The authoritative public forum
  for Metal Finishing since 1989
-----

Surface finish for parts to be electroless Ni plated Vs. adhesion

2002

My question relates to a metallic substrate surface finish prior to electroless Nickel plating. How refined can the substrate surface finish be without reducing coating adhesion. If the surface were polished either by a buff or electropolish to a 2-4 microinch surface finish, would a post plating temperature treatment be adequate to ensure adhesion?

Is the bond between the nickel and metal substrate primarily mechanical or is there a chemical component(is there an inner metallic formed during plating or the temperature treatment)?

Thank you,

Suzanne DeFriez
Goleta, California

In my experience I have been able to achieve very good adhesion with electroless nickel (NiP and NiB), even on polished surfaces. Proper chemical activation is the key, which depends of course on the composition of the underlying metallization. Electropolishing is preferable to buffing, as buffing tends to create a disturbed surface layer that may contain embedded contaminants. Whether or not intermetallics form depends on the "under-metal" composition and the temperatures to which the plated part is exposed. (see the binary phase diagram(s) for your material system.)

Erich Rubel
- Phoenix, Arizona
2002

---

one of several simultaneous responses2002

The bond should be ionic, with the coating molecules sharing electrons with the base metal. This is a very strong bond, and I don't believe that baking the part will improve this adhesion. Baking is usually done to change the characteristics of the electroless nickel, and sometimes as a way to prove that the plating does have good adhesion; baking will very often expose poor adhesion by causing blistering or peeling of the deposit.

Another test of adhesion is the bend test, where the part is bent upon itself until it breaks, and the plating/base metal edge is examined at 10X. No flaking or peeling should be observed For parts that don't bend easily, the grind test is administered, touching the plating to a grindstone at a right angle, to expose the base metal. No evidence of peeling should be observed.

  
Tom Pullizzi
Falls Township, Pennsylvania

---

one of several simultaneous responses

Yes electroless nickel can bond to a smooth substrate with good adhesion given the proper polishing.Take aluminum memory discs for instance.They are polished to a smooth finish with an abrasive media and then electroless nickel plated . They must pass a 180 degree bend test on a mandrel ( rod) and not flake or peel. There are many factors involved to get good adhesion. I would avoid buffing as it involves a waxy media that is smeared on the part. The surface imperfections are sometimes just smeared over not cut off or removed. The heat treating or baking will increase adhesion. It can also cause aluminum parts with poor adhesion to blister. The phosphorous from the EN migrates into the substrate allowing better adhesion on parts that were properly cleaned and plated. Also stresses are alleviated.

Todd Osmolski
- Charlotte, North Carolina, USA
2002

---

one of several simultaneous responses

That question is a little more involved than it seems. While a rough surface can give you a mechanical bond and increase adhesion sometimes blasting is used for that purpose to hide improper or inadequate pretreatment. For a more common metal like aluminum or steels if the pretreatment is done properly you should have good to excellent adhesion on a surface that smooth. Decorative Chrome parts are typically polished to almost a mirror finish before plating and they have excellent adhesion. I have always referred to this as a chemical or atomic bond. Now when you start dealing with less common metals such as Titanium you almost have to have a combination of a mechanical and chemical bond for good adhesion. I hope this helps.

Rick Richardson, MSF
Dayton, Ohio
2002

---

one of several simultaneous responses

Suzanne, The answer to your question is: It depends on the substrate. Most low alloy steels develop very strong bonds to electroless nickel. To give you an idea, we plate plastic injection molds and when properly pretreated the plate can resist sand blasting without blistering or chipping even though the substrate was previously mirror polished. The only way to take it off is to abrade it or "eat it" away with chemicals. Not so over stainless steel or aluminum. Copper and its alloys are in between.

Guillermo Marrufo
Monterrey, NL, Mexico
2002