Galvanised Into Action…

This is another post in the Rusting Series. Please make sure that you have read the others if you are here revising for upcoming exams – otherwise just relax and absorb.

Many metals corrode over time; they become weaker as they react with their environment and often eventually break, crumble or otherwise fail. Iron (steel) is famous amongst metals because we have a special word for when it corrodes; we say that it has rusted. Rusting only happens to iron (steel) and only happens on contact with both water and oxygen.

Iron v Steel

Iron is the name of element number 26 on the Periodic Table. It is the most abundant element on Earth, forming most of the inner and outer core. In the crust of the Earth (the bit that we can dig into and mine) it is the fourth most abundant element. Pure iron is actually quite soft, but the iron produced by a blast furnace is heavily mixed with carbon (3% to 5%) and other impurities and needs to be refined a little further to produce steel. Steel is an alloy of iron and carbon, where the carbon makes up between 0.002% and 2% of the alloy depending on the type of steel being produced. Iron with greater than 2% carbon mixed with it is known as cast iron, and is a significantly weaker material. Amazingly, steel can be up to one thousand times stronger than pure iron. In addition, other metals such as chromium, vanadium, manganese, nickel and several more may be included to modify the properties further. This allows the production of stainless steel (which doesn’t rust) or specialist steels for particular jobs (manganese steel is super-hard and used to make bulldozers and tanks).

Rusting

Iron and many forms of steel will rust when they meet water and oxygen. This is a hugely economically important issue as a great deal of care must be taken to ensure that steel in buildings, vehicles and factories does not break as a result of rusting. Rusting is speeded up by dissolved salt in the water or the presence of acid (from acid rain) as it encourages water to be a better electrical conductor. Iron has some of its electrons conducted away and they combine with water and oxygen to produce hydroxide ions. Hydroxide ions are negatively charged so they react with the iron atoms that lost their electrons (and are therefore positively charged). The iron hydroxide formed in this way dehydrates over time to form iron oxide, commonly known as rust.

Galvanising

Now to the main point of this post – how does galvanising protect against rust? Galvanising means coating in a thin layer of zinc. This is usually done by briefly submerging the entire item into molten zinc. The steel finishes with a thin coat of zinc that provides a physical barrier between the iron atoms and the air. But this is not all! The zinc will protect the steel even if the surface is scratched, exposing the iron beneath. This works because zinc is higher in reactivity than iron. When the galvanised object gets wet, electrons flow from the zinc to the iron meaning that even if the iron atoms are exposed to water and oxygen they will not react. The zinc atoms slowly become zinc hydroxide instead. This is sometimes called sacrificial protection because the zinc atoms are sacrificed to protect the iron.

This nerdy fellow (with a strangely deep voice) explains how galvanising works with some simple home experiments that we could easily repeat in the lab.

Here is a video from the industry all about ‘hot dipping’ objects to coat them in zinc. The vats of liquid zinc look rather enticing!

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Questions…

1. What are the three elements more abundant than iron found in the Earth’s crust?
2. What is the most abundant element in the Universe?
3. Suggest a physical property of a typical metal.
4. What other ways are there of stopping steel objects from rusting?
5. To what group of chemical compounds do iron oxide and zinc hydroxide belong?
6. What compound causes (a) all rainwater to be slightly acidic? (b) rain around industrial areas to be strongly acidic?