Coolant and Brass

[nitewatchman]

I am using the "Premier 600 Synthetic Coolant" sold by Tormach on my 1100. I cut a fair bit of yellow brass using this coolant and it seems to work fine and has not had a problem with turning rancid. After 8 or 9 months the coolant has turned blue-green with what I assume is copper leached from the brass.

This has not been a problem until last night. I milled a light collar frame from aluminum to mount LED Festoons around the spindle similar to Kent Meyers' concept and this required a lot of time cutting the festoon pockets. Basically the pockets sat full of puddled coolant from a long time. I noticed after I finished that in the pockets there was a dark stain starting to cover the fresh cut surfaces. After cleaning and drying this appears to be a deposit of copper plated out on the aluminum.

Any one else seen this happen before and is there a way to clean the copper away?

nitewatchman

[Gerry Sweetland]

After I silver solder or braze brass I dip it in a "pickle" a solution of citric acid (supermarket) and water. Don't remember the ratio but I'll check my notes and get back to you.

I usually machine brass (not sure if it is yellow brass or not, have to check that out too) dry or w/ a little air to clear chips, but I don't machine enough material to make many chips, pretty small parts and stuff.

But I noticed using the same coolant you are using that if I don't vacuum the chips out and clean the machine well I'll get that blue/green residue when I machine aluminum w/ flood coolant running.
Gerry

[GLCarlson]

The deposit is most likely copper, probably from a dissolved copper (II) ammine complex with one of the corrosion inhibiting components of the coolant- there's almost certainly an organic amine in the coolant. Blue-green color is pretty much diagnostic for cupric ion. You are likely seeing some copper plating out -that dark stain- as a result of the reaction of the copper in solution with bare aluminum, which is exposed during machining. Aluminum normally reacts almost instantly with air to form a layer of aluminum oxide, but immediately after cutting it'll be just the metal, and very reactive.

What to do? You might try a solution of salt and vinegar (or citric acid) in water (just use regular white vinegar and a dash of salt- don't sweat the proportions). That's a good copper oxide/copper brightener, and works by dissolving just a little bit of the copper. It's actually a very, very dilute solution of hydrochoric acid (in effect). Might dull a bright aluminum finish, but will probably remove the copper flash. Citric acid might do it, though I'd expect that to be less quick; if it worked, I'd prefer that. You might also try nitric acid with a quick exposure. Some Al alloys (5 and 7 series) can be etched by exposure to nitric acid, especially with chloride present; some are fairly indifferent at least for short exposures. Nitric acid is readily available as the 70% reagent, but aluminum reacts with all concentrations up to around 90% (above that the aluminum oxide layer is inert, oddly enough). I'd try dilute nitric, 5% or less, with a quick exposure. Don't dilute the acid with tap water- it will have chloride present.

In any case, rinse well when done, and don't let any of these solutions get near any machine ways or get aerosolized. It should go without saying that you won't add anything to the coolant, either. If you use nitric, be aware that the 70% will react with skin very quickly.

Incidentally- a little copper ion in the coolant is a pretty effective biocide. So this is a mixed blessing.

[nitewatchman]

Thanks, I'll try it.

Not a big deal since I will powder coat the part anyway, I just don't like it.

nitewatchman

[bevinp]

Wow GLCarlson,
That is a very knowledgeable post. May I inquire of your background/profession?

I have a question you may be able to answer. I am speaking of the mark that is left under my cast steel vise and the cast iron milling table of the Tormach 1100 when the vise is not removed for a few days/week. I assume it is a galvanic reaction between the dissimilar metals when separated by an electrolyte, ie cooolant. It is relatively easy to remove with kitchen scouring pad.

I have tried using oil under the vise but without success, I assume because the coolant replaces the oil. But I don't know.

Can I prevent the reaction/marking?
Bevin

[GLCarlson]

Bevinp,

I hold a doctorate in chemistry, have been a professor and a senior exec at a well-known chemical specialty company. And a hobby machinist for a half century or so.

Regarding the stain. Yep, probably electrolytic, what the electroplaters would call 'smut'. Try this: coat the underside of the vise with a thin, even layer of Krylon clear coat. Or, if you want to just see if that'd work without doing anything permanent, use a single layer of Saran wrap. Used to see people do something like this all the time for the green stain they'd get from cheap rings- except they used clear nail polish. Saran film is very consistent and very thin; I would not use other plastic films.

Possible downsides: a teeny bit of flex, probably unmeasurable, in the vise-to-table mount. And, bigger risk, trapping coolant and exhausting the anticorrosion system, which I think is no worse than your current risk since you leave your vise on the table for extended periods.

An alternative that might work: electroplate (or sputter-coat) the base with a good inert metal that forms a hard oxide layer (chromium, or aluminum). Same result, really- you'll get an inert layer in the way. There are services that coat mirrors with aluminum (for amateur telescope makers) that aren't too expensive. I bet they'd do a vise base. Don't know how durable that'd be, though.

I've always been taught to give the base of the vise a film of oil, and to tear down workholding setups as soon as done, but in a production environment -or even big project setups- that's sometimes impractical or just silly. One makes the best choices for the situation at hand.

[kevinro]

What to do? You might try a solution of salt and vinegar (or citric acid) in water (just use regular white vinegar and a dash of salt- don't sweat the proportions). That's a good copper oxide/copper brightener, and works by dissolving just a little bit of the copper. It's actually a very, very dilute solution of hydrochoric acid (in effect). Might dull a bright aluminum finish, but will probably remove the copper flash. Citric acid might do it, though I'd expect that to be less quick; if it worked, I'd prefer that.

Just curious, would Muratic acid work? I have that already (swimming pools use it for controlling the pH).

Thanks for the very informative post.

Kevin

[Geof]

......I've always been taught to give the base of the vise a film of oil, and to tear down workholding setups as soon as done, but in a production environment -or even big project setups- that's sometimes impractical or just silly. One makes the best choices for the situation at hand.

After many decades of running machine tools I gave up on trying to prevent galvanic marking between vise and table. It is not deep enough to affect anything.

Over the years the way I found best to minimize it was to coat both surfaces with a film of white petrolatum, otherwise known as Vaseline.

[Don Clement]

Bevinp,

I hold a doctorate in chemistry, have been a professor and a senior exec at a well-known chemical specialty company. And a hobby machinist for a half century or so.

Regarding the stain. Yep, probably electrolytic, what the electroplaters would call 'smut'. Try this: coat the underside of the vise with a thin, even layer of Krylon clear coat. Or, if you want to just see if that'd work without doing anything permanent, use a single layer of Saran wrap. Used to see people do something like this all the time for the green stain they'd get from cheap rings- except they used clear nail polish. Saran film is very consistent and very thin; I would not use other plastic films.

Possible downsides: a teeny bit of flex, probably unmeasurable, in the vise-to-table mount. And, bigger risk, trapping coolant and exhausting the anticorrosion system, which I think is no worse than your current risk since you leave your vise on the table for extended periods.

An alternative that might work: electroplate (or sputter-coat) the base with a good inert metal that forms a hard oxide layer (chromium, or aluminum). Same result, really- you'll get an inert layer in the way. There are services that coat mirrors with aluminum (for amateur telescope makers) that aren't too expensive. I bet they'd do a vise base. Don't know how durable that'd be, though.

I've always been taught to give the base of the vise a film of oil, and to tear down workholding setups as soon as done, but in a production environment -or even big project setups- that's sometimes impractical or just silly. One makes the best choices for the situation at hand.

A probable electrolytic cause as you suggested would have to be due to some current flowing though the coolant. The cause could be dissimilar metals ala thermocouple or poor grounding. Could a possible solution rather than insulate the part or vise would be to provide a lower impedance path for current to flow rather than through the coolant. I would suggest investigating better grounding on the Tormach. I had a similar problem many years ago with two BMW model 2002 cars that had aluminum head on a cast iron block with copper radiator and an insulating head gasket between. In both cases electrolysis had eaten away the aluminum around the exhaust valves quenching them with coolant. If it wasn’t that the exhaust valves in those cars were not satellite and sodium filled the engine certainly would have swallowed them. However on a replacement engine from Munich I solved the electrolysis problem by installing a low impedance copper braided ground strap between the aluminum head and cast iron block.

Don

[GLCarlson]

Don,

Excellent idea. Wish I'd thought of that! And I'd try your recommendation before any of the things I suggested.

One would think that bolting the vise to the table would ensure a good electrical connection (ie, zero potential difference), but 'tain't necessarily so. And if the speculation is right that the cause is electrolytic, then eliminating the p.d. should eliminate, or at least minimize, the effect.

Wonder if a sensitive meter would register a difference? I don't get one between my vise and table, but I don't have the problem, either.

[Geof]

.....One would think that bolting the vise to the table would ensure a good electrical connection (ie, zero potential difference), but 'tain't necessarily so. And if the speculation is right that the cause is electrolytic, then eliminating the p.d. should eliminate, or at least minimize, the effect.....

It doesn't quite work that way. You can actually get galvanic (electrolytic) corrosion between different regions of the same piece of material depending on the level of impurities or the stress in the material. A simple experiment in grade school chemistry shows this. A soft iron nail is bent and then left in a humid atmosphere for a few days and rusting occurs prefentially in the region of the bend where the iron is stressed.

About the only way to prevent (actually greatly reduce) some form of electrolytic corrosion between two metals in contact is to keep them perfectly dry; which is impossible.

[Don Clement]

It doesn't quite work that way. You can actually get galvanic (electrolytic) corrosion between different regions of the same piece of material depending on the level of impurities or the stress in the material. A simple experiment in grade school chemistry shows this. A soft iron nail is bent and then left in a humid atmosphere for a few days and rusting occurs prefentially in the region of the bend where the iron is stressed.

About the only way to prevent (actually greatly reduce) some form of electrolytic corrosion between two metals in contact is to keep them perfectly dry; which is impossible.

Assuming the cause is due to galvanic action may not be correct. What if the current through the coolant is due to poor grounding or electrical leakage in the Tormach electrical system?

Don