Summary for chemical drop on zinc plating Discussion
Chemical drop on alkaline zinc plated part is the issue raised here: a part plated with alkaline Zn and chromate(III) yellow developed a black spot, and the poster suspected contamination before plating. SEM-EDS comparison of the spot versus a normal surface showed lower Cr and higher Zn at the defect, with O and C tending to decrease, leading to the theory that an alkaline substance had adhered to the part and interfered with chromium(III) adhesion. A pH paper check at the defect read about pH 12, which supports the idea of a strongly alkaline contaminant. No alternate answer or correction is shown in the excerpt, so the thread mainly documents the investigation and the likely pre-plate contamination cause identified by Jon Barrows, MSF, EHSSC.
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.
Ice
Students – Thai
September 8, 2025
This is a meeting place for camaraderie & sharing, not a free consultancy. So some readers don’t engage with anonymous posters.
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
Independence MO
September 9, 2025