Question

Identify the anode in a galvanic cell of copper and brass electrodes in standard solutions of their own ions. Identify the anode in a galvanic cell of a two-phase microstructure of a 50:50 Pb-Sn alloy. Identify the anode in a galvanic cell of a lead-tin solder on a Cast iron in seawater. Identify the anode in a galvanic cell of a brass bolt in a passive 430 stainless steel plate in seawater.

Answer 1

sOLUTION:

In a galvanic cell, the redox reactions are spontaneous. So, reduction and oxidation reactions occur simultaneously without any external agent. The equilibrium is given by the standard reduction potentials, where the more positive is the greater tendency to be reduced in the cathode of the cell, and the less negative is the greater tendency to be oxidized in the anode. Then, for every galvanic cell given:

a) Copper and Brass electrodes in standard solutions of their own ions

Standard reduction potential of copper is EºCu=+0.15 V

Standard reduction potential of brass is EºBrass = -0,25 V

We see that EºCu>EºBrass and then nickel electrode is the anode of the cell.

b) A two phase microstructure of a 50:50 Pb-Sn alloy

With the same criterion of answer a, we have:

Standard reduction potential of tin is EºSn=+0.13 V

Standard reduction potential of lead is EºPb=-0.13 V

We see that EºSn>EºPb and then lead microstructure is the anode of the cell.

c) A lead-tin solder on a Cast Iron alloy in seawater

Standard reduction potential of aluminum is EºAl=-1.66 V

We already know that potentials of tin and lead are more positive, then aluminum alloy is the anode of the cell.

d) A brass bolt in a passive 430 stainless steel plate, also in seawater

Stainless steel can not be oxidized. So, between tin and copper (in the brass) Tin is the anode, due to its potential.