Help with removing tungsten oxidation 1

[onekohm]

Hello all, hope this is the right forum to ask. We use tungsten chips from a machining operation in a back-mass. The tungsten is sieved to select the correct size (around 1 mm), cleaned in an ultrasonic cleaner with hot water and degreaser several times, rinsed with water and finally MEK.

We just use a few hundred grams a year, and have been using material from the same large bucket for the last five or so years. This came right from the machine shop, with who knows what kind of cutting fluid all over the chips.

When we cleaned the last batch, there was a noticeable green look to the cleaned chips. Under the microscope there is clearly visable a crystaline green to blueish deposit on some of the chips. From searching I guess this is an oxide which has formed over time due to something in the cutting fluid. Repeating the ultrasound and degreaser cleaning does not remove the material.

I found some references to etching tungsten in preparation for bonding with caustic soda, so we've tried that and it does seem to help. We may need to experiment with exposure time and agitation. Is there something better that I am missing? I would prefer to stay away from chemistry more agressive than the MEK and caustic soda.

Thanks in advance.

 

[btrueblood]

About 90% of this will be "duh" to you onekohm, but may be useful for others reading.

First: why worry about it - the oxide is likely quite stable. If your "back mass" is sealed, or better potted in epoxy or urethane, the green stuff will likely not cause any harm, if it never gets much above 100C or so. Just looks funny.

Tungsten oxidizes slowly in room temp. air, slightly more rapidly in the presence of water. The oxide you noticed is likely a copper/tungsten/oxide complex, or possibly a sulfide/oxide complex. You could try rinsing the tungsten in dilute (1 Molar) solution of hydrochloric (aka muriatic) or acetic acid (aka vinegar) (straight oxides should dissolve pretty quickly in a weak acid). Or you may also be able to "boil off" the oxides with a propane torch. Or, toss the amount you want to use into a tumbler with some Aluminum oxide or garnet grit, and then use a gold pan or riffle box to recover the tungsten (or just rinse with a strong stream of water into a 1-gallon pail, the water needs to be strong enough to lift and wash out the grit, leaving the heavier tungsten chips behind).

Whatever you do, don't store your tungsten in open containers, instead put it in double plastic baggies (preferrably purged with nitrogen before closing), and the baggies in a sealed 1- or 5-gallon pail. Washing it right when you got it might've been a good idea too.

 

[onekohm]

Thanks for the thoughts, btrueblood. The back mass is indeed potted in epoxy in the end. The corrosion is a difference from before, and while it probably doesn't affect anything, I don't like it.

I found some other references to using hydrogen peroxide either alone or with ammonia. The straight hydrogen peroxide does a great job of cleaning after degreasing.

We are currently working on screening the bulk of the material and cleaning and storing it better protected.