Chromate rubs off of zinc plating

A. YOU HAVE NOT MENTIONED WHICH TYPE OF PLATING BATH YOU ARE USING. HOWEVER WASHING BETWEEN PLATING AND CHROMATING, AND PROPER PREPARATION OF PASSIVATION BY USING LR GRADE NITRIC ACID WILL IMPROVE. EXCESS CONCENTRATION OF CHROMATE MAY CAUSE POOR ADHESION, MAINTAIN MINIMUM THICKNESS OF 8 TO 10 MICRON OF ZINC COATING.

A. The previous responders are all quite correct. A few other points if you please. The reason it is necessary to know if you are using an alkaline or acid plating bath is that acid chloride additives tend to increase the total organics in the bath and consequently interfere with the formation of hexavalent yellow films. The solution to this is to purchase a chromate formulated to be effective on chloride zinc, rinse well and pre dip in 0.5% nitric (although sometimes 1% sulfuric acid works better!). It is also helpful to reduce the total organics in the plating bath by lowering the amount of brightener used, carbon filtration and oxidation. Further, be sure that there is no oily residue on the surface of the zinc bath (for all types of electrolytes), rinse tanks or chromate.
When using an alkaline electrolyte good rinsing and maintenance of the acidity of the acid pre dip is a must to keep from getting the chemistry of the chromate out of balance.
Finally, as mentioned in the other replies, do not overheat when drying. If work is done in a barrel do not over rotate when chromating and do not over dry. Among the other don'ts for barrel work is do not reverse direction of the dryer to aid in stopping the rotation. This causes the parts to scrape against each other and remove the chromate.

A. Mr.Faiz , In Bangalore platers are using a chrome fixer which normally fixes the passivation , even after rubbing passivation won't removed, which has to be used @3%,and as a last rinse before drying.
All the best.

A. I am a bit confused. Is the work dull that is the problem, or is the rub off of the yellow the problems. Both should be. Alkaline non-cyanide zinc will plate bright with good yellow chromate adhesion. The rinse(s) after plating should be clean and have a near neutral pH. You should then have a pre-dip tank before chromating consisting of 0.5% nitric acid. This will help remove residual alkaline and organic films from the deposit as well as brighten up the finish. A properly formulated hexavalent yellow chromate generally has a pH of between 1.8 and 2.0. If your pH is higher you have either a weak chromate or one which has been contaminated by drag over of the alkali from plating. If the latter is true, additions of acid or even chromate to maintain pH will at some point unbalance the chromate resulting in non adherent films. Finally, as has already been pointed out by previous responders, you rinsing after chromating needs to have adequate flow to be clean. A final hot water rinse (120 °F) is recommended. If you are barrel plating there can be mechanical reasons for finishing problems.

A. Hello,
This problem may arise due to 2 typical situations:
1. There is too much of carrier brightener in the alkaline zinc bath.In such a case it does not passivate properly.
2. Usually the Bright dip concentration recommended is 1% v/v so that the excess alkalinity is removed completely before yellow passivation.

Besides the above 2 reasons, there is always a likelihood of acidic fumes condensing on the passivated parts when Relative Humidity is high wiping off the passivation color easily.(this happens in small shops without separate stores for finished goods.)

A. Hi Aravinda. Are you sure you have correctly identified the problem? Before we go on, what evidence do you have that the zinc plating is depositing on, and adhering properly to, those welded areas? Good luck.

Regards,

A. Hi cousin Bharat. The first thing I would do is try doing the hydrogen de-embrittlement process before the chromating. Good luck.

Regards,

A. Hello Bharat,

As Ted said, you may have better results when the chromate doesn't pass through the baking process. AND If I were the process designer for this particular part, I wouldn't dare to bake with the chromate, because the high thickness of the coating makes it more difficult to hydrogen to escape, and chromate makes it harder. You surely will have better results in embrittlement relief baking without chromate.

Best of luck!

A. 25 microns is a lot of plating particularly if it is zinc electroplating. I would be worried about deposit ductility. I think that if you need that much coating or corrosion protection you should investigate alternative processes such as mechanical galvanizing, hot dip galvanizing, dip-spin coatings, or zinc-nickel plating.

A. Hi Suresh. The yellow chromate conversion coating as it stands is improper -- it should not easily rub off with a thumb. There are several possible and common causes. Insufficient curing time (you should not attempt that thumb test for at least 24 hours after plating), drying at too high a temperature (should not exceed 140 °F / 60 °C), or excessive brightener in the zinc plating tank for starters (plus, of course, a defective chromate bath).

In the USA we do not call zinc electroplating 'galvanization'; I'm not sure of the custom in India. In any case, it is probably not the zinc plating which is being removed with your thumb, but just the yellow chromate conversion coating (passivation). Good luck.

Regards,

A. We have encountered the same problem. Ours was insoluble oils carried from degreasing got into the zinc plating and passivation baths resulting in that the passivation could be rubbed off.

A. Hi John. Although your question is about black chromate, and this thread was previously about yellow chromate, they are similar enough that I'm sure there is good food for thought on this page, so we appended your inquiry to it. But first, are you saying you gave the parts to your customer immediately with no curing time? Chromates should cure for 24 hours before handling.

Luck & Regards,