Oxidised screws

[fabiodl]

Many screws appear yellow, like the one attached.
If I am not wrong, they are known as chrome plated. To my (very limited) knowledge, however, chrome plating is for having a shiny, silver like color.
What do they add that makes them yellow?

I had some oxidized screws (the oxide was white) from a 1982 sega console. I used acetic acid to strip the oxide. This removed the plating as well, so I plated them with nickel. This clearly changed the color to gray, and I would like to restore them to the original color, how can I do that?
 
And, is there something other than acetic acid that only dissolves the oxide but not the plating itself?

[ataradov]

The yellow color comes from yellow paling that is applied on top of galvanized zinc coating. I'm not sure what is on that yellow stuff, but it prevents zinc from corroding.

The yellow part comes after zinc plating is done as far as I know, so it is a two step process.

Although on further googling, I'm confused. It looks like it may be just one step process, but I'm struggling to find the difference.

It is not chrome. And I'm not sure how realistic it is to repeat that  process at home. It is probably easier to find new screws with required coating.

[fabiodl]

Here is where I found the term "chromated"

http://translate.google.com/translate?js=n&sl=auto&tl=en&u=https://www.neji-navi.com/contents/trivia/k_sozai.php

[ataradov]

The coating material is Zinc Chromate. But I'm not clear on the color. Some articles talk about coloring agents being added, but the coloring agent will not add any protection. So there must be some chemical difference between clear and yellow zinc coating.

[Nusa]

https://www.fastenermart.com/understanding-fastener-platings-and-finishes.html

But you might just consider getting some model paint in the closest color you can find and paint the heads.

[ataradov]

https://www.fastenermart.com/understanding-fastener-platings-and-finishes.html

Ok, so this makes sense. Chromate is applied on top of the Zinc coated parts. That's the two step process I was thinking about.

[fabiodl]

I believe it is this one
https://en.wikipedia.org/wiki/Chromate_conversion_coating
It seems a pretty nasty chemical
https://labchem-wako.fujifilm.com/sds/W01W0116-0365JGHEEN.pdf

[BradC]

https://janekits.com.au/products/150-ml-gold-bright/

[fabiodl]

Thank you BradC, that seems awesome!

[Cerebus]

Many screws appear yellow, like the one attached.
If I am not wrong, they are known as chrome plated. To my (very limited) knowledge, however, chrome plating is for having a shiny, silver like color.
What do they add that makes them yellow?

The colour or chromium depends on its oxidation state and what it's compounded with. Chromium metal is bright silver, but chromium compounds can be blue, yellow, orange or red. The classic chrome passivation coating is, as others have said, zinc chromate and that's a light yellow. There's a very strongly yellow compound, lead (II) chromate that was used as an artist's colour know as chrome yellow for years - disfavoured now because lead and chromium are somewhat toxic. There's a chromium passivation that got a lot of use on military equipment that's a dull green, almost khaki, that you'll see if you collect old military electronics - not sure of its exact chemistry.

Chromium Oxide, Potassium Chromate and Potassium Dichromate:

[Haenk]

White powdery surface is very likely Zinc hydroxide ("zinc rust") from the zinc coating of the screw (before passivation); so the passivation likely had a defect somewhere.