Zinc against corrosion
Protecting iron from corrosion
Wear protective gloves, eyewear, and a mask. Perform this experiment in a well-ventilated area.
Reagents and equipment
- 10 g sodium chloride;
- 80 mL distilled water;
- 2 zinc washers;
- 2 pieces of iron;
- 2 plastic cups;
- 5 mL 5% ammonium thiocyanate solution;
- 10 mL 10% hydrochloric acid.
Dissolve 5 g sodium chloride in 40 g of distilled water in 2 plastic cups. Add 2 zinc washers to the first cup. Immerse one piece of iron in each cup. Make sure the iron is touching the zinc in the first cup! Wait 40 minutes. Notice that the solution in the second cup has turned orange. Conduct a qualitative reaction for iron ions: add 10 mL of 10% hydrochloric acid solution to the second cup, then add a few drops of 5% ammonium thiocyanate solution to both cups. Notice that the solution in the second cup turns red.
Corrosion is a process by which materials (metals, plastics, wood, concrete, and so on) are gradually chemically destroyed by environmental factors. Corrosion carries a devastating economic loss, speeds aging, and brings destruction. Cathodic protection is one method used to fight corrosion. The metal being protected is attached to a more active metal. This experiment demonstrates the defense of iron by zinc in a sodium chloride solution imitating seawater. The zinc dissolves, and the iron is left untouched.
Zn - 2e = Zn²⁺
Iron corrodes in the solution without zinc. It dissolves, forming iron(III) oxyhydroxide:
Fe + 2H₂O - 3e = FeO(OH)↓ + 3H⁺
A qualitative reaction with thiocyanate ions can be used to prove the presence of iron(III) ions – a red complex will form. Diluted hydrochloric acid is added to dissolve the orange precipitate of iron(III) oxyhydroxide, then a few drops of ammonium thiocyanate are added.
FeO(OH) + 3HCl = FeCl₃ + 2H₂O
Fe³⁺ + nSCN⁻ = [Fe(SCN)ₙ]³⁻ⁿ
Such a “zinc defense” can serve to save ships from corrosion in such environments as seawater.