Letter 40056

Why is Zinc Iron Plating better than Zinc Plating?

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I have a doubt. How adding Iron to zinc will improve the corrosive resistance since iron oxide is a rust.

J.Vinoth Kumar
- Coimbatore, Tamilnadu, India

Hello, J. Vinoth. Please try to describe the situation you find yourself in rather than describing your question in abstract terms. It makes the questions more answerable -- which leads to greater reader participation and a better chance of you receiving the help you seek.

The reason that iron and steel corrode so readily is not because of their electrochemical activity (actually iron is less active than zinc or aluminum) but because the corrosion products are not adherent, tenacious, and impervious to further degradation. While some other metals can form a nice tight tarnish or oxidation product that seal the metal surface fairly well, the oxidation product of steel is a fluffy, powdery, porous, water retaining rust which does nothing to seal the underlying steel away from the environment. But the corrosion product of a zinc-iron alloy plating is not a fluffy rust, but a reasonably stable oxidation product.

Once that is recognized, we can go on to look at the electrochemical potential of zinc-iron and recognize it as being closer to steel than non-alloyed zinc plating. Now the galvanic potential between the steel and the plating is lower, and we have the opportunity for slower corrosion of the plating and better corrosion resistance.

Please come back with a followup if I wasn't clear. Good luck.

Ted Mooney, P.E.  finishing.com Brick, New Jersey