Sacrificial protection relies on the principle of electron loss (oxidation). Corrosion is an electrochemical process involving the loss of electrons by a metal.

Consider a scenario where iron (Fe) is in contact with zinc (Zn) in a corrosive environment (e.g., saltwater). Iron is more readily oxidized than zinc. This means that iron atoms have a greater tendency to lose electrons and become iron ions (Fe²⁺ or Fe³⁺). The zinc, being more reactive, readily loses electrons to become zinc ions (Zn²⁺).

The electrons released by the zinc flow into the iron. This influx of electrons causes the iron to gain electrons and remain in its metallic state, preventing it from corroding. The zinc atoms are oxidized (lose electrons) and become ions, dissolving into the solution. The iron, meanwhile, remains reduced (gains electrons) and is protected from oxidation.

Therefore, the sacrificial metal provides a path for the electrons released during the oxidation of the protected metal, effectively preventing the corrosion process from occurring on the protected metal. The more readily a metal loses electrons, the more effective it is as a sacrificial anode.

Control Variable: A crucial control variable is the amount of zinc coating applied to each piece of metal. The thickness of the zinc layer should be kept constant for both the iron and copper samples. This ensures that the only difference between the samples is the base metal being protected.

Independent Variable: The independent variable is the type of metal being protected. The student should have two groups: one group with iron coated in zinc and another group with copper coated in zinc. The presence or absence of iron in the base metal is what's being manipulated.

Analysis of Results: The student should calculate the percentage mass loss for each sample after a week. This is done by subtracting the mass of the sample after a week from the initial mass and dividing by the initial mass, then multiplying by 100. The formula is: Percentage Mass Loss = [(Initial Mass - Final Mass) / Initial Mass] x 100.

The student should then compare the percentage mass loss of the iron and copper samples. If the iron sample shows a significantly lower percentage mass loss than the copper sample, this would indicate that galvanising is effective in preventing corrosion of iron. The copper sample, being more resistant to corrosion than iron, would likely show a lower mass loss regardless of the zinc coating. The results should be presented in a table to clearly show the mass loss for each sample.

The rusting of iron requires the presence of three key conditions:

• Iron (Fe): Iron must be present in its metallic form.
• Oxygen (O₂): Oxygen is essential as it acts as the oxidizing agent.
• Water (H₂O): Water acts as an electrolyte, facilitating the flow of ions and completing the electrochemical process. The presence of water significantly speeds up the rusting process.

Therefore, rusting occurs when iron is exposed to both oxygen and water.