What IS that metal?

[paulca]

When you open an electrical product and it has an internal metal frame, it's often a yellow/golden looking material, often with what looks like temperature related rainbow bands of reds and even blues like Titainium.

Surely it can't be titanium?   It feels more like some form of aluminium alloy.

It's also commonly found securing transformers.

Anyone know what it is and why it's got the rainbowing?

Here is an example:
https://www.ebay.co.uk/itm/Dormeyer-DCT-20-120-Transformer-In-120VAC-Out-24VAC-20VA-w-Mounting-Bracket-/401521781402

[jpanhalt]

That example is likely zinc-plated steel (galvanized).  It can show various colors depending on the process.

[Alex Eisenhut]

It's the result of some sort of coating. eg

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

[Monkeh]

That's just steel. The yellow is a chromated zinc plating.

[RoGeorge]

Ideally that should be some magnetic shielding alloy, like Mu-metal, Permalloy or alike.
Not sure what metal is shown in the ebay pic, seen those colors often in vintage equipment.

[TimFox]

Chromated zinc and galvanized zinc are cheap anti-corrosion coatings.  Mu metal is expensive and requires annealing after forming, since work-hardening destroys its useful magnetic properties.  Cadmium plating used to be popular, but Cd is very toxic and is essentially banned now.

[paulca]

Mystery solved.  Thanks!

[Kerlin]

I was recently handling some metal coated in chromated zinc plating and I recalled that in its liquid form that chemical is extremely carcinogenic.
Are there any problems or precautions required when handling plated metal that is maybe over 5 years old?

[TimFox]

The MSDS for zinc-chromate primer has required precautions for application (as a spray paint), but no indication of problems with the cured product.  The MSDS states that there are no heavy metals and the material is non-carcinogenic.  The wet paint is quite flammable, and an eye irritant, and all sorts of other safety issues.
https://cdn.website.thryv.com/22e397b789124750bb8ff5fbfa2d09f7//files/uploaded/v-005sds448e.pdf

[ChristofferB]

Strangely enough, the datasheet clearly calls the varnish chromate, yet it consists only of solvent and CAS # 7779-90-0, which is zinc phosphate.

Perhaps the chromate designation is just a name in the trade but the chromate has been bred out of it.

Chromium is very much a heavy metal, and chromate salts are super duper carcinogenic. How it behaves in a varnish is difficult to predict, it might be fairly safe.

[T3sl4co1l]

There's also alodyne, an aluminum anodizing, and chromating I think, process.  You'd probably only see it in aerospace equipment though, and I think it has a more consistent color?

If this is like, equipment from the 70s up, yeah, most likely galvanized steel, chromate finish (and easily confirmed with a magnet or other test for steel base).

Chromium is very much a heavy metal, and chromate salts are super duper carcinogenic. How it behaves in a varnish is difficult to predict, it might be fairly safe.

Just to clarify, and add the point of interest -- chromate (VI) is the nasty one.  The metal by itself is not generally toxic, it's the oxidation state that kills in this case.

Chromate solutions, and most compounds, have a yellow to orange color, are strong oxidizers (when available for such reactions -- acidic solutions, high temperatures, and strong reducing agents, drive a reaction), and are potent carcinogens when biologically available.

Amazingly, chromium (III) (and other low oxidation states; which take on various colors, green most often, but also blue, purple and others -- hence its name!) is essentially non-toxic: chromic chloride hydrate is a green salt with (oral LD50) toxicity comparable to table salt.  I can only imagine eating that much, would be a rather unpleasant affair -- as would salt, if you can imagine eating a hundred grams or so of salt in one sitting...

And the metal of course is rather inert, largely protected by its tough oxide, hence chrome plating, and stainless steel (which contains an abundance of chromium).

Chromate isn't very common in the environment, or as a result of weathering -- stainless steel corrodes slowly in neutral conditions, and is attacked by many acids giving chrome (III) compounds.  Chromate usually has to be created intentionally, with a strong oxidizer, often under basic conditions (like bleach -- not a risk factor for stainless steel, but not recommended to go bleaching a pile of pure chromium oxide).

By far, the most common source of chromate in the environment, is irresponsible use in chemical processes, especially metal plating and finishing, and other chemical syntheses.  Hence the movie Erin Brockovich -- a sadly quite accurate telling of a true story.

Tim

[ChristofferB]

Alodine, and it seems a similar coating called iridite might be that sickly yellow/green tint of airframe parts.

Chromium(VI) is not that rare nature-wise. Lead chromate occurs naturally as crocoite, and is the classic "chrome yellow" pigment used all over in the past.

Aside from poorly controlled industrial processes, I think a significant source is also pigments and rust inhibitors.

Chromated copper arsenate was (is?) used extensively for wood protection, although the oxidation state of chromium is probably difficult to determine.

[Terry Bites]

When you phone your metal fabricator just ask for "zinc and pass".

[frogg]

I have a couple of bottles of alodine and I use it occasionally for light metal parts work.

The Hexavalent Chromium is carcinogenic, mostly because of its extreme oxidizing power and its ease of entry into cells. Its oxidizing potential is very beneficial to creating a corrosion-prevention coating on reactive light metals (Al, Mg, Zn). In order to work, it has to be mixed with something that can solubilize light metal oxides, usually a very dilute HF solution.

Yes, it is very nasty stuff, even with the very low concentrations of HF and Cr+6.

Once applied, Zinc chromate on fasteners and steel parts really isn't something you have to worry about. It is effectively totally neutralized (as opposed to, say....unreacted excess isocyanates in two-part urethane coatings.) If you're not the one in the factory making the stuff, don't worry about it.

[T3sl4co1l]

Once applied, Zinc chromate on fasteners and steel parts really isn't something you have to worry about. It is effectively totally neutralized (as opposed to, say....unreacted excess isocyanates in two-part urethane coatings.)

Yes, worth adding this to my earlier points -- many chromates are highly insoluble, making them effectively inert.  The aforementioned metal compounds are good examples, hence the value as a protective coating.  Or like the aforementioned crocoite (lead chromate, a double toxic whammy, heh), indeed crystallizing because of its low solubility; nice crystals take hydrothermal conditions and millions of years to form.

Neutralized, wouldn't exactly be my choice of word; it's still there, just unavailable under neutral conditions.  Avoid contact with strong acids, which can liberate the chromate ion.  Avoid creating dust.  Don't lick paintings made with chromate yellow. etc.

Anyway, I don't think the coating layer is all that thick, a micron or so?  Casual handling and cutting isn't a threat.  Industrial processes are the hazard.  Don't accept a job where you're sanding that shit 8 hours a day, y'know, take precautions around the plating line, etc.

Tim

[G7PSK]

The yellow is passivated zinc which also come in many other colours. https://www.ecwilliams.co.uk/zinc-passivation/