On the mechanism of oxidation of iron-16·4% chromium at high temperature

Abstract

The mode of oxidation of pure iron-16·4% chromium alloy in air at 1100°C has been studied by thermogravimetric methods, metallography and particularly electron-probe microanalysis. It has proved possible to follow in detail the processes occurring during the development of protective scale, stratified nodular growths and thick stratified scale. Electron-probe microanalysis of protective scales in cross-section and plan demonstrates that they are largely Cr 2O 3 containing small proportions of iron oxide. The subjacent alloy may be depleted in chromium to as low as 5·3% without apparent transformation of thick Cr 2O 3 scale to Cr 2O 3Fe 2O 3 solid solution rich in Fe 2O 3 or Fe Fe 2− x Cr x O 4 spinel. It appears likely that the failure of Cr 2O 3 scales under the present conditions is due to scale lifting and cracking, followed by rapid oxidation of the depleted alloy, although in some cases the initial layer may be inherently non-protective due to its containing a higher proportion of iron oxides than usual. Stratified scale usually develops by way of nodules which also exhibit internal oxidation. As the catastrophic break-through progresses, the chromium content of the inner layer is diluted to a limiting value of 20–25% chromium, outer, virtually pure, iron oxide layers develop and the depleted subjacent alloy is almost entirely eliminated. “Steady-state” conditions now largely prevail, there only being minor fluctuations in the compositions of the various layers as the total scale thickens. The contribution of the present study in understanding the mechanism of oxidation is considered in conjunction with previous investigations.