Non-stainless steels and rusting

[StringJunky]

This regards non-ss steels. Why do some steels only rust very superficially with a thin layer and others rust in a fluffy manner and rust right through? Is it the manner in which they are made/moulded or the composition or both? 

[tinkerer]

On 10/27/2018 at 8:56 AM, StringJunky said:

This regards non-ss steels. Why do some steels only rust very superficially with a thin layer and others rust in a fluffy manner and rust right through? Is it the manner in which they are made/moulded or the composition or both? 

Both, to answer simply. Most affecting corrosion, given constant conditions of surrounding atmosphere or materials, is actually the hardness of the steel. Heat treatment of alloy steels not only makes them physically stronger, but also less amenable to corrosion, as heat treatment hardens them significantly.   imp

[Akmose]

On 10/27/2018 at 8:56 AM, StringJunky said:

This regards non-ss steels. Why do some steels only rust very superficially with a thin layer and others rust in a fluffy manner and rust right through? Is it the manner in which they are made/moulded or the composition or both? 

Off the top of my head I would say grain structure. Could also be level and types of impuritys.

Do you have some examples? Is it tool steel vs a car panel? 

[studiot]

On 10/27/2018 at 4:56 PM, StringJunky said:

This regards non-ss steels. Why do some steels only rust very superficially with a thin layer and others rust in a fluffy manner and rust right through? Is it the manner in which they are made/moulded or the composition or both?

First you need to understand that there are two oxides of iron.

The red oxide we call rust and the black oxide.

These have different physical properties concerning porosity, adhesion to parent material and friability.

The black oxide is nearly non porous, adheres well to the parent material and quite stron.
So it forms a somewhat protective layer.
It is one of the reason cast iron weathers so well.

On the other hand, the red oxide is porous, poorly adhesive and friable.
So it does not form a protective layer.

It is possible to add alloying elements to steel to enhance the production of the black oxide and even add some oxides of other metals to boot.
This produces the dark reddish/blackish 'weathering steel - called 'corten' in the UK.

I once did a study of two viaducts built of corten steel, one near the coast and one inland.
The protective barrier functions well on the inland one, but the salty air of the coastal one causes the barrier to continually crumble away, depositing a dust beneath the viaduct.

[StringJunky]

24 minutes ago, studiot said:

First you need to understand that there are two oxides of iron.

The red oxide we call rust and the black oxide.

These have different physical properties concerning porosity, adhesion to parent material and friability.

The black oxide is nearly non porous, adheres well to the parent material and quite stron.
So it forms a somewhat protective layer.
It is one of the reason cast iron weathers so well.

On the other hand, the red oxide is porous, poorly adhesive and friable.
So it does not form a protective layer.

It is possible to add alloying elements to steel to enhance the production of the black oxide and even add some oxides of other metals to boot.
This produces the dark reddish/blackish 'weathering steel - called 'corten' in the UK.

I once did a study of two viaducts built of corten steel, one near the coast and one inland.
The protective barrier functions well on the inland one, but the salty air of the coastal one causes the barrier to continually crumble away, depositing a dust beneath the viaduct.

Great stuff mate. Cheers. Red oxide makes a good polish in jewellers rouge,  so. it's useful for that.   Is there any process or stuff you can add as the end user to induce that black oxide layer on the surface in non-cast steel? Shall look up Corten.

[studiot]

Only nitriding and similar.

But they still eventually rust.

https://www.google.co.uk/search?ei=AAmBXIrEIpOk1fAPzKGLWA&q=nitriding&oq=nitriding&gs_l=psy-ab.3..0i131i67j0j0i67j0l7.59530.61254..61596...0.0..0.90.662.9......0....1..gws-wiz.......0i71j0i131j0i10.y5K3bkBNV7c

https://www.google.co.uk/search?ei=lQSBXLL1O4LfxgPp8p-YBQ&q=black+finish+on+tool+steel&oq=black+finish+on+tool+steel&gs_l=psy-ab.3..33i22i29i30l3.125408.130712..130986...0.0..0.116.2022.23j3......0....1..gws-wiz.......0i71j0i131j0j0i67j0i10j0i22i30j33i160j0i13i30j0i8i13i30.u1NhXuzWpXA

 

 

30 minutes ago, StringJunky said:

 Is there any process or stuff you can add as the end user to induce that black oxide layer on the surface in non-cast steel?

Only nitriding and similar.

But they still eventually rust.

https://www.google.co.uk/search?ei=AAmBXIrEIpOk1fAPzKGLWA&q=nitriding&oq=nitriding&gs_l=psy-ab.3..0i131i67j0j0i67j0l7.59530.61254..61596...0.0..0.90.662.9......0....1..gws-wiz.......0i71j0i131j0i10.y5K3bkBNV7c

https://www.google.co.uk/search?ei=lQSBXLL1O4LfxgPp8p-YBQ&q=black+finish+on+tool+steel&oq=black+finish+on+tool+steel&gs_l=psy-ab.3..33i22i29i30l3.125408.130712..130986...0.0..0.116.2022.23j3......0....1..gws-wiz.......0i71j0i131j0j0i67j0i10j0i22i30j33i160j0i13i30j0i8i13i30.u1NhXuzWpXA

[StringJunky]

5 hours ago, studiot said:

Only nitriding and similar.

But they still eventually rust.

https://www.google.co.uk/search?ei=AAmBXIrEIpOk1fAPzKGLWA&q=nitriding&oq=nitriding&gs_l=psy-ab.3..0i131i67j0j0i67j0l7.59530.61254..61596...0.0..0.90.662.9......0....1..gws-wiz.......0i71j0i131j0i10.y5K3bkBNV7c

https://www.google.co.uk/search?ei=lQSBXLL1O4LfxgPp8p-YBQ&q=black+finish+on+tool+steel&oq=black+finish+on+tool+steel&gs_l=psy-ab.3..33i22i29i30l3.125408.130712..130986...0.0..0.116.2022.23j3......0....1..gws-wiz.......0i71j0i131j0j0i67j0i10j0i22i30j33i160j0i13i30j0i8i13i30.u1NhXuzWpXA

 

 

Only nitriding and similar.

But they still eventually rust.

https://www.google.co.uk/search?ei=AAmBXIrEIpOk1fAPzKGLWA&q=nitriding&oq=nitriding&gs_l=psy-ab.3..0i131i67j0j0i67j0l7.59530.61254..61596...0.0..0.90.662.9......0....1..gws-wiz.......0i71j0i131j0i10.y5K3bkBNV7c

https://www.google.co.uk/search?ei=lQSBXLL1O4LfxgPp8p-YBQ&q=black+finish+on+tool+steel&oq=black+finish+on+tool+steel&gs_l=psy-ab.3..33i22i29i30l3.125408.130712..130986...0.0..0.116.2022.23j3......0....1..gws-wiz.......0i71j0i131j0j0i67j0i10j0i22i30j33i160j0i13i30j0i8i13i30.u1NhXuzWpXA

Not a DIY-friendly process, nitriding.

Edited March 7, 2019 by StringJunky