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Zinc Plating
2019-05-18

Zinc plating refers to a surface treatment technique that coats a layer of zinc on the surface of a metal, alloy or other material for aesthetics, rust prevention, and the like. The main method used is hot-dip galvanizing.


Zinc is easily soluble in acid and soluble in alkali, so it is called an amphoteric metal. Zinc hardly changes in dry air. In moist air, a dense zinc carbonate film is formed on the zinc surface. In the atmosphere containing sulfur dioxide, hydrogen sulfide and marine, the corrosion resistance of zinc is poor, especially in the atmosphere containing high temperature and high humidity containing organic acid, the zinc coating is easily corroded. The standard electrode potential of zinc is -0.76V. For the steel matrix, the zinc coating is an anodic coating. It is mainly used to prevent corrosion of steel. The protective performance of the zinc has a great relationship with the thickness of the coating. After passivation, dyeing or coating of the photoresist, the zinc coating can significantly improve its protection and decorative properties.


Main Features:

1, the appearance is smooth, no zinc, burr, silver white;

2, the thickness is controllable, arbitrarily selected within 5-107μm;

3, no hydrogen embrittlement, no temperature hazard, can ensure the mechanical properties of the material unchanged;

4. It can replace some processes that require hot-dip galvanizing;

5, good corrosion resistance, neutral salt spray test for 240 hours.

Application range Steel nails, nails, fasteners, water pipe joints, scaffolding fasteners, wire rope horse steel chucks, etc.

 

Working Principle:

In the plating tank containing the galvanizing solution, the parts to be plated which have been cleaned and specially pretreated are used as the cathode, and the anode is made of the plated metal, and the two poles are respectively connected with the positive electrode and the negative electrode of the direct current power source. The galvanizing bath is composed of an aqueous solution containing a plating metal compound, a conductive salt, a buffer, a pH adjuster, and an additive. After energization, the metal ions in the galvanizing solution move to the cathode under the action of a potential difference to form a plating layer. The metal of the anode forms metal ions into the galvanizing bath to maintain the concentration of the plated metal ions [1]. In some cases, such as chrome plating, it is an insoluble anode made of lead, lead-bismuth alloy, which only acts to transmit electrons and conduct current. The concentration of chromium ions in the electrolyte is maintained by periodically adding a chromium compound to the plating solution. When galvanizing, the quality of the anode material, the composition of the galvanizing bath, the temperature, the current density, the energization time, the stirring strength, the precipitated impurities, the waveform of the power supply, etc. all affect the quality of the coating and need to be controlled at the right time.

 

Related Categories:

1. Alkaline cyanide galvanizing

2. Alkaline zincate galvanized

3. Ammonium salt galvanized

4. Potassium salt galvanizing

5. Ammonium potassium mixed bath galvanized

6. Sulphate galvanizing

7. Chloride zinc plating

 

Post-treatment of the galvanized layer:

1. Dehydrogenation

2. Passivation

3. Coloring

 

Factors affecting galvanizing:

In the actual production process, common factors affecting the speed and quality of galvanizing are:

(1) Pre-treatment is not thorough. There is an oxide film on the surface of the workpiece, which affects the normal deposition of zinc.

(2) Poor electrical conductivity. The current is consumed on the wire and the current distributed to the surface of the workpiece is too small.

(3) The workpiece has a high carbon content. High carbon steel, cast iron parts, etc. will reduce the precipitation potential of hydrogen, accelerate hydrogen evolution on the surface of the workpiece, and reduce current efficiency.

(4) The workpiece is tied too densely. When the galvanizing is partially shielded, the coating is too thin.

(5) The bath temperature is low. When the bath temperature is low, the current density of the distribution is correspondingly reduced, and the deposition speed of the plating layer is also inevitably lowered.

(6) The sodium hydroxide content in the plating solution is high. When the sodium hydroxide content is high, the current efficiency is correspondingly reduced.

(7) The additive content in the plating solution is low. Low additive content will affect the dispersing ability, and the coating will appear too thin.

(8) The estimated area of the plated part is insufficient, and the current density delivered during plating is too small.

(9) The workpiece suspension method is improper, and the distance from the zinc anode is too large, and the position should be adjusted.

(10) The workpiece is over-etched. The hydrogen evolution potential is lowered, and hydrogen evolution at the surface of the workpiece accelerates the current efficiency, thereby affecting the deposition rate of zinc. An appropriate amount of corrosion inhibitor should be added to the pickling solution. The local scale is too thick to be removed by mechanical means, and more inspection is carried out during the pickling process.

(11) Anode passivation. The effective area is reduced, affecting the normal distribution of current.

(12) The sodium hydroxide content is low. If the sodium hydroxide content is low, the current density is not high and the anode is passivated.

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