Science question

[Baron Von Murderpaws]

Why does metal rust in layers instead of just rusting as a whole piece, or rusting straight thru?

I've always wondered why this is.

 

[JGalt]

Probably water getting in a crevice and freezing. I'm no rustologist, just a rusty old man. So that's just my guess.

The answer's probably buried in this gobbledygook...

 

[jotathought]

Why does metal rust in layers instead of just rusting as a whole piece, or rusting straight thru?

I've always wondered why this is.

Metal rusts in layers due to the differential aeration, where variations of available oxygen and moisture cause localized corrosion areas. The initial rust layer acts as a barrier, but it is not completely impermeable, allowing the corrosion process to continue and form subsequent layers of rust over time.

 

[Rambunctious]

Why does metal rust in layers instead of just rusting as a whole piece, or rusting straight thru?

I've always wondered why this is.

I don't know but today I put in new grates in my weber and looked down at the base and noticed the paint was loose... I peeled it right off and underneath it was all rusted and pitted....
I went and got some rust destroyer primer scraped off all the paint and applied the primer... hopefully that fixes it and I can paint it grey to match... My guess... the cause was the severe weather we had here last winter....

 

[Rambunctious]

I found this;

Oxidation occurs over time as raw metal is exposed to air and moisture in the atmosphere. In the formation of a permanent patina, the metal's surface forms a scale, a combination of hard mineral coatings and corrosion deposits.

Stages of Rust

For some ferrous metals, rust can be a good thing.

www.reliance-foundry.com

 

[1srelluc]

Metal rusts in layers due to the differential aeration, where variations of available oxygen and moisture cause localized corrosion areas. The initial rust layer acts as a barrier, but it is not completely impermeable, allowing the corrosion process to continue and form subsequent layers of rust over time.

On guns I call that scale.

You can see it on the lock of this M1888 Springfield.

On most of the lock it protected the metal but some got through and pitted the surface.

 

[occupied]

The rust partially protects the metal from oxidation for a while until it flakes off then a new round of oxidation begins.

 

[toobfreak]

Why does metal rust in layers instead of just rusting as a whole piece, or rusting straight thru?

It can do all three. There are literally hundreds of alloys of steel, and how it corrodes is largely determined by the alloy both in formulation and quality that determines how the metal catalyzes back into iron due to the opposite electric charges of the metal and the oxygen.

 

[toobfreak]

The initial rust layer acts as a barrier

Special alloys of steel called cortenide actually seal themselves with an outer coating of rust to bar further oxidation.

 

[toobfreak]

I don't know but today I put in new grates in my weber and looked down at the base and noticed the paint was loose... I peeled it right off and underneath it was all rusted and pitted....

Must have had water that collected and laid down there.

I went and got some rust destroyer primer scraped off all the paint and applied the primer... hopefully that fixes it and I can paint it grey to match... My guess... the cause was the severe weather we had here last winter....

Be sure to seal the primer well with heat resistant paint. Those "rust-converting" primers work great but are water soluble!

 

[Kruska]

Why does metal rust in layers instead of just rusting as a whole piece, or rusting straight thru?

I've always wondered why this is.

Metal or steel is not a single solid mass as it optically appears.

Its mass consists of grains (crystallites) and each grain has a randomly created boundary, aka diameter/thickness. Randomly oriented, the grains contact each other at surfaces called grain boundaries. (lamellars). The structure and size of the grains determine the physical properties of the solid metal or steel.

Via environmental surroundings - e.g. humidity and temperature, (chemical reaction), iron oxide is created (rust) and thus converts a metal grain (very slowly) into an iron oxide grain. An iron oxide grain then expands between 7 to 11 times its original thickness. As the oxide grain expands, it forces the surrounding metal grain structure (grain boundaries) apart, thus forming into layers of rust (dead metal) also termed corrosion.

Hope that helps.

 

[ReinyDays]

Depends on the metal ... metallic iron will sit in one piece likely forever ... however it's oxide is granular and falls off the iron piece leaving more iron exposed to oxidation ...

Aluminum isn't so ... her oxide forms a tough skin over the metal, preventing any further oxidation ... fresh cut aluminum is shiny like a mirror, leave it out in the elements for a year and now it's all grey and dull ... that's the protective oxide layer ...

Stainless steel is 10% chromium ... though the iron rusts and falls off, the chromium oxide remains and protects the iron ... this layer is much thinner than a wavelength of visible light, so we can't see the chromium oxide layer ... so the metal looks like steel that won't corrode ...

 

[scruffy]

There's also another answer. Iron is paramagnetic. The molecules will align themselves when exposed to magnetic fields.