Carbonates in the Plating Bath

If you have an alkaline plating bath, then you are likely familiar with carbonates. Those of you who still have or have had cyanide plating baths are all too familiar with carbonates. In this article, we will discuss what a carbonate is, how it is formed in a plating solution, and what it does to the plating bath. We will also visit a couple of ways of dealing with these when they get too high.

What Is a Carbonate?

First things first, what is a carbonate? A carbonate is a salt of the anion CO32-. That’s all well and good for those of us who stayed awake in chemistry, but for the rest of us, what does that mean? A carbonate is simply an ion, like calcium, sodium, or potassium, which is attached to the anion CO32-. So in an alkaline plating bath, a common carbonate would be Na2CO3, or Sodium Carbonate. Now that we understand what a carbonate is, let’s look at how they are formed in the plating bath.

There are two main ways that carbonates form in the alkaline plating solution. The biggest culprit is carbon dioxide in the air. Carbon dioxide reacts with water to form carbonic acid (H2CO3). This acid will then react with the alkaline solution to form carbonates, such as Sodium Carbonate. A smaller source of carbonates is the oxygen that is evolved at the anodes. This oxygen can react with the organic compounds that are present in the solution to form carbonates. Because of this, current density on the anode will also play a factor. Higher anodic current density will lead to more oxygen being produced which will lead to more carbonates. In addition to this, air agitation will increase carbonate buildup.

How Are Carbonates Made?

So we know what a carbonate is and how it is made. But why does it matter? Well, all alkaline plating baths contain some sodium carbonates. These carbonates can be present for various reasons, and typically are around 6-8 oz/gal. Once the carbonates build above 12 oz/gal or so, we start to see some issues. These issues include slower plating speed, reduced throwing power, increased brightener usage, burned deposit in the high current density zone, plating roughness, and if the level of carbonates grows too high, the organics in the solution can float to the top, or “oil out”. Because of this, it is important to remove carbonates from solution.

How Do You Remove Carbonates?

Alright, so we can’t let the level of carbonates get too high. How do we remove carbonates from solution? There are a couple of ways to do this. The easiest is to freeze the carbonates out of the bath. To do this, you can simply put the solution outside in the winter months or in a freezer truck in the summer. It’s important to get the solution to at least 35-40 F. This will allow the carbonates to settle out and you can decant the liquid from the solution. The solid waste will need to be disposed of, and the liquid can be returned to the bath. There are also specially designed machines to aid in the removal of carbonates. You can download more information about one of these machines here

As always, we invite you to reach out to discuss carbonates in greater detail with your friendly Asterion Sales Representative.

Reduce Carbonates