Are there any metallurgists out there who can tell me how long it would take for steel to rust away, leaving nothing but dust in its place?

(Clearly different environments would take a shorter or longer time, but that should be part of the discussion...)

Quote from: VellorianAre there any metallurgists out there who can tell me how long it would take for steel to rust away, leaving nothing but dust in its place?

(Clearly different environments would take a shorter or longer time, but that should be part of the discussion...)

That is such a huge and open-ended question. Besides environmental factors there is the tempering and alloy mixture angle (stainless steel would last lifetimes in dry, cool environment) along with thickness. Because rusting only occurs at the oxygen/steel interface.

EDIT: Incidentally that is how stainless steel works. The chromium content when it oxidizes forms a thin, effectively invisible to the naked eye layer that keeps the oxygen away from the rest of the metal.  Same reason aluminum doesn't usually noticable rust.

P.S.  It wouldn't be 'dust'. It would be large, rust flakes.

Notably, you can find regularly in acheological articles photographs of thousand year old swords. They look like utter shit, but there they are.  The answer then, in as generic a terms as you postulated...

A long damn time.

If I'm not mistaken, there are iron pillars in India that have been dated to the third or fourth century BC. That's quite some time and it's not even stainless steel. I think in the right conditions, a few millennia should be possible.
(The lower end is probably a few years. Metal tools lying in the earth are gone pretty fast)

I'm not a metallurgist (nor even a mineralogist), but the responses above are all getting at the same answer -- it depends on the circumstances.

First, there's the matter of mass-to-surface-area ratio.  A piece of steel sheet that weighs 1 kg will waste away much faster than a steel sphere of the same weight.  The factor here is that far more molecular surface is exposed to the atmosphere with the sheet than with the sphere, where most of the molecular structure is safely hidden away within the mass of the body.

Next, there's the matter of the atmospheric content.  Some atmospheres are more corrosive than others.  Oxygen alone will slowly do a number on steel, but acids (like water, which acts as a mild acid) will step up the corrosive process much faster.  Some naturally-occurring chemicals in soil will increase the rate of corrosion, as will certain polutant compounds (like acid rain, which combines sulphur dioxide with water to make dilute sulphuric acid).

Last, there's the matter of corrosion layers creating a non-reactive surface on the body of steel.  In short, the first few layers of material oxidise, which effectively creates a seal around the inner body, preventing the atmosphere from reacting with any of the metal within.  Going back to our steel sheet and sphere above, the sheet will probably corrode through before developing a non-reactive layer (depending on how thick our sheet is, of course), while the sphere has enough solid mass to develop the protective layer.  This is also the concept behind galvanisation, where steel is coated with a layer of zinc -- the layer of zinc oxidises, creating a non-reactive layer of zinc oxide that protects the steel underneath.

So, in other words, there's not really a simple answer for you.  Once you've concentrated a large mass of steel in one lump, like a construction girder, it's likely going to be around for a long, long time.

!i!

in re the iron pillar of Delhi, from wikipedia:

"Metallurgists at Kanpur IIT have claimed that a thin layer of "misawite", a compound of iron, oxygen, and hydrogen, has protected the cast iron pillar from rust. According to them, the protective film took form within three years after erection of the pillar and has been growing ever so slowly since then. This information was wrongly flashed by the news media [4] based on an article that appeared in Current Science (see further). In the original article in Current Science, it has been mentioned that after 1,600 years, the film has grown just one-twentieth of a millimetre thick, according to R. Balasubramaniam of the IIT. In a report published in the journal Current Science[5]], Balasubramaniam suggests that the protective film was formed catalytically by the presence of high amounts of phosphorus in the iron — this phosphorus is as much as one per cent against less than 0.05 per cent in today's iron. There are three stages identified in the protective passive film formation, as outlined in detail in the detailed article on the corrosion resistance of the Delhi Iron Pillar in Corrosion Science, one of the reputed journals in corrosion science and engineering [6] The high phosphorus content would be a result of the iron-making process practiced by ancient Indians, who reduced iron ore into iron with low carbon content by solid state reduction by employing charcoal as the reducing agent. Modern blast furnaces, on the other hand, use limestone in place of charcoal, yielding molten slag and pig iron that is later converted into steel. In the modern process most phosphorus is carried away by the slag. Since lime was not used in the ancient furnaces, a higher amount of phosphorus remains in the material [7]. Balasubramaniam's theories have been widely published in several reputed international journals and available in the literature for review and critical analysis [8]."

Additionally, note that it is not steel, but virtually pure iron, the surface is quickly heated dry by the sun after rain, retained solar heat keeps it clear of dew at night, and it is not in direct contact with the ground. It illustrates several of the points Ian notes above. Under these very specific conditions, therefore, iron can last millenia. On the other hand, a mild steel bolt can corrode to shapelessness after a year or two in salt spray. This is an extreme example of YMMV.

-clash

The funny thing about mice's post was I had read just that wiki article not more than an hour prior... deja vu...:eek:

Really, it depends more on the conditions than anything else. even being immersed in salt water can have a lot of conditions that can alter the rate of decay.

During ww2 a lot of steel ships were sunk in warm waters and are really disintegrating rapidly, whereas the titanic sank in very cold waters and is corroding away much more slowly.

Salt, oxygen, temperature, biological activity and a dozen other things can affect the durability of steel. In the desert a piece of steel could last millennia, in the rainforest it could rust to nothing in a few years.

Quote from: Dominus NoxSalt, oxygen, temperature, biological activity and a dozen other things can affect the durability of steel. In the desert a piece of steel could last millennia, in the rainforest it could rust to nothing in a few years.

Quoted and bolded for emphasis.  I neglected the combined effect of temperature and chemical environment.

!i!

Composition also plays a role. Allotropic iron is very corrosion resistant. Mild steel corrodes if a molecule of water passes within a mile of it.

Then you have changes in local condition. A steel beam could last a long time even in damp ground, so long as conditions stay stable. But if the ground alternates between damp and dry, then the steel will degrade quickly.