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(1) Pickling
The pickling time is short and the rust removal effect is poor, resulting in missing plating.
If the pickling time is too long, it will cause over-pickling. A layer of black fine powder will appear on the surface of the workpiece after pickling, which may cause plating leakage or increase the thickness of the coating.
(2) Different acid concentrations require different pickling times.
Light rust, pickling time 10~15 minutes
Heavy rust, pickling time 15~20 minutes
The way to judge the quality of pickling is to see that there is no rust on the surface of the workpiece and it is gray-white.
High concentration of new acid is prone to hydrogen embrittlement, so when pickling with high concentration of new acid, special attention should be paid to the control of the pickling room.
(3) Rinse water
Rinsing water is mainly used to remove acid and iron salts (FeCl3+FeCl2) remaining on the surface of pickled parts.
By rinsing the pickled workpiece, the acid and iron salts brought into the plating solution can be reduced.
Too high iron salts and iron ions in the rinsing water will also cause an increase in zinc slag.
(4) Add anti-leak plating agent
When there is too much aluminum in the zinc pot, it will have a great impact on the traditional zinc + ammonium flux plating solution.
Mainly there are many leakage plating and large leakage plating area.
The traditional zinc + ammonium flux plating solution can only adapt to aluminum content below 0.02%.
When the aluminum content is too high and a large area of leakage plating occurs, the zinc-to-ammonium ratio must be changed to reduce the concentration of ammonium chloride when using an anti-leakage plating agent.
(5) Zinc liquid temperature
The higher the temperature of the pure zinc liquid, the better the fluidity of the zinc liquid, and this situation is unlikely to occur in production.
When the workpiece is pulled out, the zinc liquid easily flows back into the zinc pot, which is beneficial to thinning the galvanized layer and making the surface smooth.
The high temperature of the zinc liquid is also conducive to the iron-zinc reaction, accelerates the reaction speed of the iron-zinc alloy layer, and increases the adhesion strength of the galvanized layer.
But be aware of the "Sandelin" phenomenon. When the temperature of steel is 460~480℃, the coating thickness increases significantly.
The temperature of the zinc liquid is low, which is opposite to the above situation.
The "Sandelin" phenomenon is strictly controlled for hot-dip galvanizing of iron towers, but there is no need to deliberately require color difference for galvanizing of scaffolding.
