Iron and rust: Difference between revisions

Revision as of 12:50, 28 February 2024

Rust

What is 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 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.

Do you need moisture for rust to happen?

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 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.

@@ Line 1: / Line 1: @@
-<!--
-Most metals oxidize, but for some reason, and has similar effects,
-though for some reason, only iron oxide has a specific name.
--->
+==Rust==
@@ Line 12: / Line 8: @@
 <!--
 An oxide of rust, but it's more complex than that.
+Most metals oxidize,
+and has similar effects on most metals,
+though for some reason, only iron oxide has a specific name.
@@ Line 25: / Line 26: @@
-In the sense that you need rust for more to grow (like an organism), no.
+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.
-We just call it growing because surfaces that rust tend to be not-quite-uniformly susceptible - so a lot of it happens nearby in time.
+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.
-And rust breaks the surface (particularly protection, like paint) in a way that helps moisture and oxygen reach surface right next to it (rust is itself somewhat porous[https://commons.wikimedia.org/wiki/File:The_Art_of_Science_2018_finalists_(27870916548).jpg]).
+* when protection is brittle and flakes off, like paint
-: {{comment|(...at the same time, thick enough rust can (like any oxide layer) slow down and potentially even stop oxidation from going deeper. This tends to work better in some other metals (like aluminium) than in iron, though)}}
+: then rust taking hold in one place breaks the surface near it and expose that, because rust expands in volume and itself is [https://commons.wikimedia.org/wiki/File:The_Art_of_Science_2018_finalists_(27870916548).jpg 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 much prefers nucleation sites,
+* Rust prefers nucleation sites,
-so you have places where it forms earlier than others,
+: so you have places where it forms earlier than others,
-which means that even unprotected iron won't rust uniformly,
+: which means that even unprotected iron won't rust uniformly,
-and you can be forgiven thinking it's spreading from those sites.
+: and you can be forgiven thinking it's spreading from those sites.
-And that effect looks even clearer when that iron is largely protected (paint, zinc coating, chrome or nickel plating, etc.) but some of that protection breaks (paint is the best example because it's softer and more brittle than the others just mentioned), it will spread from that side ''because'' that break increases makes both oxygen and moisture avaiable there. If it had no protection it would be relatively uniform.
+* 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
-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.
+* 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.
@@ Line 54: / Line 62: @@
-(see also homegnous versus heterogeneous corrosion)
+(see also homogeneous versus heterogeneous corrosion)
 '''Technically'''
@@ Line 69: / Line 78: @@
 '''Practically'''
-Explanations that say "yes, iron will rust without water" often seem to mean ''visible'' amounts of water, like rain.
+Explanations that say "yes, iron will rust without water" often seem to mean "without large and ''visible'' amounts of water", like rain,
+and ignores that all air is humid.
+We ''can'' answer the question "would iron still rust in a 0% [[RH]] air environment".
+...but that answer is practically completely irrelevant, "consistently zero moisture in the air" does not exist in places that humans live.
+Basically all air anywhere has humidity.
+"But deserts" - Deoends. Some are ''near'' zero almost always, some have cold nights where whatever is there in the air condenses well.
+Searchign for images of abandoned cars in a desert shows them rusted -- but also still there. This suggests it happens -- just quite slowly.
+Some deserts have e.g. world war wreckage that still hasn't fallen apart.
-So while you can read the question like "would iron still rust in a 0% RH air environment",
-the answer to that is barely relevant.
-Because "zero moisture in the air" basically doesn't exist in places that humans live.
-Basically all air anywhere has humidity.
-{{comment|(Even deserts, which are often ''near'' zero much of the time, may have cold at night condense whatever's there fairly well)}}
-And to be fair, that's also exactly why it is basically ''not relevant'' to interpret the question like
+It also depends on the metal
-How fast depends on but it depends on the metal how fast.
 Say, aluminium oxidizes at room temperature.
 Iron too, but ''very'' slowly, and you probably wouldn't consider heat oxizing iron an issue until a few hundred degrees higher - like in soldering iron tips.