jojobas said, 'in "deep future" where all metals have corroded away', and was talking about aluminum casting. given that we have lead artifacts from level ix of çatal höyük (about 8000 years ago?) and that aluminum seems to be even longer lasting than lead, they are presumably talking about a 'deep future' of tens or hundreds of thousands of years in the future, if not millions to hundreds of millions. the 158 tonnes of steel you're talking about will almost certainly become 226 tonnes of hematite long before that. (the oldest surviving iron artifacts are only 5400 years old, the gerzeh beads, and those are entirely oxidized despite being meteoric iron and preserved in dry egypt¹.) 200 tonnes of hematite is not a massive deposit; the minas rio mine produces about 23000000 tonnes of iron every year from a single deposit, and it's far from the world's largest, so a 'massive' deposit of iron needs to be billions of tonnes, not hundreds
your hypothetical rusted pipe is an especially rich deposit, and perhaps an especially accessible one, and maybe (if kept away from oxygen, as you point out, or from water) one that doesn't require smelting, but not a massive one
the massive deposits, if human-created rather than things like untouched banded iron formations, will be things like landfills rather than oil pipelines or fleets of 100 tonne+ trucks. the apex regional landfill in las vegas currently contains 50 million tonnes of waste and may be close to a billion before it's full, and larger landfills can be built in the future
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¹ rehren et al. say, 'The ToF-ND testing for grain size and crystal lattice structure of any metallic phases present in the beads found no metallic form of iron in any of the three beads. (...) typical for the corrosion products of iron,
and the absence of metallic iron above the detectable minimum
mass of about 10 mg indicates that the samples are to more than [sic]
99.9% corroded, with virtually no metallic form of iron remaining.
The noticeable magnetism of the beads is probably due to the
presence of magnetite(...)'
your hypothetical rusted pipe is an especially rich deposit, and perhaps an especially accessible one, and maybe (if kept away from oxygen, as you point out, or from water) one that doesn't require smelting, but not a massive one
the massive deposits, if human-created rather than things like untouched banded iron formations, will be things like landfills rather than oil pipelines or fleets of 100 tonne+ trucks. the apex regional landfill in las vegas currently contains 50 million tonnes of waste and may be close to a billion before it's full, and larger landfills can be built in the future
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¹ rehren et al. say, 'The ToF-ND testing for grain size and crystal lattice structure of any metallic phases present in the beads found no metallic form of iron in any of the three beads. (...) typical for the corrosion products of iron, and the absence of metallic iron above the detectable minimum mass of about 10 mg indicates that the samples are to more than [sic] 99.9% corroded, with virtually no metallic form of iron remaining. The noticeable magnetism of the beads is probably due to the presence of magnetite(...)'