Longfellow Bridge guardrail perforation

Hundred-year-old Longfellow Bridge handrails are rusted through, perforated, where snow (pH 5.5 frozen acid rain) accumulates and melts. Used to teach atmospheric corrosion rate (~4 mil/year) and the square-root-of-time scaling.

Rust, which forms at lower temperatures in the air with the moisture around, is Fe2O3 dot a bunch of H2Os — that's hematite. There's also wüstite at higher temperatures. Magnetite has a volume ratio and coefficient of thermal expansion such that when it forms on steel, it will adhere. Rust on the other hand, when it forms, can have a four to ten times volume change depending on how many H2Os are in it and how much chlorine might be around, and it will flake off. Typical atmospheric corrosion rate of steel is about four thousandths of an inch per year. So in forty years you lose an eighth of an inch from both surfaces. If it's a big heavy steel beam — why are they going to rebuild the Longfellow Bridge? It's rusty, but it's been there for a hundred years.