Remove oil contamination by alkaline cleaning solution and remove rust by acid cleaning process to ensure that no impurity remains on the surface of the substrate and to avoid pores in the plated layer due to surface contamination.
Author: Anna
I. Optimise the pre-treatment process
Thoroughly clean the surface of substrate
Remove oil contamination by alkaline cleaning solution and remove rust by acid cleaning process to ensure that no impurity remains on the surface of the substrate and to avoid pores in the plated layer due to surface contamination.
Use ultrasonic cleaning or organic solvents (e.g. acetone, alcohol) to dissolve stubborn oil contamination and enhance surface cleanliness.
Surface activation treatment
Use chemical etching or ion bombardment on metal substrate to form microscopic rough structure and enhance the bonding force of plating; introduce active groups through plasma treatment on plastic substrate to improve the adhesion of plating.
II. Control the plating process parameters
Adjustment of current density and voltage
Adopt low current density (such as 0.5~2A/dm² for chloride zinc plating) to slow down the deposition rate of the plating layer, reduce the generation of dendritic crystals and make the plating layer more uniform and dense.
Appropriately reduce the plating voltage, to avoid the formation of loose structure due to the rapid deposition of the layer.
Control the composition and temperature of the plating solution
Keep the ionic concentration of the plating solution stable (e.g. replenish the anode metal in cyanide galvanising) to reduce the defects of the plated layer due to the fluctuation of ionic concentration.
Adjust the temperature of the plating solution according to the type of process, for example, zincate zinc plating needs to maintain the pH value of 12.5 ~ 13, and control the temperature at 15 ~ 40 ℃ in order to optimise the lattice structure.
III. Use of functional additives
Add surfactant and brightener
Add surfactants (e.g. DPE series or DE series) to the plating solution to refine the grains and reduce the porosity to improve the uniformity of the coating.
Use of chloride zinc plating process with brightening agents (e.g. CI-87) promotes the densification of the coating crystals and improves the appearance at the same time.
IV.Passivation for enhanced densification
Chemical passivation
Through chromate passivation or trivalent chromium passivation process, generate dense oxide film (such as Cr₂O₃) on the surface of the plating layer, close the microporous and enhance corrosion resistance.
Chromium-free passivation technology (e.g. silicate or molybdate systems) is used to form a homogeneous passivation layer and reduce environmental hazards.
V. Post-treatment process optimisation
Cooling and ageing treatment
Rapid cooling after plating (e.g. water or air cooling) to avoid coarse grain size due to slow cooling of the zinc layer.
Aging treatment (e.g. 80~90°C oven heating) to stabilise the structure of the layer, reduce internal stresses and improve densification.
Improvement of layer densification requires comprehensive optimisation of pre-treatment, process parameters, additive selection and post-treatment, including: substrate cleaning and activation, low-current-density deposition, application of functional additives, and passivation to close micropores. Through multi-dimensional control, the mechanical properties and corrosion resistance of the coating can be enhanced while reducing porosity.