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I have noticed that, when exposed to water for a few days, the steel in my wagon turns into rust. After several more weeks, the rust that formed is dissolved in the water, and once I pour the water out there isn't any rust where there previously was rust.

This is unusual because iron oxides and iron hydroxides (the components of rust) don't normally dissolve in water, and so the water just keeps being reduced and the iron keeps being oxidized.

How can it apparently happen such that steel turns to rust, then back to steel with no rust, in the presence of water if rust does not normally dissolve in water?

This happens in a cycle like this:

steel + water = rust -> rust + water = steel + rusty water

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Rust appears because iron is oxidized in presence of oxygen (it is a oxydo-reduction reaction, meaning an electron transfer occurs between species), and water helps this phenomenon occur. Iron loses 3 electrons to the favour of the oxygen.

In presence of water, oxygen can oxidize iron by the mean of hydroxyl group. It leads to:

$$\ce{2Fe(OH)_3 \leftrightharpoons Fe_2O_3\cdot 3H_2O}$$

where water is not the oxidizer, rather it is a mediator for oxidation.

Hydroxides have low solubility, so water can only dissolve a small fraction of it.

On the other hand, poured water can abrade rust (via mechanical stress) and remove it of from the bulk metal, leaving the surface clean for producing more rust. This is how huge rods of metal can vanish in a few months in corrosive environmental conditions (such as wet air or water).

Preventing rust requires the use of specific alloys and surface finishing. You can also add a galvanic electrode (generally made of $\ce{Zn}$, a so-called sacrificial anode) to protect submerged metallic structures such as the hull of a boat.

Therefore, in your example water is both a mediator of oxidation and mechanical abrasion agent.

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Steel isn't pure iron by a long shot! The dissolved carbon in steels creates a myriad of shorted electro-chemical cells. Any naturally occurring water is not pure and contains ionic impurities which make the water electrically conductive, and when in contact with these inherent shorted cells accelerates the consumption of the iron. The high Chromium content of Stainless steels mitigates this by isolating the carbon-iron cells from the aqueous solution. If you capture the off-gasses from water corroding steel, you will find it is $\ce{H2}$.

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  • $\begingroup$ 0.25% carbon and less in low carbon steel- which is most carbon steels- His wagon is probably made of medium carbon steel at up to 0.6%. Neutral water rarely produces hydrogen or any gas in open atmospheric corrosion more like absorbs oxygen to produce hydroxyl ions. $\endgroup$ – user2617804 Aug 5 '15 at 23:48

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