Passivation and the Copper Penny

Location:

Concepts Shown:

corrosion, passivation, anodic, cathodic

Equipment:

1. one 100mL beaker

2. 50mL of 1 M HCl

3. two pennies, year 1987 or newer

Procedure:

1) Before class, file the copper coating off one of the pennies in 4 different spots so the zinc inside is exposed. Place both pennies in the acid and let it sit for two days. Remove both pennies from the beaker and rinse with water.

2) During class, explain the corrosion procedure performed in step (1). Pass around the samples showing the zinc has corroded away from the filed sample, whereas the unfiled penny has no corrosion.

Science:

Passivation is the formation of a protective layer, known as a passive layer, on the anode which prohibits corrosion by providing a barrier between the anode and the cathode of a particular system. The acid reacted with the zinc in the filed penny to produce hydrogen gas, leaving the copper coating whereas acid had no effect on the other penny (assuming there were no blemishes on the copper coating). The copper acted as a passive layer on the unfiled penny because copper is more cathodic (+.34 V) than hydrogen ions, but the filed penny corroded because the hydrogen ions were able to contact the penny's zinc inside which is anodic relative to copper and hydrogen ions.
Oxidation: Zn Zn2+ + 2e- -.736 V
Reduction: 2H+ + 2e- H2 .000 V
______________________________________
Zn + 2H+ H2 + Zn2+ -.736 V

Remarks:

Pennies that date back before 1987 do not have zinc inside. They are all copper; consequently, the experiment will not work. Other forms of passive layers include galvanized steel and alumina films.

References:

Author:
Dave Goodman
Credits:


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