Troubles with Yellow Chromate finish

[adamthomas]

I'm having some real challenges obtaining a proper yellow chromate finish on my fasteners. My attempts at using the yellow chromate solution according to instructions is easily rubbed off of corners and threads after several days of drying time. It is not easily rubbed off of smoth surfaces but I expect that applying a wrench to them will change that. This suggests that it is merely a coating rather than a treatment. Is the Caswell formula simply a surface coating or is it supposed to interact and bond with the underlying zinc plating?

My understanding was that chromate is an oxidizer, and as such it changes the surface of the zinc plating causing it to oxidize and creating a harder and more protective surface than the zinc alone. Please educate me if that's wrong.

I have a 1.5 gal Copy Cad kit with the 110F/140ma formula and I have been very careful to work within the parameters provided with all solutions in the kit. I finally have a decent rectifier and have been able to achieve consistent plating results. Now if I could only get a more durable chromate finish I would be a happy man.

Here are the steps that produced the best chromate finish results so far, but still not what I was hoping for.

1) Parts cleaned of all dirt and grease using commercial degreaser and prepped with a wire wheel.

2) Parts are stripped down to bare steel removing any remaining rust or zinc plate using a 5% solution of muriatic acid until bubbling stops. Time varies from 10 seconds to 10 minutes.

3) Parts are thoroughly rinsed with distilled water.

4) Clean parts soaked in SP degreaser @ 180F for a minimum of 10min.

5) Parts are rinsed thoroughly with distilled water and immediately placed into plating tank.

6) Zinc plating is being done at a calculated ~140ma/sq in. @ 110F for 20min with 1/2 tsp. of brightener.

7) Parts are removed immediately and bright dipped in 5% muriatic acid solution for 3-5 seconds.

8 ) Parts are rinsed thoroughly with distilled water and immediately placed into yellow chromate solution.

9) Parts are swirled in 1.2% yellow chromate solution @ 80F for 30 seconds with medium agitation. Note: (1.2% = 1.5 fl oz./gal)

10) Parts are rinsed thoroughly with distilled water and hung to dry at room temp(62-68F).

That does it. One thing I have noticed which raises my questions about the yellow chromate being a coating rather than an oxidizer is that placing the treated bolts into 5% muriatic acid, strips the chromate off like peeling paint. In otherwords, what appears to be skin like sections of the chromate coloring come off rather than simply disolving in the acid.

Any suggestions or corrections would be most helpful.

Adam

[lcaswell]

Try skipping step 7.

The yellow chromate is a true chromate. It bonds with the zinc.

[adamthomas]

Originally Posted by lcaswell

Try skipping step 7.

The yellow chromate is a true chromate. It bonds with the zinc.

Thanks for the quick reply Lance. It's nice to see the personal ownership and involvement that you take here on the forum.

Your response was the response that I was expecting. Unfortunately, I have already tried it both with and without the bright dip. My first attempts did not include a bright dip. Later attempts included increasing the amount of agitation and heating the solution to 80F. Lastly I tried it with the bright dip and that seemd to produce the best results overall. Note that I did the bright dip not for brightness but more for insuring that the newly plated surface was prepped or activated, and ready for chromating.

I plan to run another batch this weekend and will try to replicate this again w/o the bright dip.

BTW, I am only using 1/2 tsp brightener in 1.5 gal of plating solution.

The fininsh on comercially chromated fasteners seems to be much more durable and does not come off easily, even with considerable wrenching. Is it unreasonable to expect a similar level of durability with your yellow chromate solution? How does this method differ from the methods the commercial platers use? Or does it?
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[adamthomas]

Ok, I'm finally getting some better and more consistent results. As suggested I eliminated the bright dip, although I have a suspicion that this was only part of the problem. I believe the main culprit was that I was using the same muriatic acid solution to strip the parts down to bare metal, and it was simply too dirty, having been polluted with oils from the fasteners I was stripping.

I think the second factor was that I re-did my amp calculations, creating some more accurate formulas for various well defined parts like bolts and nuts, providing more consistent results.

Lastly, it seems the chromate solution reacts better with a smoother plate. This was achieved due to a combination of the more accurate caclulations, a better more accurate rectifier, regular additions of brightener as the plate began to dull, and longer, slower plating times.

Thanks for the help guys. I'll post some pics when I'm done with this batch and find some spare time.