Galvanic corrosion area effects

Comparison of the physical appearance of the corrosion, with the area of galvanic corrosion as simulated by the BEM model, showed that the computer model provided good predictions of the effective galvanic corrosion area for each of the three different sizes of steel cathodes in 5% NaCl solution. 

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. 

Figure 6.9 Area effect of the galvanic corrosion cell on (a) steel and (b) copper plates 6...

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. 

The cause of corrosion in the domestic hot water system in the building is attributed to the improper use of dissimilar metal pipes and associated components. The temperature difference in the heat exchanger and heat circulation locations makes the corrosion more severe. An area effect is also a contributor to the corrosion. The remedial measures should be aimed at reducing the existing galvanic cell, to minimize the temperature and area effects. 

As mentioned above, the environment has a significant effect on whether or not galvanic corrosion will be a problem. For example, carbon steel will corrode rapidly if equal or larger areas of Monel 400 are coupled with it in seawater. Conversely, carbon steel is compatible with Monel 400 in concentrated caustic solutions. Even freshwater can be sufficiently different from seawater couples incompatible in seawater work well in freshwater. For example, copper-steel and aluminum-steet couples are satisfactory for handling... 

Figure 7 The effect of increasing area ratio between copper and tantalnm on galvanic corrosion (TMAH pH 10).

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... 

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

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