Breakaway scale

Both single-layer protective and duplex-layer protective scales have a PBR of 2.1, whereas breakaway scales show a PBR of approximately 2.7, containing around 23% porosityand graphitic carbon grains (up to 6% by weight) between the oxide grains... 

However, the observation of existing duplex scales (and breakaway scales) during metal oxidation rather points to the second oxidation mode. The duplex morphology... 

In practice, thermal cycling rather than isothermal conditions more frequently occurs, leading to a deviation from steady state thermodynamic conditions and introducing kinetic modifications. Lattice expansion and contraction, the development of stresses and the production of voids at the alloy-oxide interface, as well as temperature-induced compositional changes, can all give rise to further complications. The resulting loss of scale adhesion and spalling may lead to breakaway oxidation " in which linear oxidation replaces parabolic oxidation (see Section 1.10). 

Since Mn is both soluble in iron oxides and mobile to the same extent as Fe, the addition of Mn to steels has little effect on the overall scaling rate in air or oxygen. Jackson and Wallwork have shown that between 20% and 40% manganese must be added to steel before the iron oxides are replaced by manganese oxides. However, Mn supresses breakaway oxidation in CO/CO2 possibly by reducing the coalescence of pores in the oxide scale. 

The presence of small quantities of S in steels has little effect on the initial scaling rates in air, but may be detrimental to long-term scale adhesion. Sulphur has, however, been shown to be detrimental to breakaway oxidation in CO/CO2 environments. However, sulphur has been shown to reduce the total uptake of carbon in the steel under CO/C02 and reduce the scale thickening rate. In this context, free-cutting steels were found to oxidise at a significantly lower rate, as did steels subjected to pretreatment in H2S. 

Like sulphur, phosphorus appears to have little effect on the overall scaling of iron alloys in air. It may, however, play a role in suppressing breakaway oxidation in carbon steels in CO/CO2 environments. Donati and Garaud " found that the tendency for breakaway was lower over ferrite, where P segregates. To confirm this, the authors doped pure Fe with P and found that... 

Small additions of Ce have been shown to have a favourable influence on oxide growth of several Fe-Cr alloys by improving scale adherence and acting as nucleation sites for CrjO, . Levels of Ce as low as 0.024% reduce the carbon uptake of steels in carbonaceous atmospheres by several orders of magnitude. Trace concentrations of As and Sn have been found to improve the breakaway properties of mild and low alloy steels in CO/COj, whereas Cu has been found to be detrimental. ... 

Following the initial protective period, under certain conditions of temperature, alloy and gas composition , the oxidation goes through a transitional stage into breakaway. Several authors have reported that breakaway oxidation is initiated once the scale reaches a critical thickness or weight gain and only occurs below an initially protective duplex layer . 

The rapid diffusion causes impoverishment of A1 in the TiAl3 layer leading to breakaway oxidation, because the A1203 scale formed is not maintained anymore. It also causes Kirkendall voids near the coating/substrate interface during the treatment and/or oxidation. This will reduce the adherence of coating to the substrate. 

The oxidation life of ODS FeCrAl alloys is determined by their ability to form or reform a protective alumina scale, and can be related to the time required for the aluminum content of the alloy to be depleted to some minimum level [2-5]. As a result, the service life is a function of the available aluminum content of the alloys and the minimum aluminum level at which breakaway oxidation occurs. Therefore, there is a minimum cross sectional thickness which can be safely employed at temperatures above 1200°C.The major factors that result in depiction of the reservoir of aluminum in the alloy are the inherent growth rate of the aluminum oxide and the tendency for the scale to spall, which results in a (temporary) increase in oxide growth rate in the area affected by spallation. Because of their significantly higher aluminum content >28 at% compared to 9 at%), alloys based on Fe3Al afford a potentially larger reservoir of aluminum to sustain oxidation resistance at higher temperatures and, therefore, offer a possible improvement over the currently-available ODS FeCrAl alloys [61-... 

Fig. 8. Toial weight change (sample + spalled oxide) during lOOh cycles at 1200°C for Kanthal APM and undoped Fe3Al with various O contents. In long-term testing, Zr02-dispersed FeCrA1 showed excellent scale adhesion but A1203 dispersions did not improve scale adhesion and instead shortened the time to breakaway compared to a cast alloy.The extruded FAS appeared to slump and rapidly oxidize.

Breakaway oxidation was also reported by Becker et al. after prolonged exposure of TiAl in dry oxygen [4], This breakaway was attributed to a change in the scale structure. The A1,03 barrier layer was supposed to have no long-term stability. After a critical thickness is reached the alumina layer begins to dissolve and precipitates again as... 

The behaviour of iron in CO-CO2 atmospheres was studied by Pettit and Wagner over the temperature range 700-1000 °C, where wustite is stable. They found that the kinetics were controlled by reactions at the scale-gas interface, but carbon pickup was not observed. In contrast, Surman oxidized iron in CO-CO2 mixtures in the temperature range 350-600 °C, where wustite was not stable and magnetite exists next to the iron. He observed breakaway oxidation whose onset, after an incubation period, coincided with the deposition of carbon within the scale. This was explained by Gibbs and Rowlands by the penetration of the scale by CO2 which achieved equilibrium in the scale, dissolved carbon in the metal substrate, and then deposited carbon within the scale, which split open the scale and left the system in a state of rapid breakaway oxidation. The incubation period observed corresponded to the time required to saturate the metal substrate with carbon. 

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