Electronic – the identity of this shiny insulator

corrosioninsulationresistivitysafety

Shiny layer
fig: Shiny layer

I've seen many times, on metallic objects, a kind of insulator layer is given (I've tested its resistance with multimeter and it shows infinite ohms).

The layer is greenish-gold in overall color, but have colored patches that may vary from red to green.

It is seen on small transformers(above image), switches, fan-regulators, etc.

The layer change its color, and colored-bands shift their place when the parts get heated.

Now , what is the identity of this golden insulator? What is its resistivity and other characteristics?

And also, what is the identity of another, reddish-colored insulator used in electromagnetic coils? also, what is its resistivity and other characterisics

Best Answer

As others have said, the yellow color is chromate treatment on top of mild steel. Less common these days, I think, there is some environmental issues with some versions of it. It's actually quite conductive electrically, to the extent that a version of it is used on aluminum, for example, when we actually require electrical conductivity. Similar parts from Wikipedia link above:

enter image description here

The entire transformer is probably also vacuum impregnated with enamel by dunking it into a liquid in a vacuum chamber. This leaves an almost clear coating on top of the chromate (where it is thicker, as in drips, it will appear more brown). It improves the insulation of the windings, and bonds the laminations together so they are not as likely to buzz at 100Hz or 120Hz from the mains (photo of equipment from above link): enter image description here