Weathering Metals

by Don Friedman on August 9, 2017

Beautiful, right? Thanks to some old alteration work, we get to see the results of a nice little experiment in exposing metals to water.

You’re looking at three beams here: a modern steel beam that had been directly supporting a sidewalk (middle left, outlined in purple below), the original cast-iron girder that supports that beam (upper left to lower right, outlined in green below), and a steel beam lower down – possibly original, possibly new – that supports the cast iron girder (bottom, outlined in red below).

The lower steel beam has only been partially exposed to water. It’s rusted where the cast iron bears on it, almost certainly because the cast-iron girder, which has a U cross-section, was dumping water at the open joint where two of its segments meet at the lower steel beam.

The upper steel beam, as is obvious from the picture, is a mess.

The cast iron girder, which is some 150 years old, has only surface rust.

We – the design and construction community – stopped using cast iron in multiple-story buildings more than 100 years ago because of its flaws: it’s brittle, weak in tension, and of wildly uneven quality depending on the skill of the casting laborers. That said, it is far more resistant to rusting than wrought iron and steel. It rusts in a similar manner to aluminum, where the rust product stays adhered to the base metal. So once the surface rust, the process stops. Wrought iron and steel delaminate, so after the surface rusts it flakes off, exposing a fresh surface to rust, and so on.

It’s too bad cast iron doesn’t meet modern standards for structural safety, because it weathers incredibly well.

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