Galvanic Corrosion on a Garden Gate
A garden gate frame is made of mild steel. The hinges are brass (copper–zinc alloy), and the fixings are galvanised steel bolts. After one wet winter, the steel frame shows rust around the brass hinges, while the galvanised bolts remain intact but the zinc coating on one bolt has been scratched through to the steel. A neighbour suggests painting the whole gate with a zinc-rich paint and replacing the brass hinges with steel ones.
1. Explain, using electrode potentials, why rust appears near the brass hinges.
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Brass contains copper. Standard reduction potentials: Fe²⁺/Fe E° = –0.44 V; Cu²⁺/Cu E° = +0.34 V. Iron is the more reactive (more negative) metal, so when the two are in contact in the presence of an electrolyte (rainwater), iron becomes the anode and oxidises: Fe → Fe²⁺ + 2e⁻. Copper is the cathode where oxygen reduction occurs. The potential difference (about 0.78 V) drives the corrosion, so the steel near the brass corrodes faster than bare steel would. Rust (hydrated Fe₂O₃) appears at the joint.
2. Explain why the galvanised bolts survive, and what happens at the scratch.
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Zinc is more reactive than iron (Zn²⁺/Zn E° = –0.76 V versus –0.44 V for Fe). The zinc coating therefore acts as a sacrificial anode and corrodes in preference to the steel - cathodic protection. At a scratch, the exposed iron is still in electrical contact with the surrounding zinc, so the zinc continues to oxidise and the iron remains cathodic. This is why galvanised steel tolerates small scratches, unlike a simple paint layer, which fails once breached.
3. Evaluate the neighbour's two suggestions.
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Replacing brass hinges with steel removes the galvanic couple, which is sound in principle, but steel hinges will themselves need protection and may seize. Zinc-rich paint is a reasonable protective coating because it provides both a barrier and sacrificial protection, but it must be applied to clean, rust-free steel or the rust will continue underneath. The better judgement is: remove existing rust, apply a zinc-rich primer, then a topcoat, and separate dissimilar metals with an insulating gasket or washer rather than relying on paint alone.