JIM5.0

That might be my problem. I do wash my car, well I just rinsed it over the cold months, but I never rinsed the underside.

Despite there not having snow in Texas, they do salt the bridges in anticipation to the occasional freezing snaps

4LiterSonic

The steel rusts to form a sort of sheath around the pipe itself to keep it from rusting through. Cheaper than T409 and you never see it unless you're under the car, so it doesn't matter how it looks.

JIM5.0

This is correct. And it is actually an engineering industry standard practice. The outer layer of oxidization protects the inner material, and just like you said, it functions as a sheath.

A prime example of the outer layer being allowed to oxidize to protect the rest of the metal under neath is the Statue of Liberty. The outer layer of copper is allowed to oxidize thus the green color of Lady Liberty.

Also, if you look at steel girder bridges, some of them are a reddish brown. No need to paint the girders, the rust prevents the steel underneath from rusting all the way through.

HT

So when you see things that are rusted through, does that mean they've just been that way for too long, or is it a thinner metal? I've never thought about this before...pretty interesting concept.

4LiterSonic

Likely because they haven't been taken care of and been out in the elements too long.