Zinc Diecasts Again

Q: Our company plates zinc diecasts. Recently we received a new batch of diecasts from a new customer and have found that after plating they appear to have “speckles” on the surface. The process we use consists of soak and electrocleaning in two separate tanks, followed by 0.5% sulfuric acid dip and then plating. Can you give us some ideas as to what might be causing this speckled plating? K. E.

 

A: This is a problem that occurs quite often when plating zinc diecasts. In the last ten years I’ve probably commented on this problem is least a half dozen times in this column. Microscopic size pores on the surface of your diecasts probably cause the so-called speckles that you mention in your e-mail. There are some other factors that you should consider as causes of this problem.

One such factor is the casting itself. If you examine the diecasts before plating, you probably will discover that they probably contain a number of cold shots and pits. If the diecasts have not been buffed properly, the cold shots will open up and show numerous pores.

You mentioned using an electrocleaner during the cleaning process. An electrocleaner is usually a poor choice for cleaning zinc diecasts. The higher alkalinity required for good electrical conductivity could cause attack on the zinc diecasts.
You’re soak cleaner that is used prior to the electrocleaner should be copper free. If the soak cleaner contains copper, you will in end up with copper immersion deposit on the castings.

A 0.5% sulfuric acid dip is a standard dip for diecastings. However, there are better acids out there for doing this. One such as it is fluorboric acid. A 5% solution of this acid will work a lot better than what you are currently using. An added bonus is that the fluoride present in the acid will help remove silicates from the surface of your parts.

Pitting on the surface can be caused by particulate matter in your plating bath as well as tiny gas bubbles sticking to the surface of the part. The particulate matter problem can be addressed by using better filtration in your plating bath. Using a suitable wetting agent can reduce the tiny gas bubbles. Talk to your chemical vendor about obtaining a suitable wetting agent for your plating
baths.