After the products are passivated with trivalent chromium zinc water, some products will change color after being placed for a period of time. What is the reason?
Author: Marisa
After the products are passivated with trivalent chromium zinc water, some products will change color after being placed for a period of time. What is the reason?
The direct factor causing the discoloration is the organic impurities mixed in the passivation layer. During the storage period, the passivation layer itself undergoes electrochemical corrosion, and the organic impurities become reducing agents, causing the hexavalent chromium in the passivation layer to be reduced to trivalent chromium: when it is reduced to purple trivalent chromium, the passivation layer becomes dark; when it is reduced to a light blue state, the color of the passivation film becomes lighter.
After studying the cause of discoloration after passivation with trivalent chromium zinc water, the generally accepted view is that the plating solution uses thiourea and polyethylene glycol as additives. Some zincate zinc-plated layers have similar discoloration phenomena at high current density, which is also likely to be due to excessive attachment of corresponding brighteners at high current density. The potassium chloride zinc-plated color passivation layer contains many surfactants, organic brighteners or their decomposition products, which makes the color very easy to fade within the full current density range.
The fundamental way to reduce the discoloration of the passivation layer of trivalent chromium zinc water is to choose suitable zinc plating additives and try to reduce the organic inclusions in the passivation layer.