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Colors on tungsten by anodization? Answered

I want to make tungsten rods in colors. As it is possible to have titanium in MANY colors by simple variation of the thickness of oxide-layer, i wonder if the same is possible with tungsten. Somebody tried it already? What are the voltage-ranges for nice colors?

As references:

https://www.instructables.com/id/How-to-EASILY-Ano...

https://www.reddit.com/r/pics/comments/99sdse/the_...

https://www.wikihow.com/Color-Titanium

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Downunder35m

Best Answer 5 days ago

Tungsten, in a humid enviorment already oxidizes at quite low temperatures.
A quite thick oxide layer forms rapidly above 500°C.
Getting defined colors is quite hard.
Both the humidity and and oxygen concentration in your controlled enviorment must match.
The range of colors though is quite limited.
There are some research paper from the 90's that go into more detail.
You should be able to still find them these days.
If you plan to get the color only through heat and exposure to oxygen you might have a problem though.
For this you not only need very high temperatures but also a controlled way to limit the exposure to oxygen once the desired color is reached.
For example by flushing the the sealed oven with inert gas.
Uncontrolled the oxide layer grows in various colors and is quite sentive to temperature differences.
For a rod you might end with a color gradient instead, which can be quite nice too.

I never experimented with it but it should be possible to anodize Tungsten like other metals.
And under the right conditions the crystal growth should be capable of taking colors like used for aluminium.


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OrngrimmDownunder35m

Reply 4 days ago

Thanks! I dont plan to use heat and Oxigen. I plan on using a precisely controlled voltage and wet oxidation in submersion.
As showed in the video i linked with Titan.
I found a single paper on Anodising like Aluminium, but it was a special method with electro-sputtering and stuff...
But thanks! I will test and if it works, make an 'ible! :)

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Downunder35mOrngrimm

Reply 1 day ago

Don't have any tungsten scrap to try out, so maybe you can test it one day as I can't promise it will work as expected:
Mix hydrochloric acid and hydrogen peroxide.
The first should be around 20% in concentration, the second at 5-20%.
A 50-50 mix so you will end with just under 10% in concentration for the acid.
Partially submerge a tungsten piece in it and observe.
The theory behind this step is that this corrosive oxidiser will remove everything from the surface, including possible oil from fingerpints, oxides and such.
Even if you can't see any action like bubbles the submerged part should look slightly different after about 15 minutes max.
If my theory is correct here then use this piece for the next try:
Prepare an anodizing bath with sulfuric acid.
Concentration does not need to be high here, just enough to provide a proper current flow.
One electrode will be the partially oxidised tungsten, the other a slightly sanded piece of tungsten.
I am not a 100% for tugnsten if the proper oxidation or anodisation happens on the positive on negative - sorry.
Either way, once you start increasing the voltage, the amps will rise as well.
The oxidation happens already before bubbles appear but in the case of tungsten there will be no color change unless you are able to increase the current to the point where the tungsten heats up.
Again, only my theory here, but if correct then you should be able to see a slight color change at some point before all starts to boil over.
If that is the case then for once be happy ;)
Key is then to make a suitable power supply when you can use PWM pulses at the desired voltage/amps for a color change.
High cuurent in short bursts so to say.

I prefer this route for experimentation because it removes a lot of factors that otherwise can render experiments useless.
If you pay close attention to the video you linked then you can notice what I stated already.
The guy says he places the part between the two electrodes of same material.
But in reality he creates and extended pole as the wire holding the piece is always connected to the electrode in the front ;)
For the final oxidation once you found the basics working it is perfect though.
Small pieces benefit from the bigger electrode where itthey are attached to.
Prevents the current going out of control as the surface difference is minimal.
For much bigger pieces the attached electrode should be adjusted in size or if in doubt removed with just the part and a similar sized electrode on the other pole.
Also please keep in mind that using sulfuric acid for this will constantly remove the oxide layer, so it will be only visible while you oxidise and still be very faint in color - if it colors properly that is.
For the final anodising in real you need to find to find a substitude for the acid that won't affect the oxide.
It might even be required to try this for the testing already if the acid here is removing the oxide layer faster than it can form.
As said, have no tungsten to try otherwise I would jump in here for the fun of it.
The TSP might work or it might not, so be creative here!
In some case things like sodium hydroxide or even alkali salts like potassium nitrate can be used - it all depends on the metal in question and how it affects the oxide.
For a perfect situation you would select something that provides a low enough resistance while leaving the oxide layer totally unharmed.

Two things are utterly important when trying to figure out ways to anodise "unknown" metals:
a) Pure metal for electrodes and parts!
For example the common welding rods for tig welding contain added metals and other things.
They are perfect for the job but make anodising next to impossible.
I found that out the hard way when trying to anodise some rims for a scooter.
Manufacturer claimed pure aluminium rims, sadly it was an alloy of aluminium, magnesium and something else.
The result was a very pitted surface which was not taking any color at all as no aluminium oxide formed due to the contamination by the alloy of the rim.
b) Pure Di-hydrogen-monoxide for everything you dilute or use to wash parts, icluding the anodising bath.
Using tap water will ruin everything, even if properly filtered.
The common demineralised water you often find is no good IMHO either as it just has no minerals in it but does not mean it isn't full of other things you don't know about.
So distilled water should be fine.

Last words....
From what I could figure out by reading, due to the lack of available materials here, tungsten is quite different.
Basically all properly documented research for the public eye is in the following categories:
Alloys, carbides, catalysts and for its heat and chemical resistance.
The only thing that stands outhere is the properties to shield from radiation but that does not help us either.
Tungsten oxides, colorations or their technical used is from what I can find totall neglected and only appears as a by-procut from other resear as side notes.
What little I could find in terms of coloration through direct oxidation is quite limited in colors as well as color intensity.
Like when you anodise a piece aluminium and would use a quite diluted yellow for the coloration and sealing.
The blues and greens seem to only happen if the tungsten is at least above the 50°C mark but below the 80° mark.
But as said this bit I found used a sealed chamber with a controlled humidity and heat.
You would have to create the required temp either by heating the anodising bath or by using a current high enough so the tungsten part itself heats up to this temp.
Unlike aluminium where the oxide is colored later or titanium where the crystal structure changes depending on the voltage, tungsten forms different types of oxides for the different colors it can produce.
That is why the right electrolyte in the bath is so important.
One might not affect a yellow oxide but prevent a green one forming, while another might only work for blue color ranges.
Only an elctrolyte that affects none of the oxides can be used to experiment with all possible colors.
Or you would have to revert to using one electrolyte solution for every color you desire.
I guess that is why i might be worth to consider unusal things like TSP, citric acid, baking soda or even salts that are not based on metals that react directly with tungsten.
A weak electrolyte can always be compensated for by a higher voltage but a too high concentration always needs to be diluted down.
Ideally you want to find the point where the electrolyte won't heat up too much by itself, with electrodes big enough to provide enough current flow and good heat transfer to the electrolyte.
Like this you can put it all in a water bath to keep the temp low enough and constant.
Grab a notebook (paper type ;) ) and document everything!
Note the electrolyte used and its concentration.
Voltage and current tested.
Temperature!!
If you manage to get ANY coloration happening focus on the parameters.
Repeat with the same voltage, curent and temp with a freshly cleaned and HCL H2O2 treated.
It it does not repeat the same right away you know the temp of the part gave the color ;)
It you manage to get consistent results try a different electrolyte concentration and see how it affects the voltage and amps to get the same coloration as before.
Sometimes a weaker or stronger electrolyte will drastically improve on the outcome.
Same story for the voltage and current.
In some cases the right balance is important so the oxides form in the right way and form.
For example going overboard here with aluminium will create a quite fragile oxide layer that often disappears with the color when slightly polished.
Or due to the increased heat the oxide will not form on the metal but instead dissolve into the water and form the hydroxide instead, a contaminated electrolyte is the result.
It shouldn't take more than a weekend to go through a bunch of possible electrolytes and the power range a normal bench top supply can offer.

Happy experimenting! :)