sukhoi 30 on production line Why are aircraft usually coated with "yellow", while either during construction or repair and overhaul?
What exactly is the purpose of it?

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    $\begingroup$ Could you please provide the source of your image? $\endgroup$ – Federico Mar 30 '18 at 12:23
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    $\begingroup$ What type of plane is that? $\endgroup$ – dalearn Mar 30 '18 at 12:53
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    $\begingroup$ The aircraft in the picture is Su-34 $\endgroup$ – Shravan Firestoned Mar 30 '18 at 13:01

For unassembled and new planes or planes not yet painted the green/yellow color you see is the anti-corrosive coating on the aluminum

...every unpainted airplane is nominally green from being coated (typically) with an anti-corrosive green zinc chromate or zinc phosphate primer over the aluminum skins. The different shades of green simply tell you that different vendors produced the different pieces—they don’t all use the exact same primer.

Originally it was some variation on Zinc Chromate but it was found to be fairly toxic other things have been used over time which lead to the more green color and even some potentially "chrome-free" options.

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    $\begingroup$ do you mean nominally or normally? $\endgroup$ – bogl Mar 30 '18 at 11:48
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    $\begingroup$ @bogl: It's a quote from the given link. "Nominally" is in the original. $\endgroup$ – Fred Larson Mar 30 '18 at 14:06
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    $\begingroup$ To add to this, in the US, you will typically see the trade name "Alodine", which is usually called out in drawings as something along the lines of "CHEM. CONVERSION COAT PER MIL-DTL-5541 <or MIL-C-5541>, TYPE <X>, CLASS <Y>", where <X> is I for coatings with hexavalent chromium or II for coatings with no hexavalent chromium, and where <Y> is 1A for maximum corrosion protection, painted or unpainted, or 3 where the corrosion protection needs to preserve electrical conductivity. $\endgroup$ – Tristan Mar 30 '18 at 14:39
  • $\begingroup$ "nominally" would be the normal, expected word here, FWIW. $\endgroup$ – Fattie Mar 30 '18 at 14:59
  • $\begingroup$ Lamp black is added to yellow zinc-chromate to make it resistant to UV light and increase wear resistance. This results in a green color. The more lamp black that is added, the greener the color. $\endgroup$ – JScarry Mar 31 '18 at 23:12

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