Early Stages of High Temperature Corrosion in Steel Processing and Manufacturing

High temperature corrosion represents a vitally studied research field and fundamental understanding of the underlying mechanisms of material degradation is still a necessity to face upcoming challenges in modern energy politics.

Our current research activities in this field aim on a better understanding of the early stages of material degradation, especially on the changes of the surface conditions of the material under investigation. Many efforts have been undertaken to establish an accurate definition of process parameters to generate well-characterized exposures for a large variety of experimental possibilities.

Besides the unique combination at our institute, which makes this laboratory one of the flagship places in high temperature research worldwide, it amalgamates the benefits of rapid heating up to 30 °C s-1 (due to the use of an infrared-furnace), an oxygen impurity content of the used reaction gases below 3 ppm and a long-term stable dew-point (±0.2 °C). Thus the high temperature lab allows experiments covering a wide parameter field.

View of the novel thermobalance set-up for fast measurements showing an IR furnace (1) with thermobalance (2), a vacuum pump for adjusting the low-pressure conditions (3), gas humidification (4) and computer display for measurement results (5).

However, the in situ measurement of mass gain, especially during fast temperature cycles, which is a problem of significant practical importance, remained a problem that was experimentally unsolved. The reason for this is that due to fast changes of temperature in close vicinity to the sample surface, thermal drifts begin to dominate the thermobalance signal, making a clear scientific interpretation of the mass changes impossible. This problem is well known to the community for more than a couple of decades but a sound solution of such an issue has never been presented so far. Instead, trials to combine fast heating by infrared furnace with thermobalance were so far unsuccessful, i.e. results obtained with set-ups caused a lot of doubts and controversies and hence this combination is up to now only used for long-term exposures of several hundred hours.

In addition to coupling of IR-heating and thermobalance, it was therefore decided at MPIE to eliminate the content of inert gas – which often represents up to 95 % of the atmosphere – and to perform thermal exposures in a low pressure environment instead. By this procedure, we can still establish the same amounts of all reactive components and reduce the buoyancy effects by more than a factor of 10.

Initial tests with exposures of pure iron samples in an argon atmosphere, as illustrated in Fig. 2, prove the success of this technique. Whereas in both experiments a clear reduction of the initial drop in the recorded mass signal has been observed, the fluctuations in a steady gas flow could also be reduced by several orders of magnitude. This enables extremely accurate measurements of in situ mass changes down to 0.1 µg. Please note, that a low oxygen contamination in the argon gas causes a residual mass increase and that this can be clearly seen at 30 mbar (Fig. 2, left), whereas this is not possible at ambient gas pressures.

Pressure dependence of the thermogravimetric signal during a fast heating process in stationary argon atmosphere (left) and with continuous gas flow of 20 l h-1 (right).

This method provides a unique contribution to elucidate the kinetics of oxidation in the early stages and closes the gap between experimental observations on a large time scale and other measurements of the initial stage behavior. Hence this technique represents an important scientific progress in the field and helps to shed light on early stage material degradation, both from a scientific point of view as well as for many industrial processes.

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