The Chemistry of Rust (Oxidation)

That reddish-brown crud we call "rust" is all around us, yet we probably rarely think much about it. It turns out what we call rust is a chemical process that combines iron (Fe) and oxygen (O) to form iron oxide. Thus, by studying rust we are studying chemistry!

The chemical formula is:  4Fe + 3O2 = 2Fe2O3

What is happening? During this reaction the iron atoms are passing electrons to the oxygen atoms, a transfer that is called oxidation. In the process the atoms are bound together.

rust-nails

Rust Experiments

Because it is a slow process, doing experiments with rust takes a few days.

1. What rusts? (Preliminary free exploration)

Gather:

  • paper clips, small bolts, metal washers and any other small metal objects to check for rusting - let the children brainstorm and gather samples as appropriate
  • include some items that probably won't rust such as pennies or brass brads
  • container to hold water
  • water

Place a sample of all the objects in a container of water and check them every day for a few days. Leave the rest of the objects nearby or in a similar dry container to compare what happens. See which objects start to show signs of rust and which do not. Let the children touch and smell the objects that have rusted. Do they feel different? Do they smell? Do they look different?

2. What environmental conditions are needed for iron to rust?

Can iron rust in dry air or is water needed? Does the presence of acids, such as acid rain, speed up rust? What about salt? Do the salty roads in winter or salt spray from the ocean really make cars rust faster? What happens when the tannins in tea meet iron/rust? Let's find out.

Gather for each participant:

  • fine steel wool (from paint stores or home supply centers- see note below)
  • water
  • white vinegar
  • table salt
  • teaspoon measure
  • tea bags, hot water and container for making tea
  • tape and marker for labels
  • 5 beakers or similar containers
  • paper and pen or pencil to record results

steel-wool

Note:  Why fine steel wool? The coarser steel wool you get to clean dishes is stainless steel, which is resistant to rust. For another experiment, get samples of both and try them side by side.

Note 2: The tea isn't central to the question, but does react quickly which may engage impatient youngsters who might otherwise lose interest. You may definitely omit it.

Prepare the tea by soaking one or two tea bags in hot water in a container such as a tea mug for about three minutes. Stir briskly and discard tea bags.

Make saltwater by adding 2 teaspoons of salt per 8 ounces of water and stirring.

Label the containers:

  1. water
  2. saltwater
  3. vinegar
  4. tea
  5. air

Pour 4 ounces (1/2 cup) or roughly 120 ml of water into the first container. Add 4 oz or 120 ml of saltwater to the second container. Add 4 ounces white vinegar to the third container and 4 ounces of tea to the fourth. Leave the 5th container dry.

Break off pea to marble-sized balls of steel wool and roll into 5 small balls. Try to use a consistent amount for each container. Drop the steel wool into each container. Some may float, which is okay.

tea-and-water-pretest

Rust experiment, before set-up.

tea-darker-after15

Check what is happening after 15 minutes.

tea-left-water-right

After 15 minutes the tea probably has started to darken. The steel wool will have turned black. In the photograph above the steel wool that was in the tea is on the left and steel wool that had been in plain water is on the right.

What is happening? The tannins in the tea are reacting with the iron and rust in the steel wool to make iron tannate. Iron tannate is very stable and people are investigating its use to prevent metals from rusting.

Check again after 24 hours.

rust-at-24hours

The tea, on the right, has turned black with a concentration of iron tannates. The water, on the left, and the saltwater (not shown) are turning brown and the steel wool is beginning to rust.

The vinegar (center) is still clear and the steel wool is not showing rust. Why not? One reason might be that the vinegar has been setting on a shelf in a closed jar and might not have much oxygen in it. How would you test this?

The dry steel wool is not rusting either. Even though the chemical equation shows that only iron and oxygen are needed, the chemical process actually needs some water or another catalyst to be present to get the reaction going.

Record your results again after 48 hours. What has changed? Use your results to plan more experiments.

Can you tell me...

why we paint metal objects like the San Francisco bridge?

_______________________________

A word of caution to educators:

During preparation for this post I came across a couple of references to experiments that promised "fast rust." These experiment required mixing bleach and vinegar. Mixing bleach and vinegar is not a good idea! The acid reacts with the bleach releasing chlorine gas. In small amounts the chlorine gas reacts immediately with the iron to give iron chloride, which looks like rust. If you add an excess amount, however, toxic chlorine gas might possibly be released.

About.com has more information

38 thoughts on “The Chemistry of Rust (Oxidation)

  1. Anon

    Hi.
    Just wondering where you got this information from:
    "In small amounts the chlorine gas reacts immediately with the iron to give iron chloride, which looks like rust."
    Because according to wikipedia, iron(II) chloride is a white or green salt, and iron(III) chloride is green or purple/red. Also, iron chloride is soluble, so how did you manage to see the colour?
    Sorry if I've misunderstood something.

  2. Post author

    I actually found the information that the reaction might produce iron chloride on this forum: http://www.scienceforums.net/topic/22109-bleachvinegar-and-steel-wool/ (scroll down near bottom)

    If you check this forum, again near the bottom, AJKOER suggests "My view on the reaction of HOCl and Fe is temporary formation of Fe(ClO)2, which decomposes forming FeCl3 and liberating some Cl2. " http://www.sciencemadness.org/talk/viewthread.php?tid=19477

    I did not see the color, because I didn't want to mix bleach with an acid. I was just reporting what was shown in videos posted on the Internet of the experiment, and questioned its safety. If you have more information, I would appreciate clarification.

  3. Madalynn Sierra

    hi, I love this idea I'm in the 5th grade and my teacher said this was an amazing project. This really helped me understand what rust was. So thank you!!!!!!!!!!!!!!

  4. kelly

    i am helping my daughter with her science project. we took three glass jars filled one with vinegar one with water and one with bleach . we put a steel scrubbing pad in each jar. we set one of the sets in direct sun one in the shade and one set in total darkness i can not find anything that she can read to understand why the bleach is breaking down faster. please help.

  5. Post author

    It is my understanding that the bleach (formula Na-O-Cl) produces oxygen atoms when it breaks down as well as causes oxidation (removes an electron from the iron atom). Oxygen is needed to produce rust (fe2O3).

  6. Sophia Flanagan

    Hey!

    For a school chemistry practical we did something similar to this, investigating the effects of corrosion with different variables etc. I just need some help with some of the discussion questions we were given if possible.
    I have to explain why objects corrode slowly when painted, and slowly in the desert. I also have to write the formula for rust and explain why metal object fall apart when they rust.

    Sophia Flanagan

  7. Post author

    Sophia,

    Sorry for the late reply, your question got caught in the spam filter.

    Okay, the answers to these questions are in the post. Painting an object helps keep out the oxygen needed for oxidation or rusting to occur. Painting also keeps out the water. The absence of or low levels of water in the desert also helps prevent or slow rusting.

    The equation for oxidation of iron to iron (III) oxide or rust is 3 Fe + 2 O2 → Fe3O4

    The reaction can be speeded up by water, salt or by burning iron wool.

    The iron (III) oxide molecules do not bind to each other like iron metal does, which is why the object falls apart.

  8. Post author

    Emma,

    I'm not sure if this is what your instructor is looking for, but iron plus oxygen in salt water becomes iron (III) oxide or rust. The salt water acts as a catalyst.

  9. Erik

    I have to do this lab where we write down the chemical eqaution of vinegar and of steel wool and then we have to write the chemical equation of their product so i was just wondering what the chemical equation of their products is

  10. Post author

    Hi Erik,

    The chemical reaction is the standard chemical reaction for rust (which you can find in the comments above). The vinegar is not actively involved in the reaction from the standpoint that it ends up in the products. It serves to remove the coating from steel wool so the oxygen can get to the iron in the steel, and it may also serve as a catalyst. Rusting takes place a lot faster in water and acidic conditions.

  11. Aye Myat

    Can vinegar only help the rate of corrosion because I don't want to mix it with bleach ( it's too dangerous) ?

  12. Post author

    Aye,

    When we did the experiment in straight vinegar, the steel wool did not rust as quickly as it did in water, which is likely because it formed iron acetate rather than iron oxide. The vinegar may act as a catalyst in some cases if the steel wool is coated to prevent rusting. If you think it may be coated, briefly dip the steel wool in vinegar, dry it and then add it to water.

  13. amy

    I am doing this experiment for my school project and 1 of the varibles is vinegar. I understand that steelwool usually comes with protective coating that requires to be removed using vinegar. In this case, it will not be a fair experiment as all the setup will have vinegar in them. Pl help.

  14. Post author

    Amy,

    It depends on what question you are trying to test. If you are trying to test whether vinegar is necessary for the steel to rust (because it has coating that needs to be removed), then you will need to have one treatment that leaves the vinegar out -just water and steel wool. However, it you are testing another question, such as whether salt speeds up rusting, then it is okay to include vinegar in all the treatments, so they are all treated the same except the amount of salt. Does that make sense?

  15. Jordan

    Hi, I came across this site for a slightly off-topic reason, but I'm hoping I can still get some help from someone. I work at a coffee shop, and I happen to like science, so when I found a patch of irritated skin (contact dermatitis or maybe a chemical burn?) on my wrist under my watch after my first day working at my new coffee-shop job, I began to wonder what could've happened.

    I did some research, and it seems to me that some of the bleach water we use to sanitize the rags we use to clean surfaces at work got caught under my watch (which has a stainless steel back, and which I wore for my 9 hour shift that day) might've done it. When I got home, I noticed that my skin was very dry, red, irritated, and looked like I'd maybe gotten a chemical burn. At first I just thought some bleach got under my watch and did something to my skin. We use about 1-2ish caps full of bleach per gallon-ish of water, so it's sort of a strong solution and I didn't think it'd be that far fetched.

    I figured the prolonged exposure was what caused the irritation, as the bleach water spent 9 hours sitting between my skin and my watch-back, with a number of repeated exposures over the course of my shift. Fast-forward two months later and I don't wear a watch at work anymore period after finding out that stainless steel can be corroded by bleach, and that probably caused the problem, not just the bleach. My watch's back doesn't seem to have any dark spots or obvious pitting on it, but there is some beige-ish colored stuff stained onto it in a very thin layer (looks like some sort of light film of oxidization (maybe?) and I've been searching to see what happened.

    If anyone here thinks they can help me ID what happened to give me this burn (it's still there, but I'm seeing a doctor about it soon), I'd love some feedback about the science of what happened, if anyone can help, please? I'm having a hard time getting a straight answer about what the reaction product of bleach and stainless steel would be, specifically the oxidization rxn or other sort of corrosion. Formulas, equations, all of that would be a huge help. I really am mostly just curious about what happened from a chemistry standpoint, and from the comments I've seen, you all seem like you can help!

    Thanks in advance!

  16. Post author

    Jordan,

    Because this is a medical issue, you need to consult a doctor. Bleach alone can irritate the skin or you may have allergies. In any case, you need to see a medical professional.

  17. Miira

    I don't know if someone already answered your question but I believe we paint metals to prevent oxygen and acids from hitting the metals directly to slow down the rusting process/

  18. Pingback:

  19. Well m gettin in trouble with it..is it the real equation of rust so whatz this ...Fe+CuSO4 -----) FeSO4 + Cu...isn't it rust..

  20. Post author

    Sakshi,

    No, FeSO4 is iron(II)sulfate, which when anhydrous (no water) is a yellow powder. Rust is reddish brown.

  21. Post author

    Bee,

    Adding salt increases the rate of reaction at first by increasing the conductivity of the water and thus the rate the electrons are moved, but only to a point. There is a maximum value where adding more salt actually slows the reaction by interfering with the amount of oxygen the water can hold.

    Does that answer your question?

  22. bee2

    What happening is that salt reacts with oxygen molecules. Therefor, there are less reactions between hydrogen and oxygen. Is the chemical formula Na20

  23. Post author

    bee2

    It is likely sodium oxide in water (which this would be) forms sodium hydroxide, so it would not be the way it works. You are talking about at high concentrations?

  24. Franco

    Hi guys

    I have a task to do and one of the questions is: what is the difference between the conditions in which a) magnetite and b) rust forms. Can someone help me plz?

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