Chromia-forming alloys have good resistance to oxidizing agents such as O2, CO2, … It is accepted that the protection of these alloys is always due to the chromia layer formed at the surface of the alloys, which acts...Chromia-forming alloys have good resistance to oxidizing agents such as O2, CO2, … It is accepted that the protection of these alloys is always due to the chromia layer formed at the surface of the alloys, which acts as a barrier between the oxidizing gases and the alloy substrates, forming a diffusion zone that limits the overall reaction rate and leads to parabolic kinetics. But this was not verified in the study devoted to Inconel®625 the oxidation in CO2 that was followed by TGA, with characterizations by XRD, EDS and FIB microscopy. Contrary to what was expected and accepted in similar studies on other chromia-forming alloys, it was shown that the diffusion step that governs the overall reaction rate is not located inside the chromia layer but inside the alloy, precisely inside a zone just beneath the interface alloy/chromia, this zone being depleted in chromium. The chromia layer, therefore, plays no kinetic role and does not directly protect the underlying alloy. This result was demonstrated using a simple test that consisted in removing the chromia layer from the surface of samples partially oxidized and then to continue the thermal treatment: insofar as the kinetics continued without any change in rate, this proved that this surface layer of oxide did not protect the substrate. Based on previous work on many chromia-forming alloys, the possibility of a similar reaction mechanism is discussed. If the chromia layer is not the source of protection for a number of chromia-forming alloys, as is suspected, this might have major consequences in terms of industrial applications.展开更多
The first chromia-pillared layered tetratitanate was prepared by the reaction of layered tetramethylammonium tetratitanate with chromium (III) acetate [Cr(OAc)3] aqueous solution and subsequent calcination of the resu...The first chromia-pillared layered tetratitanate was prepared by the reaction of layered tetramethylammonium tetratitanate with chromium (III) acetate [Cr(OAc)3] aqueous solution and subsequent calcination of the resultant solid product in air at 400°C. The obtained chromia-pillared layered tetratitanate has an interlayer distance of 1.06 nm and a high thermal stability up to 600°C. It was also found that calcination in N2 led to the chromia-pillared layered tetratitanate with relatively higher Brunauer-Emmett-Teller (BET) specific surface area (93.9 m2·g?1) and smaller average pore diameter (4.44 nm) than that in air (82.0 m2·g?1, 7.61 nm). Both Br?nsted and Lewis acid sites (mainly Lewis type) are present on the chromia-Pillared layered tetratitanate (500°C, N2, 8 h) and strong enough to still remain a small proportion of pyridine upon outgassing at 250°C. Moreover, ammonia temperature-programmed desorption (NH3-TPD) result showed that there were three NH3 desorption peaks at 160, 200 and 315°C, respectively. The corresponding acid amount is 41.3, 73.9 and 290.7 μmol·g?1. The total acid amount is 405.9 μmol·g?1.展开更多
文摘Chromia-forming alloys have good resistance to oxidizing agents such as O2, CO2, … It is accepted that the protection of these alloys is always due to the chromia layer formed at the surface of the alloys, which acts as a barrier between the oxidizing gases and the alloy substrates, forming a diffusion zone that limits the overall reaction rate and leads to parabolic kinetics. But this was not verified in the study devoted to Inconel®625 the oxidation in CO2 that was followed by TGA, with characterizations by XRD, EDS and FIB microscopy. Contrary to what was expected and accepted in similar studies on other chromia-forming alloys, it was shown that the diffusion step that governs the overall reaction rate is not located inside the chromia layer but inside the alloy, precisely inside a zone just beneath the interface alloy/chromia, this zone being depleted in chromium. The chromia layer, therefore, plays no kinetic role and does not directly protect the underlying alloy. This result was demonstrated using a simple test that consisted in removing the chromia layer from the surface of samples partially oxidized and then to continue the thermal treatment: insofar as the kinetics continued without any change in rate, this proved that this surface layer of oxide did not protect the substrate. Based on previous work on many chromia-forming alloys, the possibility of a similar reaction mechanism is discussed. If the chromia layer is not the source of protection for a number of chromia-forming alloys, as is suspected, this might have major consequences in terms of industrial applications.
基金Project (No. 29573108) supported by the National Natural Science Foundation of China
文摘The first chromia-pillared layered tetratitanate was prepared by the reaction of layered tetramethylammonium tetratitanate with chromium (III) acetate [Cr(OAc)3] aqueous solution and subsequent calcination of the resultant solid product in air at 400°C. The obtained chromia-pillared layered tetratitanate has an interlayer distance of 1.06 nm and a high thermal stability up to 600°C. It was also found that calcination in N2 led to the chromia-pillared layered tetratitanate with relatively higher Brunauer-Emmett-Teller (BET) specific surface area (93.9 m2·g?1) and smaller average pore diameter (4.44 nm) than that in air (82.0 m2·g?1, 7.61 nm). Both Br?nsted and Lewis acid sites (mainly Lewis type) are present on the chromia-Pillared layered tetratitanate (500°C, N2, 8 h) and strong enough to still remain a small proportion of pyridine upon outgassing at 250°C. Moreover, ammonia temperature-programmed desorption (NH3-TPD) result showed that there were three NH3 desorption peaks at 160, 200 and 315°C, respectively. The corresponding acid amount is 41.3, 73.9 and 290.7 μmol·g?1. The total acid amount is 405.9 μmol·g?1.
