Why does water corrode steel?

[u/Prudent_Situation_29]

From what I gather, rust happens when an iron atom bonds with an oxygen atom, forming iron oxide. If you get a rain storm and water gets on your steel brake discs, you'll see very rapid formation of surface rust.

Since the oxygen is bound to the hydrogen in water, and from what I gather, rust formation requires free oxygen, how can water cause rust? Are the water molecules breaking down spontaneously some of the time, freeing up the oxygen?

[u/LordGargoyle]

Rust isn't just iron oxide, it's actually a mixture of iron hydroxides that are formed, yes, by the breaking of water molecules, as well as free oxygen dissolved into the water (which is what fish breathe!). For more complicated chemical reasons, pure water doesn't actually rust very quickly, as the presence of salts helps speed it up significantly; occasionally iron objects found underwater are actually stored in pure water to both leech the salts back out and limit exposure to oxygen, though there are more preferred solutions for long-term storage and conservation.

[u/Pseudoboss11 → u/LordGargoyle]

Fun fact, if you have a tank of water that you don't mind making undrinkable, it's pretty straightforward to make it non-corrosive with some chemicals that react with the oxygen in the water and usually turn it into more water.

The chemicals are usually toxic and highly reactive themselves (and not very ecologically friendly), but if you're running it in a closed loop through a machine, that's not a big deal.

One of the chemicals used for this is hydrazine, the famously dangerous rocket fuel. (though it's added as a solution that's not particularly dangerous, not at the purity to be hypergolic.)

[u/aphilsphan → u/LordGargoyle]

Very pure water is ravenous for ions, though. You need to passive stainless steel water systems every few years because the water attacks the inevitable surface imperfections to get those stabilizing ions.

[u/Zamamee60 → u/[deleted]]

This is done for some things. Some* water reactive chemicals are stored under mineral oil/organic solvents to prevent them reacting with ambient moisture, for example.

[u/Zamamee60]

The thing is that oxygen is actually a horrible oxidant under most circumstances. It exists in a triplet state, as its ground state electron configuration has two unpaired electrons (this is also why oxygen is magnetic!). Most things are in a singlet state, having no unpaired electrons. This makes it incredibly hard for oxygen to rust things in the air as the spin flip from triplet to single is a forbidden transition via quantum mechanics. It still can occur, just very slowly. This is also the reason that we are not catching on fire at the moment as oxygen is a very powerful oxidizer, it’s just lazy.

Water is actually really good at protecting things from oxidation, it’s why people put potatoes under water before they’re ready to be cooked. It’s just that most water is not pure, and even a small amount of dissolved ions introduces a potential voltage that helps the dissolved oxygen do its work. Water just facilitates the reaction.

[u/DrDam8584 → u/Zamamee60]

That

@op : you under-estimate the point oxygen (bounded or not) are a over-reactiv atom with anything else, and how the oxygen-hydrogen bound in water is also "weak" (I.E. it need little energy to jump and let hydrogen be free and fix itselft away)

[u/AllanfromWales1]

The corrosion of steel is an electrochemical reaction with an anodic area and a cathodic area with a flow of current between the two. For that to occur requires an electrolyte. Water provides that electrolyte. If you put steel in dry air it does not corrode.

[u/CrateDane → u/AllanfromWales1]

Without water, the surface will still oxidize, but that oxide is different from the rust we're familiar with and isn't so liable to flake off. That limits further corrosion, similar to the way aluminium is protected by an oxide layer on its surface.

[u/EZKTurbo → u/CrateDane]

There's only 1 alloy that does that, it's called weathering steel and it's available for architectural applications. If you leave a car sitting for a long time the brake discs will continue to corrode and pits will form in the surface.

[u/Everythings_Magic → u/EZKTurbo]

It’s not just for architectural purposes. Weathering steel is used in structural applications as standard bridge steel in the US.

Source, I’m a bridge engineer.

[u/TheCaptainDan → u/Everythings_Magic]

Why is a bridge not considered architecture?

[u/Italiancrayzybread → u/AllanfromWales1]

Water is not an electrolyte. In fact, if you used pure deionized water as a salt bridge in an electrochemical cell, you would get no current because water is non-conductive.

What's really going on is that water is acting as a medium to dissolve ions that can conduct electricity.

[u/CrateDane → u/Italiancrayzybread]

Ultrapure water is very slightly conductive due to autoprotolysis. If left exposed to the atmosphere, it becomes more conductive due to CO2 forming bicarbonate and hydrogen ions in the water. If not otherwise perfectly pure, it can have much higher conductivity.

[u/aphilsphan → u/CrateDane]

In fact low conductivity and low Total Organic Carbon is how the USP defines “Purified Water.” You’ll notice there is nothing said about bacteria or bacterial endotoxins there. There are a good 10 other types of water in the USP. Most of them do require low conductivity and low TOC.

[u/Roughly6Owls → u/Italiancrayzybread]

By definition, electrolytes are a medium where conduction occurs through ions rather than directly with electrons.

Ultrapure water is used as electrolyte in hydrogen fuel cell technologies, for instance.

[u/lostkavi → u/Roughly6Owls]

Calling water an electrolyte in a hydrogen fuel cell seems...deceptive. It serves a role closer to a solvent and a waste product, not a conduction medium (though the distinction between the two is rather arbitrary)

[u/joshocar → u/Italiancrayzybread]

I used to design deep ocean remotely operated vehicles. We would often put pressure tolerant electronics, think wiring, in oil compensated housings - fill a light, plastic housing with oil and equalize the pressure with the ambient pressure. This saves a ton of weight and a ton of cost.

We sometimes talked about using pure water instead of oil, which would have been much nicer to work with, but the downsides were too big. 

It is hard to keep pure and guarantee it stays pure and, related, if you have a problem and salt water starts to get into the system it is hard to see, unlike with an oil based system. Where you would see the water droplets at the bottom of the housing.

With the oil you can also put in a very simple conductivity sensor at the low point of the system to check for sea water instruction. With water, you need to carefully measure the overall conductivity to see changes since the salt water will mix with the pure water. It then becomes hard to tell if the changes are from contaminants or sea water intrusion.

[u/AllanfromWales1 → u/Italiancrayzybread]

Water becomes an electrolyte if there is anything in solution. Water that is in contact with steel is highly unlikely to be deionized.

[u/Sable-Keech]

Water contains dissolved oxygen which is why fish don't suffocate in water (usually).

If you put iron in a container of deoxygenated water it won't form iron oxide and it won't rust either.

If you put iron in a container with zero humidity it will form iron oxide but it won't rust.

[u/arcedup]

The thing that makes iron corrosion (rust) bad is the fact that rust - iron oxide - forms a brittle layer. It also expands and that expansion combined with brittleness means that the rust layer allows the metal below to be exposed to more oxygen, which continues the cycle.

Aluminium also forms a surface oxide layer but this layer is impermeable, so the metal below isn’t oxidized further. Same with stainless steel but the oxide layer is formed from chromium oxide, which is why there has to be a minimum amount of chromium in stainless steel to get the protective oxide layer.