Electro zinc plating process
1. Concept of electroplating zinc
Electrogalvanizing: It is a process that uses electrolysis to form a uniform, dense, and well-bonded metal or alloy deposition layer on the surface of the workpiece. Compared with other metals, zinc is a relatively cheap and easy-to-plat metal. It is a low-value anti-corrosion electroplating layer and is widely used to protect steel parts, especially to prevent atmospheric corrosion, and for decoration. Plating technologies include tank plating (or rack plating), barrel plating (suitable for small parts), automatic plating and continuous plating (suitable for wires and strips).
2. Classification of electroplated zinc
At present, domestic electroplating solutions can be divided into four major categories:
(1) Cyanide galvanizing
Since (CN) is highly toxic, environmental protection has put forward strict restrictions on the use of cyanide in electroplating zinc. The development of electroplating zinc plating solutions that reduce cyanide and replace cyanide is continuously promoted, requiring the use of low cyanide (micro cyanide) electroplating solutions. .
After electroplating using this process, the product quality is good, especially the color plating, and the color remains good after passivation.
(2) Zincate zinc plating
This process evolved from cyanide galvanizing. At present, two major factions have formed in China, namely: a) the "DPE" series of the Wuhan Materials Protection Institute; b) the "DE" series of the Radio, Film and Television Institute. Both are zincate zinc plating with alkaline additives, with a pH value of 12.5~13.
Using this process, the coating lattice structure is columnar, has good corrosion resistance, and is suitable for color galvanizing.
Note: After the product comes out of the tank -> water washing -> light extraction (nitric acid + hydrochloric acid) -> water washing -> passivation -> water washing -> water washing -> ironing and drying -> drying -> aging treatment (80~90℃ in the oven.
(3) Chloride galvanizing
This process is widely used in the electroplating industry, accounting for up to 40%.
After passivation (blue white), zinc can replace chromium (comparable to chrome plating). Especially after adding water-soluble varnish, it is difficult for laymen to distinguish whether it is zinc plating or chromium plating.
This process is suitable for white passivation (blue white, silver white).
(4) Sulfate galvanizing
This process is suitable for continuous plating (wires, strips, simple, thick and large parts and components), and the cost is low.
3. Electro-galvanizing process
(1) Electro-galvanizing process flow
Taking galvanized iron alloy as an example, the process flow is as follows:
Chemical degreasing → hot water washing → water washing → electrolytic degreasing → hot water washing → water washing → strong corrosion → water washing → electroplating of zinc-iron alloy → water washing → water washing → light extraction → passivation → water washing → drying.
(2) Preparation of electrogalvanizing bath
Preparation of plating solution (taking lL as an example):
a. First add 1/3 volume of pure water into the plating tank;
b. Dissolve sodium hydroxide with 1/3 pure water (it will generate heat when dissolved, so be careful);
c. Use a small amount of water to make zinc oxide into a paste, then add more pure water and stir thoroughly. Slowly add the stirred zinc oxide to the dissolved sodium hydroxide solution, stir while adding, until it is fully complexed and then added to the plating tank;
d. When the temperature of the plating solution drops below 30~C, add 85g of Baser and stir thoroughly;
e. Dissolve 15mL BaseF in 15gBaseR, and then add the mixture into the plating tank;
f. Add 4mL of H-O624, stir thoroughly; add water to the required volume;
g. Add brightener ZF-105A and ZF-105B; stir thoroughly.
Black passivation process: water washing → light extraction → water washing → black passivation → water washing → post-processing → drying.
(3) Factors affecting electro-galvanizing
a. Effect of zinc content
If the zinc content is too high, the brightness range is narrow, and thick coatings are easy to obtain, and the iron content in the coating is reduced; if the zinc content is too low, the brightness range is wide, it takes a long time to reach the required thickness, and the iron content in the coating is high.
b. Effect of sodium hydroxide
When the sodium hydroxide content is too high, it is easy to burn during high-temperature operation; when the sodium hydroxide content is too low, the dispersion ability is poor.
c. Effect of iron content
If the iron content is too high, the iron content in the coating will be high, and the passivation film will not be bright; if the iron content is too low, the iron content in the coating will be low, the corrosion resistance will be reduced, and the color will be olive-brown.
d.The influence of brightener
If ZF-IOOA is too high, the coating will be brittle; if it is too low, there will be no coating in the low current area and the passivation color will be uneven; if ZF-100B is too high, the coating will be brittle; if it is too low, the entire coating will not be bright.
e. Effect of temperature
If the temperature is too high, the dispersion ability is reduced, the iron content in the coating is high, the corrosion resistance is reduced, and the color of the passivation film is uneven and blooming; the temperature is too low, the high current density area is burned, the coating is brittle, and the deposition rate is slow.
f. Effect of cathode movement
Cathode movement must be used. If the movement is too fast, the coating in the high current density area will be rough; if it is moved too slowly, air flow may occur and there will be no coating locally.