The oxide is produced by the reduction of CO2 to form the adherent black oxide, magnetite (nominally Fe^Oj). The rate of formation of this scale is tem­perature-dependent and like any chemical reaction increases with temperature. The early experimental evidence suggested that a protective oxide would be built-up and would reach a limiting thickness which was catered for in design. The unforeseen aspect of steel oxidation was the so called ‘breakaway’ behaviour.

The stages of oxidation are illustrated by Fig 3.70. The protective layer is about 50 microns and is strongly adherent to the metal surface. On mild steel, given sufficient time, a porous sub-layer of oxide is formed and this bursts through the protective layer to form excrescences. As this transition stage proceeds, more and more of the protective layer is broken down until the full breakaway condition is reached, when the metal will continue to oxidise and is wasted away.

The oxidation rate in the breakaway condition is temperature, gas coolant composition and material composition dependent. These features are illustrated by Figs 3.71 and 3.72. Except for very low silicon con­tent steels, the oxidation rate is low at temperatures below 360°C and roughly doubles for every 20°C increase in temperature. Hence the reduction of maxi­mum gas temperatures to 360°C in 1968. The carbon monoxide and hydrogen content of the coolant gas also influences the rate of oxidation (the lower the lex els, the less the oxidation). However, low values of these constituents increases the corrosion of graphite, і. e., the core, so that in controlling gas composition a balance has to be made between these two conflict­ing requirements.