Alkaline Non-Cyanide Zinc Plating: Yellow Patches Fix

Summary for alkaline non-cyanide zinc plating Discussion

alkaline non-cyanide zinc plating can show yellow lines or yellow shading after nitric acid bright dip and blue chrome passivation, especially during drying. Nagaraj L. asked about the cause in a process using zinc plating, nitric acid bright dip, a blue chrome passivation bath held at pH 2.2 to 2.6, and a hot air dryer. Willie Alexander suggested first adding a rinse after the nitric acid bright dip and before the clear chromate, and noted that if the clear chromate contains chromic chloride, HCl should be used to adjust pH unless the supplier bulletin says otherwise. He also said that if staining appears mainly at the lower end of the pH range, the bath should be maintained accordingly. Another responder said the post-treatment type was not stated and recommended using a good supplier’s trivalent passivation and keeping all parameters within specification. The thread does not give a single confirmed root cause, but points to rinsing and passivation control.