Coating on older DEC slides

Noel Chiappa jnc at
Thu Jan 1 15:34:13 CST 1970

Thanks to everyone for trying to help, but I'm not sure we've cracked it yet.
This coating is _not_ a lubricant: it's a hard, thin even coating over the
entire piece; it cannot be easily scraped off (e.g. with a fingernail), but
requires a knife tip or somesuch. So I think that rules out the molybdenum
disulfide suggestion.

A few comments to various replies:

    > From: Rod Smallwood

    > It might be cadmium

I don't think so; cad-plating produces a surface which is either silver, or
yellowish (cadmium dichromate). (The yellowish coating one often sees on
later DEC equipment is probably not cad, but zinc-dichromate; see here:

for a good intro to distinguishing platings. This makes it sound like we
might be dealing with some sort of phosphate plating.)

    > From: Mike Ross

    > I do hope not; that stuff is *seriously* toxic.

Indeed: "Cadmium is very toxic and should not be used on any part intended
for use where direct food contact could occur"; but it is/was used to plate
things (especially in the aerospace industry, where it reduces the tendency
to galvanic corrosion when placed in contact with aluminium).

    > From: Al Kossow

    > molybdenum

Did you mean molybdenum disulfide (which I don't think it can be, for the
reasons given above), or some other form?

    > From: Chuck Guzis

    > If this is a small job and you need a solid coating, have a look at
    > some of the moly-teflon rattle-can gun coatings from Brownell. You
    > spray them on, then bake them. The finish is very tough and pretty much
    > foolproof.

Right, but I'm not just looking for _a_ coating, I would really prefer to use
_the_ coating that DEC did originally (if it's still available, of course).

    > I've got a few HP slides from a storage array and I'll swear they're
    > just plain old electroplated zinc.

Some off-brand slides are zinc; and, as I said, I think a lot of later DEC
stuff (racks and frames) is dichromate zinc.

    > From: William Donzelli

    > [dry moly disulfide] can be mixed with a resin binder and thinly
    > painted on surfaces, like rack slides.

Right, but this coating doesn't look like it was painted on. It's got a
finely textured surface that is rougher even than flat paint.


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