No description available.

This work was undertaken to investigate the transpassive behaviour of the anodic film on two Fe-Cr alloys, namely Fe18Cr and Fe18Cr2Mo in acidic aqueous media in the pH range 0.5 to 3.8. Two electrochemical techniques were used, namely cyclic voltammetry and chronoamperometry. The two primary experimental variables in the cyclic voltammetric experiments were pH and sweep rate (2 - 800 mV/s). The main variables in the chronoamperometric experiments were the size of the potential step, the number of potential steps and the starting and ending potentials. Secondary experimental variables were temperature (25, 90°C), rotation rate (0, 150 rad/s), and the artificial addition of cations (Fe2+, Fe3+ and Cr3+) to some of the solutions. A voltammetric anodic peak, referred to as peak A, occurs in the transpassive region of the above Fe-Cr alloys, followed by a region of secondary passivity and then oxygen evolution. It was this peak that was investigated by cyclic voltammetric methods. The peak A current response was independent of rotation rate at pH 3.8 but was dependent on rotation rate at pH 0.5 with solutions of intermediate pH showing a gradual change in rotation rate dependence. This indicated a predominantly solid state process in less acidic solutions (pH 2.4 & 3.8) whereas in strongly acidic solutions (pH 0.5) the action of ions in solution must contribute significantly towards peak A processes. A method was developed to correct the peak A current response for the current due to oxygen evolution. The results of this method indirectly confirmed the hypothesis that more than one oxidation process contributes to the peak A current response. A diagnostic plot for diffusion control was applied to the peak height of peak A. The diagnostic involves plotting the peak height over the square root of the sweep rate versus the square root of the sweep rate. A process under diffusion control would give a horizontal line for this diagnostic plot. At pH 0.5 and at slow sweep rates (less than or equal to 60 mV/s) the diagnostic plot gave a positive deviation from the horizontal and this deviation was enhanced as the temperature was increased. As the pH was increased (towards pH 3.8), the deviation from the horizontal at slow sweep rates gradually became negative and this deviation was again enhanced when the temperature was increased. This phenomenon is explained in terms of the role of the hydronium ion. From the addition of Fe2+, Fe3+, and Cr3+ to pH 0.5 and pH 3.8 solutions it was noted that ferrous ions increased the peak A current response more than chromic ions of the same concentration. Ferric ions slightly decreased the peak A current response. Based on these results, reports in the literature, and the apparent role of the hydronium ion, a partial scheme was proposed in order to explain the role of Fe and Cr, from the alloy substrate, in the anodic film in the transpassive region. In chronoamperometric experiments, stepping to the transpassive region confirmed the phenomenon of the rising transient. A quantitative nucleation model - which was based on previous models from the literature - was generated. The model was successfully fitted to two rising transients, one from the pH 3.8, and the other from the pH 0.5 solution. The model also allows for the presence of a pre-existent laver at the starting potential of a chronoamperometric experiment after the electrochemical cleaning procedure. The model incorporates both diffusion controlled and charge transfer controlled steps. A key concept in the model is that of nucleation and "slow death" of corrosion pits growing into the electrode. "Death" of a pit occurs when it is covered by a nucleating and or growing passivating film. The rising transients were only obtained on Fe-Cr alloys (with one exception) when stepping to the transpassive region and also only in solutions where peak A was obtained in a cyclic voltammetric experiment. The exception to this was that in the pH 0.5 solution and at 90°C, rising transients were obtained when stepping to the passive region. This did not occur at 25°C. Rising transients were also obtained on pure iron when stepping to the passive region. In addition to the rising transient, a reverse rising transient was discovered. This reverse rising transient (which generated a cathodic current) was obtained when stepping the potential cathodically from the transpassive region. It was shown that the occurrence of the reverse rising transient was dependent on the presence of a stable, transpassive anodic film before the potential step. One indirect result from the discovery of the reverse rising transient was that it indicates that secondary passivity exists at least 200 mV into the oxygen evolution region. / Thesis (M.Sc.)-University of Natal, 1993.

Ferritic chromium steels are important structural materials for future nuclear fission and fusion reactors due to their advantages over traditional austenitic steels, including low swelling rates, better thermal fatigue resistance, and lower thermal expansion coefficients. Radiation-induced segregation or depletion (RIS/RID) of solute atoms at grain boundaries is considered to be a potentially significant phenomenon for structural materials because of its potentially detrimental role in affecting microstructure and furthermore mechanical properties. However, the behaviour of Cr at grain boundaries in ferritic steels is not well understood. Both segregation and depletion of Cr at grain boundary under irradiation have been previously observed and no clear dependency on irradiation condition or alloy type has been presented. Furthermore, ferritic alloys are known to undergo hardening and embrittlement after thermal aging in the temperature range of 300-550DC and this phenomenon is related with a and a' phase separation occurring in the solid solution. However the low temperature a-a' miscibility gap in the currently used phase diagram is extrapolated from high temperature results and conflicts with many experimental observations. To understand the Cr behaviour at gram boundaries in ferritic steels under irradiation, a systematic approach combining SEM/EBSD, FIB specimen preparation and APT analysis has been developed and successfully applied to a Fe- 15.2at%Cr to investigate the effect of pre-irradiation chemistry, grain boundary misorientation, impurities, irradiation damage, irradiation depth, and other possible factors to get a better understanding of RIS/RID phenomena. Both low sigma boundaries and randomly selected high angle boundaries have been investigated in detail. Systematic differences between the behaviour of different classes of boundaries had been observed, and the operating mechanisms are also discussed in this thesis. The maximum separation method has been applied on APT data to study the C- enriched clusters and Cr-enriched clusters, which were not directly visible on the atom maps. The composition of the Cr-enriched clusters was consistent with a' phase and the irradiation was found to accelerate the nucleation rather than the growth of these clusters. Such results provided important information in re- determining the a-a' phase boundary.

No description available.

A dissertation submitted to the faculty of Engineering, University of the Witwatersrand, in fulfillment of the requirements for the degree of Master of Science in Engineering. / The role of chromite dissolution in the smelting of ferrochromium was investigated with the object of improving the throughput and chromium recovery of the process. The solubility of a typical Transvaal chromite in silicate slags with cao/si02 ratios from 0,03 to 0,55 at 1550·C and 1650·C was determined. Synthetic slags were melted in porous chromite crucibles and the slag underwent repeated reactions with the chromite grains as it penetrated the crucible wall. Finally the slag came into equilibrium with -''the ,original chromite towards the outer part of the crucible wall. Microprobe analysis of this slag yielded the maximum or saturated solubilities of the chromite constituents in the slag. The solubility of cr203 was found to be low (-1 per cent) while the remaining components' solubilities (A1203 -16 per cent, FeO)T 12 per cent and MgO-8 per cent) were significantly higher. CaO/Sio2 ratios greater than 0,1 lowered the solubility of Mg0 significantly (e.g. from 14,1 per cent to 5,8 per cent at 1650·C). The complete dissolution of this chromite in these slags requires the slag to contain less than the solubility limits of each of these species simultaneously. It was recommended that the best way to achieve this in practice would be a well-stirred slag bath containing suspended carbon particles, in which a cao/Sio2 ratio of less -than 0,1 was maintained. Further work to investigate the effect of slag composition (including Na20 and CaF2) on the kinetics of chromite reduction in such a system was recommended. / AC 2018

M.Sc. (50/50) Research project submitted to School of Chemical and Metallurgical Engineering, Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africa July 2017 / This study and the work done involves investigating the effects of different parameters on the decarburization process of high carbon ferrochromium melts to produce medium carbon ferrochrome, and takes into account the manipulation of the different parameters and thermodynamic models based on actual plant data. Process plant data was collected from a typical plant producing medium carbon ferrochrome alloys using AODs. The molten alloy was tapped from the EAF and charged into the AOD for decarburization using oxygen and nitrogen gas mixtures. The gases were blown into the converter through the bottom tuyeres. Metal and slag samples and temperature measurements were taken throughout the duration of each heat. The decarburization process was split into two main intervals namely first stage blow (where carbon content in the metal bath is between 2-8 wt. % C) and second stage blow (carbon mass% below 2 wt. %). The first and second blow stages were differentiated by the gas flow rates whereby the first stage was signified by gas flow ratio of 2:1 (O2:N2), whilst the stage blow had 1:1 ratio of oxygen and nitrogen respectively. The effect of Cr mass% on carbon activity and how it relates to rate of decarburization was investigated, and the results indicated that an increase in Cr 66.54 – 70.5 wt. % reduced carbon activity in the metal bath from 0.336 – 0.511 for the first blowing stage. For the second blowing stage, the increase in Cr mass % of 67.22 – 71.65 wt. % resulted in an increase in C activity from 0.336 – 0.57. The trend showed that an increase in chromium composition resulted in a decrease in carbon activity and the same increase in Cr mass% resulted in reduced carbon solubility. Based on the plant data, it was observed that the rate of decarburization was time dependent, that is, the longer the decarburization time interval, the better the carbon removal from the metal bath. An interesting observation was that the change in carbon mass percent from the initial composition to the final (Δ%C) decreased from 10.18 – 8.37 wt. % with the increase in Cr/C ratio from 8.37 – 10.18. This effect was attributed to the chromium affinity for carbon and the fact that an increase in chromium content in the bath was seen to reduce activity of carbon. It was also observed that the effect of the Cr/C ratio was more significant in the first stage of the blowing process compared to the second blowing stage. A mass and energy balance model was constructed for the process under study to predict composition of the metal bath at any time interval under specified plant conditions and parameters. The model was used to predict the outcome of the process by manipulating certain parameters to achieve a set target. By keeping the gas flow rates, blowing times, gas ratios and initial metal bath temperature unchanged, the effect of initial temperature on decarburization in the converter was investigated. The results showed that the carbon end point with these parameters fixed decreased with increasing initial temperature, and this was supported by literature. The partial pressure of oxygen was observed to increase with decrease in C mass % between the first and second blow stages. For the second stage blow the partial pressure changed from 5.52*10-12 – 2.1*10-10 and carbon mass % increased from 0.754 – 2.99 wt. %. A carbon mass % of 7.87 had an oxygen partial pressure of 4.51*10-13 whilst a lower carbon content of 1.53 wt. % had an oxygen partial pressure of 8.06*10-11. The CO partial pressure however increased with increase in carbon composition in the metal bath. When the oxygen flow rate increased, a corresponding increase in the carbon removed (Δ%C) was observed. For the first stage of the blowing process, an increase in oxygen flow rate from 388.67 – 666.5Nm3 resulted in an increase in carbon removed from 5.06 – 7.28 wt. %. The second blowing stage had lower oxygen flow rates because of the carbon levels remaining in the metal bath were around +/- 2 wt. %. In this stage oxygen flow rates increased from 125 – 286.67 Nm3 and carbon removed (Δ%C) from 0.16 – 2.093 wt. %. The slag showed that an increase in basicity resulted in an increase in Cr2O3 in the slag. As the basicity increased from 0.478 – 1.281, this resulted in an increase in Cr2O3 increase from 0.26 – 0.68. Nitrogen solubility in the metal bath was investigated and it was observed that it increased with increasing Cr mass %. The increase in nitrogen solubility with increasing Cr mass % was independent of the nitrogen partial pressures. / MT2018