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How long does it take for iron oxide to form on iron?

Hi,

I'm doing a science fair project on what affects the process of rusting. What I need to know is what is the average time it takes for iron (a nail specifically) to begin rusting. I would be very grateful if someone were to answer my question. :)ally(:

Ally M [last name deleted for privacy by Editor]
- Bowie, Maryland, US
2002

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You're probably supposed to do the experiment yourself and observe, thus finding the answer to your question. But a guideline won't hurt. The surface cleanliness is very important, otherwise the air (and humidity in the air) can't contact the actual iron. Given a totally cleaned, fresh surface, you are likely to be looking at minutes or hours, rather than days or weeks.

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
2002

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The rate at which iron will rust is very dependent on lots of factors. The main ones are the relative humidity, the purity of the iron and the cleanliness of the surface. There is (reputedly) an iron monolith in India that is many hundreds of years old that has never rusted; this is because the relative humidity is very low and the iron is very pure. There are claims it was made by an extra-terrestrial life form, but you can believe that or not, as you wish. Most iron contains tramp metals or impurities that can set up galvanic cells and these will promote rusting, so it is best to have pure metal. As far as cleanliness is concerned, if the iron is coated with a protective coating such as oil or grease, it will take longer for the water in the air to get to the iron, so it will not rust very fast. However, if you get a piece of pure iron and thoroughly clean it in a good aqueous wash (preferably including an acid dip between the wash and final rinse), you will see the surface turn a very pale green in front of your eyes - this is ferrous oxide/hydroxide. Normal rust is brown coloured and is due to the formation of ferric oxide/hydroxide mixture. Rust also needs oxygen to form. Let me suggest an experiment you could do, but it would only be fair if you report your findings on this website. Get four pieces of iron, say some cheap nails, all from the same batch, but make sure they are not galvanized or stainless steel. Thoroughly clean them up with some emery paper or other suitable abrasive system. Put them each into different glass jars. Leave one alone, put some tap water into the second, put some boiled tap water into the third (having boiled it for at least five minutes) and put the same boiled water into the fourth, but tip a good layer of mineral oil or melted wax onto the top of the water in the fourth jar. Check them every day and assess the level of "rustiness"; this can be done by their change in colour. Watch what happens to the metal and look for anything strange. Now explain your observations.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2002

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Q. I similarly wanted to know if it was feasible to do an iron rusting experiment in a lab class so I think I'm going to have to do a couple of different stages of a nail in water to get my students to see the process. Thank you for the replies everyone.

Lucy Crockford
- Newport, Shropshire, UK
October 2, 2014