"Andy Dingley" wrote in message
...
On Mon, 1 Mar 2004 17:39:53 -0000, "Ed Rear"
wrote:

Why don't manhole covers, drain grids, etc, rust away?

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.


--
Smert' spamionam

Thanks very much for that Andy, I was sure I would find the answer here.
Ed