Metal Working Fluids

I see that you had visitors but nobody reacted so that i try to give you an explanation.

Only a hypothesis: cast iron contains also graphite particles which are black (carbon).

Steel has carbon bounded to iron and other metals, aluminum has no carbon. It could be possible that those particles give the color grey and / or black according to the quantities of graphite in the irons used for the parts machined.

I was also thinking on those lines. But somebody suggested that it might also be because of the presence of sulphate reducing bacteria that converts sulphates to sulphides which is also black. Can this be true as well

Of course there are always several possibilities. But where do come sulphates in big quantities (relatively) ? In some free cutting steels there is sulphurs as well to be found. Do you notice same results when such steels are used?

I dont see similar thing happening in Steel. The only change that i see in steel machining is sometimes when adequete coolant flooding is not done during machining, there is a distinct smell and the coolant colour changes but not to the extent of cast iron.

The gray is particles of the cast iron, the black is an effect of oxidation

I'm with bwire on this one.

oxidation.

milo

"I dont understand why this happens? Any inputs would be much appreciated."

Machining C.I. is a dirty business as anyone who had done so will attest. Your emulsion is picking up the carbon particles from the machining operation. Nothing more nor less than that.

Placing a sediment tank into the filtering system with a magnet may resolve

For iron particles yes it could help but not for graphite, at least as far as i know.

I find it likely, that for cast iron carbon will turn the metal working fluid gray/black. On the other hand the carbon is tightly bound into steel and not present in aluminium.

Kind regards

T. Sobisch

At one stage in life I was a research chemist who formerly blended industrial oils for a major oil company. It seems to me that when machining ferrous metals, be they cast iron, or steel, compounds containing sulfur were needed in the machine oil to protect the machine tool surface and make the work piece neater (more precise) and easier to manufacture. When various sulfur compounds are used in machining fluids the sulfur combines with the iron in the metal to form iron sulfide. This occurs at the site of high temperature and high pressure on the very edges of the tool that come in contact with the work piece. Iron sulfide is black, hence the appearance of a dirty gray, or black to your machining fluid. Iron sulfide is very much like graphite in the sense that it a lubricating property at the site of formation (tool edge contact area). This is very good because it keeps the cutting edge of the machine tool very sharp. The tool would dull quickly without it! The water base, synthetic fluid base, or oil base, fluid will act to keep the tool cool, this too is important! I would examine the chemistry of the cutting fluid to better understand your perceived problem. That is, if you think the problem of gray or black appearing color is one of chemistry (by the way, it usually is) and not of one related to shaving fines creating the color. If the problem is related to fines, usually filtering can clean up the fluid by removing the fines, --and hence the color. Hope this helps.

Okay and when iron sulfide is exposed to air it is oxidized back to iron oxide and retains its dark color.

Special care should be taken when exposing large quantities of iron sulfide to air, explosive results can occur termed pyrophoric iron oxidation.

My experience machining cast irons proved compressed air to be far better than a fluid coolant.

Welcome to Cr4 amocoguy

This Sounds reasonable, and your experience is certainly relevant,

BUT may i respectfully ask

"if it is sulfur from the cutting fluid combining with iron in the iron, why does the sulfur in the cutting fluid not then also combine with the iron in the steels that he is also cutting to make similar black particles?" Fe is Fe. and there is more iron in steel than iron in iron on a weight percent basis. His original post states he does not get this effect when cutting steels...

Looking forward to more of your posts in the future.

milo

I agree that there are most probably as always several effects which lead to the colour change.

I think that according to the description the graphite has a stronger effect.

With respect to filtering i would like to mention that graphite particles can be very small and usually filters for cutting fluids are not so fine to hold particles in micrometer range.