Corrosion and galvanic protection

Corrosion is essentially an electrochemical process, due to the tendency of metals to dissolve in solutions and form electrolytic cells when coupled with other metals.

The more negative the reducing potential of a metal, the greater its capability to reduce hydrogen ions or another metal with a potential less negative.

Zn + 2H3O+ Zn2+ + 2H2O + H2 ↑.

Fe + 2H3O+ Fe2+ + 2H2O + H2 ↑.

In these examples metals dissolve in the solution, at different rates according to their standard potential.

Copper is resistant to the attack of acids since the potential of the redox couple Cu/Cu2+ is positive against the redox couple H2 / 2H+ ± 0 V.

The standard potential of the redox couple Fe/Fe2+ is – 0.44V whereas the Cu/Cu2+ one is + 0.34V; iron will therefore dissolve in solution and reduce the copper ions (this is a visible process the solution takes on a yellowish colour).

The corrosion process of base metal in acid solution and some salt solutions can be very fast.

Two metals held together by crocodile clips and immersed into the solution are equivalent to a short circuited galvanic cell. In this way, the metal with more negative potential dissolves in the solution and its electrons flow towards the other metal where they can reduce hydrogen ions near to the other metal. The presence in the shortcircuit cell of the more base metal prevents the corrosion of the less base metal until the first one is dipped in solution. This feature is helpful to protect the less base metal against corrosion.

• 2 × Crocodile clips — 2531.52
• Copper electrodes — 4716.21
• Zinc electrodes — 4716.16
• 3 × Iron electrodes — 4716.23
• 4 × Test tubes — 1473.60
• Test tube rack — 1680.06

• Copper sulphate solution, dilute — R:22-35/38-50/53 S:22-60-61
• Hydrochloric acid, approx. 10% — R:34-37 S:26-45

• Report your observations on what has happened to the electrodes in the test tubes, and state which metal is corroded more easily.
• Make your observations about the metal electrodes and water.