain Monitoring in High Temperature
Oxidation/Corrosion Environments

Primarily, the Microbalance is used for continuous weight-gain monitoring of alloys at high temperature in a specific environment (e.g. 1000°C, 1 atm. dry oxygen) and on cooling to room temperature. The kinetic data obtained play an integral role in the general understanding of the high temperature oxidation/corrosion processes involved and the performance of scales or coating both at temperature and when cooling.

The output data from the microbalance is fed via a CI Disbal control unit onto a PC, which presents the kinetic data in the form of weight change versus time.

Measurement of Oxide Formation
High Temperature Oxidation

CI Robal control units (the predecessor of the DISBAL) and 5g Vacuum heads formed an integral part of a programme that commenced in 1979, in a nuclear laboratory, to research the effects of high temperature oxidation on fuel cladding alloys in Advanced Gas Cooled Reactors (AGRs).

In a highly corrosive environment involving continuous exposure to oxidisation coolant gas at high temperatures for periods up to five years, niobium stabilised 20% chromium / 25% nickel stainless steel, from which cladding tubes are made, must act as an impermeable barrier between the uranium oxide fuel and the flowing carbon dioxide based coolant. It is, therefore, important to have a detailed knowledge of the mechanism of high temperature oxidation processes.

Oxide formation on a metal surface results in a weight loss. The continuous measurement of such weight changes using a thermobalance based on a CI microbalance enables the effects of these processes to be monitored precisely.

The weight gain / temperature data from 8 thermobalances were fed directly into a data logging facility which provided an analysis of oxidation kinetics and spallation / temperature relationships. Weight changes of the specimens were monitored continually for 1500 hours and also shorter 24-hour periods under the steady state, isothermal and transient conditions.