Corrosion & Degradation of Materials

1. Define corrosion as it pertains to materials.

Solution;

Corrosion is the deterioration of a material resulting from chemical attack by its environment.

2. What are some of the factors that affect the corrosion of metals?

Solution;

Some of the factors that affect the corrosion of metals are: the temperature and concentration of the reactants and products of a chemical attack; the state of mechanical stress in the metal; the presence of erosion in the metal; and the energy state of the metal.

3. A standard galvanic cell has electrodes of zinc and tin. Which electrode is the anode? Which electrode corrodes? What is the emf of the cell?

Solution;

Zinc has a more negative Eo potential of –0.763, as compared to tin’s potential of –0.136, and will thus serve as the anode. The zinc corrodes in an oxidation reaction.

The emf of the cell is obtained by adding the half-cell reactions together:

Anode reaction: Zn → Zn²⁺ + 2e^- E^o = - 0.763 V

Cathode reaction: Sn²⁺ + 2e^- → Sn E^o = - 0.136 V

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E^o_cell = - 0.627 V

4. Consider a magnesium-iron galvanic cell consisting of a magnesium electrode in a solution of 1 M MgSO4 and an iron electrode in a solution of 1 M FeSO4. Each electrode and its electrolyte are separated by a porous wall, and the whole cell is at 25°C. Both electrodes are connected with a copper wire.

i) Which electrode is the anode?

ii) Which electrode corrodes?

iii) In which direction will the electrons flow?

iv) In which direction will the anions in the solution move?

v) In which direction will the cations in the solution move?

vi) Write an equation for the half-cell reaction at the anode.

vii) Write an equation for the half-cell reaction at the cathode.

Solution:

i) The magnesium electrode is the anode.

ii) The magnesium electrode corrodes since the anode in a galvanic cell corrodes (oxidizes).

iii) The electrons will flow from the anode, Mg, to the cathode, Fe.

iv) The SO_4- anions will flow toward the magnesium anode.

v) The cations will flow toward the iron cathode.

vi) The oxidation reaction that occurs at the magnesium anode is: Mg ® Mg²⁺ + 2e⁻

vii) The reduction reaction that occurs at the iron anode is: Fe²⁺ + 2e⁻ ® Fe

5. Describe two methods by which cathodic protection can be used to protect steel pipe from corroding.

Solution:

Cathodic protection of steel pipe can be achieved by (i) an external DC power supply, or by (ii) galvanic coupling with a metal, such as magnesium, that is more anodic than the steel.

Cathodic protection of an underground tank by using impressed currents

Cathodic protection of an underground pipeline by using sacrificial Mg anode