Inhibition, corrosion polarization curve

Abstract The flotation mechanism is discussed in the terms of corrosive electrochemistry in this chapter. In corrosion the disolution of minerals is called self-corrosion. And the reaction between reagents and minerals is treated as inhibition of corrosion. The stronger the ability of inhibiting the corrosion of minerals, the stronger the reagents react with minerals. The two major tools implied in the research of electrochemical corrosion are polarization curves and EIS (electrochemistry impedance spectrum). With these tools, pyrite, galena and sphalerite are discussed under different conditions respectively, including interactions between collector with them and the difference of oxidation of minerals in NaOH solution and in lime. And the results obtained from this research are in accordance with those from other conventional research. With this research some new information can be obtained while it is impossible for other methods. 

Keywords corrosive electrochemistry corrosive potential corrosion inhibition polarization curves Electrochemistry Impedance Spectrum... 

Figtire 7.12 is the polarization curves of pyrite electrode in xanthate solution with different concentration for dipping for 48 hours. Electrochemistry parameters determined by the computer PARcal are listed in Table 7.2. Inhibiting efficiency can be calculated by Eq. (7-7), Rp- is the polarization resistance after adding collector, Rp is the polarization resistance without collector. It can be seen from Fig. 7.12 and Table 7.2 diat, with the increase of xanthate concentration, corrosive potential and corrosive current of the pyrite electrode decrease gradually while polarization resistance increases, indicating the formation of surface oxidation products. 

Figure 7.41 is the polarization curves of sphalerite-carbon combination electrode in different collector solution at natural pH. The corrosive electrochemistry parameters are listed in Table 7.8. These results show that xanthate and dithiocarbamate have distinctly different effects on sphalerite. The corrosive potential and current of sphalerite electrode are, respectively, 42 mV and 0.13 pA/cm at natural pH in the absence of collector, -7 mV and 0.01 pA/cm in the presence of xanthate, and 32 mV and 0.12 pA/cm in the presence of dithiocarbamate. The corrosive potential and current decrease sharply with xanthate as a collector, indicating that the electrode surface has been totally covered by the collector film from the electrode reaction. Xanthate has big inhibiting corrosive efficiency and stronger action on sphalerite. However, the corrosive potential and current of sphalerite electrode have small change with dithiocarbamate as a collector, indicating that DDTC exhibits a weak action on sphalerite. 

From the slope of the polarization curve and its variation with time (exposure time of the iron electrode), information on the kind of inhibition can be gained. An inhibition of anodic processes decreases the ia versus E current density and increases the corrosion potential correspondingly, an increase in cathodic inhibition causes a decrease in the i. and lowers the corrosion potential. 

The effect of ultrasound on the process of tellurium anodic dissolution in alkaline solutions was studied by the method of plotting polarization and galvanostatic curves [148]. Tests were made in NaOH solutions (concentrations of 0—20 g/L), subjected to the action of ultrasound at a frequency 17.5 kHz and using Te electrodeposited under ultrasound. The anodic polarization curves plotted without ultrasound and in its presence shifted with increased NaOH concentration towards negative values as a result of the increasing rate of Te anodic dissolution. The presence of ultrasound inhibited the process of Te anodic dissolution, probably due to the desorption of OFT anions from the anode surface. This sonoelectrodeposited Te thus showed greater corrosion resistance in alkaline solution than that deposited... 

This simple model of the inhibitor action which is based essentially on the potential independence of the inhibitor adsorption is, however, often not applicable. Kaesche (15) indicates that the corrosion inhibition of pure iron in sulfuric or perchloric acid by phenyl-thiourea strongly affects the slopes of the polarization curves, leaving the corrosion potentials almost unchanged Fig.7. In fact, the polarization curves for the inhibited situation do not exhibit real Tafel behavior. This behavior finds a partial explanation in the fact that the mechanism of the hydrogen evolution appears to be changed in the presence of... 

For optimum inhibition, the concentration of passivator must exceed a certain critical value. Below this concentration, passivators behave as active depolarizers and increase the corrosion rate at localized areas, such as pits. Lower concentrations of passivator correspond to more active values of the oxidation-reduction potential, and eventually the cathodic polarization curve intersects the anodic curve in the active region instead of in the passive region alone (Fig. 17.1). 

Polarization curves (Fig. 4.8) also meant that the anodic process of the MC alloy was inhibited. It attributed to a protective product film formed on MC alloy surface. Therefore, for thorough understanding of the microcrystallization effect on corrosion resistance ofAZ91D alloy, it is necessary to study the nature of the corrosion product film on MC alloy. 

Table 4.1 demonstrated that the open circuit potential of Mg-Gd-Y alloy shifted to the noble direction with the decrease in TEL thickness. Generally, the increased corrosion potential attributed to the acceleration of cathodic process or the inhibition of anodic process. The polarization curve results implied that the cathodic process was significantly inhibited under the TEL condition. So, it could be predicated that the inhibition of anodic process under TEL might be the proper explanation for the increasing of corrosion potential. 

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