Hamish Coutts wrote:
but do some 'stainless' systems not have a higher % of mild steel in them to make the material easier to work? ergo, some stainless systems can show corrosion fairly quickly. It's just that they corrode a hell of a lot slower than their mild steel equivalent.
I am not a metallurgist but.....
It depends on the type of stainless - basically two types are commonly used for exhausts, austenitic and ferritic
If the stainless is is in the ANSI 300 series then the stainless is austenitic and the ones usually used for exhausts (304 or 316) have 18% Chromium and 8% Nickel, with a bit of molybdenum added in 316.
If it's in the ANSI 400 series then the stainless is ferritic with 409 being commonly used for exhausts. 409 only has about 11% Chromium and very little nickel. In service they will usually form a rust like coating fairly quickly but they are easy to weld and even press into shape and are subsequently used in a lot of production vehicles for catalyst housings and silencer boxes.
Most exhausts you see on sale for the aftermarket will probably be 304 or 316 but one or two places are offering exhausts with some 409. The reason the 300 series types will corrode on the welds is usually down to a couple of major reasons. The weld will often be done with no shielding gas on the rear of the weld area (either the weld needs to be done in a completely inert atmosphere or you feed gas in via bungs and purge all the air out before you commence welding) - I suspect this is quite common having briefly seen a couple of production facilities in the UK. The other reason is incompatible filler wire is being used. But there are lots of other reasons too, I know someone involved in welding research and he reckons you could write a book on how not to weld or utilise stainless - with exhausts being featured in every chapter!
To my mind stainless exhausts are a huge can of worms - don't ever buy one without a full guarantee as likely as not you WILL need it sooner rather than later and always keep a spare one to fit (in mild steel) to fit on the car just in case