CED coating has pinholes

A. Hi Govind.

There is obviously nothing wrong with seeking a "root cause", but we must not misinterpret what the phrase means. It does not mean that there is only a single possible cause for pinholes, and what it definitely does NOT mean is what ONE thing can we change that will fix everything even if we continue doing five other things wrong :-)

As Larry Durney tells us in the previously-mentioned "Trouble in Your Tank", one key to troubleshooting is "Obey the Letter of the Law", and others are "Look at the Record" and "Walk the Line". Figuring out what has changed is usually a faster route to a fix than a theoretical study, but there are many hints on this page as to the possible causes of pinholes if you determine that no parameters you have been monitoring have changed.

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 2021

A. CED Can be done on hexavalent yellow passivated part. If it is as plated or trivalent passivated, carry out tricationic phosphating then go for CED Coating.

Nithin reddy
- Bangalore, India
June 26, 2011

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Hi Avinash. Nithin's response suggests that you should take a different route depending on whether you are hexavalent chromating vs. trivalent chromating. Let's not move on to hypothetical questions about Zn buildup from phosphating until the inquiry is made less abstract by answering Nithin's question of whether you are trivalent chromating or not. Thanks!

Luck & Regards,

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

A. Maybe it's the angle from which the photo was taken... but I can't see a weir to remove the surface foam being produced by the coating process?

Terry Hickling
Birmingham, United Kingdom
August 17, 2011

A. Bubble on top of coat bath. You need a weir box to roll the top of the tank to. You also need to determine where the air bubbles are coming from.

Adam Adkisson
- Mission Texas
August 19, 2011

A. Most important to avoid pitting at the bottom - clearance of work piece to bottom sparger pipes should be minimum 200 mm!

Sara Michaeli
Tel-Aviv-Yafo, Israel
October 30, 2011

A. Hi KC.
I once had the opportunity to look at an ED film (40X, I think) before the curing process and 'lo and behold ', they were full of pin holes! It is only through the curing process, the higher temperature makes the film more 'fluid', those pin holes were closed up, so to speak.
It may be possible your pin holes problem are curing oven-related rather than ED tank-related?

SK Cheah
- Penang, Malaysia
November 1, 2011

A. First ensure that all the bath parameters are within specification and pH meter, Current-voltage meters are working properly.
Pinholes at the bottom of tank may be due to excessive gassing, i.e., due to high current density which further may be due to high solids content or suspended particles or too low pH.
Also, get solvent content tested by your paint supplier.

Manoj Kushwaha
plating & painting - Rampur, India
November 27, 2011

A. Check your solvent (BC) content. If it is on higher side it will definitely create pin holes. Check also ramp up time for rectifier voltage.

Manish Dhyani
- Noida, India
January 25, 2012

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? Kindly provide details of the pre-treatment process that is done prior to E-coat.

Have you checked the component after pre treatment and before E-coat for presence of oil on its surface?

Gurvin Singh
Mohali, Punjab, India
February 1, 2012

A. Hi Ashena, hi Senthil.

As you see, we appended your inquiries to a thread which already addresses this issue and it should solve your problem. If not, please give us the details of your situation which make it different. Thanks!

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 16, 2012

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Hi again cousin Senthil. 8 readers already offered their specific suggestions on what needs checking, so simply saying that "all parameters are within spec" probably won't move the discussion forward and resolve your problem. Please try to comment more directly regarding those previous suggestions. Thanks!

Luck & Regards,

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

? Hi Virender. Is that a semicircular rust spot at 8:00 o'clock where the recessed diameter meets the ID of the flange? If it is, while you may well have electrocoating problems, the pretreatment has to offer a properly prepared surface first. What is the substrate and the pretreatment? Thanks.

Luck & Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
December 2015

March 1, 2016

? Hi Mr. Virender,
My questions:
1. Tried on other substrates or materials? same defect appeared?
2. Any solvent/oil floating on bath surface during ED process?
3. Ensure no stubborn oil & grease on part surface?
4. After rework process part proceed to ED happened same defect?
Above questions might lead you to certain ideas to proceed.
Thanks.

KC Tan
- Ulu Tiram, Malaysia

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March 20, 2016
A. The problem is in the bath composition; you have to add hydrogen scavenger to the bath like nitroethane or hydrogen peroxide in the ratio 4% -- this is because hydrogen evolves during the process making film defects and the surface of the work piece not fully painted which causes free zones that makes rust and pits in the film so you have to eliminate defects by hydrogen scavenger.

Thanks

Ahmad abo awf
- cairo, egypt

A. Hi Ganesh. We appended your inquiry to a thread with lots of great info on the subject. Please review it, then expand upon your question with details if this thread doesn't solve it for you. Good luck.

Regards,

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

A. For the Pin hole problem:

1. check that all parameters are in range (PT line, voltage, ED temperature, pH, N/v content, conductivity, dipping time & cleaning frequency)
2. check the circulation in ED bath
3. check the hanging positions of part
4. check the proper insulation removed before part loading

To avoid this problem ... then install filtration method in ED bath.

Amir Jamadar
- India, Maharashtra, pune
March 2, 2019

A. Hi Ryan. We may be having a little bit of language difficulty because I don't understand what you mean by "roof". Are you coating discrete parts or reel-to-reel? Maybe you are saying the coating is good at the beginning of a reel, but full of pinholes by the end?

Also, we may not be agreeing on how a DI rinse water tank is supposed to function. Conventionally, you don't fill the tank with DI water and dump it when it gets too contaminated. Rather, you constantly recirculate the water from the tank through a de-ionizing unit, and back to the tank so it remains de-ionized to the same degree all the time. What you change is the DI cylinder when it becomes exhausted, not the water in the tank.

Please clarify these things for us. Thanks.

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 2019

A. Hi again, Ryan. Thanks for the explanation about the roof, it is very clear now!

A whole pretreatment and electrocoating line is a complex arrangement of equipment and chemistry, and I cannot pretend to understand everything that is going on ...
... but if you already know that the problem gets much worse as the DI rinse gets contaminated but you cannot take the necessary measures to keep the DI tank from getting contaminated even knowing that, what can be suggested? Trying to change the electrocoating formulation or application parameters so it is more dirt-tolerant doesn't sound promising to me. Maybe another reader has an idea though.

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 2019

A. Hi again Ryan. Hopefully another reader will jump in because I am not an e-coating expert and I don't pretend to know exactly what is causing those pinholes. But the fact is that contamination can cause them or at least aggravate them, and you already realize that and have demonstrated it.

Why exactly a dirty DI rinse accentuates the problem may not be known now or ever; a certain amount of knowledge is simply empirical and we can't necessarily trace it all the way back to first principals: it might be dirt interfering, it might be that the higher conductivity is allowing galvanic currents, it might be a reaction with some very specific contaminant.

If you have another theory of what is causing it, and feel that a dirty rinse merely accentuates rather than causes it, that's fine. But wet metal finishing processes are quite complex and as Larry Durney writes in "Trouble in Your Tank", an important starting point in any troubleshooting exercise is "Obey the Letter of the Law". When you know that contamination in the rinse tank is substantially aggravating the problem, you need to fix that problem before you look for others. Good luck!

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 2019

A. Hello Ryan. Change the dip D.I. water rinse to SPRAY rinse or Add fresh D.I.water SPRAY while taking out car body from the dip rinse tank. The sludge from phosphate tank remains at flat portion and that's the reason for pin holes (What I think is those are not pin holes but are CRATERS that formed due to impurities in water.)

Avinash Vidhate
- NASHIK, Maharashtra, India
August 1, 2019

A. Hi Sunny. If the above discussion does not suggest to you what to do next, I think we'll need photos and more words before additional suggestions can help much. But please clarify what you mean by "casting material" -- cast iron parts, diecast zinc parts, pressure cast aluminum parts? Thanks!

Luck & Regards,

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