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pH fluctuation in nickel sulfamate bath

2004

I am using a non bright ductile nickel sulfamate bath but even though the pH is advised to be in the range of 3.5 to 4.5, we find that over time, it drifts lower to around pH 2. We increase our boric acid level to about 40g/l and it stayed fine for about 2 months.

Now, it is back to around pH 2.

Can anybody help to explain the mechanism and possible solution to maintain the pH?

Thank you.

Roland Teh
plating shop - Penang, Malaysia

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2004

You are right to increase the boric acide to about 40g/l. This will stabilise the pH by its buffering action.

However, since the pH has come back down to about 2, I would suggets one of three things. Firstly, the boric acid has depleted - not a normal thing to occur, but possible. Secondly, your anodes have passivated and the anodic reaction is not the dissolution of nickel, but the evolution of oxygen or possibly chlorine, depending on the bath composition. This will effectively increase the hydrogen ion content and hence the pH will fall. Thirdly, you are topping up the tanks with chemicals that have too high free acid in them. First shot, I would go on the passivating anodes.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK