Andy Dingley wrote:
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
snip
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

To add to this, there is also steel designed for exposure.
Google for Corten.

Not stainless, just composed to rust very slowly.