Effect galvanic coupling

An interesting effect is sometimes observed when cupronickels are galvanically coupled to less noble materials. The corrosion rate of the active metal is increased and the corrosion rate of the cupronickel is diminished, as expected. The diminished corrosion rate of the cupronickel can, however, diminish its fouling resistance since reduced production of copper ions lowers toxicity to copper-ion-sensitive organisms. 

Galvanic effects If niobium is cathodic in a galvanic couple the results can prove disastrous because of hydrogen embrittlement. If niobium is the anode in such a couple it anodises so readily that no damage occurs and the galvanic current drops to a very low value due to the formation of an anodic oxide film. 

Care should be exercised in the use of dissimilar metals in contact or in close proximity. If dissimilar metals must be used, they should be insulated from one another so far as is practicable. Alternatively, if they cannot be insulated, the use of a middle piece with a suitable potential may be effective . In any event, where a galvanic couple exists, the more active metal should have the greater exposed area. 

It is often desirable to know something about the probable distribution of galvanic effects in a galvanic couple. This will be determined, of course. 

Finally, the increase in the corrosion rate of the steel and the reduction of the corrosion rate of the copper upon coupling can be easily seen in the results they demonstrate the usefulness of cathodic protection and the detrimental effect of improper control of galvanic couple corrosion. 

The distribution of cation concentration across the cut-edge is in agreement with studies of a Fe/Zn galvanic couple [13], Diffusional effects are in evidence in figure 2(b) where the highest iso-contour of cation concentration can be seen to exist within a pit formed by dissolution from a much smaller dendrite. A reduced zinc loss can be seen to be offset by the more tortuous route from the pit, impeded further still by closer proximity of cells containing corrosion product. 

When two metals or alloys are joined such that electron transfer can occur between them and they are placed in an electrolyte, the electrochemical system so produced is called a galvanic couple. Coupling causes the corrosion potentials and corrosion current densities to change, frequently significantly, from the values for the two metals in the uncoupled condition. The magnitude of the shift in these values depends on the electrode kinetics parameters, i0 and (3, of the cathodic and anodic reactions and the relative magnitude of the areas of the two metals. The effect also depends on the resistance of the electrochemical cir-... 

In essentially neutral environments (pH = 7), in contact with air, the controlling reaction is the reduction of dissolved oxygen. For Case IV, the effects of galvanic coupling under conditions of oxygen diffusion control are analyzed by reference to Fig. 4.23. Again, metal B is repre-... 

Example Calculations of Corrosion Potentials, Corrosion Currents, and Corrosion Rates for Aerated and Deaerated Environments, and the Effects of Galvanic Coupling... 

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