Mek-a-nik wrote:Chris wrote:Ever wonder why the exhaust system tends rot out AFTER after the cat ?
I'm a little bit behind in my reading, bringing this up again, and maybe a little off track. Just wanted to say that I think they rot out after the cat because the material is heavier (thicker) in the front of the system and the moisture inside the pipe(s) doesn't evaporate as well further back because the system is cooler towards the rear.
There was something that one of my mentors told me about back when i was first starting out. He was a pretty smart guy and had an understanding of not just the mechanics of things but the science & chemistry behind it. We were talking about catalytic converters one day. Back then, in the early eighties, they were kind of a new thing. He didn't have a very high opinion of them because he felt that it was just swapping one environmental problem for another.
He asked if i had ever noticed that sharp acrid smell coming from the exhaust. I said yes, and he went on to explain that all fuel has some degree of sulphur in it. An unintended consequense of the catalytic process was that it combined this sulphur with oxygen to form sulphur dioxide. Combine that with water vapor and you get a weak sulphuric acid.
Years later the whole "acid rain" thing came to light.
Most recently, they've been redesigning the diesel truck engines to run on
ultra low sulphur diesel fuel and adding catalytic converters for that reason. If the sulphur isn't in the fuel in the first place then sulphuric acid pumping out of the tailpipe won't be a problem.
That was the long answer.
Short answer ...... you're right about the water vapor condensing but it's got a little something extra in it that makes it corrosive.