Finishing terminology: "mandrel swage", "aluminized", "dichromate"

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.

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.

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.

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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.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey