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topic 291

Zinc Chromate plating surface quality


(1996)

Q. Hi guys,
First your page is great!

I'm a not plater / finisher type, but I have a friend and old student who has a subcontract plating operation that is giving him fits, to the point that he asked me for help. Neither of us and his subcontractor have much high quality plating experience, other than my electroless nickel/copper on diamond materials, and that's rather specialized.

His problem deals with Zinc Chromate plating of cast iron parts.

The problem is that the output parts require a smooth surface finish, about 24 rms or less, the parts entering the plating process are 16 rms or less in surface finish.

The current part often soon after startup develop rougher, rougher surfaces exceeding 64 rms.

I have suggested:
- Periodic Reversing of the electrical potential.
- A double cycle of the plating process,
The First for color or maximum coating thickness.
The Second for producing a uniform surface.

He has yet to try either of these.
Thanking you in advance for any suggestions.

Gordon Smith


A. The chromate is thin enough that it is not likely to be having any measurable effect on surface roughness, so I think you are on target looking at the zinc; but I would certainly suggest taking roughness measurements on a part after cleaning/acid dipping, and before plating, to be sure the plating is the problem.

I would do my best to make sure the brighteners/levelers are optimum and that the filters are working perfectly before proposing other changes.

I think your ideas have merit, but I don't think they are going to work to your friends total satisfaction because plating nearly always roughens a surface. Unfortunately, I don't have any experience taking surface roughness measurements, to be able to quantify this assertion. Hopefully one of our readers can offer a more quantitative answer!

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


Desert Island Consultancy

(or, How would you solve the problem if you were on a desert island)

A. Dear Mr. Smith:

1) What preplate cycle do you use for the parts; times, concentration, temperature

2) The preplate cycle does not increase RMS?

3) For zinc plating: What is the average current density in amperes/square foot?

4) What kind of zinc plating solution are you using?

Please post the latest laboratory analysis.

5) What is the RMS profile over the part from lowest to highest current density.

6) Is the problem intermittent?

7) Check RMS before and after chromating to eliminate/implicate one or both.

pooky
tom pullizi signature
Tom Pullizzi
Falls Township, Pennsylvania

Now what did I do with that machete?..

You have a machete?, Why, when I was on a desert island, blah, blah,blah..


Looks like my partner is working too hard. --Ted


(2000)

Q. Help please! I am looking at importing some wire shelving from Asia.

The price is very competitive but the USA customers say that the epoxy coating must be applied over a zinc chromate substrate to prevent chipping off of the epoxy and/or corrosion or rust etc.

Can someone please help me understand what this zinc chromate process is? Also--- don't you think that the makers of the shelving, who have chrome plated, stainless steel and epoxy coated shelving should be able to apply this substrate? Is it complicated or expensive?

Thanks-- Peter

Pete Hymans
- Roseville, California


November 15, 2008

A. Hi, Pete. Epoxy is fine, but organic coatings like epoxy require pretreatment before the coating, and one of the excellent pretreatments is zinc plating plus chromate conversion coating. One reason it is good is that zinc plating is a sacrificial coating which will protect the article from rusting even if the epoxy coating is scratched.

Yes, your shop should be able to do this. Zinc plating plus chromate conversion coating is not an exotic process and perhaps a little easier than chrome plating.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


Zinc Plating
by Geduld





Electroplating Engineering Handbook

January 4, 2012

Q. Respected sir
I want to know how to manage zinc plating quality, and step by step working procedures.
Why variations in the zinc plating thickness? and material evaporation?

SAJI ELDHO
- dammam, saudi arabia


January 5, 2012

A. Hi, cousin Saji.

We admire your thirst for knowledge, but people spend YEARS writing their step by step procedures for plating and quality assurance. You would need to read several plating books for that depth of information. As a starting point, can you us whether you do rack plating or barrel plating; what general types of parts you plate (fasteners, aerospace components, fencing that will be painted); whether these parts will see a benign indoor environment or hostile outdoor exposure; and whether you do cyanide zinc plating, acid zinc, or alkaline non-cyanide? Must the chromates be RoHS compatible?

The primary thing that affects thickness is that the amount of plating deposited is directly proportional to the current density per Faraday's Law of Electrolysis. Electricity takes the path of least resistance, so there will be little current flowing to recessed areas, and the plating will be thinner there unless you can correct it with shields or auxiliary anodes.

Sorry, I don't understand "material evaporation" -- please clarify. Thanks.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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