Electroless Nickel anti-tarnishes, post dips, chromates

A. Hi Raj,
According to "Electroless Plating" [on eBay , Amazon, or AbeBooks affil links] by Mallory & Hajdu, silicates, waterglasses, and waxes are additional potential post-treatments. But your posting is abstract, so we don't know if you need a special appearance, conductivity for electrical purposes, whether you are doing rack or barrel plating, whether the whole component can be treated or areas need masking, etc. But you at least have your list of alternatives now :-)
Luck & Regards,


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

A. Mr Khozem. British Aero Space has made a test. When using a high phosphorus 10-14 % with 40 micron nickel deposit. You should not rinse the part in NaOH dip after passivate in chromic acid. I hope Jim can explain it better for you.

Best Regard

A. A NaOH dip may accelerate the corrosion of the Nickel depending on the Phosphorus content.

A Chromic acid dip will reduce the activity of the freshly deposited Nickel.

It's not uncommon for poorly rinsed parts to tarnish and stain due to the high activity of the new coating. The Chromic acid will help slow down the oxidation of the Nickel by leaving a very thin film on the surface. There are other products that may help without the use of Chrome. Most Nickel suppliers will have these.

A. Khozem, Anders and Jim are right. You cannot dip the chromated parts in alkaline solutions this is not because of the substrate materials but chromium itself. Trivalent chromium hydroxide or oxide is amphoteric, that is, it dissolves in both acid solution and alkaline solution.

Ling

A. It is not common, but not rare, to dip nickel plated parts in a dilute solution of chromic acid. It does very little for the nickel since it is dilute. It does minorly help form an oxide layer, but it is close to irrelevant. The more common use is a "cheat" when a customer has specked out a plate that is too thin. The chromic solution forms a very thin chromate coating on the steel, thru the pores, which lasts just long enough to keep tiny rust spots from forming until it has been on his shelf for a couple of weeks. JIT users have it gone before that happens. It is a lousy cure, but the customer is always right, even when he is wrong.

 Thanks everyone especially Mr Todd for posing his problem which answered a lot of my possible questions !

Is there yet any other process unexamined to minimize corrosion in DC plated Nickel ? Is pulse plating the only other answer or can we look for any other post treatment /sealing ?

thanks in advance ,

A. To my knowledge you cannot do a chromate conversion coating on nickel. However, it would not surprise me to be advised that there is chromate in the tarnish-ban post treatments sometimes done on nickel. I think you should inquire of Technic or another supplier of nickel and electroless nickel post-treatments as to whether there is chrome in them and how long they can be expected to keep the nickel from tarnishing.