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I tried one experiment. I started with ordinal yellow zinc screws.

The screws were sumberged in bath of two parts of $4.7\%$ solution of $\ce{NaClO}$ and one part $\ce{NaOH}$. The screws turned silverish in color. Then, I submerged them into vinegar. Vinegar turned them black. I am now wondering what actually happened. The bath of $\ce{NaClO}$ and $\ce{NaOH}$ is used to blacken the steel, but it doesn't work with zinc. The vinegar can be used to remove zinc.

But why it was black after that bath?

I was expecting that it would need another bath in $\ce{NaClO}$ and $\ce{NaOH}$ to become black. Another control screw which wasn't in first bath was just stripped of zinc layer and got a steel color.

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    $\begingroup$ Yellow? Zinc is anything but yellow. $\endgroup$ Apr 18 at 19:49
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    $\begingroup$ E.g. reliablefasteners.com/documents/docsGr/101/631/7/1016317/… $\endgroup$
    – Roman
    Apr 18 at 19:53
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    $\begingroup$ I still stand by my statement. Certain allows of zinc can look like that; zinc alone can't. $\endgroup$ Apr 18 at 20:12
  • $\begingroup$ Your zinc is probably an alloy called brass, which is yellow, And brass is made of half zinc, half copper. When dipped into a $\ce{NaClO}$ solution, the copper atoms are oxidized, and transformed into copper oxide $\ce{CuO}$ which is a silverish then black powder $\endgroup$
    – Maurice
    Apr 18 at 20:24
  • $\begingroup$ The screws you show are brass. There are many brass alloy compositions, not just half copper and half zinc. You might want to look up brass in wikipedia. $\endgroup$
    – Ed V
    Apr 18 at 20:40
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ZnO solubility

These screws are steel with thin cover of zinc with colored oxide layer, probably done by anodizing, I think. At first you dissolved zinc oxide with NaOH. This removed the colored layer and exposed zinc, making it look metallic. Then you reacted zinc with vinegar. Zinc itself is more reactive with acids. The zinc acetate that could be forming is transparent, so that doesnt explain the black color. If black color is present everywhere in solution, it could be that zinc was not pure, and as zinc was being chipped away by vinegar impurities colored the solution black. Or, if black was present only on the screw itself it could be that steel formed iron oxide on its surface. In both cases black is likely non zinc metal oxide. Most likely copper or iron.

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