Chemical Drop/Black Spot on Alkaline Zinc Plated Part

Q. A chemical drop problem occurred on a piece plated with alkaline Zn and chromate (III) yellow. The hypothesis is that this likely occurred during the process before the piece was plated. Based on SEM-EDS results comparing the chemical drop point to a normal surface, the drop point showed low Cr and high Zn, while O and C tended to decrease compared to the normal surface. Therefore, it is suspected that an alkaline substance had adhered to the piece, reducing the Chromium(III) adhesion. A pH paper test on the chemical drop point revealed a level 12 alkalinity.

- How should I check to see what solution or chemical the chemical drop is from?
- I don't know what caused this chemical drop, whether it was a drop, a mist, or something else. What should I do to find out?
- I have simulated a chemical drop in the lab by dropping a solution from the plating bath or by simulating bubbles and then placing a test strip against the bubbles. When the bubbles burst, a black defect appeared, similar to a real chemical drop. However, it did not appear as a round spot and the same size as the actual Chemical drop.

A. You have made a reasonable conclusion that it was a drop of something that is alkaline. Why would you need to know specifically what chemical caused the defect? A caustic etch would be strong enough, but it could be something else.

Jon Barrows, MSF, EHSSC
Kansas City
September 9, 2025