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I think all cars exhaust pipes get very rusty after not very long. Why is that?

I know that the pipe gets extremely hot, but how come that makes it rust more than any other part of the car?

  • A lot depends on your location - if the roads are salted where you are, that will contribute massively to the speed with which the exhaust will rust. I have a couple of cars that are still on the original exhausts after 18-20 years, but they both ran in a salt-free environment. – Timo Geusch Jan 2 '13 at 0:03
  • My MR2 still has its original exhaust system (22 years old now) and is run year round (I've had holes in the floor replaced 3 times). My (18 year old) Eclipse which is rarely run in the Winter is now on it's third exhaust system (1 stock, then an aftermarket stainless steel, now a custom built stainless steel) due to rusting them out. – Brian Knoblauch Jan 2 '13 at 12:43
  • 1
    This is a very broad statement ... many exhaust systems today are made out of stainless steel ... even those coming out of the factory. Several of my newer GM vehicles I've had used stainless. Will last for years. – Pᴀᴜʟsᴛᴇʀ2 Oct 15 '15 at 1:11
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Various reasons:

  • It is close to the ground - where you have water, salt, grit etc.
  • It is typically not made of rustproof metals, but is considered a consumable, with portions like the catalytic converter being relatively short lived, so spending lots extra to protect it would be pointless.
  • The exhaust gasses include corrosive gases such as sulphur dioxide, which becomes an acid when mixed with the water in the exhaust

That said, most of my exhaust (the portion after the catalytic converter) is stainless steel, so it doesn't rust and looks pretty nice when I polish it. It cost a bit though.

4

You will see people quote,"Driven daily" when selling a car.That's a major issue if a car also sits on grass,has low miles for the year and the stated acids/water sit usually at the joints and rots them.Recently after a short drive I tried to use JB weld and the amount of water seeping from the flange made it impossible to stick.A mechanic told me years ago if you're going to the store take the long route as short drives are killers of exhausts, stainless or not.

  • Welcome to mechanics.stackexchange.com! Do you mind clarifying this a bit? Are you saying that a reason for exhaust pipes rusting out is just taking short drives? Sounds like a plausible answer, but would you mind making the answer a bit more precise? – anonymous2 Feb 3 '17 at 18:13
  • @anonymous2 I believe he's saying that cars are driven infrequently, or stored for long periods, tend to develop rust, not that taking short drives. My dad bought a mid-90s Z28 about two years ago that had under 5000 miles on it. Every rubber seal was brittle, leaked like a sieve, not a lot of rust (but we're in Texas). – 3Dave Feb 3 '17 at 20:13
  • Anonymous2 - yes, short drives kill exhausts much faster than long drives - as the cat doesn't get a chance to heat up properly, so you end up with acid sitting in the exhaust. On longer journeys everything heats up so no liquids remain in the exhaust, as Shobin said. – Rory Alsop Feb 28 '17 at 23:03
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As Rory states the primary reason for the exhaust corrosion are that the exhaust gases react with the cold silencer when starting the engine and condense to form acids which corrode metal.

I would not say that salted roads are the only reason since in my country they dont salt the roads.

Another variable depends on the manufacturer's choice of using the material to make the exhaust manifold and silencer. Some vehicles have higher quality alloys which do not corrode.

Would I or should I be worried? Heck NO! Unless you are going to use this car for track day and racing purpose the rust on the exhaust should definitely not cause an issue and should last the lifetime of the car.

However , in case of an accident the rusted exhaust might give way easily since its already weakened.

You can combat the rust by using solutions such as using vinegar and sandpaper to remove rust and applying a coat of WD 40

3

Because it's not painted – it gets too hot for paint.

Other surfaces that can rust are usually covered in paint, chrome or other coating (even the car's underside) to seal the iron from the corrosive environment of this planet – rust will also develop if you scratch the paint:

scratched paint with rust

That said, stainless steel doesn't need paint. The DeLorean DMC-12 had unpainted stainless steel body panels:

enter image description here

As to why stainless steel exhaust pipes don't seem to make economical sense, I don't know, but there are aftermarket parts.

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Stainless pipes will last the life of the car however the junctions, especially the lowest one on the car, will rot anyway because corrosives collect there and moisture lingers. If you combine excessive movement with this you get a possible failed junction so it is very important to replace those inexpensive rubber hangars ( OE please!).

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