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Problem in hard chrome plating an ID

We are doing hard chrome plating on internal surface of stainless steel screens, used as filters for various applications. The thickness of plating is around 150-200 microns. The material is S.S 316, and the screens are electro polished (EP) before hard chrome plating (HCP). There is a gap of 4-5 days between EP and HCP .The components are of various dimensions, typically 1000mm X 1000mm lg.
The hard chrome plating is done in a conventional bath (chromic acid 250-275 gms/ltr, sulphate 2.5-3. gms/ltr at a C.D of 1-2 Amps/sq.inch. While we get a satisfactory plating on most of the components, occasionally we face a problem of partial deposition. Either a small patch in the low C.D area remains unplated, or only the H.D area gets plated.
all other parameters like fixturing , electrical connections and bath impurities are being monitored. At present we are reverse etching the component in the plating bath for 1-2 min at around 1amp.sq.inch We feel that this could be problem of passivation of S.S surface during EP, and would like to know the remedy for it.
Thanks,

Prashant Soman
plating shop - Maharashtra, India
February 12, 2009

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Hi, Prashant. It sounds to me like you are not providing enough current density. 1 amp per square inch is too low for chrome plating. 2 amps per sq. in. may be enough if you don't let it drop below that.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 12, 2009

we have gone at 2 amps/sq. inch ,but still the problem persist.do you think EP is creating a problem? if so how can we over come
thanks

further to my question ,whether the process of EP will cause any additional passivation of the SS316 surface, which has to be taken care of during HCP

Prashant Soman
plating shop - Maharashtra, India
February 14, 2009

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February 16, 2009

Hi, Prashant. Chrome will not deposit at a low current density. If it is not depositing, the current density is too low. You have to either get the anode closer to the low current density area or you have to increase the current.

Regards,

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

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EP definitely will cause passivation, but that is not your problem. A clean passivated part will plate and then it will chip or peel. No plate is you do not have enough current density in the low areas, such as the center and around wire intersections that overlap one another.

I used about about 4 amps per sq inch for chrome plating. I also used conforming anodes where I could get the low current areas to plate, without excessive plating in the high areas and without burning on sharp edges.

James Watts
- Navarre, Florida
February 16, 2009

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shall try the same. in case you have any advice for removing the passivation layer formed due to EP prior to HCP , do tell us
thanks again

Prashant Soman
plating shop - Maharashtra, India
February 18, 2009

It sounds like your chrome:sulphate ratio is incorrect (besides your current density is too low).

George Brackett III
- Maine
April 8, 2009

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April 12, 2009

thanks for the reply
what would be the correct parameters, we should keep to get the desired result
Regards

Prashant Soman
- Ambarnath, Maharashtra, India