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Old 28th July 2008, 06:12 PM   #1
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Default Yellow zinc passivation of steel

Hey All,

Yellow zinc passivated mild steel seems to be quite a common chassis material - maybe not so much now (apparently it's not very green?).
Anyway.. does anyone have a recipe for doing it yourself?

I'm not looking for 'kits', or dips that have trade names that kide their true chemical ingredients - I want to know the real chemistry of the process, but I can't find any internet sites that describe it.
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Old 29th July 2008, 01:33 AM   #2
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Default yellow chromate

It's a chromate conversion coating produced on the zinc from an acidified hexavalent chrome solution, with certain activators added (acetate, formate, chloride,fluoride, nitrate, etc.)

These are all proprietary solns. and I'm not at liberty to divulge their formulations, but a soln. of sodium dichromate in weak nitric acid migt be a starting point for a zinc plated chasis.

Why do you want this? it's mostly used as an undercoat for painting nowadays.

John L.
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Old 29th July 2008, 10:26 AM   #3
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Maybe I've got the wrong process name. The finish I am thinking of doesn't take paint. It's a golden-yellow finish with pinkinsh purple areas - almost like an oil film on water. The colour patterns make it look like it was diiped - they sort of follow water run marks.

The reason I want it, is that I have some 'old' 19-inch rack cases made like this - I've cut them up to make new chassis, but I need to do something to the cut edges to prevent corrosion, and make them look nice - preferably something that matches the original.
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Old 29th July 2008, 04:17 PM   #4
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I believe the process you are looking at is cadmium plated steel. It can leave a yellow/iridecent finish like you describe.

I doubt if you can DIY. Its hard (near impossible) to find local shops that will do it these days, as the chemicals used are under tight EPA controls.

You might try inquiring at places that chrome plate motorcycle parts. Lots of times they also do cad plate as a sideline.
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Old 29th July 2008, 04:48 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

I was under the impression this was the cheapest form of stainless
steel used in a lot of far eastern stuff - for example tape decks.
AFAIK it is not surface treatment, the small parts being punched out.

/sreten.
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Old 29th July 2008, 04:56 PM   #6
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Default done this for.. oh 4 decades now

Nope...

All of the finishes you see are conversion coatings of one form or another

Iron/steel can be chromated or phosphated, zinc, cadmium, copper, aluminum etc. can be chromated to varying degrees, and can indeed be an undercoat for subsequent painting.

The process provides additional corrosion protection over the bare metal, depending on the soln. composition, alloy, etc.

Much of the "older stuff" with the greenish yellow/pinkish finish is yellow chromate, usually formed over zinc or cadmium plating.

Not something a DIY-er will probably be successful at, although as I stated b4, you can try with acidified sodium dichromate / HNO3 solutions.

John L.
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Old 29th July 2008, 05:32 PM   #7
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Default Re: done this for.. oh 4 decades now

Quote:
Originally posted by auplater

Much of the "older stuff" with the greenish yellow/pinkish finish is yellow chromate, usually formed over zinc or cadmium plating.
I believe John L is right with chromate, it's the kind of metallic yellow with a slight neon green hint coloured finish

Click the image to open in full size.

http://en.wikipedia.org/wiki/Passivation

http://www.finishing.com/117/28.shtml <----- This is the place to ask about finishing, if you can do it someone here probably has done it...

http://www.caswellplating.com/ <----- sell various finishing kits for home scale work that can achieve professional results more than once and don't involve industrial equipment. Don't know if they have a chromate kit, but you can ask - they'll be much more geared towards your needs than any industrial supplier, and probably more helpful

From memory, I think phosphorus forms phosphoric acid at some percentage when dissolved in water. Phosphorus is a primary ingredient in meth synthesis, so if you're going to buy it expect questions and possibly visits. Although, if you have a genuine use, you have nothing to worry about in that respect. It is also available from strike anywhere matches of coarse. Phosphorus, and most things containing it, are toxic (not just harmful).

John
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Old 29th July 2008, 05:33 PM   #8
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It's always been my understanding that the iridescent coating used on steel was an aluminum coating. See here:

http://www.finishing.com/121/17.shtml

Not something uou could do yourself, it would appear (although look at some of the exotic things people do on this forum).

If it's just the cut edge as you suggest, I would simply grind/file/sand/buff smooth and then hit it with a sealant such as lacquer or clear nail polish to prevent corrosion. The alternative is to clean up the edge and take the whole piece for nickel or chrome plating.

Also, be aware that some of the things discussed here, like cadmium and hexavalent chromium are quite toxic and constitute a health hazard and may create hazardous waste that shouldn't be dumped down the drain.

--Bukapound
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Old 29th July 2008, 05:40 PM   #9
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Quote:
Originally posted by Buckapound

Also, be aware that some of the things discussed here, like cadmium and hexavalent chromium are quite toxic and constitute a health hazard and may create hazardous waste that shouldn't be dumped down the drain.
^This is true, and not in the sense that bleach is hazardous, you really need to take care with metal finishes.

Not sure about an aluminium coating. Zinc is the coating used on most steel. You can get things galvanised if you just find the local galvinisers and get friendly. They usually do industrial sized batches of railings and other stuff like that, but could probably be talked into doing a chassis. They just hook them onto a conveyor and it goes through a huge zinc bath that's continually kept molten - usually 24/7. You need to make sure you can't end up with pockets of trapped air if you galvanise anything, that's why fence and gate posts have holes drilled in them somewhere - to let the air pocket inside the single ended tube escape as it goes into the bath, allowing the inside to be coated.

Personally, it'd be cheaper and easier to just take it to anywhere that might be equipped with a spray booth or powder coat oven and get it painted. There are even guides for a cheapo DIY powder coating gun and videos of the results on YouTube - it uses a tiny, $5-10, over the net, high voltage generator for air ionisers I think, the diode multipler kind. You can cook the powder in a kitchen oven (raid one from the dump).
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Old 29th July 2008, 06:21 PM   #10
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Those yellow bits in the photo look like what I have, though mine is more 'yellow'. If anyone has ReVox machines, they are done like that inside.

Phosphoric acid is very easy to get here - just about every hardware store sells it. It works very well funtionally, but it doesn't give a 'pretty' result.
I do have already a bottle of potassium dichromate, so I'll give that a try - thanks for the suggestions all.

Since I only want (at the moment) a very little done, it doesn't make sense to get someone to do it for me, or to buy a kit that produces many litres.
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