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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

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    \$\begingroup\$ google transformer varnish coating \$\endgroup\$
    – JIm Dearden
    Jun 3, 2016 at 14:11
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    \$\begingroup\$ Yeah, it's just a form of varnish. Mainly to prevent corrosion, but the fact that it insulates (protecting from an accidental loose wire, etc) is a happy coincidence. Also, in some cases, the entire transformer may be dipped in the varnish to "lock together" the turns of the coils, preventing annoying noise and possible mechanical wear due to electromagnetic forces on the coil windings. \$\endgroup\$
    – Hot Licks
    Jun 3, 2016 at 16:26
  • \$\begingroup\$ Related: with the second-part of the question (vernish or polymer coating on magnet-wire); electronics.stackexchange.com/questions/34040/… , and electronics.stackexchange.com/questions/34743/… \$\endgroup\$
    – Always Confused
    Jul 19, 2016 at 13:20

4 Answers 4

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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

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That is common chromate surface treatment. Seen in a great many metal utility items. Not intended to be an "insulator" but perhaps non-conductive as a side-effect.

The red surface of magnet wire is the enamel insulation which is applied as a coating (vs. being an extruded plastic outer sheath as most other wire uses. They use a very thin enamel insulation to get many windings into the space available in a transformer, coil, solenoid, motor, or whatever. It is called "enamel" which it probably was in early days. But in modern times, it is a more sophisticated plastic coating of perhaps several layers. Note that red is only perhaps the most popular color. Magnet wire comes in several other insulator colors also.

Ref: https://en.wikipedia.org/wiki/Chromate_conversion_coating

Ref: https://en.wikipedia.org/wiki/Magnet_wire

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    \$\begingroup\$ Thanks very much, but "enamel" (coating) of which substance? i'm telling about chocolate-red shiny layer used in choke coils,etc. However, i've heard that motor-coils are vernished with resin etc. \$\endgroup\$
    – Always Confused
    Jun 3, 2016 at 11:47
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    \$\begingroup\$ Likely several different kinds of polymer compounds are used. Depending on what they need for mechanical toughness, voltage rating, etc. The specific color is likely just an un-controlled side-effect of the composition of the coating ingredients. There is no particular "color-code". Note that many coils (transformers, motor coils, etc) are also fully immersed in varnish (or a modern polymer) and put in a vacuum chamber to fully infuse the coils and mechanically lock the coils together and prevent buzzzing or humming. \$\endgroup\$
    – Richard Crowley
    Jun 3, 2016 at 11:51
  • \$\begingroup\$ no no i didnt mean any color code. i just wanted to know the chemical substance. thanks. \$\endgroup\$
    – Always Confused
    Jun 3, 2016 at 11:56
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    \$\begingroup\$ No. That is why the magnet wire is INSULATED with the "enamel" (actually polymer plastic) coating. So that each winding is actually insulated from its neighbors. If you used BARE uninsulated wire, then you would be correct that it would be just the equivalent of a metal cylinder. \$\endgroup\$
    – Richard Crowley
    Jun 3, 2016 at 12:04
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    \$\begingroup\$ Note that chromate surface treatment IS conductive. If your example is NOT conductive, then you probably have a conformal coating over the entire transformer, including the chromate-treated steel frame. They commonly dip the entire transformer as illustrated by @Spehro Pefhany. \$\endgroup\$
    – Richard Crowley
    Jun 4, 2016 at 18:38
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Chromating is an anti-corrosion surface treatment for metals. It is not used for insulation. The colour can vary a bit, depending on the process and base metal condition. That's normal.

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

Chromate conversion coating. More comonly known as yellow chrome. Wikipedia article

I've tested its resistance with multimeter and it shows infinite ohms

Push harder with your multimeter pins or scratch the yellow surface with a knife first and you will measure short circuit instead though the steel underneath the yellow chrome.

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  • \$\begingroup\$ Thanks for informing the pressure-characteristics. does the chromate layer change resistivity on pressure? or it is just closer contact of meter lead through minute pores? (since porosity is a property of any substance). \$\endgroup\$
    – Always Confused
    Jun 3, 2016 at 11:53
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    \$\begingroup\$ @AlwaysConfused I think what winny was trying to say is that by pressing harder with the tips of your probes you should be able to actually pierce the treated skin of the metal. I don't know how hard you'd have to press, but you can achieve the same thing by scratching the surface with the tips of your probes. \$\endgroup\$
    – David Wilkins
    Jun 3, 2016 at 12:42
  • \$\begingroup\$ @DavidWilkins. You are correct. I've updated my answer accordingly. \$\endgroup\$
    – winny
    Jun 4, 2016 at 17:12
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    \$\begingroup\$ The transformer is most likely "vacuum impregneted" where they immerse the entire finished transformer in some kind of polymer which coats ALL the surfaces, including the chromate-finished mounting bracket. The insulating layer (from the dipped coating) has nothing to do with the yellow color of the chromate treatment of the metal. Those are two completely separate things unrelated to each other. \$\endgroup\$
    – Richard Crowley
    Jul 19, 2016 at 16:49

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