Breakaway oxidation

If a compact film growing at a parabolic rate breaks down in some way, which results in a non-protective oxide layer, then the rate of reaction dramatically increases to one which is linear. This combination of parabolic and linear oxidation can be tenned paralinear oxidation. If a non-protective, e.g. porous oxide, is fonned from the start of oxidation, then the rate of oxidation will again be linear, as rapid transport of oxygen tlirough the porous oxide layer to the metal surface occurs. Figure C2.8.7 shows the various growth laws. Parabolic behaviour is desirable whereas linear or breakaway oxidation is often catastrophic for high-temperature materials. 

At ambient temperatures beryUium is quite resistant to oxidation highly poHshed surfaces retain the brilliance for years. At 700°C oxidation becomes noticeable in the form of interference films, but is slow enough to permit the working of bare beryUium in air at 780°C. Above 850°C oxidation is rapid to a loosely adherent white oxide. The oxidation rate at 700°C is paraboHc but may become linear at this temperature after 24—48 hours of exposure. In the presence of moisture this breakaway oxidation occurs more rapidly and more extensively. BeryUium oxide [1304-56-9] BeO, forms rather than beryUium nitride [1304-54-7] Be2N2, but in the absence of oxygen, nitrogen attacks beryUium above 900°C. 

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... 

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 . 

Weight-change data for most of the alloys at temperatures below 900°C did not show a breakaway oxidation period at least till about 150 hours. However, a common microstructural observation made in all the alloys that were oxidized at temperatures of 900°C or less was that the surfaces of all the alloys were characterized by cracks parallel to the surfaces just below the oxide layer as illustrated in Figure 6(a). Cracks form just below the surface oxide layer (see figure 6(a)) and these cracks appear mostly in the silicide phase, though one can see them in the P phase as well to a lesser extent. This... 

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-... 

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... 

Generally, further investigations on the mechanism initialing rapid breakaway oxidation in 02-H20 appear to be necessary. 

After initial protective oxidation of 7-TiAl rapid breakaway oxidation follows in moist oxygen. In general, the oxidation is faster in moist than in dry oxygen. The oxidation rale increases with increasing p,70 and decreasing pQ2, which may indicate surface reaction control by H2 dissociation, retarded by adsorbed oxygen. 

Figure 10-44. Breakaway oxidation wave front structure morphological changes in the oxide, non-homogeneous stress distribution in oxide layer.

Baranov, I.E., Fridman, A., Kirillov, LA., Rusanov, VD. (1991), Breakaway Oxidation Effect and Its Influence on Severe Nuclear Accidents, Kurchatov Institute of Atomic Energy, Moscow. 

Laboratory oxidation-corrosion data indicate that extrapolation of short-term oxidation-corrosion data to yearly rates is difficult. These extrapolations are necessary to provide a basis for comparing oxidation-corrosion data obtained from variable CGA exposure times. Extrapolated data, particularly at high H2S concentrations in the CGA atmosphere, should be employed with caution. Long-term kinetics of the oxidation-corrosion process can result in transitions in corrosion behavior to high rates not predictable by short exposures. Similar behavior, breakaway oxidation, occurs in air primarily at temperatures above 2000 F. 

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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