If you have two metals in contact, with an electrolyte present, how do you determine which metal will corrode?
The answer lies in a table based on electrochemical potential. This gives the likelihood of a particular metal to react, compared to a standard.
This table lists the reactions as half equations. Each half equation has an associated electronegative potential.
The table lists the reactions that occur at the anode. The likelihood of the reaction occurring is measured compared to the H2 reaction.
A metal higher in the series has a higher corrosion resistance than one below it in the series.
From the table we can see that gold (Au) has the highest corrosion resistance, and that if we were to join a steel pipe (Fe) with a copper fitting (Cu) then it is the steel that would corrode because it is below copper in the series.
Examples
This table can also be used to determine if a certain metal will corrode in a particular environment. To see if a particular metal will corrode in water, for example, we would take the cathode’s reaction potential and subtract the anode’s reaction potential. If this number is positive then corrosion will occur.
Q. Will iron corrode in water?
A. If iron is corroding then it has to be the anode. We therefore look at the equation: Fe —» Fe2+ + 2e-
The water reaction will occur at the cathode: O2 + 2H2O + 4e- —» 4(OH)-
Note that wet corrosion requires both water and oxygen.
So we take the cathode potential and subtract the anode potential:
EoV cathode (water) minus EoV anode (iron) =+0.401- (-0.440)= +0.841V, therefore iron corrodes in water.
Q. Will silver corrode in water?
A. Again, silver would be the anode and water the cathode.
EoV cathode (water) minus EoV anode (silver) = +0.401- (+0.799)=-0.398V, therefore silver will not corrode in water.