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My largest car maintenance cost is exhaust pipes, mostly catalytic converters. I do not think this is the case for most people. What am I doing wrong?

I tend to buy cheap cars (<£500), change the oil myself, get the brakes changed when they squeak and change the car if there are any repairs that cost more than the car. I do not drive slowly, such that I am not surprised that I get through tires and brakes a little quicker than other people. However by far my largest maintenance cost is exhaust pipes (mainly catalytic converters).

It is always the connecting pipes that corrode and/or break, rather than the actual catalytic element. I do not see an obvious link between driving style and exhaust pipe corrosion, and I am not aware of any preventative maintenance that could affect the exhaust pipe. On one occasion I went through three in the course of 4 years on the same car, and in two of those cases there was very little corrosion, the pipe sheared above the catalytic converter. I live in the UK, but not near the sea.

Is there something I can do to make my exhaust pipes last longer?

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    Welcome to Motor Vehicle Maintenance & Repair! In a nutshell, the easiest way to get the exhaust done once and not worry about it again is to replace the exhaust with something which doesn't rust ... namely stainless steel. You'll pay a little bit more for it up front, but you won't have to worry about it rusting again. Nov 30, 2021 at 11:59
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    @Paulster2! That comment looks suspiciously like an answer.
    – psaxton
    Dec 1, 2021 at 3:44
  • Are you within a mile of coastal or tidal waters? Do they salt the roads there? Dec 1, 2021 at 23:35
  • Sounds like each time you replace an exhaust, you're doing on the budget side of things and buying a mild steel exhaust system. Next time spend a few quid more on a full stainless system. Dec 2, 2021 at 7:05
  • If you're seeing breakage with little corrosion, I'd be surprised if stainless would be much help. Stainless is (usually) a little more brittle than mild steel so if something is moving excessively it's more likely to fail
    – Chris H
    Dec 2, 2021 at 9:29

7 Answers 7

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Another cause of rapid exhaust system rusting is driving patterns which don't warm up the vehicle thoroughly. In this case, a "thorough" warmup means the entire exhaust is hot enough to cause all moisture within to evaporate. Depending upon the vehicle, this might take 10 miles of driving, the sort of distance necessary to warm the oil (not the coolant) to a steady temperature. If the vehicle is driven a lesser amount, moisture will remain in the system, and its presence will accelerate corrosion.

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3 in 4 years? That sounds like the engine mounts were loose or broken and putting undue strain on the pipe.

Another possibility is hitting the exhaust on sleeping policemen - are the roads in good condition - lots of potholes etc can also cause flexing damage.

Making sure all the mounting points and rubbers are in good condition is another thing to check.

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  • In answer to your question, I do not think it is sleeping policemen, I do not have any on my usual routes and always go very slowly over them. There are quite a few potholes around, but not exceptionally nationally and that is the same for other people around here.
    – User65535
    Nov 30, 2021 at 12:43
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    For any non-Brits who may be scratching their heads, a "sleeping policeman" is a speed bump.
    – J...
    Dec 2, 2021 at 21:08
  • I strongly prefer speaking proper English - the kind we speak here in the USA. But I gotta admit: The dialect you speak over there has some awfully amusing traffic-related idioms. "Sleeping policeman" indeed! "Lollipop lady" is another one.
    – davidbak
    Dec 3, 2021 at 18:32
  • @davidbak and the easy spelling you need: “gotta”…
    – Solar Mike
    Dec 3, 2021 at 18:50
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For more than 25 years most auto exhaust systems have been 13 chrome stainless. I have never seen one corrode significantly; I have driven at lease 3 vehicles to about 130,000. So unless you are using carbon steel exhaust or leaded gasoline it is a very unusual problem. Leaded gasoline had bromine added to carry lead out of the engine, it made acid which ate through exhaust systems in a year, regardless of mileage. Little pipe corrosion and " pipe sheared" is a welding problem (not corrosion). The converter shells are 13 chrome ; if welded to carbon steel, it is difficult to avoid a hard brittle weld that can fracture. The 13 chrome used for exhaust is ferritic; that means it is low carbon and will not harden. A carbon steel pipe has carbon ( no surprise) but with no alloy like chrome so it will not harden when cooled. When ferritic 13 chrome is welded to carbon steel , there is combination of carbon and chrome in the melted weld metal. The weld metal will harden when it cools . You can select filler metals to try to avoid this but by far the best solution is to use ferritic stainless pipes when welding to converters. Also , filler metal can cause brittle problem when both components are ferritic stainless. The standard recommendation is 430 ferritic filler; but because of your problems I would use a nickel filler, Inco A. It is expensive but you don't need much.

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    I don't know where you live, but in the UK most exhausts certainly are not stainless. It's possible to buy stainless exhausts, sure, but in 30 years of driving I've never owned a car with one.
    – Graham
    Dec 2, 2021 at 9:10
  • So left hand drive (US) come with stainless and right hand drive (UK) come with carbon steel ? Dec 2, 2021 at 16:32
  • US-delivered left-hand drive cars don't have stainless steel exhausts either. Exhaust systems are usually made from the cheapest junk metal available - this is why places such as Midas can offer exhaust component replacement for as long as you (the purchaser of the exhaust system) own the car - because they're selling you crap at a premium price and betting you'll get rid of the car before their junk rusts out. Dec 2, 2021 at 18:32
  • If you want to know if you have stainless , swab the pipe with concentrated nitric acid; stainless has no affect, carbon steel gets dark. Dec 3, 2021 at 15:44
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I replaced the same section of rusted pipe over and over again (two feet behind the catalytic converter), whereas the rest of the system, including the mufflers, lasted for decades. I found it was due to water collecting there; I could understand/guess that the water puddled there due to the angle of my pipes. Also, my daily commute was under 15 minutes in an area that sees all kind of weather, so they didn't get as warmed up as I would have liked.

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Welds on steel exhausts are not protected

Exhausts are normally constructed from galvanised steel. Welding breaks through the galvanized protective layer though, so every weld in an exhaust is a corrosion point. It would be possible to galvanise the pipe on the outside after welding, and exhausts fitted by manufacturers usually are, so that the car can complete a typical 5-year warranty period without failure. The welds are still less well protected though, so this is still normally where they fail.

Replacement exhausts will not have this done though. It costs money, so if you buy a cheap exhaust then you get a cheap exhaust. The result is near-immediate corrosion around the welds. You might get 10 years from the original exhaust on a car. You'd be very lucky to get 4-5 years on a replacement.

The alternative of course is to go for stainless steel. This doesn't corrode in the same way, so it will probably outlive the car. Mainly this is useful for "weekend" sports cars which have low mileage but are expected to stick around for many years, but it's occasionally fitted to more "prestige" car brands in manufacture, simply because the cost of the car and their reputation justifies it.

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The ultimate fix for failures caused by corrosion, is a stainless steel exhaust system.

As for causes:

  1. Driving on salty roads (near the sea? Or harsh winters?)
  2. Many short journeys (exhaust doesn't get hot enough, water and exhaust acids don't get expelled but sit around in the exhaust rotting it from the inside out)
  3. High sulphur fuel (creates more acids when burned)
  4. The exhaust is getting scraped on the road (a car built for flat smooth roads being used on rural ones). Its not getting scraped hard enough to rip it off the car, but it breaks once there is some corrosion present.

When I moved to a rural location, I lost two exhausts to 4. in a year (ripped right off the car). The answer was to trade the car in for one with bigger wheels.

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  • Rural location? poor driving as I live in a rural location and never lost an exhaust yet...
    – Solar Mike
    Dec 3, 2021 at 12:44
  • Try driving a sporty Seat Leon (designed for AutoStrada and Autobahns) down the terrible single-track road North out of my village. It has a bump in the middle with grass growing on it, because tractors and 4WDs have caused subsidence on both sides. Its in absolutely terrible potholed condition, but one mile on it saves 8 miles if one is heading North (the better roads head West and South-East). A Vauxhall Astra has no problems with it. Bigger wheels, more ground clearance. You'd like to drive with one set of wheels on top of the hump but the road isn't wide enough to do that!
    – nigel222
    Dec 3, 2021 at 13:01
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It's caused by the combustion products.

When hydrocarbons combust, they produce two main combustion products: carbon dioxide and water (plus some pollutants that are being removed by the catalytic converter). Carbon dioxide, as we know, causes climate change. Water on the other hand causes rust in your exhaust pipe.

There is no possibility to prevent the exhaust pipe from rusting apart from a stainless steel exhaust pipe and/or driving patterns that warm up the entire exhaust pipe above the boiling point of water. Water will be in huge amounts in the exhaust, and if the exhaust pipe isn't boiling the water away, it will condense and cause rust.

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    So, why have I still got the original exhaust on my car after 15 years? Something does not add up here if the combustion products are so corrosive...
    – Solar Mike
    Nov 30, 2021 at 17:47
  • @SolarMike Is your car periodically warmed up completely? That could ameliorate the results I described, which, for the record, occurred in the early 1980s. Nov 30, 2021 at 23:14
  • Fresh water doesn't cause corrosion. You need salt for that.
    – d-b
    Nov 30, 2021 at 23:36
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    There's never fresh water in an exhaust system. It's always contaminated with combustion byproducts and materials from the chamber and exhaust. Even gasket material contributes. Nitrogen oxides are always present to some extent, since air is mostly nitrogen. These dissolve in water, making it acidic. Sulfur is rarer these days but still present to some extent, and there are many other sources of impurities, from gas additives to metals from the combustion chamber, oil, and of course whatever happens to be in the air that you're driving through.
    – barbecue
    Dec 1, 2021 at 17:10
  • This answer isn't complete but it's not wrong. Combustion products play a significant role in exhaust corrosion, always have.
    – barbecue
    Dec 1, 2021 at 17:13

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