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Leopard spots on brass and stainless steel after ultrasonic cleaning

Q. Dear all,

We operate electroplating lines for taps and bathroom accessories and are experiencing a defect during the ultrasonic degreasing stage on satin brass and satin stainless steel substrates. After ultrasonic degreasing, white, diffuse, leopard-like spots appear on the surface. These spots are not removed by either the subsequent acid neutralization step or by anodic degreasing, and they lead to poor results after electroplating.

We have confirmed that the defect is generated during the ultrasonic degreasing process because, when we skip this step, the spots do not appear (although this can obviously cause other problems due to insufficient cleaning). We have tried changing the degreasing chemistry (both alkaline and acidic), as well as varying temperature and treatment time, but without improvement.

Interestingly, not all satin brass and satin stainless steel parts are affected. Only a small percentage show the defect, and they tend to be the same items repeatedly. We have also noticed that the problem is more frequent on low‑lead brass and on AISI 304 stainless steel. A bright nickel deposit can help to visually mask the defect due to its leveling and brightening effect, but we cannot use bright nickel in satin finishes without losing the desired satin appearance.

Here is a photo of the defect on stainless steel:

Do you have any ideas about the root cause of this problem and possible solutions?

Thank you in advance for your help.

Federico Pizzetti
- Florence, Italy
December 19, 2025

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