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topic 38946

True that cast iron can't be Zinc Plated?


(2006)

Q. Chemistry student at the Budapest Technical Institute working with some plating.

I have heard that you can only plate cast iron, malleable iron and carbo-nitrided steel with an acid zinc plating solution and that you can't use alkaline or cyanide zinc plating solutions. Is this true?

Thomas Kolossa
Student - Budapest, Hungary


(2006)

A. Yes, that's essentially true, Thomas. For reasons which I confess to having never actually studied, the presence of the carbon in alkaline conditions causes the evolution of hydrogen in lieu of the deposition of zinc. According to standard potentials, hydrogen should evolve before zinc deposits, but it doesn't actually happen in acid baths, nor in alkaline baths when no carbon is present. But in alkaline baths with carbon present the "hydrogen over-voltage" is dissipated, and hydrogen evolves instead of zinc depositing :-)

Before the development of practical acid zinc plating in the mid 1970s, such items often would be cadmium plated instead of zinc plating them.

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



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Zinc plating cast iron

(2006)

Q. In endeavoring to plate cast iron parts with alkaline zinc we are getting good thickness in the high/mid CD but no deposit in the low current density areas.
Can anyone suggest a cleaning cycle, activator or strike to overcome this problem?

Geoffrey Whitelaw
Geoffrey Whitelaw
- Port Melbourne, Australia


(2006)

A. Hi Geoff. An acid zinc strike should do it, I think.

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


Zinc Plating
by Geduld

(2006)

A. A light sandblast will help because after casting it forms a film and at most times it won't plate at low current density areas. Second, you can make up a cadmium strike!
That will definitely work. It covers it immediately!

Willie Beneke
- South Africa


September 29, 2008

A. As a zinc plater myself I know the difficulty in plating cast iron especially when using cyanide zinc. But the way I find to get around this is to give it a strike of cyanide copper then use a scotch-brite to rub the surface down and it should zinc plate fine.

Michael Jones
- Brisbane, Queensland, Australia


thumbs up signHi again. As we see, although the 3 responders suggested 3 different strikes, but they all suggested striking with some other bath because where there is carbon, hydrogen is released in lieu of the deposition of zinc; it apparently is hard to overcome, especially at low current density.

Regards,

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



To minimize searching and offer multiple viewpoints, we've combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.



Alkaline Zinc plating on top of Acid Zinc plating for cast iron materials

April 24, 2008

Q. I am Yamuna from CHENNAI from Plating department . Sir I need a clarification regarding Alkaline zinc plating of the cast iron parts.The above attached component is made of cast iron material MES M - 01 -200 ( FCD450 ).

Kindly let me know Alkaline Zinc Plating Process is possible for the above mentioned component.

My chemical supplier said that Alkaline zinc plating is possible on cast iron by first plating the component with Acid zinc plating then with Alkaline zinc plating .

Please explain.

Thanking You

With Regards

J.Yamuna Kanagam
Plating shop engineer - Chennai, Tamil Nadu, India


May 9, 2008

A. Dear Ms Yamuna,

It is advisable to use acid zinc before alkaline zinc as cast iron can be attacked easily by alkaline solutions.

Also please do take care of your cleaning process as the job needs to be treated in low concentration of alkaline cleaners.

Maybe two dilute baths instead of a single concentrated one if you need better cleaning.

Regards
All the best,

S.V. Aurobind
plating process supplier - Bangalore, India



May 12, 2008

A. Dear Aurobind Venkataraman,
It is my understanding that cast iron is mainly composed of iron and carbon and neither of these elements are significantly attacked by alkalies. On the other hand, acids attack iron. Could you or someone else please explain to me why or by what mechanism will cast iron be attacked by alkaline solutions and not by acidic zinc? Thanks,

Guillermo Marrufo
Monterrey, NL, Mexico


May 13, 2008

A. Hi Yamuna,

Generally Zinc plating on cast iron is done using an acid chloride electrolyte. This is because of the unique leveling property of this bath. Zinc ions from this high efficiency bath try to cover the pits traveling at a greater speed in the first few moments and then start leveling the deposit. So it is advisable to stick to acid chloride Zinc plating on castings. Cyanide/Alkaline bath does not have this property as required to plate cast iron.
Regards

t k mohan
T.K. Mohan
    plating process supplier 
Mumbai, India



May 15, 2008

A. Dear Readers,

Since cast iron has lot of carbon the formation of smut is the problem. Further I think Ms Yamuna needs excellent coverage property for her calipers and alkaline zinc is best suited for her.

Regards,

S.V. Aurobind
plating process supplier - Bangalore, India


May 17, 2008

A. Dear Ms. Yamuna,

Acid-chloride zinc process is best suited for cast components like calipers and housings. A few platers give a flash coating of acid zinc and then do alkaline-zinc plating.

Hope this helps.

Vara Ramesh
chemist - chennai, Tamil Nadu


October 29, 2010

A. Hi yamuna

As per our experience acid chloride zinc is better than alkaline zinc plating.

The throwing power of acid zinc is high when compare to alkaline.

cast iron is porous component; during acid pickling the acid may get into the gaps of cast iron; because of this the alkaline media won't accept the coating over the cast iron.

bright_murugan
BRIGHT C.MURUGAN
METAL FINISHERS - Chennai, TamilNadu, India


thumbs up sign  Unlike the knowledgable people responding before me, I've never personally plated any such parts; all I have is book knowledge.

I believe that they are all correct in their assessment that cast iron should not be directly alkaline zinc plated, but I think the actual reason is that under alkaline conditions the carbon spots will cause hydrogen to be reduced instead of zinc, leading to poor or no zinc deposition at the low current density areas, and an unsatisfactory porous deposit (if any).

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Will Rogers 1922
"Well all I know is what I read in the papers"
-- Will Rogers

----
Please also see letter 23909, "Problems in zinc plating of cast iron".



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