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Thread: Titanium or Stainless steel?

  1. #1
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    Titanium or Stainless steel?

    Hi all,

    I have a question about material choice for a bleeding nipple going into aluminum. The existing one (zinced steel) is fused in the aluminum hole. Removing it strips the aluminum thread.

    The choice of material I have is 316 grade stainless steel or grade 5 Titanium.

    Given that the torque on the bleeder is only 20Nm the only concern is corrosion resistance and especially galvanic corrosion from the road salt.

    Which one would you chose for this application?
    Should I be worried about galling with stainless steel?

    Thanks!

  2. #2
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    Have you thought about a thread insert?

  3. #3
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    Yes, I did but there is very little wall material around the nipple for drilling for an insert. Then there is the risk of the drilled hole not be axially aligned with the vale seat. Kind of too much risk to take.

    One other idea I found in the meantime is to still use zinced steel (zinc is very close to aluminum in galvanic table) and use Tef-Gel on the threads.

    Maybe there are two problems: corrosion of the valve itself and galvanic corrosion of aluminum when used with practically any metal. I think I saw signs of both (some rust and lots of aluminum oxide) on the valves that I removed.
    If that's the case then I can use titanium valve and some lube to protect the threads. Thread locker or antiseize may not work due to presence of brake fluid and the cleaning agents from wheel cleaners.

    What do you think?

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    How hot does the part get? Can it be a burly engineered plastic?

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    Principle Engineer Cragyon's Avatar
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    Zinc or stainless should be fine unless you're operating in a high salt environment.

    Usually, there is a thread coating on the fitting threads to help minimize corrosion also it's common to see a cap placed over the grease fitting to help keep the elements off..

  6. #6
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    Temps can go quite hot. Using plastic is not easy due to the existing design that I cannot change. I can only change materials.

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    Quote Originally Posted by Cragyon View Post
    Zinc or stainless should be fine unless you're operating in a high salt environment.
    Usually, there is a thread coating on the fitting threads to help minimize corrosion also it's common to see a cap placed over the grease fitting to help keep the elements off..
    During winter there is a lot of road salt that I read is more aggressive than the sea salt due to high concentration.

    Yes, usually there is a coating, and actually originally there was black e-coat primer. But that is now gone because of corrosion. After cleaning the threads there is bare aluminum on the threads. Unless I redo the entire finish (chromate, e-coat, paint/powder coat) I'll have to use something like a grease\sealer\threadlocker. Given that the nipples are often opened and closed for bleeding, threadlocker is out. So the only options is grease\sealer. The question is which such coating is not affected by brake fluid?

    There are rubber caps but they only cover the nipple ends not the nipple threads.

  8. #8
    Administrator Kelly Bramble's Avatar
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    I have a boat - it gets launched in salt water. The calipers are aluminum and the bleeder valves are steel and coated. Brakes should only be bled once maybe twice in the useful life. I always water rinse off the brakes after being in salt water and then occasionally coat the brake housing with silicon spray. Eventual corrosion is normal for any brake application unless one uses stainless in all materials.

    I did have a thread problem once that was crated by over-torque the bleed valves. I replaced the calipers and bleed valves 0 everything works fine.

    I think it's more common to have a over-torque then a corrosion issue in my experience.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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    My thinking was the same about corrosion vs. overtorque. I have aluminum calipers on a Volvo also made by Brembo and no significant issues after many years in PNW. Also no issues reported on the Internet.
    But the Subaru calipers I'm talking about are different. There are so many reports about the same issue dealing with corrosion. Granted that some of them might also be overtorque but still so many.
    I got 3 sets of used calipers hoping to swap parts and get one good pair (no luck, I'll still need to completely rebuilt and refinish). All of them exhibit corrosion even the ones that I got from Japan.

    I think I'll go with some stainless steel nipples as they are half the price of titanium. I'll also add some coating on the threads like Tef-Gel to prevent water getting in the threads and causing galvanic corrosion.

    I'm still thinking if I should do an e-coat on the aluminum threads first.

  10. #10
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    Quote Originally Posted by dorucn View Post
    Temps can go quite hot. Using plastic is not easy due to the existing design that I cannot change. I can only change materials.
    Have you considered thermoset plastics?

  11. #11
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    I know there are brake pistons made of phenolic plastic. I'm not familiar with how the parts are made but I suspect, because these are resins, they are not machined out of a solid piece.

  12. #12
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    Thermoset plastics are typically formed by compression molding at temperature.

    If you are interested in a high temperature plastic that you can machine for prototypes, look at Dupont Vespel which can be had in stock sized pieces for machining.

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