Yellow Chromate Adhesion Problem

A. Hi Joshua.

1. Neither will do any good and either may do harm, but some production shops do zinc plating and chromating without a nitric acid dip in between, so it's certainly possible to do without if necessary.
2. Absolutely. We attached your question to one of several long discussions on the subject.
3. Larry Durney answered all such questions with the same first answer "Obey the Letter of the Law" -- follow the instructions exactly, not approximately; even more important for a beginner.
4. No, heat is not necessary for curing; the parts should be dried, but anything over 140 °F (60 °C) is destructive.

Luck & Regards,

A. Zinc is not ductile, which is why cadmium was traditionally used for such applications as hose fittings destined for crimping.

Still, and this is just intuition and experience, the drying temperature is probably the critical factor here. I say that simply because irrespective of the other factors you vary in the list that you gave us: let the temperature get too high and any of those chromates will dehydrate and slough off. It's a common problem.

Ted,

Thank you for your response.

We are now drying these parts at ambient temperature, but even so this has not eliminated the problem.

We have had some success by introducing a fairly strong water based lacquer dip. Parts from within one batch which have been lacquered seem OK whilst unlacquered parts still exhibit the sloughing off problem after compression. We are not sure if the lacquer holds the chromate on or prevents the chromate film from dehydrating and becoming brittle.

A. Richard,

This is just a thought, but you might look into the possibility that your compression process is building up large amounts of heat which could be destroying the chromate film. Look closely at every step these parts take in the production process after chromate.

A. Dear Richard,

You may try a post-dip that improves adhesion of chromates.

Go in touch with Atotech Australia.

I am not sure if it will help, as the problem you describe very much depends upon the mechanical and physical conditions during further processing of the plated article.

regards,

A. It is not true that one picks the zinc plating process based on reliability of chromate application. Rather, one picks alkaline zinc for difficultly shaped work because the throwing power is much better, and acid zinc where full brightness is required (if we disregard economic and wastewater treatment issues that also affect the question).

Each process utilizes organic brighteners that can discolor the chromate or cause poor adhesion, and most suppliers of proprietaries will offer chromates with somewhat different formulations depending on which brightener residue you are dealing with. Usually, making allowance for a nitric acid dip between the zinc and the chromate offers the required flexibility in the cycle.

A. A very common problem is too much brightener. It will cause terrible adhesion and a nitric dip does virtually nothing when it is excessive.

A. Hi Nathaniel , The problem with Acid Zinc solutions is that the resultant Zinc plate contains a lot of Occluded "Brightener & Wetter" on the surface and if you do the conventional thing and use a pre - dip of Nitric acid it will often send that brightener Film Hard, the answer is to use a pre - dip of Hydrochloric acid and a yellow chromate specially designed ( Built around Hydrochloric acid ) for use after Acid Zinc. You can use yellow chromates based on acetic acid ⇦this on eBay or Amazon [affil links] , but then you would use a pre-dip of acetic acid. Process sequence should be similar to :-

• Acid Zinc Plate
• Air Agitated Static drag - out
• 2.5% Hydrochloric acid
• Air Agitated Cold water Rinse ( double Counterflow preferred)
• Yellow Chromate
• Air Agitated Cold Water rinse ( double counterflow preferred)
• Warm Water rinse ( not above 55 °C )
• Dry

Best regards

A. The deposit from an alkaline cyanide free deposit will provide better chromate adhesion as compared to an acid chloride deposit. This is due to the superior brightness and excellent grain refine of the acid chloride process.

Typically, a plater should consider the following suggestions to improve yellow chromate adhesion:

1. Operate the bath at optimum or slightly above optimum Carrier/Wetter/Makeup levels to improve solubility of the aldehyde based Brightener
2. Operate the bath at minimal Brightener levels to avoid over brightening of the deposit
3. Treat the bath "as needed" with activated carbon or potassium permanganate ⇦this on eBay or Amazon [affil links] to remove dragged-in organic contaminants and excessive brightener content
4. Use a yellow chromate that is specially formulated to provide good receptivity/adhesion on acid chloride plated work

A. You have not mentioned the Zinc Plating Process being used? Is it Acid Chloride Based Zinc plating process as this process has some problems related to adhesion of the chromate, You must use special proprietary chromates for Acid Chloride Zinc process. The Alkaline Cyanide Free Zinc plating process is also the best process to use for good yellow chromate adhesion. Post plate rinsing (use multiple rinses of good water) and Nitric Predip can play a role, please look into this area as well.

Best of Luck,

• Topic 45598 "Chromate rubs off of zinc plating" or