jweb12

iTrader: (2)

This may be a real dumb question but, isn't stainless steel not supposed to rust?

I changed out my race pipe for a hfc this afternoon and while I was under the car I took a good look at the rest of the exhaust (ATLP quads) to make sure everything looked as it should. I'm seeing a decent amount of rust, quantity not severity. It's mostly at the flanges but also pretty regularly along the pipes, looks like a camo pattern on the pipes actually.

I guess a little rust is considered acceptable by "stainless" standards? I've only got about 1,000 miles on the exhaust so I didn't expect it to look like that. I have gone through a couple snows (and salt) since installing the system.

Thanks.

LoveMyTL-S

Here ya go:

Stainless does not "rust" as you think of regular steel rusting with a red oxide on the surface that flakes off. If you see red rust it is probably due to some iron particles that have contaminated the surface of the stainless steel and it is these iron particles that are rusting. Look at the source of the rusting and see if you can remove it from the surface. If the iron is embedded in the surface, you can try a solution of 10% nitric and 2% hydrofluoric acid at room temperature or slightly heated. Wash area well with lots and lots of water after use. Commercially available "pickling paste" can also be used.

Jesstzn

Stainless steel does not stain, corrode, or rust as easily as ordinary steel (it stains less), but it is not stain-proof.[2] It is also called corrosion-resistant steel or CRES when the alloy type and grade are not detailed, particularly in the aviation industry. There are different grades and surface finishes of stainless steel to suit the environment to which the material will be subjected in its lifetime. Common uses of stainless steel are cutlery and watch straps.

Stainless steel differs from carbon steel by the amount of chromium present. Carbon steel rusts when exposed to air and moisture. This iron oxide film is active and accelerates corrosion by forming more iron oxide. Stainless steels have sufficient amounts of chromium present so that a passive film of chromium oxide forms which prevents further surface corrosion and blocks corrosion from spreading into the metal's internal structure.

There is many grades of SS and the grade used in car exhausts isn't very high grade .. more of a selling point than a point of cosmetics.

I have exhaust tips made of 316L SS and they won't even pick up exhaust carbon ..


Note

Type 409—cheapest type; used for automobile exhausts; ferritic (iron/chromium only).


Here is an example of the grades.

Stainless steel grades
100 Series—austenitic chromium-nickel-manganese alloys
Type 101—austenitic that is hardenable through cold working for furniture
Type 102—austenitic general purpose stainless steel working for furniture
200 Series—austenitic chromium-nickel-manganese alloys
Type 201—austenitic that is hardenable through cold working
Type 202—austenitic general purpose stainless steel
300 Series—austenitic chromium-nickel alloys
Type 301—highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than 304.
Type 302—same corrosion resistance as 304, with slightly higher strength due to additional carbon.
Type 303—free machining version of 304 via addition of sulfur and phosphorus. Also referred to as "A1" in accordance with ISO 3506.[11]
Type 304—the most common grade; the classic 18/8 stainless steel. Also referred to as "A2" in accordance with ISO 3506.[11]
Type 304L— same as the 304 grade but contains less carbon to increase weldability. Is slightly weaker than 304.
Type 304LN—same as 304L, but also nitrogen is added to obtain a much higher yield and tensile strength than 304L.
Type 308—used as the filler metal when welding 304
Type 309—better temperature resistance than 304, also sometimes used as filler metal when welding dissimilar steels, along with inconel.
Type 316—the second most common grade (after 304); for food and surgical stainless steel uses; alloy addition of molybdenum prevents specific forms of corrosion. It is also known as marine grade stainless steel due to its increased resistance to chloride corrosion compared to type 304. 316 is often used for building nuclear reprocessing plants. 316L is an extra low carbon grade of 316, generally used in stainless steel watches and marine applications due to its high resistance to corrosion. Also referred to as "A4" in accordance with ISO 3506.[11] 316Ti includes titanium for heat resistance, therefore it is used in flexible chimney liners.
Type 321—similar to 304 but lower risk of weld decay due to addition of titanium. See also 347 with addition of niobium for desensitization during welding.
400 Series—ferritic and martensitic chromium alloys
Type 405— ferritic for welding applications
Type 408—heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.
Type 409—cheapest type; used for automobile exhausts; ferritic (iron/chromium only).
Type 410—martensitic (high-strength iron/chromium). Wear-resistant, but less corrosion-resistant.
Type 416—easy to machine due to additional sulfur
Type 420—Cutlery Grade martensitic; similar to the Brearley's original rustless steel. Excellent polishability.
Type 430—decorative, e.g., for automotive trim; ferritic. Good formability, but with reduced temperature and corrosion resistance.
Type 439—ferritic grade, a higher grade version of 409 used for catalytic converter exhaust sections. Increased chromium for improved high temperature corrosion/oxidation resistance.
Type 440—a higher grade of cutlery steel, with more carbon, allowing for much better edge retention when properly heat-treated. It can be hardened to approximately Rockwell 58 hardness, making it one of the hardest stainless steels. Due to its toughness and relatively low cost, most display-only and replica swords or knives are made of 440 stainless. Also known as razor blade steel. Available in four grades: 440A, 440B, 440C, and the uncommon 440F (free machinable). 440A, having the least amount of carbon in it, is the most stain-resistant; 440C, having the most, is the strongest and is usually considered more desirable in knifemaking than 440A, except for diving or other salt-water applications.
Type 446—For elevated temperature service
500 Series—heat-resisting chromium alloys
600 Series—martensitic precipitation hardening alloys
601 through 604: Martensitic low-alloy steels.
610 through 613: Martensitic secondary hardening steels.
614 through 619: Martensitic chromium steels.
630 through 635: Semiaustenitic and martensitic precipitation-hardening stainless steels.
Type 630 is most common PH stainless, better known as 17-4; 17% chromium, 4% nickel.
650 through 653: Austenitic steels strengthened by hot/cold work.
660 through 665: Austenitic superalloys; all grades except alloy 661 are strengthened by second-phase precipitation.
Type 2205— the most widely used duplex (ferritic/austenitic) stainless steel grade. It has both excellent corrosion resistance and high strength.

paliknight

wait so does steel or iron that is galvanized in zinc rust also?

jweb12

iTrader: (2)

Thanks for the responses guys. What I have is basically a combination of rust and staining. The flanges are rusted, the piping is stained. It's still minor considering the exhaust has only been on a few months but I wouldn't have expected it to be at this level already. The rust is not sheeting or delaminated, just surface right now but it's only going to get worse obviously.

Not really concerned with the staining since it's under the car and barely noticeable looking from the rear. The rust is what concerns me. Have to look into this some more.

jweb12

iTrader: (2)

It can. It depends how good the coating is. Once you get a weak spot in it, the coating is basically useless. While you're out driving sometime, look at the W-beam traffic barriers, typically along highways. Usually, they are clean but sometimes you'll see small or even large areas of rust. Don't confuse this with weathering steel barriers which are rusted throughout. Those are made to form a uniform oxide coating that actually protects the steel (residential areas typically use these barriers since the rust color is "better" to look at than the metallic finish). Part of my job is inspecting bridges so I see a lot of different materials with varying amounts of corrosion....

jweb12

iTrader: (2)

Damn edit rule, meant to add "just not stainless steel" to the end of that.