Corrosion electrochemistry curves

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... 

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. 

Diagram, after U. R. Evans, a pioneer in corrosion electrochemistry. Any corroding metal should, under open-circuit conditions, exist at corroding at a rate which can be predicted ft-om the Evans Diagram. The value of will depend on the position of the curves. The anodic curve is a function of the type of metal and the charge it receives when it goes into solution. The cathodic curve is a function of the type of cathodic reaction, the type of surface on which that reaction takes place, the concentration of reacting species, and other factors, most of which are dependent on the environment in which the metal sits. 

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. 

An additional interpretation issue involves the presence of oxidation reactions that are not metal dissolution. Figure 28 shows polarization curves generated for platinum and iron in an alkaline sulfide solution (21). The platinum data show the electrochemistry of the solution species sulfide is oxidized above -0.8 V(SCE). Sulfide is also oxidized on the iron surface, its oxidation dominating the anodic current density on iron above a potential of approximately -0.7 V(SCE). Without the data from the platinum polarization scan, the increase in current on the iron could be mistakenly interpreted as increased iron dissolution. The more complex the solution in which the corrosion occurs, the more likely that it contains one or more electroactive species. Polarization scans on platinum can be invaluable in this regard. 

An -> ideal nonpolarizable electrode is one whose potential does not change as current flows in the cell. Much more useful in electrochemistry are the electrodes that change their potential in a wide potential window (in the absence of a - depolarizer) without the passage of significant current. They are called -> ideally polarized electrodes. Current-potential curves, particularly those obtained under steady-state conditions (see -> Tafel plot) are often called polarization curves. In the -> corrosion measurements the ratio of AE/AI in the polarization curve is called the polarization resistance. If during the -> electrode processes the overpotential is related to the -> diffusional transport of the depolarizer we talk about the concentration polarization. If the electrode process requires an -> activation energy, the appropriate overpotential and activation polarization appear. 

The principles that govern electrochemistry at semiconductor electrodes can also be applied to redox processes in particle systems. In this case, one considers the rates of the oxidation and reduction half-reactions that occur on the particle, usually in terms of the current, as a function of particle potential. One can use current-potential curves to estimate the nature and rates of heterogeneous reactions on surfaces. This approach applies not only to semiconductor particles, but also to metal particles that behave as catalysts and to surfaces undergoing corrosion. 

In corrosion and in electrochemistry, the potential sweep technique is commonly used to measure polarization curves, and the result is referred to as potentiodynamic polarization curves. Equation (5.98) offers a criterion for the selection of the maximum sweep rate while still working under steady state conditions with respect to mass transport. As a rule of thumb, for a value of > 20 the error in the measured steady-state limiting current is less than 1% [7]. Equation (5.98) shows that to attain a steady state, the sweep rate must be the slower the larger the diffusion layer thickness, in other words, the weaker the convection. If, for example, D = 10 m s, = 2 and 5= 10 pm, the sweep rate must not exceed 15 mV s ... 

Electrochemistry of the metal corrosion (dissolution) under an applied potential is presented in the form of a typical polarization curve. The origin of the common three regions (activation, passivation, and transpassivation) is explained. 

Concerning anti-corrosion properties, Tafel curves allow the calculation that the corrosion current for this kind of coating is slightly below that for classical Ce-based ppHMDSO films. Images of HMDSO treated samples after 25 days in a salt spray chamber are shown in Fig. 12.11(c,d) which compare the resistance of HMDSO plasma treatments with and without ethanol. An increase in corrosion protection is afforded by the presence of solvent, since pitting corrosion is reduced. This last result corroborates those obtained by electrochemistry. 

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