Reasons for Blackening of Zinc Edges in Alkaline Galvanizing
Author: Robby
Blackening of zinc edges in alkaline galvanizing can be caused by several reasons:
Improper passivation: Passivation is required after galvanizing to enhance corrosion resistance. If the passivation solution contains impurities or if the treatment time and temperature are not properly controlled, this may result in blackening of the edges of the galvanized layer.
Excessive current density: In the edge or raised part of the workpiece, the electric field is not uniform, and it is easy to produce a high current density, which leads to faster deposition of zinc, and may entrain impurities and form a looser structure, thus blackening the edge part.
Poor quality of additives: If the alkaline galvanizing additives used are of poor quality, it may lead to blackening of the edges of the galvanized layer.
Improper suspension of workpieces: If the workpieces are hung with too many contact parts or hung in an unstable manner, a dead zone of electric current may be formed in the localized area, resulting in the blackening of the edges due to the deposition of too much zinc layer.
Excessive organic impurities and brighteners: When there are too many organic impurities and brighteners in the bath, the rate of hydrogen precipitation increases and the deposition rate slows down. Organic impurities are mixed in the plating layer, affecting the arrangement of the structure of the plating layer, the crystal structure becomes disordered, resulting in blackening of the plating layer.
Lead ion pollution: When the content of lead ions in the galvanizing solution exceeds a certain value, the dispersion ability of the plating solution decreases, and the plating layer will appear black or gray streaks after the dilute nitric acid out of the light.
Solutions to alkaline galvanized zinc edge blackening
Optimize passivation treatment: Ensure that the passivation solution does not contain impurities, control the treatment time and temperature
Adjust the current density: Avoid high current densities when designing the workpiece or use a current regulator to control the current density during the plating process.
Use high quality additives: Choose high quality alkaline galvanizing additives and check and replace them regularly.
Improve workpiece suspension: Ensure that workpieces are securely suspended to avoid localized current dead spots.
Control organic impurities and brighteners: Clean the bath regularly to reduce the content of organic impurities and brighteners.
Prevent lead ion contamination: Use high quality zinc anodes and regularly test and control the lead ion content in the plating solution.