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Pinholes in copper-nickel-chrome plating

2003

We manufacture a range of wheel bolts that are made from manganese steel that are hardened & tempered to 32 Rc. The dome head of the bolt is then 220-grit flap wheeled, and then polished to a mirror finish.

The bolts are then sent to the chrome platers to be:

Flash copper
20-25 microns bright nickel
Flash chrome
De-embrittled for 8 hours at 180 °C

Approx. 20% of the items always tends to have small pinholes on the domed surface of the bolt. It tends to be only covering about 20% of the dome but always in a patch - not over the whole of the domed face, but always usually over to one side of the face.

We approached the heat treatment company and they suggested tempering in a nitrogen atmosphere to try and reduce any oxidation. Ironically the first batch went through 100% with no pinholes. We have just had a second batch plated and the pinholes have appeared yet again. The strange thing is that it is only on one type of bolt (out of about 4 different types sent). All of the bolts have been made from the same batch of material.

Could it possibly be an inconsistency with the polishing operation that is causing these pinholes to appear?

Many thanks in advance,

Paul Hill
precision engineering - London, UK

Just a quick update. I have been talking to the metallurgist at the steel company and they tell me that the material will have manganese sulphate inclusions - he thinks this is what the problem may be. Is there any process that can help over come this? Any comments greatly appreciated.

Paul Hill
- London, UK
2003

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Paul,

Try to demagnetize this bolt before chrome plating steps also check the trivalent chrome ratio to sulfuric acid in your chrome solution.

Regard,

Anders Sundman
4th Generation Surface Engineering
Consultant - Arvika,
Sweden
2003

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Paul. Sorry I mean the content of trivalent chromium in the solution, but also make a check of the ratio between sulfuric acid and chromic acid.

Anders Sundman
4th Generation Surface Engineering
Consultant - Arvika,
Sweden
2003

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Dear Mr Paul,

The fact that it appears on one side of the dome, and on one type of bolt, looks like the problem is Nickel bath based. Probably you need to ramp down the current and increase the use of additive ratio to brightener ratio.

This could be the pitting caused by brightener sticking to the bolt as there is insufficient wetter/additive to dissolve the brightener fully in the nickel bath. please come back if this doesn't solve your problem.

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

2003