Cathode:anode ratio

In all cases of localized corrosion, the ratio of the cathodic to the anodic area plays a major role in the localized dissolution rate. A large cathodic area provides high cathodic currents and, due to electroneutrality requirements, the small anodic area must provide a high anodic current. Hence, the local current density, i.e., local corrosion rate, becomes higher with a larger cathode/anode-ratio. 

Steel can be obtained due to the small cathode/anode ratio. This macrocell monitoring system has been installed since 1990 into tunnels, bridges, foundations and other structures exposed to aggressive environments [5]. Similar systems have been developed and are on the market (6). 

Cathode-anode area ratio and spacing Current density... 

Since copper-sheathed cables are also coated with plastic, the ratio of cathode/anode area (SJSJ is very small so that there is not an increased risk of corrosion of the lead-sheathed cable by the electrical connection between the cable sheathings according to Eq. (2-44). 

A similar effect can be produced if a crevice is present in the steel, since the geometry of the system is such that whereas oxygen can diffuse readily to the metal surface outside the crevice it can only gain access to the metal within the crevice through its very narrow mouth (Fig. 1 A6d), and the large cathode anode area ratio leads to localised attack of the metal within the crevice. 

It enables greater cathodic/anodic surface area ratios to become active in corrosion processes, thereby promoting pitting mechanisms in vulnerable materials. 

FIGURE 2.7. Double potential step chronoamperometry for an EC mechanism with an irreversible follow-up reaction, a Potential program with a cyclic voltammogram showing the location of the starting and inversion potentials to avoid interference of the charge transfer kinetics, b Example of chronoamperometric response, c Variation of the normalized anodic-to-cathodic current ratio, R, with the dimensionless kinetic parameter X. 

FIGURE 2.12. Double potential step chronoamperometry for an ECE (dashed line) and a DISP (solid line) mechanism. Variation of the normalized anodic-to-cathodic current ratio, RDps = [—ia(2tR)/ic(tR)]/(l — l/y/2), with the dimesionless kinetic parameter X — ktR. 

As with the other reaction schemes involving the coupling of electron transfer with a follow-up homogeneous reaction, the kinetics of electron transfer may interfere in the rate control of the overall process, similar to what was described earlier for the EC mechanism. Under these conditions a convenient way of obtaining the rate constant for the follow-up reaction with no interference from the electron transfer kinetics is to use double potential chronoamperometry in place of cyclic voltammetry. The variations of normalized anodic-to-cathodic current ratio with the dimensionless rate parameter are summarized in Figure 2.15 for all four electrodimerization mechanisms. 

Kendig and Leidheiser (16) electrochemica1ly evaluated thin (9 micron) polybutadiene coatings on steel. They concluded that movement of the corrosion potential in the noble direction was indicative of an increasing cathodic/anodic surface area ratio. Oxygen and water penetrate the coating to produce the cathodic reaction at the metaI/coat ng interface. 

The effect was more pronounced at the starting potential than at the finish potential. Leidheiser suggested that the best performance is obtained when the cathode/anode surface area ratio is the same as the uncoated metal. Inadequate performance is obtained when the cathode/anode area ratio becomes larger. Qur work agrees with Leidheiser s hypothesis. The B210/GBL coatings have rest potentials less noble than the B40 coated steel panels and perform best in the salt fog environment. 

In terms of nobility, the open circuit potentials for the B210/NVP system at all temperatures are more positive than the B40 panels. The open circuit potentials for the B210/GBL system at all temperatures are generally more negative than the B40 panels. The open circuit potential trends for the B210/GBL system mimic the B210/NVP system. However, the effects are not as pronounced. We propose that in the GBL system there is a reduction in the cathode/anode area ratio as suggested by Leidheiser (16). 

Chlorosilanes Ratio B/A Cathode Anode Electrolyte Product (Yield %) Reference... 

Steel socket welds is a good example of rapid local corrosion. The potential difference between the anodic and the cathodic states drives the corrosion cells (this is an example of galvanic corrosion). Corrosion due to adjacent active-passive sites can be particnlarly rapid if the corrosion cell has an unfavorable anode/cathode area ratio. 

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

Minimize cathode-anode area ratio to minimize galvanic coupling and galvanic... 

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