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Dealing with rusty exhaust pipes

This is a discussion on Dealing with rusty exhaust pipes within the General forums, part of the Caferacer.net Forums category; Originally Posted by o1marc The prep work would be the same as any new pipe would be. Sand blast with the proper media and profile ...

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  1. #21
    Senior Member DesmoDog's Avatar
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    Quote Originally Posted by o1marc View Post
    The prep work would be the same as any new pipe would be. Sand blast with the proper media and profile and shoot the ceramic. It is more worth than spending big money to replace them. Your last few posts have made me lose any confidence in what you say on the subjects, you seem to be greatly misinformed.
    What exactly is "profile"? Sanding all the rust pits out to get a smooth surface?

    And I don't know who you're buying "stainless steel" from but in 25 years of designing exhaust systems for OEM automotive companies I've never seen any grade of stainless that gets red rust on it. Kinda defeats the purpose of using stainless, don't you think? But then we wouldn't waste time or money on a material that shitty anyway so I guess it could exist.

    But thanks for straightening me out in you oh so holier than thou way.
    -Craig
    The Mighty Monza Jr. Thread: https://www.caferacer.net/forum/proje...r-project.html

  2. #22
    Senior Member o1marc's Avatar
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    Quote Originally Posted by DesmoDog View Post
    What exactly is "profile"? Sanding all the rust pits out to get a smooth surface?

    And I don't know who you're buying "stainless steel" from but in 25 years of designing exhaust systems for OEM automotive companies I've never seen any grade of stainless that gets red rust on it. Kinda defeats the purpose of using stainless, don't you think? But then we wouldn't waste time or money on a material that shitty anyway so I guess it could exist.

    But thanks for straightening me out in you oh so holier than thou way.
    I didn't think my reply was a "oh so holier than thou way" at all. I was just trying to educate you on a matter you seem to think you know it all on.
    Profile is also known in the paint industry as "tooth" or a texture proper for the best adhesion of the paint, powder, or ceramic coating.
    Here is an excerpt from a paper on "Rust On Stainless Steel":
    Rust on Stainless Steel


    What Causes Rust on Stainless Steel Surfaces?
    Rust is formed on the surface of stainless steel when a condition develops inwhich the metal molecules at the surface are not sufficiently alloyed withchromium to create or maintain the required oxide layer. Contrary to popularopinion, surface rusting does not occur when stainless steel is “sensitized” sincethe bulk of the metal has adequate chromium to prevent surface rusting;sensitized stainless steel only corrodes along the grain boundaries. Rustingphenomenon discussed in this paper is a surface phenomenon only.
    The simplest condition under which rusting can occur on stainless steel is whena piece of ordinary carbon or low-allysteel is rubbed against the surface of anotherwise corrosion-resistant piece of stainless steel. See Figure 1. The ironfrom the ordinary steel will rub off onto the stainless steel surface as a film ofunalloyed steel, and, after exposure to moisture in the atmosphere for a fewdays, that unalloyed steel film will form ugly rust. This happens because theunalloyed steel film on the stainless steel has little or no chromium, so the filmtransforms to ordinary “red rust”. Once the shallow film of iron that has“contaminated” the stainless steel surface has oxidized, corrosion stops providedthe stainless steel surface remains exposed to the atmosphere and is not wettedon a nearly continuous basis. The rusted surface looks bad, but, the chromiumin the stainless steel under the rust film forms a suitable corrosion-resistantoxide layer, and further corrosion does not continue once that film of unalloyedsteel turns to rust.
    When ordinary steel is rubbed off on stainless steel, the material which istransferred to the stainless steel is generally referred to as “free iron.” This kindof “contamination” of stainless steel is also known by the misnomer of “carboncontamination.” This term is a misnomer because it is not the carbon thatcauses the corrosion but the unalloyed iron from which the steel is made.
    Corrosion of “free iron” on stainless steel is always faster than corrosion of theunalloyed steel itself because the free iron “contamination” is anodic to thestainless steel, so it corrodes to protect the stainless steel, just like zinc corrodesto protect carbon steel when zinc is used as galvanizing.
    A second means of forming a rust film on stainless steel occurs during weldingwhen using a process that depends on flux for shielding, such as SAW andGMAW using flux-cored wire. These fluxes tend to be easy-to-remove or self-peeling, and, although the weld surface may look clean, there is a thin residuallayer of iron-rich material on the surface. This layer rusts easily if the weldmetal surface is not abrasively or chemically cleaned after welding. The nature ofthis rust is similar to that formed by contact contamination as discussed in theabove paragraph.




    See the whole paper here:
    http://www.sperkoengineering.com/html/Rust.pdf
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    I carry a gun because I'm too young to die, too old to take an ass whooping, and a cop is too heavy.

  3. #23
    Senior Member DesmoDog's Avatar
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    Quote Originally Posted by DesmoDog View Post
    I'm not all that familiar with ceramic coatings but I doubt it would be worth the money on a used rusty system? I would think the prep work to get it to stick would be expensive?
    Quote Originally Posted by o1marc View Post
    I didn't think my reply was a "oh so holier than thou way" at all. I was just trying to educate you on a matter you seem to think you know it all on.
    The phrases "I'm not all that familiar with" and "I would think" say to you that I think I know it all? Seriously? Also note the QUESTION MARK at the end of both of those phrases. For future reference, I don't typically end a statement with a question mark if I'm trying to imply I know it to be true.

    What are you trying to show with the paper you quoted? Did you read it? It says rust found on stainless is due to the crap it's been contaminated with. If you don't want it to rust, don't contaminate it. Don't drag iron over your stainless parts, or fail to clean off the flux after you weld it if you don't want to see red rust on it. Kinda like don't spray silicone lubes on your powdercoating equipment if you don't want to see fisheyes in your coatings. The fisheyes are due to a mistake, not the powder itself. The rust on stainless is due to a mistake when you process it. And when I say rust I mean red rust. Not oxidation. Of course it oxidizes.

    And yes, you can polish just about any metal to some level of sheen. I shouldn't have worded that the way I did and in fact I've polished carbon steel for temporary solutions. In production applications where you want the finish to have a nice luster and a useful life, 300 series stainless is a far better choice. We would never try to polish a lesser grade. You can polish carbon steel and tell yourself it's just like chrome if that makes you happy. You can probably even fool a lot of people with it. Until it gets hot. Or wet. (Though stainless will change color when it gets hot too)

    I'm also surprised you wouldn't charge more to prep a rusty pipe than a new one. The last thing I had powder coated still showed pits where the rust had been (I told the guy not to worry about it, the bottom of the battery tray isn't going to show), I would have thought ceramic coating would still show the pits too but apparently I'm wrong on that? (<- note the question mark)

    Wait, I see where I went wrong. I questioned that the prep to get it to stick would be expensive. Not the prep to get it to look unpitted. My bad. I just assumed anyone who went to the effort of ceramic coating a pipe would want it to have a smooth surface when they were done, and the coating wouldn't take care of that on it's own.
    -Craig
    The Mighty Monza Jr. Thread: https://www.caferacer.net/forum/proje...r-project.html

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  5. #24
    Senior Member o1marc's Avatar
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    Desmo, I'll only make comment on a few things you stated as it seems no matter what I say I'm going to take shit for it. The paper was in reference to your comment of "I've never seen any grade of stainless that gets red rust on it.". It was the words "rust ON IT" that the paper explained why mine are all red rusty. I agree good clean stainless will not red rust, but will oxidize or corrode.
    No one that I know that does ceramic coating will fix the pitting in pipes. Ceramic must be applied to clean virgin metal, so no fillers can be used to fill the pits and if you try and sand the pits out you probably won't have much useable pipe left to coat. Actually nothing should be used on the pipe ager blasting,A complete ceramic job will be prepped by blasting and coating inside and outside the pipes. If they are pitted outside there is probably a better chance they are pitted inside as well. Powder coating won't fill the pits either without multiple coats of powder with sanding between each coat till they are gone. This adds considerable expense to the coating job. As far as "I doubt it would be worth the money on a used rusty system", a used rusty system may have nothing more than surface rust and after blasting will be the exact same pipe as if you had blasted a brand new pipe. So the benefits or worth would be exactly the same or more so for the used pipe because it could be had for less money than a new one. A used pipe would be cheaper to ceramic coat than a new chrome pipe because you can't coat over the chrome, it would need to be removed or blasted to the proper texture for the ceramic and that texture is hard to do on chrome plating with out going through it and then having to remove it completely.
    www.CreativeCandy.org

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