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Problems in zinc plating of cast iron



A discussion started in 2001 & continuing through 2017 -- add your Q to bring it back to the Hot Topics page.

(2001)

Q. Hi,

We are doing Zinc Plating on Iron Casting (Potassium Chloride Bath). The component is machined after plating. After 4 to 5 hours of machining, stain marks develop from the plating edges and moves inside the machined area. We had been plating the same component for the past year without this problem.

Trials conducted:

1) Fresh plating solution prepared
2) Plating solution of another company tried out
3) Electrolytic cleaning included.

Any suggestions would highly be appreciated.

Thanks,

Rajesh Kumar
- India



Problems in acid zinc plating on cast iron

(2001)

Q. DEAR SIR,

I HAVE GOT A PROBLEM, I AM DOING ZINC PLATING WITH YELLOW PASSIVATION ON CAST IRON BODIES IN ACID ZINC BATH. IN THE PROCESS THE PLATING IS GOOD BUT LATER ON THERE ARE PITS OBSERVED AND THE YELLOW PASSIVATE FADES. KINDLY GUIDE ME THE EXACT PROCESS TO FOLLOW FOR PLATING ON CAST IRON BODIES. MY PROCESS IS AS UNDER.

1.CLEANING IN ALKALINE SOLUTION
2.WATER RINSE
3.ACID PICKLING (HCL)
4.WATER RINSE
5.WATER RINSE
6.ELECTRO PLATING IN ACID ZINC BATH
7.WATER RINSE
8.DIP IN YELLOW PASSIVATION
9.WATER RINSE
10.NATURAL DRY

AFTER FOLLOWING THIS PROCESS THE RESULT ARE NOT SATISFACTORY.PLEASE HELP ME IN THIS MATTER.

KIND REGARD,

XAVIER FERNANDES
INDUSTRIAL PAINTING - HUBLI, KARNATAKA, INDIA


Zinc Plating
by Geduld

(2001)

A. You could try doing anodic cleaning in NaCN solution after acid pickling and prior to plating it may help; please let me also know if you succeed.

Viney Chaddha
- Gurgaon, India


(2001)

A. You have not mentioned the grade of Casting, assuming the grade to be 25, before Yellow passivation dip in 2% HNO3 and rinse Dip in Yellow passivation and spin dry in a Centrifugal dryer at about 50 °C.

MAHENDRA gARGATTI
- Belgaum, Karnataka


(2001)

A. Many castings are porous. It is common for chemistry to leak back out of these pores after finishing, causing the described types of failures. One technique which I have used in the past has been to alternatively dip the finished part in clean cold, then hot rinse water tanks, repeating several times. This will cause the metal casting to expand and contract slightly, thereby drawing in and expelling the water, and rinsing to some extent the pores. Thorough drying must be achieved with time in a hot air box, fan-forced type dryer.

Brian A. Calver
- Grimsby, ON, Canada



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(2003)

Q. I have inconsistent plating results when sending out gray cast iron valve bodies for zinc dichromate plating. There are two reoccurring problems. The first is a lack of plating coverage in low current areas on the exterior of the casting. An example would be the inside corner of the port flange on the valve body. The more complex the shape of the valve the more problems I have with plating coverage. Are there suggestions I can give the plater to reduce these occurrences?

The second problem is in the appearance of the body after plating. White chalky spots show on the body surface. The appearance will continue to deteriorate over time. I have been told that this is chemical bleed out from the plating process. I have tried impregnating the casting with a Loctite LRTC product with little improvement. Is there any preparation that I can do to prevent bleed out and staining from occurring?

John R [last name deleted for privacy by Editor]
fluid products mfgr. - Mauston, Wisconsin, USA


A. Hi, John. The impregnation should help on the bleedout issue. But you can't give the plating shop a pass; it is they who must satisfactorily plate the parts.

It is possible that the shop you are working with uses an alkaline zinc plating process whereas acid zinc plating may give better coverage on cast iron. In any case, careful rinsing will minimize staining, alternately rinsing in hot and cold water can be effective in getting the plating solution out of the pores.

Regards,

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



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White spots on zinc plated cast iron bodies

(2004)

Q. Hallo,

I'm looking for some help on a yellow passivation treatment on a cast iron finished body (EN-GJL250). During the zinc plating process it looks to reach a good result. After some weeks, on the body surface, come out little white spots. These spots become larger during time and our customer reject all supplied components. Our supplier cannot solve this problem and we are in trouble. Is it depending on the material composition? Could anyone help us?

     

Thanks in advance, best regards,

Pingani Davide
fluid power engineer - Vezzano s/c, Reggio Emilia, Italy


(2004)

A. This kind of spotting out is normal for castings that will be zinc plated. Degassing or vacuum impregnation may help minimise but will not be a 100% fool proof solution to this problem. Try E- Coating the part instead.

khozema
Khozem Vahaanwala
Saify Ind supporting advertiser
Bangalore, Karnataka, India

saify logo


(2004)

A. We have had many years experience with plating cast iron, what we have found that this defect can arise when the part is overpickled. His results in surface porosity and the corrosion is from inside out. Try using a weaker pickle or utilize inhibitors to limit metal attack.

Martin Sullivan
- Merthyr Tydfil, United Kingdom


(2004)

Q. Thanks for your interest, I've already spoken with our customer and he told us that he use a hydrochloric pickling bath for about 15 minutes. I asked him what he think about and he said that we cannot decrease pickling time 'cause our body needs it. Which kind of inhibitors shall we use? Shall we decrease our level of acidity or do we need another kind of acid bath?

Thanks a lot,

Pingani Davide [returning]
fluid power engineer - Vezzano s/c, Reggio Emilia, Italy



Can't get shiny consistent zinc plating on spheroidal graphite cast iron

July 22, 2015

Q. Hi,
Trade in truck spare parts.
Of of the components that fits into the wheel, is a bracket that weighs 0.600 KGS, made of SG iron graded casting. It's NOT machined at all, hence its got the grainy surface.

We've tried our level best to bring about a shiny zinc surface to it and have taken following measures:

1) Shotblasted the components with fine grand for like 45 minutes per batch
2) Use Electrogalvanising tank/dip style (not in barrel)

But somehow, we are never able to achieve the shiny surface. The final appearances are like this:

1) kind of average zinc finish
2) yellow spots on surface here and there

The only solution to this is that we've to use a Chrome based spray paint which matches the overall finishing, but is damn expensive.

Please suggest where can we be going wrong?

Guneet Sahni
- Jalandhar, Punjab, India


August 12, 2016

A. If you cannot seal the porosity adequately and your client will allow a copper plate, castings can quite often be sealed with a cyanide copper plate and then subsequent zinc plating. Cyanide copper is nearly 100% efficient and there for similar to acid zinc the castings will usually cover, and being highly conductive the zinc deposit will readily plate over the copper. In the galvanic series zinc preferentially corrodes to both iron and copper and therefore the zinc will still corrode preferentially. Iron on the other hand corrodes preferentially to copper.

The only two drawback here is possible migration of the zinc and copper at the interface that over considerable time may spot out, and the other would be how critical the dimensional issue may be with say a .0002" copper deposit in addition to your zinc deposit.

Jon Hibbard
- Albert Lea Minnesota USA



Zinc Plating on FM Steel and Ductile Iron Issues

23909
August 2, 2017

Q. We are running chloride zinc on some parts. The parts are coming out bright and plated perfectly. 2 days later, they are spotting and rusting like there is chemical leaching out or pores much like a bad casting will do. Any suggestions on how to stop this or fix it?

Brandon Boyd
Plating Facility - Holdenville, Oklahoma USA

August 8, 2017

? Hi Brandon!

What type of steel parts? Is this cast ductile iron or SAE 1212/similar? Leaded steel?

Can you give us a hint of how you prepare the parts prior to plate them? Maybe we can help you that way.

Regards!

Daniel Montanes
TEL - N FERRARIS - Canuelas, Buenos Aires, Argentina

----
See also letter 38946, "True that cast iron can't be Zinc Plated?"

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