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Thread: Material Compatibility (galling)

  1. #13
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    I went to the Wiki link in post 8 and found this sentence:

    "Galling does not occur on carbon steel."

    Oh, yes it does. You can disbelieve me and trust Wiki if you like, but I learnt the hard way.

    An open mind is a virtue...so long as all the common sense has not leaked out.


  2. #14
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    yeah i read that too and was a bit confused, i have had steel gall before. I wonder why they only mention stainless and aluminum in that wiki page, surely it applies to all metals, and maybe even plastics?

    well i hope 7075 and steel won't gall, ill have some oil in it so i don't think ill have to worry.



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    Quote Originally Posted by skmetal7 View Post
    .... I wonder why they only mention stainless and aluminum in that wiki page, surely it applies to all metals, and maybe even plastics?

    well i hope 7075 and steel won't gall, ill have some oil in it so i don't think ill have to worry.
    Stainless and aluminum are the two most gall prone metals in my experience so maybe that is why the Wiki focuses on them. But I think they have the explanation a bit wrong. They are correct regarding these metals having a tightly adhering oxide layer on the surface; that is what makes them corrosion resistant. Because the oxide sticks it protects the metal underneath from further corrosion atack because it is an impermeable barrier; atmospheric oxygen cannot get through.

    Possibly you have heard about passivating stainless steel? This is a process in which the stainless steel is immersed in a mixture of strong oxidising acids to create a thicker than normal oxide layer and enhance the corrosion resistance.

    You have almost certainly heard about anodizing aluminum; this is an electrochemical process to create a thicker oxide layer for improved corrosion resistance. I did mention hardcoat anodize in my first post and this is a proc`ess that very greatly enhances the thickness so when you have hardcoat anodized aluminum alloy sliding against un-anodized you do have two 'dissimilar' metals in that one has a much harder surface than the other. Also because the oxide layer is analogous to a ceramic it is not a metal so you don't get the welding effect.

    You probably have a good chance of success with the 7075 on steel; not guaranteed, mind you, cast iron would be better.

    An open mind is a virtue...so long as all the common sense has not leaked out.


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    Quote Originally Posted by skmetal7 View Post
    ...and maybe even plastics?....
    I skipped over the plastic comment.

    In my experience; yeah, sort of. Much, much worse than galling really; more like wholesale fusion welding.

    Look back in my first post where I mention localized hotspots causing brittleness. The brittleness occurs because most metals are good conductors of heat so the local hot spot is 'quenched' by the heat flowing into the surrounding metal.

    Plastics are not good conductors of heat and if you run a plastic shaft in a plastic bearing at any sort of speed it can turn itself into a blob of molten spinning plastic. Well, it tries to do that but what happens is the shaft tears off.

    An open mind is a virtue...so long as all the common sense has not leaked out.


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Material Compatibility (galling)
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