Concept
Electroplating: It is the process of using electrolysis to form a uniform, dense, and well-bonded metal or alloy deposition layer on the surface of a workpiece.
1. Electro-galvanizing
(I. Overview
Compared with other metals, zinc is a relatively cheap and easy-to-plat metal. It is a low-value anti-corrosion plating layer. It is widely used to protect steel parts, especially to prevent atmospheric corrosion, and for decoration. Plating technology includes Tank plating (or rack plating), barrel plating (suitable for small parts), automatic plating and continuous plating (suitable for wires and strips).
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, and has continuously promoted the development of electroplating zinc plating solutions that reduce cyanide and replace cyanide. It is required to use low cyanide (micro cyanide) electroplating solutions. . After electroplating using this process, the product quality is good, especially color plating, and the color remains good after passivation.
2. Zincate galvanizing:
This process evolved from cyanide galvanizing. Currently, there are two major factions in China: the "DPE" series of Wuhan Materials Protection Institute and the "DE" series of Radio and Television Institute. They all belong to zincate zinc plating with alkaline additives; the pH value is 12.5~13. Using this process, the coating lattice structure is columnar, has good corrosion resistance, and is suitable for color galvanizing.
Typical recipe:
NaOH-------------110~120g/l
ZnO---------------11~12g/l
94------------------5~7g/l
94 is the product code name for the combination of "DPE-Ⅱ" and ethanolamine.
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 (80~90ºC 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). When the customer has no special requirements, it is best to choose silver passivation (the color remains relatively stable).
Typical recipe:
KCl----------------------180~220g/l
ZnCl---------------------65~75g/l
H3BO3------------------25~30g/l (buffer). PH value:5~5.5
CI-87--------------------15~20g/l (brightener).
4. Sulfate galvanizing
This process is suitable for continuous plating (wires, strips, simple, thick and large parts and components). low cost.
Typical recipe:
ZnSO4----------------------300~500g/l
H3BO3---------------------25~30g/l. PH value: 4.5~5.5
Sulfur zinc--------------------15~20g/l (brightener).
(2) Three major links that should be strictly controlled in electro-galvanizing:
1. Pre-treatment of electroplating:
Electroplating pre-treatment is the basis for electroplating and the key to ensuring product quality. If the substrate to be plated is not processed to the specified requirements before electroplating, even if there is a good electroplating solution, appropriate electroplating parameters, equipment to regulate the electroplating parameters, and highly skilled personnel, it is impossible to obtain an electroplated layer that meets the quality requirements. . Before electroplating, not only the grease present on the base metal and foreign objects that affect the adhesion and other quality requirements of the coating must be removed, but also its surface oxide must be removed to ensure that the surface has a specified cleanliness and a certain activity to ensure that the coating is consistent with The substrate is firmly bonded, and sometimes some other special pre-plating treatments are required according to the required appearance of the electroplated layer.
Implementation standard: GB/T12611-90
2. Electroplating process control
The use conditions and service life of electroplated zinc metal or components are closely related to the thickness of the electroplated layer. The stricter the use conditions and the longer the service life, the thicker the required electroplated zinc layer should be. For different products, the thickness of the electroplating layer for the expected service life must be determined according to the specific environment of use (temperature, humidity, rainfall, atmospheric composition, etc.). Blind thickening will cause various wastes. However, if the thickness is insufficient, the expected service life requirements will not be met. Different manufacturers, based on their own equipment conditions, must first compile a relatively complete and reasonable process flow when determining the type of plating, clarify the plating parameters, control the concentration of the plating solution, and carry out standardized operations.
3. Post-plating treatment:
Electroplating post-processing (passivation, hot melting, sealing and hydrogen removal, etc.) for the purpose of enhancing the protective performance, decoration and other special purposes of plated parts. After galvanizing, chromate passivation or other conversion treatments are generally performed to form a corresponding type of conversion film, which is one of the key processes to ensure post-plating quality. After passivation, it is best to carry out aging treatment (70~80ºC in the oven). Passivation can be divided into the following forms:
Color blunt: HNO3, H2SO4, CrO3 (three acids are indispensable)
Blue white blunt: F+Cr
Silver blunt: Ba+Cr
Black blunt: Cr+Ag or Cr+Cu
Golden yellow dull: Cr+reducing agent
Color passivation: suitable for zincate galvanizing. After passivation, the surface of the parts will be red, green, slightly yellow (Cr+6 red, Cr+3 green), purple cannot appear (the appearance indicates that the passivation film layer is loose). The simplest method is to rub your fingers back and forth on the surface of the part several times without any discoloration (fading).
Blue-white passivation: Since the fluoride in the passivation solution gradually decreases over time, the blue color on the surface of the parts will gradually become lighter, and the color of workpieces produced in the same shift will not be maintained well. Therefore, the passivation process must be strictly controlled and the thickness must be paid attention to.
Silvery white blunt: does not change with time, and the color maintains good consistency.
(3) Product quality inspection
1. Strictly implement the current national standards:
(1) <Metal covering, zinc plating on steel> GB/T 9799-1997 eqv250 2081:1986
(2) <Chromate conversion coating for electroplated zinc and electroplated cadmium layers> GB/T 9800-1988 eqv250 4520:1981
(3) <Artificial Atmosphere Corrosion Test: Salt Spray Test> GB/T 10125-1997
2. Inspection requirements: Both parties should sign a technical agreement before production. In principle, the agreement should be followed.
(1) Large items: Each item must be inspected and no inspection is allowed.
(2) Small parts: Sampling inspection method is adopted, and 5% of each batch is selected. If 1% is unqualified, an additional 10% will be selected. If 1% is still unqualified during the additional sampling, the entire batch will be unqualified.
Inspection procedures:
Appearance inspection: Inspect according to the drawings, process and standards, and keep a sample standard with the manufacturer (sealed with varnish or plastic film) as a comparison sample.
There should be no obvious coating defects on the main surface of the electroplated parts, such as blisters, pores, roughness, cracks or local lack of coating, except for inevitable coating defects caused by defects in the base metal. The plated parts should be clean and free of coating defects. Damage. The coating should be bright (unless otherwise specified).
Coating thickness inspection: Use a thickness meter for inspection (user's requirements are the standard).
(Generally specified minimum local thickness)
General purpose coating thickness: б=8~12um
For outdoor use: б=18~22um
Bolts and nuts, etc.: б=3~6um
The functional positioning of the coating should be reasonable: after galvanizing, if there are no special requirements, the brightness should not be too high. If it is too high, the brittleness of the coating will increase and the color will change quickly. It must be reasonable and the amount of brightener should be controlled.
Deep in the blind hole, there should be no plating leakage, scorching, or peeling. Color differences are allowed, and there may be slight charring on the sharp corners of large items.
3. Inspection method:
Under natural scattered light, or under the illumination of two 40-watt fluorescent lamps, at a distance of 500mm (observed with the naked eye), in this state, any problems detected are quality problems; if no problems are detected, they are allowable defects.