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Q: Alum. Stove top burner

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  • Q: Alum. Stove top burner

    more of a curiosity than anything else ...
    I've got a "modern" countersunk (countertop) stovetop with
    cast iron rack on top.

    the burners are stamped stainless except for the center one (biggest)
    which I'm pretty sure is cast aluminum.

    by "burner" i mean the ring that sits on top of the jets -- has
    the holes all 'round.

    long story short, the wife spilled something on that center
    (aluminum) burner and it turned black. I put it in the lathe and
    brought it back to a dull shine with some sand paper.

    its been back on the stove for about a week and its turned
    yellow. deep straw colored yellow. nothing else has been
    spilled on it so far

    just curious -- is this normal for cast aluminum? I've never seen
    Al change colors like this (and I've done quite a bit of aluminum
    welding, forming, etc). if its a surface oxide, shouldn't it be white?

  • #2
    Aluminum oxide is actually colorless unless doped with other elements. The white corrosion products on aluminum are aluminum hydrate and hydroxide. Natural gas contains a very small amount of sulfur which might be enough to create deposits of aluminum sulfide. Aluminum sulfide is also colorless but when exposed to water (cleaning?) it hydrates to aluminum hydrate and releases trace amounts of hydrogen sulfide. Hydrogen sulfide will then react with the aluminum hydrate to color it yellow/brown in a process similar to anodising where the colorant is trapped in the hexagonal structure of the aluminum hydrate. Hydrogen sulfide is also used to color aluminum foil yellow.
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    • #3
      just wanted to thank you for the insightful answer as usual.
      you've got me "wiki-ing" now.