Why are my aluminum parts turning black?

Last month I setup my Bredgeport clone for flood coolant, and have been using it daily! Last week I was milling some bronze, it turned my coolant from clear/yellow to clear/green!

Now when I machine aluminum the parts are turning brownish black!

Have I contaminated my coolant?
I have been using the synthetic stuff from Enco.

It would be nice to color my aluminum parts, but it is not a uniform discolorization, its only where the coolant puddles or runs over the parts!

I tryed to remove the stain using scotchbrite under running water, but it is not an easy task! This also leaves the parts frosted, and not as shiny as my other parts!

Any ideas, or should I just pump it out and scrub the sump inside the mill's casting? (Not looking forward to that):devious:

Eric

If it were a sulfer type bronze, it would not surprise me if the coolant has become acidic. I'd just change it out with new, not that much gain in scrubbing it out, but your call. Maybe one flush with water and a neutralizer and check the PH if need be

But you should charge more for that rare Patina coating!:D

DC

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Originally Posted by widgitmaster

Last month I setup my Bredgeport clone for flood coolant, and have been using it daily! Last week I was milling some bronze, it turned my coolant from clear/yellow to clear/green!

Now when I machine aluminum the parts are turning brownish black!

Have I contaminated my coolant?
I have been using the synthetic stuff from Enco.

It would be nice to color my aluminum parts, but it is not a uniform discolorization, its only where the coolant puddles or runs over the parts!

I tryed to remove the stain using scotchbrite under running water, but it is not an easy task! This also leaves the parts frosted, and not as shiny as my other parts!

Any ideas, or should I just pump it out and scrub the sump inside the mill's casting? (Not looking forward to that):devious:

Eric

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Most likely you should flush out your resevoir and replace you coolant.Word to the wise though.Coolant will discolor aluminum even if it isn't contaminated.When machining aluminum it's good practice to clean you parts in hot soapy water afterwards.

I cannot hazard a guess on what might have happened but I can tell you that Shell Dromus B coolant can be used for brass, aluminum or leaded steel on the same machine without getting any strange problems. It does not cause any discoloration on aluminum and if it is used at a 1 in 10 dilution the coolant oil film that is left behind when the water evaporates off the leaded steel parts stops the part getting a film of rust. At this dilution there is also no problem with rust on vises or machine table. Aluminum parts going for anodizing can be washed with just hot water to remove coolant residues but a bit of dish soap does help.

My guess is that the copper in the bronze became oxidized or sulfated, and is in solution. Since aluminum is more reactive than copper, the oxygen or sulfate transfered to the aluminum. Or some similar chemistry.

for the coolant to cause any amount of corosion the consentrate would have to be very low , i would try the suggestion posted earlier and check the ph level , it could be acidic if its been sitting , the only time i recall aluminum parts going black on me ,i soaked them in some water with some degreaser soap which turns out it was acidic , that was bad (nuts)

I was having the same problem (but parts were turning brown), so I replaced my coolant yesterday, ran 50 parts and the problem is gone.

I think it was contaminated by tap fluid. I tried to dilute it out by running the coolant low and refilling it, but it didn't work.

Luckily the coolant tank was only 30 gallons, but it still wasn't fun.

I always put my alluminum parts in mineral spirits after machining.Cleans them right up!

I had similar problem with my aluminum parts turning brown and after changing the coolant several times it did not improve. We found that the cabinet had been resealed with black RTV and this was the problem. I replaced the RTV with clear silicone and the problem went away.

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Originally Posted by JWOOD

I had similar problem with my aluminum parts turning brown and after changing the coolant several times it did not improve. We found that the cabinet had been resealed with black RTV and this was the problem. I replaced the RTV with clear silicone and the problem went away.

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That did not sound like a fun experience to troubleshoot.