Anodes area effect, galvanic corrosion

Fig. 6.6 Effect of the cathode-sacrificial anode surface area on galvanic corrosion of a tin-platinum galvanic couple.

Figure 3.13 Effect of cathode anode area ratio on corrosion of zinc-platinum galvanic couples...

Area effects in galvanic corrosion are very important. An unfavorable area ratio is a large cathode and a small anode. Corrosion of the anode may be 100 to 1,000 times greater than if the two areas were the same. This is the reason why stainless steels are susceptible to rapid pitting in some environments. Steel rivets in a copper plate will corrode much more severely than a steel plate with copper rivets. 

Bimetallic corrosion in atmospheres is confined to the area of the less noble metal in the vicinity of the bimetallic joint, owing to the high electrolytic resistance of the condensed electrolyte film. Electrolytic resistance considerations limit the effective anodic and cathodic areas to approximately equal size and therefore prevent alleviation of atmospheric galvanic corrosion through strict application of the catchment area principle. 

A detailed discussion of galvanic corrosion between dissimilar metals in contact in a corrosive environment has been given in Section 1.7, but in the case of coating discontinuities the effect of the anode/cathode area relationship and the nature of any corrosion products formed at small discontinuities may modify any choice made on strict considerations of general galvanic corrosion theory based on the potentials of the coating and substrate in the environment under consideration. 

Experience shows that increasing the cathode-to-anode area ratio increases the rate of consumption of the anode and decreases the corrosion rate of the cathode, but the galvanic series alone would not allow a quantitative analysis of these effects. Inspection of Fig. 32 reveals that the abscissa has been changed to current from current density. When dealing with unequal areas, such a transfor-... 

Large Cathode and Small Anode. Such a situation is known to lead to an acceleration of corrosion of the anode. If a small section of iron or steel pipe is in contact with a large piece of copper pipe, the area effect will accelerate the corrosion of the iron pipe that acts as an anode of a galvanic cell. 

Fig. 8 Effect of the area of the cathodic reaction (the fully exposed surface) relative to the area of the anodic reaction (the occluded region). An increase in the dissolution rate of the crevice will occur (just as in galvanic corrosion) owing to need to satisfy the summation of the anodic reaction rates equaling the sum of the cathodic reaction rates.

Case Study 6.1—Effect of the Ratio of the Surface Area of the Cathode to the Surface Area of the Sacrificial Anode on Galvanic Corrosion of the Tin-Platinum Galvanic Couple... 

E6.6. Sn and Pt are immersed in an acidic solution with unit hydrogen ion activity. Using the electrochemical parameters listed below, construct the Evans diagram and evaluate the effect of the cathode-sacrificial anode electrode surface area ratio on galvanic corrosion of a tin-platinum galvanic couple (see Case Study 6.1). 

The wire-on-bolt test, described in ASTM G 116 (Practice for Conducting the Wire-on-Bolt Test for Atmospheric Galvanic Corrosion), has been used with standard materials as an atmospheric corrosivity test under the names of the CLIMAT and ATCORR tests. This test consists of wrapping a 1.0-m length of wire of the anodic material around a threaded bolt or rod of the cathodic material. Post-test evaluation is typically done by mass loss only. Since the wire diameter is much smaller than the 0.5-cm galvanic interaction distance in the atmosphere, the effective tmode-to-cathode area ratio is well below 1 1, making this a fast test. A typical exposure duration is 90 days. The short duration is... 

The coupling of dissimilar alloys in conductive, corrosive solutions such ts seawater is called galvanic corrosion and can lead to accelerated corrosion of the more anodic, electronegative alloy and protection of the more cathodic, electropositive alloy. The extent of galvanic corrosion depends on factors such as (1) the effective area ratio between the anodic and cathodic members of the couple (2) solution conductivity (3) flow characteristics of the solution (4) temperature (5) system geometry (6) the potential difference of the dissimilar alloys (7) solution composition and... 

The remote crevice assembly technique (see Chapter 19) is a research tool that allows one to separate the anode and cathode areas of a crevice corrosion test sample so that the current flowing between them can be measured with a zero-resistance ammeter. This technique is similar to the dual cell method, and it lends itself well to studies of microbial effects on crevice corrosion [7]. It allows direct measurement of microbial effects on both the initiation time and propagation rate for crevice attack, provided again that a suitable control experiment without the microbial influence can be done concurrently. The scime technique of separating the anode and cathode can be used to study the influence of microbes in biofilms on galvanic corrosion [li]. 

A variety of standard electrochemical methods may be used to probe the corrosion behavior of electroplated coatings with a particular focus on assessing the effects of galvanic coupling between the coating and the substrate (ASTM G 5, G 59, G 61 , G 82 °, G 102", and G 106 ) [22], In accordance with the mixed potential treatment of galvanic corrosion, the corrosion potential and polarization resistance are expected to be a sensitive function of the anode/cathode area, i.e., p>orosity [13,14,20,23],... 

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