Iron and rust

Rust

What is rust?

https://en.wikipedia.org/wiki/Rust

Does rust spread?

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

In the sense that only rust will cause more rust to grow, like an organism, no.

Some cases looks like that. Others do not.

In the sense that rust alters a surface to take hold near it, possibly.

Consider:

• surfaces with minimal protection tend to have their last bit of protection be worn out at roughly the same time

so a lot of rusting spots happens at roughly the same time, which can looks a lot like an organic kind of growing

• when protection is brittle and flakes off, like paint

then rust taking hold in one place breaks the surface near it and expose that, because rust expands in volume so breaks nearby paint

also, rust itself is somewhat porous

that break increases both available oxygen and moisture, this will cause spread locally

• you could say rust prefers nucleation sites,

so you have places where it forms earlier than others,

which means that even unprotected iron won't rust uniformly,

and you can be forgiven thinking it's spreading from those sites.

• a thick enough oxide layer actually slow down and potentially even stop oxidation from going deeper.

This is less a thing with iron rusting, and more a thing with some other metals, like aluminium

• temporary protection works decently too

One of the reason you are supposed to oil cast iron pans is that it won't rust as easily - the oil will block moisture and oxygen by just being there instead of it (and being somewhat hydrophobic).

• things like zinc coating, chrome or nickel plating, might fail more in spots like described above

• water pipes have an interestingly different mechanism of failing - but are rarely iron anymore

Is rust bad for you?

Short answer:

iron oxide itself is not harmful

ingesting a lot of rust may increase the need for your body to remove it - but it's pretty good at doing that at small amounts we usually get

rust happens most in moist places, which also breeds things that are potentially harmful

Say, rusty nails and surfaces can be bad, but this is because places that collect rust are typically also dirty and moist, which is a good place for bacteria to grow to large numbers.

Bacterial spores such as those causing tetanus will take better on rough surfaces with dirt and moisture around - which happens to include rusty nails. (verify)

So stepping on a rusty nail in an old dirty moist run-down industrial building is a little riskier (and perhaps more likely) than stepping on a rusty nail in your room.

(Even then, a wound bleeding will often wash such things away - that's part of the point of bleeding. That said, e.g. tetanus is handier to prevent than to cure, so at least check to try to catch it early - stiffness of the jaw and neck are early indicators.)

On the other hand, a rusty water cooker, just using it will kill most anything, so it would be safe from bacteria - that's going to be just iron oxide.

(An iron / cast iron pan will sometimes be doing chemistry, which can matter, but barely so if you clean it well and/or use it frequently. )