Sacrificial protection means protecting iron by attaching a more reactive metal, such as zinc or magnesium, which is oxidised in place of the iron.
Sacrificial protection means protecting iron from rusting by attaching a more reactive metal, such as zinc or magnesium, which is oxidised in place of the iron. Malay: perlindungan korban · Chinese: 牺牲保护.
The idea rests directly on the reactivity series. When a more reactive metal is in contact with iron, that metal loses electrons more readily, so it is oxidised first and the iron is left as the site where reduction happens instead. The reactive metal is slowly eaten away, or sacrificed, while the iron is protected. As long as some of the sacrificial metal remains, the iron does not rust, even if the coating is scratched, which is an advantage over methods that rely only on a barrier.
Example. Blocks of magnesium or zinc are bolted to the steel hulls of ships and to underground pipelines. The magnesium is oxidised (Mg to Mg2+ plus 2e-) and dissolves gradually, while the iron of the hull stays intact; the blocks are replaced when they are used up.
Confusion to avoid. Do not confuse sacrificial protection with a simple protective coating such as paint or oil. A barrier only works while it is unbroken, but sacrificial protection keeps working even where the surface is exposed, because it depends on reactivity, not on covering. Also make sure the sacrificial metal is more reactive than iron; a less reactive metal would make the iron rust faster.
Link to galvanising. Galvanising, coating iron with zinc, uses both ideas at once: the zinc is a barrier and, if scratched, a sacrificial metal.
Why the reactive metal goes first. Because the attached metal is higher in the reactivity series, its atoms give up electrons more readily than iron atoms do, so it becomes the anode of the pair and the iron becomes the cathode. The iron is therefore forced to be the site of reduction, not oxidation, and it cannot rust while any of the sacrificial metal remains to be consumed.
Explaining sacrificial protection with the correct electron transfer connects rusting to the reactivity series and to cells, and it is a common application question in SPM Chemistry.
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