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-----Chromate sloughs off on slightly stressed metal.
We plate hose couplings with .40 mils. of zinc and finnish up with a layer of chromate 2 millionths thick. The problem we are having is , where the coupling have been bent or flared open (slightly stressed areas)the chromate keeps sloughing off even after a longer drying peroxide than is recommended. The parts go through several phases before zinc and chromate plate. They generally are inducted on the fingers of the collet to a rockwell hardness of around 80 c scale ,and then put on a flair press to be slightly flared open for the hose to fit into the coupling. It is where the slight bend from flaring that our chromate "peels" or rubs off. Our customer will not take this. The zinc is plenty to protect the part but aesthetics seem to be a major part of our customers appeal for our product as well as the quality. If there is anyone that had dealt with a similar problem or knows how to solve this, please fill me in. Thanks and have a splendid day.
James D. Browning- Oxford, Mississippi
1999
Similar parts were traditionally cadmium plated; and because of the greater ductility and lubricity of cadmium, it was easier. Still, when chromate sloughs off it usually has been dehydrated. I would suggest drying less, not more. No hotter than 140 degrees F.
Ted Mooney, P.E.
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finishing.com - Pine Beach, New Jersey
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You may also want to pay attention to your chromating time and the concentration of the chromate. We have seen better results with a chromate that is less concentrated and by not applying a heavy chromate. This seems counter-intuitive but it's worked for us...
Good Luck.
Megan Pellenz
- Syracuse, New York
1999
My company manufactures tubing assemblies. We use zinc alloy plating (quite similar to plain zinc) and have a chromate conversion coating on top of the plating. We flare, bend, and crimp these parts after plating quite a bit. Post-forming parts the way we do definitely damages the coating and decreases the corrosion resistance of our product. However, it is much more economical for us to post-form these parts because of our manufacturing processes. We communicate this to our customer and work with them on this trade-off.
We have found that when our plating thickness is too high, we will get chipping/flaking. I think this may be the same phenomenon you are dealing with. A co-worker of mine has called it the "Tinkerbell" effect (fairy dust). We try to keep the plating thickness high enough so that we get adequate corrosion protection but also low enough to prevent this flaking. Sometimes this is hard because some of our parts have complex geometries, which causes the plating current to vary, and this causes the plating thickness to vary.
I hope this helps!
Tim Neveau
Rochester Hills, Michigan
1999
1999
Hi James ,
What type of solution are you plating the Zinc from ? If it is Acid Chloride or Alkaline Non - Cyanide then you should be using the following sequence after plating ,
1 ) Double rinse ( air Agitated for rack plating )
2 ) 2--> 5 % Hydrochloric acid dip ( concentration depends on drag - out )
3 ) Double Rinse ( as above )
4 ) Hot water rinse ( not more than 55 °C )
one further word , the use of Hydrochloric acid NOT nitric is important , also important is the formulation of the Chromate itself, I recommend chromates built around Hydrochloric Acid rather than nitric for both the Chloride & Alkaline non - cyanide solutions .
I am sure that if you use these tips all your days will be splendid ! regards
John Tenison - Woods
- Victoria Australia
Thanks for all the good input. I'll look into these and see if it has any affect. hanks
James D. Browning- Oxford, Mississippi
1999
We see a lot of that on some brackets we plate near the mounting holes. If you are using a chloride zinc go as easy on the brighteners as possible and still get a bright finish. The film created by the brighteners is hard for the chromate to adhere to. We tried using both types of acid mentioned but it seems to give our parts a slight haze. We have a lot of success just leaving the parts in the chromate drag out for a few minutes prior to chromating. It seems to help dissolve any residue left by the brighteners. We also put small amounts of acetic acid ⇦ on eBay or Amazon [affil link] in our chromates to keep the film from becoming too brittle(hey, don't laugh it works)
Jim Conner
Mabank, Texas USA
1999
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