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The Behavior of Solutions
Published in David R. Gaskell, David E. Laughlin, Introduction to the Thermodynamics of Materials, 2017
The thermodynamic activity of a component in any state at the temperature T is formally defined as being the ratio of the fugacity (see Section 8.7) of the substance in that state to its fugacity in its standard state. For the species or substance i,
Non-isothermal kinetic studies on the carbothermic reduction of Panzhihua ilmenite concentrate
Published in Mineral Processing and Extractive Metallurgy, 2019
Wei Lv, Xueming Lv, Xuewei Lv, Junyi Xiang, Chenguang Bai, Bing Song
In previous studies, the reduction kinetics of ilmenite concentrate was studied by some researchers. However, the apparent activation energy values reported were different from those obtained in this study. Manganese oxide inhibited the reduction kinetics of the ilmenite, yielding a low reduction extent, and magnesium oxide has a larger effect on the reduction kinetics than manganese oxide (Gupta et al. 1989, Merk and Pickles 1988, Wang and Yuan 2006). Compared to that reported in the study of Wang et al. (Wang and Yuan 2006), the activation energy obtained in this study is larger, because of the high content of impurities such as magnesium, manganese, and calcium oxides in the raw materials. The magnesium concentration lowered the thermodynamic activity of Fe2+, rendering its reduction progressively difficult. The content of magnesium oxide in the Panzhihua ilmenite concentrate is 35 times larger than that of the Bama ilmenite concentrate. Iron oxide and magnesium oxide can form a spinel like structure, which makes the iron oxide much more stable. In addition, the reduction products can form M3O5 (M represents a combination of Mg, Ti, Mn, and Fe). Compared with the others studies (El-Guindy and Davenport, 1970; Elhussiny and Shalabi, 2012; Gou, Zhang and Chou, 2015b; Li et al, 2013; Wang and Yuan, 2006), the remarkable difference may be due to the different experimental conditions, such as temperature, raw materials, the size of reductant, experimental apparatus, and the pressure of carbon dioxide.
Cathodic arc deposition of NiCrAlY coating: oxidation behaviour and thermodynamic
Published in Surface Engineering, 2019
J. Khakzadian, S. H. Hosseini, K. Zangeneh Madar
where ΔG° is the standard Gibbs free energy for the oxide formation per 1 mol O2, R is the gas constant (8.314 J mol−1 K−1), T is the temperature reaction in Kelvin, and a is the thermodynamic activity. As shown in Figure 3 at 1000°C, ΔG° for the formation of Al2O3, TiO2, Cr2O3, and NiO was −851.2, −741.3, −538.2, and −250.2 kJ mol−1 (O2), respectively. Thermodynamic calculations for the oxidation of the Ni-20Cr-xAl-0.5Ti (x = 1–11) alloy at 1000°C are presented in Table 3. The values of ΔG are plotted in Figure 8. The activities of oxide and O2 were assumed to be 1. As can be seen in Figure 8, the Gibbs free energy of the formation of Al2O3 was lower than that of TiO2 for different Al concentrations. In this case, the driving force for the formation of Al2O3 was higher than that of TiO2, and the Ti element could not be oxidised.
Effect of stress on the crevice corrosion of P110 steel in the CO2-saturated NaCl solution containing thiosulphate/H2S
Published in Corrosion Engineering, Science and Technology, 2021
Lijin Dong, Xiaolong Zhang, Qinying Wang, Huaibei Zheng, Qinglong Lu, Bingbin Wang
In addition to the environment and material factor, the corrosion and crevice corrosion behaviour can be affected by the applied stress, which is typically generated from the formation pressure and liquid column pressure. For example, Bao et al. [18] suggested that the increase of tensile stress decreased the open circuit potential of carbon steel but enhanced the surface thermodynamic activity and corrosion rate in CO2-dissolved solution. In addition, both the anodic and cathodic reactions were accelerated by the tensile stresses while the effect on the latter was more marked. Yang et al. [19] found that the inhomogeneous elastic stress on the Q235 steel could cause galvanic corrosion and initiate localised corrosion. Research on N80 carbon steel in CO2-saturated solution containing acetic acid also revealed that there was a synergistic effect of stress and crevice on corrosion [20]. The increase of applied stress enhanced the stress concentration at the crevice mouth, resulting in more negative shift of potential. The decrease of potential inside the crevice could increase the galvanic corrosion driving force between the steel inside and outside crevice, and therefore led to more severe crevice corrosion of steel. A more recent investigation of corrosion behaviour of 13Cr stainless steel in 3.5 wt-% NaCl solution suggested that accumulation of H+ and Cl− ions inside crevice was accelerated by applied stress, which may promote the transition from metastable to stable pit growth and thus initiated crevice corrosion [21]. Additionally, crevice induced sulphide stress cracking of super duplex stainless steel in H2S/CO2 sour environment and yielded to lower cracking resistance than without crevice [22].