Reaction rate, chromium content

As discussed (vide supra) disproportionation and isomerization are often competitive reactions. The reaction rates of both types depend on the temperature and the catalyst used for the reaction. Chromium(III) oxide on support, or without, favors the disproportionation of 1,1,2-trichloro-1.2,2-trifluoroethane to give l,l-dichloro-l,2.2,2-tetrafluoroethane and 1,1,1.2-tetra-chloro-2,2-difluoroethane whereas with aluminum trifluoridc the isomerization is favored.24 The higher the chlorine content of the molecules the greater is their reactivity. 

It is interesting to note the effect of chromium content on reaction rate at high pressures (,—500 p.s.i.g.). Experiments (5) were carried out with normal air-activated catalysts (Figure 4). Catalysts were used with chromium contents ranging from 0.7 to 0.0005 wt. % of the total catalyst. Results of one-hour ethylene polymerization tests at 132°C. and 450 p.s.i.g. with these catalysts, activated at 500°C., are given. As the concentration of chromium was decreased, catalyst charge was increased to compensate for poisoning of catalyst sites by trace impurities and to keep total rate of production about constant. 

The effect of the chromium content of the alloy on corrosion in boiling acids is shown in Table 4.7 along with the data for carbon steel and low-carbon and low-nitrogen 35% Cr alloys. The data show that the corrosion rates of 18 Cr-8 Ni (Type 304) is lower than Type 430 and 446 that is devoid of nickel. The nickel is the alloy probably reduces the rate of hydrogen evolution reaction. The molybdenum in Type 316 alloy was found to be useful in protection from pitting by chloride ions. 

In order to substantiate this measure of chromia area, the rates of carbon monoxide oxidation over the various catalysts were measured. It was found that the alumina portion of the surface could be rendered inactive by selective poisoning with water and, under these conditions, the reaction was catalyzed exclusively by the ehromia surface. Since the activation energy was independent of the chromium content, it was reasonable to expect a linear variation of specific activity (i.e., activity per unit total surface area) with the fraction 0 (Table I) of the total surface contributed by the chromia phase. In Fig. 3 the specific rate is... 

These estimates give only an upper limit of oxygen permeation rate because surface exchange reactions may result in some suppression of the overall transport. For the conditions typical for syngas generation (pO2=0.21 atm, pOj = 10 atm, 950°C), the results for the membranes with L = 0.1cm and different chromium contents are shown in Fig. 3. In the calculations at high pressures the ion conductivity values were assumed to be nearly equal to those at low pressures. Fig. 2. It is seen that the permeation rate in chromium doped samples is smaller than in the parent ferrite. Nonetheless, it may achieve a value of about 4 ml cm min in the sample with y = 1, which corresponds to the syngas production rate of about 20-25 ml-cm -min in the methane partial oxidation process. In combination... 

Small Cr contents increase the rate of reaction, but at 20% Cr, the reaction rate starts to decrease and exhibits a minimum value at 25%-30% Cr (Figure 20.62). The minimum value depends on the pressure. More chromium is needed to stabilize a protective film since the diffusion coefficient of chromium in cobalt is lower than for chromium in nickel. However, since the adhesion strength of the film on Co-Cr alloys is poorer than on Ni-Cr alloys despite the identical oxidation rate of the Cr-containing Co alloys with Cr203 protective film, the practical oxidation resistance is lower. Other alloying elements, as Figure 20.62 shows, have little influence on scale resistance. 

With oxides containing higher chromium contents, the Lomi process proceeds very slowly since vanadous picolinate is not able to reduce Cr(III). If the chromium concentration in the oxide exceeds 15%, the dissolution rates will become unacceptably low. Therefore, in such applications, the Cr(III) must first be oxidized to Cr(VI) by an appropriate preoxidation step (AP and NP steps, see below). In order to save this time-consuming additional step, attempts have been made to replace V(II) by another reducing cation. Only a combination of Cr(II) with nitrilo triacetic acid or with EDTA shows a faster reaction, but it suffers from insufficient thermal stability thus, it cannot be used in the decontamination of systems and circuits. 

In addition to zirconium oxidation, the chromium content of the stainless steel internals of the reactor pressure vessel can also be oxidized by steam in this phase of the accident, resulting in the production of additional hydrogen. Since the rate of chromium oxidation is lower by a factor of 2 to 3 than that of zirconium under comparable conditions and since the temperatures of the core internals are distinctly lower than that of the fuel rod claddings, the contribution of this reaction to total hydrogen production during this phase of the accident can be neglected. 

Oxide formation leads to a decrease in the overall oxidation rate, according to Eq. (3). The value of in this equation [which is the same as in the crack propagation rate Eq. (5)] varies with the alloy chemistry [e.g., chromium content for a denuded grain boundary of type 304 stainless steel, or sulfur content for low-alloy steels (Fig. 11)], electrode potential, and anionic activity, and this can be related to e.g., solid-state oxide growth, dissolution-precipitation reactions, and oxide breakdown [1,11]. Thus, all of the parameters in Eq. (5), apart fi om can be quantified for the crack tip system, which can, in turn, be related to de able or measurable bulk system conditions. 

Decarburization occurs in steels and cast irons in hydrogen gas by the reaction of H with C in the steel. The decarburization rate is primarily dependent on the diffusion rate of C in the steel, but is also affected by the carbon content of the steel, alloying elements in the steel, such as chromium, impurities in the hydrogen, and of course time and temperature. Carburization of steels, the reverse of decarburization, is usually conducted at temperatures of about 900°C, but decarburization can occur at temperatures as low as 800°C. " ... 

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