Materials polarization curves

If the polarization curve for the anode is known, the coupled potential may be measured to predict corrosion performance. The couple current is the current on the anodic material polarization curve at the measured coupled potential. Corrosion rate may be calculated from the couple current determined in the same way as in the previous paragraph. 

Graphs of operating potential versus current density are called polarization curves, which reflect the degree of perfection that any particular fuel cell technology has attained. High cell operating potentials are the result of many years of materials optimization. Actual polarization curves will be shown below for several types of fuel cell. 

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

A mixed polarization diagram (where the polarization behavior of the two different electrodes is represented) for the sphalerite-hypersteel combination is given in Fig. 1.10 (Vathsala and Natarajan, 1989), in which the cathodic polarization curves for the sphalerite and the anodic polarization curves for the hypersteel ball material are seen to overlap. The active nature of the ball material is evident. The current values were observed to be lower in the absence of oxygen which indicated a lower anodic dissolution of the hypersteel grinding medium in the absence of oxygen. 

A cell with a capacity of 1 L was made of mild steel. An amorphous carbon rod (diameter 25 mm length 15 cm) was used as anode, the inside wall of the cell as cathode and a platinum wire was used as reference electrode. The anode compartment of the cell was separated from the cathode compartment by a skirt of steel welded to the cell cover. The anode gas was passed through a tube filled with tablets of NaF to absorb anhyd HF gas and then led to a gas sampler. Fluorine was detected with K.I soln. After the starting material was added into the molten KIIF2/HF salt, the electrolyte was pre-electrolyzed at a low current density until NF2 was detected, and then current efficiency of each product and polarization curves by galvanostatic or potential sweep method were determined (Table 1). At optimum conditions the current efficiency of NF3 was 55%. 

One example of the application of polarization curves in a predictive manner involves their use in galvanic corrosion. Galvanic corrosion occurs when two dissimilar metals are in electrical and ionic contact as is schematically shown in Fig. 29. Galvanic corrosion is used to advantage in sacrificial anodes of zinc in seawater and magnesium in home water heaters. It slows corrosion of millions of tons of structural materials. The darker side of galvanic corrosion is that it also causes major failures by the accelerated dissolution of materials that are accidentally linked electrically to more noble materials. 

Consider the two materials whose polarization curves are shown in Fig. 31. Both the polarization curves and the Evans lines are shown for both materials. Material 1 is the more noble material (i.e., it has a more positive Ec0II) and has a lower circuit corrosion rate when it is uncoupled. If the surface area of the two materials is the same and the materials are coupled, then the two material-solution interfaces must come to the same potential. In a manner identical to that used for the example of iron in acid used to introduce Evans diagrams, the potential and current at which this condition is met can be found by applying the conservation of charge to the sysytem ... 

As discussed in detail in Chapter 2, the corrosion potential is determined by the intersection of the sum of the anodic Evans lines and the sum of the cathodic Evans lines. For active-passive materials, the only new wrinkle is the increased complexity of the anodic line. Since the anodic line is not single-valued with respect to current density, three distinct cases can be considered. In all cases, the condition E /a = X Ic determines the position of the corrosion potential, and the condition im = z a - ic determines the appearance of the polarization curve... 

Under reducing conditions (e.g., in acids such as HC1), the predominant cathodic reaction is hydrogen evolution as shown in Fig. 5. This combination results in a polarization curve in which all of the parameters characterizing passivity can be measured as shown in Fig. 5. If a material were to be used under these conditions, nothing would be gained from its ability to passivate. 

In the presence of oxidizing species (such as dissolved oxygen), some metals and alloys spontaneously passivate and thus exhibit no active region in the polarization curve, as shown in Fig. 6. The oxidizer adds an additional cathodic reaction to the Evans diagram and causes the intersection of the total anodic and total cathodic lines to occur in the passive region (i.e., Ecmi is above Ew). The polarization curve shows none of the characteristics of an active-passive transition. The open circuit dissolution rate under these conditions is the passive current density, which is often on the order of 0.1 j.A/cm2 or less. The increased costs involved in using CRAs can be justified by their low dissolution rate under such oxidizing conditions. A comparison of dissolution rates for a material with the same anodic Tafel slope, E0, and i0 demonstrates a reduction in corrosion rate... 

Figure 5 Schematic Evans diagram and resulting potential-controlled polarization curve for a material that undergoes an active-passive transition and is in a reducing solution. The heavy line represents the applied currents required to polarize the sample.

Figure 6 Schematic Evans diagram and resulting potential-controlled polarization curve for a material that undergoes an active-passive transition and is in an oxidizing solution. The heavy line represents the applied currents required to polarize the sample. If the sample did not undergo an active-passive transition, it would corrode at a much higher rate in this solution, as is indicated by the intersection of the dotted line and the cathodic curve.

Figure 8 Schematic Evans diagram and potential-controlled polarization curve for a material/environment combination that exhibits a cathodic loop. Note that the direction of the applied current changes three times in traversing the curve.

Figure 24 Schematic Evans diagram and polarization curve illustrating the origin of the negative hysteresis observed upon cyclic polarization for materials that do not pit. Line a represents the (unchanging) cathodic Evans line. Line b represents the anodic Evans line during the anodically directed polarization, while line c represents the anodic Evans line for the material after its passive film has thickened because of the anodic polarization. The higher corrosion potential observed for the return scan (E (back)) is due to the slowing of the anodic dissolution kinetics.

From polarization curves of the type shown in case 3, three important parameters can be determined ECOSI, Ebth and In the literature there exists a nearly infinite number of variations of nomenclature, many of which are shown in Table 2. The interpretation of cyclic polarization curves has been and continues to be a subject of great controversy. The classic interpretation of case 3 would be that the potential of a material must exceed EM for new pits (or localized corrosion sites) to nucleate, but that at potentials between EM and En existing pits can propagate. At potentials below En all localized corrosion sites repassivate. Thus, from a design or material selection perspective, a material will perform well if its Econ is kept below This criterion can be met by environment... 

Figure 28 Schematic Evans diagrams and polarization curves for a material in a solution containing a redox couple that acts as a chemical potentiostat. The i used in the Evans diagram for the O/R redox couple is that relevant to the material of interest. In the absence of the redox couple, the material obtains Ec, i. In the presence of the redox couple, the material obtains Econ2. If Econ2 is above the pitting potential, the material will be rapidly attacked.

Figure 30 Schematic polarization curves illustrating the origins of the ability of the oxalic etch test and acid ferric sulfate test to differentiate sensitized (represented by the Fe-lOCr-lONi) from unsensitized (represented by the Fe-18Cr-10Ni) material.

Controversy concerning the interpretation of cyclic polarization curves has raged for many years. Of particular interest is which (if either) of the two potentials can be used for material selection and mitigation strategy decisions. The classic interpretation is that a material s potential must exceed Ehl[ in order to initiate pits, but if flaws were introduced into the surface in any way, they could propagate at all potentials above Ew. Thus Eq, could be used in design as a protection potential. 

Most modern industrial materials are designed to be passive i.e., covered by an adherent, chemically inert, and pore-free oxide that is highly insoluble in aqueous solutions and hence dissolves at an extremely slow rate. Examples would be modern stainless steels, nickel-chromium-molybdenum, and titanium alloys. The concept of passivity is often defined by reference to the polarization curve for metals and alloys in aggressive acidic solutions, Fig. 22. This curve defines the potential regions within which the alloy would be expected to corrode actively or passively. 

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