Is rust contagious?

[srah1953]

If a rusted shackle is attached to a galvanised chain or anchor will the rust spread?
Thanks

 

[coreng]

That's not the way it goes... Read http://www.galvatest.com/WP_Cathodic_Protection.pdf to learn a little more about corrosion.

 

[reeac]

No, if anything the galvanising will protect the shackle - if there's some electrical continuity there.

 

[electrosys]

In your case, no - but there are cases where rust can indeed be contagious.

When rusting occurs to iron or steel, Iron Oxide is produced at the metal's surface. 'Normally', such surface oxidation should protect underlying metal from further oxidation - but (brown) Iron Oxide is a significantly larger molecule than metallic Iron, so when that oxide is formed, the surface layer swells and eventually cracks, allowing oxidation to then occur to the metal under the oxide layer. That sub-layer will in turn then oxidise, swell and crack, and so on.

[which is why it is necessary to clean steel back to bare metal before painting it, and why painting over rust will normally not work. The logic behind converting brown Iron Oxide to black Iron Oxide by the use of Phosphoric Acid (Jenolite and similar) is that black Iron Oxide (FeO) is a physically much smaller size molecule than brown Iron Oxide (Fe2.O3), and less likely to cause further oxidation by the expansion process explained above.]

So - if the steel which is oxidising (rusting) is in constrained contact with other steel - the classic case would be a bolt in a hole, or two layers of steel rivetted or bolted together - as the rusting metal expands with chronic corrosion, that expansion will begin to physically lift-off any protective coating which may previously have been intact on the second piece of metal. So - in a sense - the rusting process will indeed have become 'contagious'.

 

[vyv_cox]

In your case, no - but there are cases where rust can indeed be contagious.

When rusting occurs to iron or steel, Iron Oxide is produced at the metal's surface. 'Normally', such surface oxidation should protect underlying metal from further oxidation - but (brown) Iron Oxide is a significantly larger molecule than metallic Iron, so when that oxide is formed, the surface layer swells and eventually cracks, allowing oxidation to then occur to the metal under the oxide layer. That sub-layer will in turn then oxidise, swell and crack, and so on.

[which is why it is necessary to clean steel back to bare metal before painting it, and why painting over rust will normally not work. The logic behind converting brown Iron Oxide to black Iron Oxide by the use of Phosphoric Acid (Jenolite and similar) is that black Iron Oxide (FeO) is a physically much smaller size molecule than brown Iron Oxide (Fe2.O3), and less likely to cause further oxidation by the expansion process explained above.]

So - if the steel which is oxidising (rusting) is in constrained contact with other steel - the classic case would be a bolt in a hole, or two layers of steel rivetted or bolted together - as the rusting metal expands with chronic corrosion, that expansion will begin to physically lift-off any protective coating which may previously have been intact on the second piece of metal. So - in a sense - the rusting process will indeed have become 'contagious'.

All sounds very knowledgeable - unless you know that rust is not iron oxide at all, it is a mixture of hydrated iron oxides, Fe2O3.xH2O, (FeO(OH) and Fe(OH)3).

It is quite common to see a nice bright galvanised shackle on some rusty chain, so I suggest the answer to the OP's question is mostly no. In theory the zinc on the shackle is protecting the chain in this example, but this assumes electrical conductivity between each link and the shackle, a fairly unlikely situation.

 

[electrosys]

All sounds very knowledgeable - unless you know that rust is not iron oxide at all, it is a mixture of hydrated iron oxides, Fe2O3.xH2O, (FeO(OH) and Fe(OH)3).

I thought I'd pitched the explanation at about the right level for this forum - didn't know I was expected to provide an undergraduate level post.

Bloody nitpicking - get a life.

 

[KellysEye]

I wouldn't call rust contagious but if you don't grind to bare metal and paint properly the area of rust will increase every year.

 

[vyv_cox]

I thought I'd pitched the explanation at about the right level for this forum - didn't know I was expected to provide an undergraduate level post.

So for this forum it's OK to give incorrect information?

 

[Conachair]

So for this forum it's OK to give incorrect information?

Well I prefer the big words to being dumbed down.

Anyway, everything I've seen on the subject says that phosporic acid converts the rust to ferric phosphate, FePO4. Whatever it does it works for me.