Ohmic drop crevice

Two special electrochemical cells are used for XRD and XAS measurements. In one case a polymer membrane is pressed on the specimen surface after its electrochemical treatment to reduce the water layer on top, but still permitting potential control during the measurements. In an other case the beam penetrates an electrolyte layer in front of the electrode, which corresponds to the specimen s dimensions, but which is thick enough to reduce the danger of ohmic drops and crevices. Beam lines often provide the exact orientation of the samples with the cell by a goniometer. For XAS measurements a special low cost refraction stage has been constructed which permits the orientation of the sample within 0.01 degrees and which has been used for the study of several systems [108]. 

The crevice corrosion occurred even at constant pH and zero chloride concentration, and was shown to be caused by the ohmic drop placed by the local electrode potential existing on the crevice wall in the active peak region of the polarization curve [148]. This can involve an increase in the... 

Ignored by most implementations of the CCS framework, ohmic drop can not only lead to passive-to-active transitions, but also can, in the context of environmental cracking, make hydrogen evolution, and therefore embrittlement, more viable at the crack tip. The IR framework has been successfully demonstrated in several model metal/environment systems [34, 35], and has been invoked to rationalize the practically important case of the crevice corrosion of Alloy 625 in chlorinated seawater [32, 33]. 

Pickering and coworkers [31, 34, 35] have demonstrated both experimentally and computationally that for systems that meet the criteria of the IR theory, lA is predicted. The amount of potential drop increases as one moves into the crevice because of the current leaving the crevice. If the geometry, solution conductivity, and passive current density of the material in the environment conspire to create sufficient ohmic drop, then the potential of some portion of the material within the crevice falls to the primary passive potential. Under these circumstances, the passive film is not stable and active dissolution occurs. The potential difference between the applied potential and the primary passivation potential is referred to as IR. Deeper still into the crevice the ohmic drop leads to decreased dissolution as the overpotential for the anodic reaction decreases. Thus, ohmic drop is responsible for the initiation and stabihzation of crevice corrosion according to this model. 

A paradox thus exists in crevice corrosion. The theory that can explain one of the most commonly observed phenomena (lA) is of restricted applicability, whereas the theory that cannot rationalize lA is thought to occur more widely. Kelly and Stewart sought to resolve this paradox by considering both ohmic drop and chemical changes. A set of boundary conditions was selected for which neither the CCS model nor the IR model alone would predict lA. The electrochemical boundary conditions were based upon measurements for stainless steel in solutions simulating occluded conditions. 

Figure 17 shows the evolution of the potential profile within the crevice. Initially, the profile is flat. All points within the crevice are polarized to the value at the mouth. Within a short time, the currents increase due to the pH changes and larger ohmic drop occurs. After 82 s, the potential at the base of the crevice is more than 300 mV below that at the mouth. Over the next 480 s the potential profile flattens somewhat, and the maximum potential difference is slightly more than 250 mV. In addition to this decrease in potential drop, an inflection point can be observed in the profile at a distance of approximately 0.15 cm from the mouth. 

Fig. 17 Potential of the material as a function of depth into the crevice and time. Note the increase in the potential (decrease in the ohmic drop) between 82 and 662 s that results from increased solution conductivity within the crevice [30].

In fact, anodic polarization generally increases the rate of the crevice attack. The only situation for which anodic polarization would be useful in preventing crevice corrosion is for systems that can be completely described by the ohmic drop model and for which no changes occur in either the occluded solution composition or the electrochemical behavior of the material in the occluded region. 

Therefore the reference electrode is placed with its Haber-Luggin capillary as close to the electrode surface as possible, thus measuring the electrode potential without large deviations due to a possible ohmic drop within the electrolyte. However, it should not get too close in order to avoid the formation of an artificial crevice. As a general rule, the distance should be two to three times the diameter of the capilleuy. The remaining ohmic drop AUa may be compensated for by up to more than 90% by a feedback loop of the potentiostat. 

Activation polarization is usually the controlling factor during corrosion in strong acids since both and iR are relatively small. Concentration polarization usually predominates when the concentration of the active species is low for example, in dilute acids or in aerated waters where the active component, dissolved oxygen, is only present at very low levels. The ohmic drop will become an extremely important factor when studying corrosion phenomena for which there is a clear separation of the anodic and cathodic corrosion sites, for example, crevice corrosion. The ohmic drop is also an important variable in the application of protective methods such as anodic and cathodic protection that forces a potential shift of the protected structure by passing a current in the environment. 

It is usual to consider that various classes of anodic mechanisms exist depending on the range of potential with respect to the passivity domain. Active dissolution taking place at potentials preceding the passivation on a film-free surface is of major importance for the homogeneous corrosion in weakly oxidizing media such as acidic solutions of stable anions (e.g. sulfuric, perchloric, phosphoric, hydrochloric). Localized corrosion in pits, crevices, cracks, etc. is also assirmed to proceed through active dissolution stabilized at passive potentials by ohmic drops and/or local chemistry. 

Passivity breakdown is thought to occur either when the corrosion potential of the crevice surfaces is located in the active peak due to ohmic drop (Fig. 21b) or when there is no more active-passive transition on the anodic curve. 

Figure 21 (a) Evolution of the anodic characteristic of passivated alloy when decreasing the pH and increasing the chloride content, (b) Activation inside a crevice when the corrosion potential is located in the activity peak due to ohmic drop in the crevice. 

The role of IR drop in crevice initiation is not clear. Different authors [58,60,61] observed crevice initiation on stainless steels at a very low IR drop level. It is clear that initiation processes can be separated into two classes (a) those that operate at relatively high potentials (pitting in the crevice gap), which cannot be enhanced by laige ohmic drops, and (b) those that occur at low potential (general passivity breakdown), which are favored by large IR drops. However, on stainless steels and nickel-base alloys, there is at present no direct evidence to support the last t5q)e of process, mainly because high free surface potential always enhances crevice initiation of passive alloys. 

The migration cmrent from the crevice gap increases dramatically and this also increases the ohmic drop a role of the ohmic drop in the control of the propagation rate is consistent with a maximmn of corrosion rate located near the crevice mouth as observed by several author [3,19]. One must notice that ohmic drop is not limited to the solution inside the crevice. Significant ohmic drops may also occm in the bulk solution near the crevice mouth, particularly in dilute environments. 

Crevice corrosion occurs in restricted transport zones where the access of reactants and the elimination of corrosion products can be very slow. The detrimental effect of a crevice is related to a very small volume of solution in contact with very large metal surfaces. The restricted transport path between crevice and bulk solution is responsible not only for low diffusion exchanges but also for the buildup of a potential difference between the free surfeces and the crevice that becomes more anodic (ohmic drop effect). 

Whatever the initiation mechanism, propagation causes further evolution of the local environment, which eventually becomes saturated (with concentrations estimated to be several mol/L of chloride and metal cations). At this point, a salt layer may form on the corroding surfaces. Thus, active corrosion in the crevice is rarely controlled by an activation process but rather by the ohmic drop and, for well-developed crevices, by cation transport through the salt layer, the thickness of which determines the corrosion rate. 

The related ohmic drop AU = iR may lead to an appreciable difference between the actual potential and that chosen for the experiment. It can be minimized by an appropriate position of the Haber-Luggin (HL) capillary of the RE, close to the surface of the working electrode (WE). However, it should not be too close in order to avoid the partial blocking of the metal surface and the formation of crevices. As a compromise, the distance should be about three times the diameter of the capillary. The ohmic drop may be also compensated electronically to about 90% of its value by an appropriate circuit built in the potentiostat as shown later in the block diagram of Figure 1.26a. The ohmic drop may be minimized by a small size of the electrode. 

Several experiments [58-60] with in situ local measurement of pH and/or chloride concentration with microelectrodes showed that the large changes in crevice pH and chloride content mentioned in the former paragraph occurred not before but after passivity breakdown. In these experiments [58,61], the potential difference between crevice and free surfaces (i.e., the ohmic drop) is often very low before initiation (few mV) and becomes larger only after crevice initiation. 

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