How to Keep a Plating Bath from Slowing Down
Alkaline plating baths form carbonates over time, and these carbonates cause the baths to become less efficient, resulting in slower plating speed. Asterion’s Roger Sowinski outlines three options for removing carbonates.
Q. Why is the plating time in my alkaline plating baths increasing? How do I fix this problem?
A. When it comes to plating, time is money, and nothing is more frustrating than your plating bath slowing to a crawl. The good news is that you can return the plating speed to its original levels; the less-than-good news is that it can be quite the chore.
Among the causes of slower plating speeds or loss of efficiency in alkaline plating is the build up of carbonates in the bath. Alkaline plating baths (both cyanide and non-cyanide) will form carbonates over time, and carbonates cause the bath to become less efficient, which manifests itself as a slower plating speed. These carbonates are formed when the alkaline solution is exposed to the carbon dioxide in the air, and this is the primary reason why employing air agitation in an alkaline plating bath is rarely, if ever, recommended.
So, your plating bath has high levels of carbonates in it—how can you remedy this? There are a few options.
1. Freeze the carbonates out of the solution. This often is the most effective option for removing the carbonates, which will begin to freeze into a crystalline state at around 35-40ºF. First pioneered by “old school platers” decades ago, freezing remains a viable carbonate-removal method for some, particularly in northern climates in the winter. Specifically, this method involves pumping the plating bath into totes and setting them outdoors overnight in sub-40ºF temperatures. The next morning, the totes are brought indoors and the liquid is decanted back into the plating tank. This can yield roughly 50 percent return of the plating bath and removes the majority of the carbonates. Next, the bath is rebuilt and work can continue, with plating speeds returned to those of a relatively new bath.
What do you do if your plating shop is located in a climate that almost never experiences sub-40ºF temperatures? I have heard of shops renting freezer semitrailers and using them to freeze the totes, and shops that have built or bought walk-in freezers for the same purpose. Unfortunately, both options carry additional costs over using the great outdoors.
Freezing out the carbonates does have limitations, however. For starters, the crystallized carbonates can form into a rock-hard mass, and the solidified carbonate masses are considered hazardous waste since complete removal all of the liquid chemistry from the crystals is not guaranteed. You can also lose half of the plating bath volume or more through this freezing method, requiring new make-up or replenish bath. Depending on the size of your tank, this could result in fairly significant cost.
2. Purchase equipment specially designed to remove carbonates from plating solutions. This is a better option, as this type of equipment offers batch-treatment chilling cycles that remove the carbonates as a low-moisture solid while simultaneously returning the liquid portion of the plating solution back to the process tank. Most importantly, the solid-waste carbonate is void of heavy metals and can be classified as non-hazardous.
3. Simple dilution. This is the least labor-intensive method for removing carbonates, but it also is the least effective. Simple dilution means removing some of the solution, say 25 percent, and replacing it with water and the appropriate amounts of plating chemicals. The idea is, if you remove 25 percent of the plating solution, approximately 25 percent of the carbonates in the bath also are removed. This method is not as work-intensive as the other two options, however, you will need to do it fairly frequently, and it may result in higher chemistry costs. Overall, this is my least favorite method, and I would only recommend it if all other options are off the table.
In summary, carbonates will build up in your alkaline plating bath, and these carbonates must be removed in order to keep the bath plating at a reasonable speed. To that end, there are several distinct methods you can use. Freezing and dilution offer varying degrees of reliability and costs; equipment specially designed for carbonate removal is the best option available today. I encourage you to review all options with trusted chemical and equipment providers.
An alternative product for passivation...
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