Summary for dichromate finish Discussion
Mandrel swage is explained as swaging a part over a mandrel, meaning a form is driven into or used inside the workpiece to shape or expand it; Ted Mooney gives the example of forming copper tubing ends for hydraulic connections so rings or fittings cannot pull off. On aluminized, Ted says the term traditionally meant hot-dipping in molten aluminum, similar to galvanizing in zinc, though he notes it is sometimes used more broadly for any aluminum application method, including sputtering, Ivadizing, and electroplating. A later reply from W. Carl Erickson addresses dichromate: he says sodium bichromate/dichromate in sulfuric acid was used as an effective deoxidizer, but because it is a carcinogen it has been replaced in some shops by less satisfactory alternatives such as hydrogen peroxide. He adds that the resulting steel finish coloring is commonly called dichromate, and another reply notes dichromate is still widely used in aluminum pretreatment and zinc plating, with environmental controls and industry efforts driving change.
Q. Dear Sir:
Please advise the meaning of “Mandrel Swage” and “Aluminized”. Thanks a lot!
Best Regards, Alex
Alex Au
– Hong Kong
A. Hi, Alex. I’d be confident that ‘mandrel swage’ means to swage (form) over a mandrel (a form); this sounds like something you would do, for example, in making hydraulic connections from copper tubing: you might drive a teardrop shaped mandrel partially into the end of a copper tube to stretch the copper tubing to prevent brass rings or other features from being able to pull off over the end of the tubing.
To me, ‘aluminizing’ always meant hot dipping in molten aluminum, much like galvanizing is hot dipping in molten zinc; but I have recently heard it used in the broader context of applying aluminum to a surface via any aluminum application method, including sputtering, Ivadizing, and electroplating. Good luck.
Ted Mooney, P.E.
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Q. Can anyone tell me where the term dichromate came from? I have always thought the term meant a “colored conversion coating”.
Don Penzenik
– Elkhart, Indiana
2000
A. Hi Don. ‘Dichromate’ means Cr2O7-2; so it’s not so much that the term means “colored” conversion coating, but rather that it is a conversion coating that is colored because of the nature of the chemical that is is.
Ted Mooney, P.E.
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2000
A. My company used sodium bichromate (aka dichromate), as Ted describes above, added to sulfuric acid (until about 25 years ago). It is an effective deoxidizer, but is also, unfortunately, a carcinogen, so has been replaced with other, less satisfactory deoxidizers, such as hydrogen peroxide, at least in our business. We still use it carefully in the metallurgical lab for sample etching, however. We saw no residual on copper alloys, but on steels, it leaves the unique finish coloring commonly referred to as “dichromate”. I do not know how finishers applying this finish now have gotten past the environmental concerns, but would be interested in learning.
W. Carl Erickson
– Rome, New York
2000
2000
Dischromate is still very widely used in aluminum pretreatment (Mil-C-5541) and by zinc platers to prevent white rust on zinc plated parts, Carl. The finishers using it have met the traditional toxicity concerns via waste water treatment, exhaust ventilation systems and scrubbers, the wearing of gloves by operators, etc.
As the result of European vehicle end-of-life directives, movement by American vehicle manufacturers to eliminate it as well, concern on the part of some people that contact with the finished surface might possibly be slightly carcinogenic, and intense work by the airline-aerospace industry to try to eliminate virtually all use of chrome, the situation is changing massively as we speak. Today’s principal thrust seems to be more toward eliminating it from finished products rather than eliminating operator exposures during production.
Ted Mooney, P.E.
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finishing.com – Pine Beach, New Jersey
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