Catastrophic oxidation

But they all oxidise very rapidly indeed (see Table 21.2), and are utterly useless without coatings. The problem with coated refractory metals is, that if a break occurs in the coating (e.g. by thermal fatigue, or erosion by dust particles, etc.), catastrophic oxidation of the underlying metal will take place, leading to rapid failure. The unsafeness of this situation is a major problem that has to be solved before we can use these on-other-counts potentially excellent materials. 

Catastrophic oxidation requires the presence of Na2S04 and Mo, W, and/or V. Crude oils are high in V ash will be 65% V2O5 or higher. V can be alloyed in metal. A galvanic cell is generated ... 

Generally, the most important reaction is that of tantalum with oxygen, since it tends to form oxides when heated in air. Reaction starts above 300°C and becomes rapid above 600°C . The scale is not adherent, and if the oxidised material is heated above 1000°C oxygen will diffuse into the bulk of the material and embrittle it. At 1200°C catastrophic oxidation attack takes place at a rate of about 150 mm/h Oxygen is not driven off by heating alone, but in vacuum above 2300°C it is removed as a suboxide. The first step of the conversion mechanism of tantalum into oxide was shown to occur by the nucleation and growth of small plates along the 100) planes of the BCC metaP. ... 

Tungsten metal is stable in dry and humid air only at moderate temperature. It starts to oxidize at about 400 °C. The oxide layer is not dense and does not offer any protection against further oxidation. Above 700 °C the oxidation rate increases rapidly, and above 900 °C, sublimation of the oxide takes place, resulting in catastrophic oxidation of the metal. Any moisture content of the air enhances the volatility of the oxide. 

Alloys containing molybdenum can potentially experience catastrophic oxidation. The super-austenitic stainless steels such as Alloy AL-6XN, a 21Cr-25Ni-6.5Mo-N alloy (UNS N08367), are an example. A heavy molybdenum oxide scale forms, usually as a result of an improper heat treatment or a severe thermal excursion. Removal of such scales prior to service (or return to service), usually by pickling, prevents this problem. 

Catastrophic oxidation caused by refractory-metal additions... 

Tvmgsten additions have not been observed to cause catastrophic oxidation of Ni- and Co-base alloys, perhaps because of the higher melting temperatures of the tungsten oxides as compared with molybdenum oxides, e.g., Tnp = 1745 Kfor WO3. However, W additions have been observed to induce some scale breakdown on Ni-Cr alloys. Additions of W to Co-Cr alloys appear to be beneficial in decreasing the transient oxidation period and promoting the formation of a continuous Cr203 layer. 

Catastrophic oxidation is not defined by any particular rate law. The term refers to oxidation that continues to increase as a function of time. Metals that demonstrate Hnear oxidation rates at lower temperatures have the potential to undergo catastrophic oxidation at higher temperatures. The oxidation reaction is an exothermic one, generating... 

For chemiluminescence experiments, the procedure is similar to the aforementioned OIT experiments, in that the time to catastrophic oxidation of the... 

The oxidation behavior of vanadium is very poor and one strategy for improvement was the addition of Al to form an external AI2O3 scale. Addition of up to ca. 45 at.% Al and ternary addition of Cr and Ti did not overcome the significant problem that V2O5 melts at 690°C (Keller and Douglass, 1991). This melting results in catastrophic oxidation at temperatures of 700-1000 °C. 

Figure 6-35. Optical macrograph showing the catastrophic oxidation-induced failure of a Cr-ZrCr2-based alloy after six-cycle, 120-h screening exposure at 1100°C in air. Note that the coupon was initially a solid 1-cm diameter, 1-mm thick disk (Brady, 1997).

Alloys containing molybdenum, specifically t) es 316 and 317 austenitic stainless steels containing 2 and 3% molybdenum, respectively, are subject to catastrophic oxidation. Under some conditions, there is a selective oxidation of molybdenum with rapid loss of volatile M03. 

The kinetics of oxidation of tantalum in pure oxygen have been studied at temperatures up to 1400°C and at pressures ranging from less than 1 to over 40 atm (0.10-4.05 MPa). The reaction is initially parabolic, with a transformation to linear rate after a period of time. Increasing the temperature not only increases the rate of oxidation, but also decreases the time before the reaction changes from parabolic to linear behavior. Above about 500°C and pressures from 10 mm Hg to 600 psi (1333 Pa to 4.13 MPa), the transition occurs almost immediately. From 600 to 800°C, the oxidation shows a pronounced increase in rate with pressure above 0.5 atm (0.05 MPa). At 1300°C and 1 atm oxygen pressure, tantalum oxidizes rapidly and catastrophically, but at 1250°C, the metal oxidizes linearly for a short time, then catastrophically. Unlike tantalum-oxygen reactions, however, tantalum-air reactions do not exhibit catastrophic oxidation at temperatures as high as 1400°C. 

Linear rate law and catastrophic oxidation. The linear rate law [Eq. (3.14)] is also an empirical relationship that is apphcable to the formation and buildup of a nonprotective oxide layer ... 

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