Are stainless exhausts rustproof?
Discussion
I'd always assumed that stainless steel exhausts have three advantages:
1. Lighter
2. Improved performance / better sound
3. Rustproof
Am I wrong on the third point? I'm seriously considering getting one for my S1, so I did a search on this forum and found a couple of reports of them rusting.
If so, what kind of a life expectancy do they have? I was hoping that the extra cost for stainless would be partly offset by a longer life than my standard exhausts which I seem to be replacing every 2 to 3 years.
1. Lighter
2. Improved performance / better sound
3. Rustproof
Am I wrong on the third point? I'm seriously considering getting one for my S1, so I did a search on this forum and found a couple of reports of them rusting.
If so, what kind of a life expectancy do they have? I was hoping that the extra cost for stainless would be partly offset by a longer life than my standard exhausts which I seem to be replacing every 2 to 3 years.
The way I understand it (and I could be wrong) is that it's the lack of chromium added to the steel that still allows it to rust. You need a minimum of about 15% chromium added to stop the steel from staining, so basically cheap Chinese stainless products are made with a reduced amount of that element. This goes for not just exhausts but most stainless products we buy in the UK these days.
A good manufacturer will use a good quality metal and therefore you won't have that problem.
As always it boils down to you get what you pay for!!
A good manufacturer will use a good quality metal and therefore you won't have that problem.
As always it boils down to you get what you pay for!!
Edited by ROB1979 on Friday 8th May 15:08
You're actually kind of wrong on all 3, aftermarket exhausts aren't really lighter or better sounding because they're made of stainless (which generally isn't particularly strong), it's because they are designed to be that way. Stainless is 'more' rust resistant than many other steels but this varies considerably with alloy spec as the last poster states, and other things like the welding and post-weld heat treatment can have a massive effect as well so it's impossible to generalise. Titanium is lighter, more resistant but costs more.
One of the byproducts of the combustion process is water, as well as nitrogen and sulfur compounds. A carbon steel exhaust will rust in the air, whereas a stainless exhaust will not (depending on its composition, as said above), so the main reason for them is that they look nicer and last longer, without really giving any performance benefits (due to the material of construction).
I am surprised you only get 3 years out of an exhaust. Do you do short runs in your car? (which worsens the corrosion problem as the exhaust does not get to reach full temperature, meaning condensed water remains in the exhaust, rusting it through from the inside. The formation of acids as a result of the reaction between the water and the nitrogen & sulphur compounds worsens the problem)
Mike…
I am surprised you only get 3 years out of an exhaust. Do you do short runs in your car? (which worsens the corrosion problem as the exhaust does not get to reach full temperature, meaning condensed water remains in the exhaust, rusting it through from the inside. The formation of acids as a result of the reaction between the water and the nitrogen & sulphur compounds worsens the problem)
Mike…
Edited by mike_knott on Friday 8th May 16:18
Depends what you are calling stainless...
there are *many* grades of stainless steel,
see http://en.wikipedia.org/wiki/Stainless_steel
OEM stuff will have light surface rust, but will take an age to actually rust though, most aftermarket stuff is 304(L) and 316(L), these will discolour, but not actually rust as such.
there are *many* grades of stainless steel,
see http://en.wikipedia.org/wiki/Stainless_steel
OEM stuff will have light surface rust, but will take an age to actually rust though, most aftermarket stuff is 304(L) and 316(L), these will discolour, but not actually rust as such.
stainless can rust very easily if not produced properly or a rubbish material spec is used. Get at least a 316 or even 321 grade. 309 is at the lower end and will not last as long. Fabrication is key too. If a shop makes it and it's not quarantined from carbon steel contamination (especially grinding sparks), the carbon steel will spot corrode the stainless. Good shops will segregate, then even passivate (a pickling type acid does this) - not only gets rid of the carbon, but shines it up really well
The internationally agreed lower limit for Chromium in "Stainless Steel" is 11.5%. The cheap grade of Stainless used in auto exhausts - Type409 has 12%. It is magnetic and it will deteriorate over 5-7 years. The better Stainless systems are made of Type304, a so-called 18/8 material. It is non-magnetic (generally)and has 18% Chromium, (which is very brittle), along with 8-10% Nickel - added to soften things up a little - making bending/fabrication work easier. This will outlast *any* car it is fitted to - incl Ali and composite. It's big of some exhaust manufacturers to offer a 25 year corrosion guarantee on their products.
Type 304L (L for LOW Carbon) is not commonly used for auto exhausts. Type 321 was developed during WW2 to stop piston-engined aircraft exhaust manifolds from cracking. It's not readily available in the UK. There is no such material as "Aircraft Quality" Stainless Steel. If I thought there was any auto-exhaust, HF-welded Stainless used in the construction of the aircraft I was booked on, I wouldn't even check in!
The beauty of Type304 is you can *always* bring it back to a new, polished finish (unlike Type409)if you have the time/patience. Heat-staining will always appear on an exhaust. It *is* oxidisation but it's only a surface condition. Stain Less Steel? - Possibly.
Type 304L (L for LOW Carbon) is not commonly used for auto exhausts. Type 321 was developed during WW2 to stop piston-engined aircraft exhaust manifolds from cracking. It's not readily available in the UK. There is no such material as "Aircraft Quality" Stainless Steel. If I thought there was any auto-exhaust, HF-welded Stainless used in the construction of the aircraft I was booked on, I wouldn't even check in!
The beauty of Type304 is you can *always* bring it back to a new, polished finish (unlike Type409)if you have the time/patience. Heat-staining will always appear on an exhaust. It *is* oxidisation but it's only a surface condition. Stain Less Steel? - Possibly.
Many thanks for all the helpful replies.
The Elise is my daily driver, so it does do a lot of short runs which probably explains the limited life of my standard exhausts.
Think I'll do some proper research into the grade of stainless steel used and the length of the warranty offered before I take the plunge.
The Elise is my daily driver, so it does do a lot of short runs which probably explains the limited life of my standard exhausts.
Think I'll do some proper research into the grade of stainless steel used and the length of the warranty offered before I take the plunge.
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