Black Corrosion Product on Electroless NiP in PBS

[RJKing]

Hello,

Electroless Ni-P is a new product for me, but I have been assigned some research regarding the process. One of the qualitative preliminary tests I am running is to submerge samples in saline solution to analyze potential corrosion behavior. What I have noticed is that after about 2 days in solution the samples have turned black, showning a flaky black corrosion product. The samples are plated from homebrew solution (we are in preliminary R&D), but the resultant plating is around 8-9% P, confirmed by EDS. I know e-less NiP normally acts as a barrier, but the samples I have are not isolated from the coating (ie the coating is only applied to part of the sample). The substrate is more noble than the NiP in the galvanic series. The coatings are typically about 8 um thick.

My questions are:

-Is the black corrosion product nickel oxide? nickel phosphate? Is it a passive film that has dropped the corrosion rate? Or is it a soluble corrosion product that will continue to eat through the entire coating?

-Is the galvanic action the key issue here? I have tried to create samples that form a more complete barrier with the substrate. They still turn black in PBS.

-Is P content the key factor here? I have seen some literature that says increasing the P content will decrease corrosion rate, but will it stop/change the corrosion reaction so it no longer turns black?

-Will altering the thickness improve the situation? I have tried preliminary runs with larger thicknesses but it seems the thicker the coating, the worse the blackening.

-Is there any literature regarding the blackening of eless NiP in salt solutions and how to control it? I have found some regarding the blackening affect of nitric acid, but none from a salt solution. Is the process the same?

Thank you for any help you can provide on this interesting problem.

 

[EdStainless]

What is the substrate?
What is your aging process on the NiP?

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[RJKing]

Currently there is no heat treatment. I was under the impression the heat treatment would worsen the corrosion affect due to amorphous -> crystalline transition and formation of grain boundaries to attack.

The substrate is cobalt-chrome, not because it is what will be on the final product, but because we currently had excess material from other studies.

 

[EdStainless]

Comments.
You coating sounds thin. I recall using 25-35 micron thickness.
Yes, aging reduces corrosion resistance.
High P coatings tend to be less pourous than low P.

How about your solution, any S compounds in it? The black could be just about anything as a fine poweder. Even pure Ni will look black as a powder. You need to have some tested.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm