zeroptzero

oh yeah, given that picture I can see how the small inlet pipe could come loose away from the muffler if the welds fail. Cool info.

JDR159

Stainless steel exhausts can be chemically damaged two ways.
1. SS welds will rust unless they've been passivated. Passivation removes the iron (Fe) that is present in the weld. The iron comes from the high heat (melting) of the materials. SS can be passivated using acids to etch the iron. I wouldn't recommend trying this at home unless you know what I'm talking about.

2. Exhaust gases combining with water vapor (also in the exhaust) to create sulfuric acid that condenses on the inside of the exhaust. This happens during startup/cold driving when everything is cold enough to allow condensation. If you drive long enough (probably the Honda suggested 10 miles for extreme conditions) the exhaust gets hot enough to essentially dry out the exhaust and keep more acids from forming.

You can see the condensation when you first start up: It's the water dripping out of the tailpipes..

SpitfireS

This flange doesn't look stainless

These cracks were a surprise to me.
It looks like the rust expanded from the inside, you can see the bulge.
IMO its not standing water creating rust as this is a vertical seam.

Almost through but I didn't hear any noise.
The carbon build-up probably sealed it.

MY'00 OEM exhaust.
Used as a DD, on salted roads.

zeroptzero

thanks for the pics SpitfireS, that is pretty ugly. Driving in salt road conditions is probably the biggest factor, mine is 7 years old and it doesn't have a speck of rust on it yet, it practically looks new but it has never seen a winter. So I was curious how long it might last like that. Thanks forf the info.

slalom44

All exhaust systems on all cars sold in the United States since the early 80's have been stainless. That's because of government emissions requirements mandating that the entire exhaust system last for at least 8 years/80,000 miles.

"Stainless" doesn't mean stain proof. All components contain at least 11% chromium to improve high temperature oxidation and corrosion resistance.

There are two sections of an exahsut system; the "hot end" (the resonator & converter up to the manifold) and the "cold end" (the exhaust pipe after the resonator, the muffler and tailpipe). The cold end is produced from stainless grades similar to 409. I say "similar" because in Japan they don't produce 409. This is fully stabilized ferritic stainless steel that is weldable (with no chromium carbide precipitation in the heat affected zone or weld nugget), therefore weld integrity is very good. That doesn't mean that welds can't fail.

The hot end is primarily made of grades similar to 439, 18CrCb and 18SR. These have 18% chromium, therefore much better high temp oxidation resistance.

The general failure mechanism of the hot end is usually high temperature oxidation and/or fatigue. High temps caused by lengthy high engine speeds and rich fuel mixtures will accelerate oxidation of stainless. The higher the temperature, the longer the time at temperature and the more cycyles of high temperature, the more oxidation occurs. This isn't "rust" per se, it's more like a gray scale that forms. As the hot end oxidizes, there is less structural steel remaining, and fatigue can set in. Keep in mind that the strength (and fatigue resistance) of stainless steel decreases dramatically at high temps.

The cold end uses less chromium because it doesn't see the temperatures of the hot end. If you have a lot of short trips, moisture remains in the muffler along with dilute sulfuric and carbonic acid (products of combustion). Over time, they tend to rust out the muffler and tailpipe from the inside. The exhaust pipe (between the resonator & muffler) may fail from either corrosion or oxidation.

Lots of highway miles will allow an exhaust system to last much longer since this keeps moisture out when the car is not running.

BTW: I'm a Metallurgical Engineer, and a former Market & Product Development Engineer for a stainless steel company. I'm no longer involved with these products, but for a time I was very involved with the stainless exhaust systems of all domestic auto manufacturers.

SpitfireS

Just by looking at it: I have a hard time believing that flange at the hot side is any sort of stainless steel.



The pipe in the same picture does exactly show the color you mention, grayisch.

slalom44

You're right. It doesn't. Some OEMs have used carbon steel flanges instead of stainless, however I recall they were never welded to the exhaust pipe - the pipe was flared and the flange pulled the two sections together.

In this case, there appears to be a weld bead. My assumption would be that the pipe was made from the Japanese version of 18CrCb (18% chromium) and the flange was made from their version of 409Ni (a little over 11% chromium with some nickel for toughness). The relative difference in chromium with the high temps and environmental exposure would result in accelerated rust & oxidation of the flange. I've examined hundreds of exhaust systems from junkyards in the past, and that's very typical.

davidc1

So, Slalom44, tell me if I just messed up.

I've got a Skunk2 exhaust. They say it's stainless steel. They have a bracket with their logo welded to connect the two sides of the exhaust. After less than a year, the weld cracked and a hole formed.

Skunk was going to warranty it for me since it's less than a year old, until they found out I just bought the car used a week before, and since I was not the original owner they refused.

So I drove it a while longer, the bracket fell off, and then i got a hole about 1/2" x 1 1/2".

Went by a muffler shop, showed the hole to them, said they could fix it for $20, and instantly brought out an arc welder and it was done in less then 5 mins. I asked if they have to use any special stuff cause it's stainless, they guy sort of muttered something I didn't get, but said they weld stainless all the time.

Do I have to worry about them doing it correctly? It now looks like a big scab, but at least no hole.

davidc1

Also, for what its worth, OEM systems in my 30 yrs of driving, usually last much longer than the twice as expensive aftermarket brands.

slalom44

First of all I should say that almost all aftermarket exhaust systems are Aluminized Type 1 Carbon Steel tubing. Aftermarket exhaust manufacturers don't produce stainless exhaust systems. That's why they only last for a couple of years. They rust out from the inside due to corrosion from condensation. All exhaust systems were made that way prior to the 70's. That's why they didn't last.

And don't expect the exhaust shops to tell you the truth about this. Most of them really don't know. In the past I have played dumb and asked about whether their exhaust systems are stainless, and they said "yes". Let's face it - they're muffler mechanics, not Engineers.

I'm not sure if Skunk2 uses a 300-series or 400-series stainless in their exhaust systems. Almost all OEM exhaust systems are made from 400-series (except for an occasional back plate on the muffler due to cosmetics). This is easy to weld to carbon steel, so it can easily be repaired with a stick (or MIG) welder and possibly a small plate to fill the gap. Unfortunately it is highly unlikely that they used stainless filler rod or a stainless plate to fill the gap. It is very likely that in a few years it will fail at that spot. Unless you took it to a place that really knew their metals, it is very unlikely that you would have had it repaired correctly.

If it was made of a 300-series stainless (like 304L or 316L), it would have been more difficult for them to weld. You can tell if it's 300-series by putting a magnet to it - if it doesn't stick or if the magnetic field is weak, it's probably 304L. IMO, 300-series is not necessarily better than 400-series stainless for durability. They have different strengths and weaknesses. But 300-series stays looking nicer longer on the cold end (mufflers).