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E-coating over Zinc or Zinc Cobalt Plating

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Q. We have a small EPD Line at our company. There is a part that we receive that is a powdered metal part. The parts are plated with a zinc plating. We do not pretreat the parts, only wash with DI water before the parts go into the the EPD tank.
After the parts are cured we do a cross hatch test and the adhesion has always been an issue. What kind of things could we do to improve the adhesion on these parts?



Tom Evans
- Anaheim California
August 9, 2022

A. You have not mentioned if the part is passivated before ED coating.If the parts are passivated, remove the passivation or tell your part supplier to avoid passivation process.
And as suggested in this thread by Mr. Khozem dip those part in phosphating tank & proceed as your regular process instead of just washing those parts in DI water.
This is surely going to help you.

Avinash Vithal Vidhate
- Nashik, Maharashtra, India.

Q. The parts are steel with a BARREL ZINC PLATE CLEAR TRIVALENT
coating before we receive them. I have a tank with Iron Phosphate heated to 130 Deg.

Would you recommend we try immersing the parts in the Iron Phosphate for 1-3 minutes before EPD Coating?

Tom Evans [returning]
- Anaheim California
August 16, 2022

A. I would suggest you to remove that passivation and then get it coated through your line.
Right from the first tank to the last tank

Avinash Vithal Vidhate
- Nashik, Maharashtra, India

A. Hi Tom. Avinash is surely right! Zinc plating is almost always chromate conversion coated because as a final finish it would otherwise tarnish and get white rust. But chromate conversion coating is sometimes hexavalent, sometimes trivalent, sometimes thick film, usually thin film, usually topcoated with something if trivalent thin film, etc.

All of these options don't always cause adhesion problems, but they are wasted effort that achieve nothing for you while presenting opportunities to introduce poor adhesion. Get your plater to skip the chromating entirely and ship the parts to you immediately, then do as Avinash suggests :-)

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

thumbs up sign Thank you very much Ted.

Avinash Vithal Vidhate
- Nashik, Maharashtra, India

? Any improvements if you have followed the suggestions suggested by me?

Avinash Vithal Vidhate
- Nashik, Maharashtra, India.

A. You answered your own question when you said that you do not pretreat the material before coating.
Any zinc coating (electroplating, galvanizing, sherardizing etc) needs a competent pretreatment before overcoating. Such pretreatments might include zinc phosphate, chromate or some of the dry in place polymers available. Can be spray or dip applied, some require temperature.

Geoff Crowley
Crithwood Ltd.
supporting advertiser
Bathgate, Scotland, UK
crithwood logo
September 26, 2022

Q. Thank You for your feedback. The parts we received have a barrel rolled zinc plated. This is our pretreatment process before EPD. We have different tanks setup on our line.

BioPrep 20/100 Bath: 3-4 minutes soak 120 °F
DI Water Rinse: Dip 2-3 times
DI Water with Stabilizer T: Dip 2-3 times
DI Water Rinse: Dip 2-3 times
DI Water/Rinse Aid: Dip 2-3 times

Coating tank: ClearClad DC 296 Black
Cure Oven: 365 °F 35 Min (per mfg spec sheet)

Tom Evans
- Westminster, California
September 27, 2022

A. Hi Tom.
I don't know what "Stabilizer T" is -- who makes it, and what's it supposedly for? Since I don't know what it is, I have to suspect it as a problem.

But we seem to be going in circles; I would again suggest asking the plating shop to NOT chromate the parts, and then go into that iron phosphate tank that you spoke of. BioPrep 20/100 is a mild cleaning solution but it does not comprise the 'pretreatment' that Geoff speaks of. Your present 'pretreatment' is proprietary trivalent chromating and proprietary topcoat sealing and it's very possibly not highly compatible with E-coating.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

⇩ Closely related postings, oldest first ⇩

Q. What causes the parts to be peelers after E-coat? After alkaline zinc plating then they are transfered to other location for E-coat process.

popat patel
Popatbhai B. Patel
electroplating consultant - Roseville, Michigan

A. Dear Mr Popat Patel;

Trivalent Passivated parts will present no problem, or you might have to phosphate over the zinc plated parts to avoid peeling.

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind
supporting advertiser
Bengaluru, Karnataka, India
saify logo

A. Hi. Mr. Vahaanwala's advice sounds good, and logistically thoughtful as well. Although phosphating is probably the best pretreatment before painting or e-coating, zinc plated parts are almost universally chromate conversion coated instead ...
... so try to minimize problems by using trivalent chromates, which are more heat resistant than older hexavalent chromates. But if it's actually practical, phosphating is probably a better solution than chromating :-)

(Update 2010: Products finishing has a very good pair of articles on this in their finishing clinic. The first [] says that phosphating rather than zinc plating is the right answer, whereas a revisit [] says e-coating on top of zinc plating can actually work fine while offering greater corrosion resistance).

Luck & Regards,

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Multiple threads were merged: please forgive repetition, chronology errors, or disrespect towards other postings [they weren't on the same page] :-)

Q. I'm new to the auto parts supplier business and our company is trying to supply a small part that is specified to have a Zinc Cobalt and Trivalent Clear Chromate base coat and a Cationic Electrocoat top coat. Two prospective suppliers of e-coating are quoting "Cathodic" electrocoating verses "Cationic" electrocoating, is there a difference? And how difficult is it to apply e-coat to a Zinc Cobalt and trivalent clear chromate base coat?

Dan W. Kava
Quality Manager - Cleveland, Ohio USA

A. I don't think there is any difference between the two processes, just semantics. Years ago there was anodic electrocoat, but these days I believe it is all or virtually all cathodic. But as for e-coating over the trivalent chromate, start your conference meeting with the plating shop and their supplier to resolve the issues immediately :-)

There is a good chance of significant trouble, and if the plating shop and e-coater are separate shops I can almost guarantee it.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Multiple threads were merged: please forgive repetition, chronology errors, or disrespect towards other postings [they weren't on the same page] :-)

Q. Hello ... I am in manufacturing and manufacture a line of under automotive chassis components. I'm looking for an attractive black finish with excellent corrosion resistance and am wondering if e-coating can be applied as a top coat to typical clear zinc. The reason I'm interested in this is that a few of these parts have threaded features and build up is a concern. Is a zinc base with the threaded features masked and then an e-coat a top coat an option?

Todd Lohse
Manufacturer (job shop machining) - Elbow Lake, Minnesota, USA

A. Yes, E-coat can be used as a topcoat over zinc plating and yes, the threads can be masked if desired. This is used for automotive components that require excellent corrosion resistance such as chassis and under-hood areas exposed to salt spray.

Toby Padfield
Automotive module supplier - Michigan

A. E-coat is used extensively for this very purpose. It is very stable coating over Zinc but in case you are doing Zinc plating, you are do away with that step. If your threads have a tolerance of 0.8 mil / 20 microns per side you may not need to mask the threads.

A few trials will answer most of your questions.

Gurvin Singh
Mohali, Punjab, India

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