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Stainless Steel FAQ or WHY IS MY SS EXHAUST RUSTING?!?!
There is a ton of misinformation on evom about stainless steel, and I wanted to put together a FAQ of sorts and hopefully set the record straight. My goal is to reduce the amount of misinformation floating around on the board about the material and it's care.
So is stainless steel really rustproof? Short answer: no. Stainless doesn't really rust in the same way that carbon steel does (scaling, flaking). What you normally see in stainless steel is iron surface contamination, though more serious contamination can and will occur as the reaction is self-catalyzing and can rust right through if it's left untreated. I'll talk about treatment and cleaning in a little bit. What makes stainless steel "stainless"? Stainless steel is basically made stainless by the presence of chromium in the alloy. To qualify as "stainless" steel, it must contain at least 10.5% chromium. Chromium reacts with oxygen in the air to create a sort of "self-healing" chrome-oxide surface layer that resists further contamination (rusting) by oxygen. This layer is not bullet-proof, and can be permeated. The presense of iron actually prevents this protective barrier from re-forming. My stainless steel [insert part here] has rust on it, how do I clean it? Whatever you do, DO NOT USE CARBON STEEL BRUSHES/WOOL/WHEELS. As I said before, the oxide barrier can be breached and needs to be treated correctly, otherwise you'll think you just cleaned up your part, only to have it rust again on you soon after. You need to remove all contamination from the material with stainless steel brushes or wool. In addition, the brush/wool you use should NEVER have been used on a carbon steel part. It's possible to contaminate a stainless part from a new stainless brush, if the brush does not have at least the same corrosion resistance as the base material. I would personally use nylon, brass, or aluminum brushes and make sure you only use them on stainless. Take care to store your brushes where they're not likely to get rust on them. If you do clean stainless with contaminated brushes, you can still remove contamination with a pickling paste or through a nitrate or citric acid bath. I like citric acid because it's 100% environmentally friendly and doesn't require special equipment to handle. It'd on the FDA's GRAS (generally regarded as safe) list. Blasting can also be done with clean silica or garnet sand, or shot, grit, or cut wire blasting can be done so long as the material is also stainless and in equal corrosion resistance and composition to the material being blasted. Once the part is free from surface contamination, the protective layer can "heal" and your part is again rust resistant. How can I prevent contamination? The most common types of contamination I've seen personally on our cars is rust that formed from improperly treated welds, and contamination due to close proximity to carbon steel parts and fasteners. To prevent contamination on welds, the weld needs to have formed using a filler material with the same composition as the base material. If the welds are to be grinded, follow the precautions above. During welding, there should be no carbon steel part within 25mm (about an inch) of the weld, or contamination of the weld could occur. For example, holding your material in a steel fixture or vice should be a no-no and not only can cause contamination of the material, but the weld its self if the point of contact is sufficiently close to the weld. Never use carbon steel straps or clamps on stainless. Never allow direct contact of stainless and steel parts, particularly if the carbon steel has contamination of it's own as the iron oxide WILL contaminate your stainless part. Never grind carbon steel in the presence of stainless. Just the particles flying through the air can result in contaminaton. Always use stainless fasteners in the proximity of stainless parts. It's very easy for a carbon steel bolt to transfer iron oxide to a stainless part and begin the corrosion process. Is there anything to consider for high heat applications? Yes there is! Any part that sees regular heat in the range of 900-1400°F (482-760°C) (hey, like exhaust manifolds!) is in danger of something called carbide precipitation (CP) or "weld decay" if an alloy not specifically formulated for CP resistance was used. I personally had a SFP manifold on my Talon whose welds rotted clean through due to what I believe to be CP specifically. What happens is when the material is heated, there is a risk of the chrome forming chrome carbides Cr23C6 with any carbon present along the austentic grains. This causes the depletion of the chromium in the austentic grains which causes the protective layer of chrome-oxide to deplete. One way you combat this effect is to only use low-carbon grades of stainless steel (L) and when welding, make sure the gas content is less than 5% CO2. Another way is to use alloys specially formulated for CP prevention, like 321. What's the "L" in 316L, 304L, etc? As I mentioned above, the L designates a low carbon content (.03% MAX) in the steel. Normal content is .08% max and some grades can have up to .15%. L grade steel is useful for increased weldability and increases CP resistance in high heat applications in the range of 900-1400°F (482-760°C). Not all stainless steel is produced as a low-carbon grade because it's more expensive and decreases the hot strength of the steel. Steel can also be "dual grade" i.e. it has the physical properties of the straight grade but technically it's L grade steel with all the advantages. What's the difference between 304, 316, 321, and 347 grade stainless steel? 304 contains 18% chromium and 8% nickel 316 contains 18% chromium, 10% nickel and 2% molybdenum. The moly in 316 is to aid in resisting chlorides. 321 is similar to 304, but with the addition of titanium for increased CP resistance and higher operating temps 347 is like 321 but with niobium instead of titanium. 321 can't be transferred across a welding arc, but 347 can, and has the same properties as 321. Therefore, 347 is mostly used to weld 321. What is the deal with 3XX and 4XX grades of stainless? The 300 family of stainless indicate the presence of nickel in the alloy and are also referred to as the "austenitic" family. The 400 family is subdivided into 2 different families based on structure. One is called "ferritic" (409, 430, 434, and 439) which cannot be hardened by heat treatment. The other is called "martensitic" (403, 410, 416, 420, and 440 A,B,C). Is stainless steel magnetic? 300 series steel is not magnetic, but 400 series is. Just because a magnet stuck to your exhaust, that doesn't mean you should beat down the door of your muffler shop becuse you think they ripped you off and gave you a mild steel exhaust. What does "CRES" mean? CRES stands for Corrosion RESistant, and while all stainless steels are CRES, not all steels that are CRES are stainless, such as galvanized steel. What's this about "pickling" and "acid passivation"? Pickling is basically the removal of free iron by using an acid solution or paste. Because there are safety and environmental concerns depending on the type of acid used, it is beyond the scope of this writing. Acid passivation is immersion in an acid dip that frees contaminants from the surface of the part and promotes very rapid development of the chrome-oxide layer. The composition of the bath is dependent on the chrome content of the steel and is beyond the scope of writing. Many citric acid solutions, for example, are proprietary and the composition of the solution is not as well documented as for nitric passivation solutions. I'm saying that you need to find some yourself and follow the instructions on the bottle. |
awesome info
thanks for the education |
No problem. If this can do more good in another section feel free to move it. I mostly just read this section so that's where I posted it {thumbup}
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Great info, LOL @ Screen name
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Originally Posted by chaotichoax
(Post 7603763)
awesome info
thanks for the education Real Good stuff |
Just a quick addition to this:
For exhaust manifolds where intergranular corrosion may be possible due to Chromium Carbide precipitation, it is better to use 321 SS. It uses a titanium addition which acts as a carbide stabilizer. Also, 321 has a higher hot strength than 304. So, if you are buying an exhaust manifold made out of 304L, maybe ask the vendor if they can do it out of 321 instead. Eric |
Great info! Learned alot!
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Thanks man, good info! Bookmarked...
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I searched a bit but couldn't find the exact grade brush.
where can you buy the tools at and what do you recommend product wise for cleaning? |
The example that I put earlier is more of a concern to industrial applications, and just goes to show that you can contaminate one type of stainless with another type of stainless. It's much harder for a consumer to get handheld brushes in any alloy you want. I would personally not use stainless brushes at all. I would use aluminum > brass > nylon depending on how stubborn the crap I'm trying to clean off is. Strangely enough I can EASILY find 316L wire for stainless brush manufacturing, but no handheld brushes that actually use it. Must be just for industrial applications.
If you must use stainless brushes, perhaps for weld prep or cleaning, the general rule is to find a brush with at least the same corrosion resistance and a similar alloy as the base material. I added this to the OP as it makes more sense than providing an extreme example that no one will encounter. |
Wow, lots of great information. Thanks for the post.
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Originally Posted by l2r99gst
(Post 7604461)
Just a quick addition to this:
For exhaust manifolds where intergranular corrosion may be possible due to Chromium Carbide precipitation, it is better to use 321 SS. It uses a titanium addition which acts as a carbide stabilizer. Also, 321 has a higher hot strength than 304. So, if you are buying an exhaust manifold made out of 304L, maybe ask the vendor if they can do it out of 321 instead. Eric |
Great read. Very good information. While a lot of it is targeted towards manufacturing, it's still great to know for the average DIY type of person.
One thing that really gets to me though is the complete lack of regulation on imported materials. MOST of the stuff that comes from China and a lot from India does not meet specification. I have to imagine that because of the price of Chromium and Nickel, they are probably leaving out a lot of it. They are probably also not keeping the carbon content down. Using a good supplier that you trust to only use material that meets specification will cost you more, but WILL be better in the long run. What ever it is, it's crap material. Take an ebay stainless part and toss it on a car for a year in any kind of corrosive enviroment. Take a close look and 9 times out of 10, you'll find intergranular corrosion and pitting in the base metal, don't even get me started on the HAZ. GOOD 304 stainless WILL NOT do this under normal conditions that cars encounter. I live in Utah, home of the salted roads. GOOD 304 stainless will hold up for YEARS here without even surface corrosion, much less it breaking down the passivation layer and causing sub-surface corrosion. The typical China made stuff will have all kinds of corrosion issues after a single winter. Inconel 625 is about 3x the cost of 304 stainless which puts it right up there with titainum. It's the best, hands down for exhaust components that are subjected to very high temps and pressures (pre-turbo). If you were building the ultimate exhaust components, I'd use 625 on the manifold and everything post turbo I'd use titainum for weight savings. The titanium cat back in just tubing is going to cost you roughly $60 a foot in 3", and good luck finding mandrel bends in that size. The 625 is available in mandrel bends from BurnsStainless is header primary sizes. |
Originally Posted by 03whitegsr
(Post 7606680)
Awesome info
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Great info! Its one of those things thats not typically put out for all to see.
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