More On Barrel Sulfamate Nickel

In the February 2007 issue of PRODUCTS FINISHING, I answered a question regarding laminations in barrel sulfamate nickel plating. One of our readers took a few moments to e-mail me some additional information about the problem and how to solve it. First the original question and my answer.

Q: We use a nickel sulfamate plating bath to plate low stress micro-layers. We normally operate the plating bath in a temperature range of 50–60°C. Recently the temperature was allowed to reach 70°C due to a broken thermostat. Since the temperature upset, our plating has been pitted, burnt and stressed. Did we ruin the plating bath because of this temperature upset? Can the plating bath be salvaged? E. B.

 

A: The short answer to your question is this: The plating bath is probably ruined. As the temperature of the plating solution increases above 60°C the rate of nickel sulfamate hydrolysis increases rapidly to give you ammonium and sulfate ions. A decrease in pH will also speed up the process. The hydrolysis reaction is:

Ni(NH2SO3)2 + H2O ? NiSO4 + (NH4)2SO4
The reaction is not reversible so the ammonium and sulfate ions have to be removed. Unfortunately, there is no way that I am aware of for removal of the ammonium ions. Sulfate ions can be removed using barium sulfamate.

An excellent paper on nickel sulfamate plating baths by Dr. S. A. Watson is available at the Nickel Institute web site. The web address for this paper is:
www.nickelinstitute.org/index.cfm?la_id=1&search=true&keywords=10052&ci_id=6579&Submit.x=11&Submit.y=7.

The paper contains a number of useful tables and figures. For example, the table below shows the effect of temperature and pH on the ammonium ion concentration.

Nickel Sulfamate Concentration, g/liter	Temperature °C	pH	Increase in ammonium ion concentration g/liter, per 336 hours
600	65	4.0	0.035
600	70	2.0	8.5
600	70	4.0	8.2
600	70	5.5	0.1

As can be seen a change in temperature from 65–70°C increases the rate of ammonium ion formation by a factor of ~ 240. The bottom line is that one must pay careful attention to both temperature and pH.

The additional comments are as follows:
“Sulfamate nickel is a more passive deposit during plating as compared to Watts type formulations. As such, it is very susceptible to laminations during the make and break contact in barrel plating. pH is important in barrel plating to help eliminate laminations. We recommend 2.8–3.2 for best results.

“Bromide additive must be carefully controlled and would not be my recommendation for use. Chloride or proprietary additives are much better suited for this process. Bromides can cause titanium anode baskets to dissolve if not kept full with nickel.”

My thanks to D. D for passing on the additional information.