For the following pairs of alloys that are coupled in seawat

For the following pairs of alloys that are coupled in seawater, predict the possibility of corrosion; if corrosion is probable, note which metal/alloy will corrode. Aluminum and cast iron Inconel (contains nickel) and nickel Cadmium and zinc Low-carbon steel and copper Write the possible oxidation and reduction half-reactions that occur when magnesium is immersed in: (a) an HCI solution containing dissolved oxygen, and (b) an HCI solution containing dissolved oxygen and Fe^2+ ions. In which of these solutions would you expect the magnesium to oxidize most rapidly? Why?

Solution

9. (i)

For the given problem in order to make predictions, we need to use the galvanic series.

General thumb rule is if both of the alloys in the pair reside within the same set of brackets in this table, then galvanic corrosion is unlikely. However, if the two alloys do not lie within the same set of brackets, then the alloy appearing lower in the table will experience corrosion.

(a) Aluminum-cast iron - corrosion is possible, and aluminum will corrode.

(b) Inconel-nickel- corrosion is unlikely, both alloys appear within the same set of brackets (in both active and passive states).

(c) Cadmium-zinc - corrosion is possible, and zinc will corrode.

(d) Low-carbon steel-copper - corrosion is possible, and the low-carbon steel will corrode.

(ii). (a) In an HCl solution containing dissolved oxygen, possible reactions are

Mg Mg²⁺ + 2e^- (oxidation)

4H⁺ + O₂ + 4e^- 2H₂O (reduction)

(b) In an HCl solution containing dissolved oxygen and Fe2+ ions, possible reactions are

Mg Mg²⁺ + 2e^- (oxidation)

4H⁺ + O₂ + 4e^- 2H₂O (reduction)

Fe²⁺ + 2e^- Fe (reduction)

The magnesium would oxidize most rapidly in the HCl solution containing dissolved oxygen and Fe²⁺ ions because there are two reduction reactions that will consume electrons from the oxidation of magnesium.