Iron and rust

Rust

Does rust spread?

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In the sense that you need rust for more rust to grow, like an organism, no. One of the cases looks like that. Others do not.

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 happens at roughly the same time, which looks a lot like that 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 and itself is somewhat porous

since that break increases makes both oxygen and moisture avaiable, locally, this will be only local, and spread

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

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

• 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.

Is rust bad for you?

Short answer:

iron oxide itself is not harmful

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

For example, a a rusty water cooker is no risk, because it's purely rust, and the frequent boiling will kill most anything.

(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. )

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.

A wound bleeding will will wash things away - that's part of the point of bleeding. That said, e.g. tetanus is easier to prevent than to cure. At least try to catch it early - stiffness of the jaw and neck are early indicators.