[ELI5] High rises steel rust under rain during construction. Will there be any structural issues?

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I often see high rises during construction sitting in rain and storms for months especially in places like New York. You can often see rust forming. Aren’t those rust going to cause structural issues?

In: Engineering

Not a structural engineer or materials scientist by any means, but from what I know, that is simply surface level rust that doesn’t affect the structural integrity of the steel beams.

The rust you see is just surface rust, similar to the underside of most vehicles. Structural issues would only develop if water was sitting on it constantly with no way to escape.

To start, rust is bad because it has a different density from iron, expanding and causing cracks that furthers the corrosion by exposing new surface.

Second, many metals form protective oxide layers, like aluminum, which is harder and prevents any further corrosion.

Steel is an alloy of iron, and some versions handle corrosion better than other steel alloys. To most structural steels, some surface rust patina is considered normal and mostly an issue with welding and finishing. Structural rust fractures should only happen if corrosive conditions persist in some spot for an extended time, which could be due to pooling water or galvanic reactions with other metals.

https://en.wikipedia.org/wiki/Weathering_steel an example of steel meant to form a rust patina.