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Electroless Nickel Plating: Want Consistent Darkening

Q. Hello,
I have an extensive background with anodizing (Al & Ti) and I can carry a conversation with some electro plating processes, but I haven't much experience with Black Electroless Nickel with a mid-phosphorus and am asking the community for some insight.

We have steel panels with a 120-grit grain on them and have them EN plated. The panels then go into an oxidizer that turns them darker. I won't say black here because we are getting a range of colors from our plater that go anywhere from a bright silver (very little darkening) to a dark bronze color. Sometimes we will get something that could be considered black, but most of the time we get a bronzy/charcoal color.

I am trying to understand the mechanism that is creating the black color. I have been told that the darkness is a function of the amount of phosphorus in the nickel bath and that having more phosphorus would lead to a lighter panel, having less would create a darker panel. I'm not sure if this is the case (it's just what I've been told), but if it is and a mid-phosphorus bath could be anywhere from 5%-9% phosphorus, then there would (could be) quite a range of colors produced since going from 5% to 9% phosphorus is nearly doubling its concentration.

What exactly is going on in this process that is creating the black color?

Why do we see such a large amount of variation in color?

Supposing the color is from the phosphorus concentration, and it could be 5%-9% how in the world could 2 different platers on opposite sides of the world ever obtain the same color consistently?

Thanks

K. Bodily
- Gardnerville, Nevada
May 29, 2024