Yellow Chromate Adhesion Problem
We are attempting to set up a yellow-chromate on acid rack zinc plate. We have read that it is better on alkaline zinc as oppose to an acid zinc due to chromate adhesion issues with the acid zinc. What can we do to eliminate chromate adhesion issues and problems on acid zinc . ThanksNathamiel Adekoya
- Binghamton New York
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.
Ted Mooney, P.E.
Pine Beach, New Jersey
A very common problem is too much brightener. It will cause terrible adhesion and a nitric dip does virtually nothing when it is excessive.James Watts
- Navarre, Florida
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 , 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 )
- Victoria Australia
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:
- Operate the bath at optimum or slightly above optimum Carrier/Wetter/Makeup levels to improve solubility of the aldehyde based Brightener
- Operate the bath at minimal Brightener levels to avoid over brightening of the deposit
- Treat the bath "as needed" with activated carbon or potassium permanganate to remove dragged-in organic contaminants and excessive brightener content
- Use a yellow chromate that is specially formulated to provide good receptivity/adhesion on acid chloride plated work
- Brunswick, Ohio
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