Summary for zinc plating leaching Discussion
Leaching in this zinc electrolyte plating thread refers to a post-treatment used on older hexavalent chromate finishes to bleach out the yellow/amber color and make the coating look clearer. Luiz Souza quoted an internal process sheet describing “leaching (incandescent passivity)” after zinc electrodeposition, washing, nitric acid, and chromatic passivation, and asked whether it was a passivation step, whether it was still used, and whether it might be hexavalent chromate conversion coating. The answerer explained that the yellow/amber/honey color was the natural appearance of effective hexavalent chromates, and that immersing parts in hot water or a special leaching bath was a common way to reduce that color. He cautioned that if the color is gone, corrosion resistance may also be reduced, though that may be moot because the specification is likely obsolete. He added that industry has largely moved to trivalent chromates with topcoats, which are clear and do not need leaching, and suggested ASTM B633 as a starting point for a new spec.
Q. Hello.
I’m a engineer working on a fastener specification, but I have a limited knowledge about coatings.
I came across a fastener coating specification (A relatively old specification, from 1998) that states:
Electrolytic zinc coating + Leaching
I have no problem with the zinc specification, but what could be this Leaching post-processing?
Many other questions arise in my head: Is it some type of passivation? What is its purpose? Is it still used nowadays? There are better options? Is “leaching” a known and correct term? May it be a hexavalent chromate conversion coating?
This leaching follows a company’s internal standard. Here are some additional details about it:
– Observation: Treatment as final operation of a component, piece or fixing element already submitted to a superficial treatment. As a post treatment, the surface protection characteristics applied on the component can’t be damaged.
– Product 1
– Chromatic White
– Water dilution: g/L 1-40
– Application Temperature °C: ambient temperature
– Immersion time sec: 5-30
– Allowed agitation: Light
– Reservoir material: Carbon steel, polyethylene or PVC
– Product 2
– Chromatic yellow
– Water dilution: g/L 7,5-45
– Application Temperature °C: 16-43
– Immersion time sec: 5-30
– Allowed agitation: mechanical or solution movement
– Reservoir material: Steel, polyethylene, PVC or ceramic tank
– As it is alkaline salt, this product must be used only with safety gloves, safety eyeglasses, and
the necessary safety equipment for individual and collective protection, avoiding any contact of
the product with eyes or skin. In the case of contact, wash the area immediately with clean water
and go to the doctor.
– Processes sequence:
– 1 Electrodeposition
– 2 Washing at ambient temperature
– 3 Washing at ambient temperature
– 4 Nitric acid, 2-5 %
– 5 Washing at ambient temperature
– 6 Passivation with product 2
– 7 Washing at ambient temperature
– 8 Leaching with product 1
– 9 Washing at ambient temperature
– 10 Washing at ambient temperature
– 11 Washing at ambient temperature
– 12 Drying, centrifugal process or other.
– Notes:
– If balanced properly, it doesn’t remove the chromate layer, giving a better resistance to corrosion.”
Thank you very much for your attention.
Luiz Souza
Engineer – Sao Paulo, SP Brazil
January 16, 2025
A. Hi Luiz.
The effective hexavalent chromates were yellow/amber/honey colored — the natural color of hexavalent chromate. It was fairly common to bleach out the yellow color, turning them more of less clear, by immersing in either hot water or a special leaching bath. Some people say if the color is gone the corrosion resistance is gone, but that’s probably a moot point for you because this is probably no longer an acceptable specification anyway.
With few exceptions the world has moved on to trivalent chromates because of the toxicity of hexavalent chromate. So, depending on who your customer is, you probably need to switch to a trivalent chromate with topcoat. These are clear anyway and do not require leaching.
If you don’t wish to retain a finishing consultant, a good starting point for a new specification is probably ASTM B633.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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