Cathodic Protection

Sacrificial anodes are used to protect another metal from corroding. A metal with a more negative standard reduction potential is attached to the desired metal to protect it. This creates a galvanic cell were the desired metal is the cathode. The sacrificial anode will corrode leaving the cathode intact.

To show this, two cells were set up using iron nails and either zinc or nickel. The electrodes were connected by copper wire and placed in ~500mL of tap water.

Metal Standard Reduction Potential (V)
Iron -0.44
Nickel -0.25
Zinc -0.76

 

Since Zinc has a more negative standard reduction potential than Iron, it corroded. Iron has a more negative standard reduction potential than Nickel so Iron corroded in that cell.

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Figure 1 Iron and Zinc cell. Zinc corroded although it is hard to tell since zinc oxide is white

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Figure 2 Iron and Nickel cell. Iron is corroded.

Rusting

2Fe(s) + 3O2(g) à Fe2O3(s)

We showed this reaction using iron nails in water. Most nails come with a galvanized coating to prevent rusting. On half the nails this was removed by soaking in 12M HCl. The nails were washed and sanded lightly to remove any coating of rust. One nail with anti-rust coating and one without were placed in beakers containing ~400mL distilled water, tap water, boiled tap water, 1M NaCl, or 1M HCl.

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The nails in 1M NaCl rusted the most, followed by tap water, then boiled tap water. The nails in the distilled water rusted the least. The nails in 1M HCl didn’t form Fe2O3, they caused the solution to turn green. This showed that removing impurities and oxygen from the water slowed the rate of oxidation.


Chemical Used                                                                Amount                                                                     Cost


NaCl                                                                            ~30g                                                                       ~$0.06

HCl                                                                            ~40mL                                                                     ~$4.70


 

Total cost of experiment $4.76.