Sorry about this being off topic, but I'm sure someone here will come up with the answer. Why don't manhole covers, drain grids, etc, rust away? Many of these have been out in all weather for 50 years or more. I have asked a couple of friends with engineering backgrounds, but they couldn't come up with the reason. Ed.
Steel plate ones do. The old cast iron ones do, but they're so thick that there's plenty of material to last a long time. Cast iron manhole covers in roadways more than likely acquire a coating of oil which will protect them to a large extent as it is polished in to the surface by tyre action.
Because they're cast iron not steel.
Some metals (like copper, lead or zinc) are fairly inert. They just aren't going to "rust", whatever happens. They usually form a visible surface film of some sort, but even the pure metal is pretty unreactive.
Others, like aluminium, titanium and the chromium in stainless steel are "reactive" metals. They react so fast that they form an oxide layer just on exposure to the air (or maybe with a bit of deliberate help, a process called "passivating"). This oxide layer is then itself inert and also seals the underlying metal from further reaction.
Steel may also form an oxide layer, albeit slowly, but crucially it's still permeable. The steel continues to rust away underneath its rust layer. Eventually you'll get a hole.
Steel is a mixture / compound of iron and carbon. Whether it's a compound or a mixture is another question that's far too complex to go into here. But it's a very close combination of the two, and the resultant alloy is pretty homogeneous. Cast iron has a different crystalline structure to steel. Rather than the carbon being combined _within_ the grains, it tends to sit as separate particles _between_ the iron grains, usualy along with some silica and other slaggy materials. When the iron rusts, it rusts at the surface and consumes the available pure iron, leaving behind a surface layer that's now relatively high in the slag. This layer is relatively impermeable and doesn't rust at all quickly.
Cast iron still rusts, but it does so very, very slowly. If you agitate the surface, then you can scrub this resistant layer away and you can have fast-rusting cast iron. For street furniture though, the alloy (and its treatment) are chosen so that this layer is robust and is buffed smooth by traffic, not scraped off.
Wrought iron is almost unheard of these days, but like cast iron it carries its non-iron parts separated out into slag grains, and again it's resistant to serious rusting. If you find an old anchor or chain of it, you'll see a pronounced "wood grain" effect developing after some decades.
To add to this, there is also steel designed for exposure. Google for Corten.
Not stainless, just composed to rust very slowly.
[snip]
That was a great explanation. Thank you.
Now a question on why it is that we don't see Wrought Iron these days? I will hazard a guess something to do with cost, but it would be good to know.
Thanks Rob
Thanks very much for that Andy, I was sure I would find the answer here. Ed
Cripes.
J.B.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.