The concept of electroplating
[Edit this paragraph]
It is the process of plating a thin layer of other metals or alloys on certain metal surfaces using the principle of electrolysis . During electroplating, the coating metal serves as the anode and is oxidized into cations that enter the plating solution; the metal product to be plated serves as the cathode, and the cations of the coating metal are reduced on the metal surface to form a coating. In order to eliminate the interference of other cations and make the coating uniform and strong, a solution containing coating metal cations must be used as the electroplating solution to keep the concentration of coating metal cations unchanged. The purpose of electroplating is to deposit a metal layer (deposit) on the substrate to change the surface properties or dimensions of the substrate. Electroplating can enhance the corrosion resistance of the metal (coated metals are mostly corrosion-resistant metals), increase hardness, prevent wear, and improve Conductivity, lubricity, heat resistance, and beautiful surface.
Electroplating
[Edit this paragraph]
Using electrolysis to deposit a metal coating with good adhesion on mechanical products, but with different properties from the base material. layer of technology. The electroplated layer is more uniform than the hot dip layer and is generally thinner, ranging from a few microns to dozens of microns. Through electroplating, decorative protective and various functional surface layers can be obtained on mechanical products, and workpieces with wear and processing errors can also be repaired. Most of the coatings are single metals or alloys, such as zinc, cadmium, gold or brass, bronze, etc.; there are also dispersion layers, such as nickel-silicon carbide, nickel-fluorinated graphite, etc.; and cladding layers, such as copper-on-steel. Nickel-chromium layer, silver-indium layer on steel, etc. In addition to iron-based cast iron, steel and stainless steel, the base materials for electroplating also include non-ferrous metals, such as ABS plastic, polypropylene, polysulfone and phenolic plastics. However, plastics must undergo special activation and sensitization treatments before electroplating.
Principle of electroplating
[Edit this paragraph]
In the plating tank containing the electroplating solution, the parts to be plated that have been cleaned and specially pretreated serve as the cathode , the anode is made of plated metal, and the two poles are connected to the positive and negative poles of the DC power supply respectively. The electroplating solution consists of an aqueous solution containing metal plating compounds, conductive salts, buffers, pH adjusters, additives, etc. After energization, the metal ions in the electroplating solution move to the cathode under the action of potential difference to form a plating layer. The metal of the anode forms metal ions into the plating solution to maintain the concentration of metal ions being plated. In some cases, such as chromium plating, an insoluble anode made of lead or lead-antimony alloy is used, which only serves to transfer electrons and conduct current. The concentration of chromium ions in the electrolyte needs to be maintained by regularly adding chromium compounds to the plating solution. During electroplating, the quality of the anode material, the composition of the plating solution, temperature, current density, power-on time, stirring intensity, precipitated impurities, power supply waveform, etc. will all affect the quality of the coating and need to be controlled in a timely manner.
Electroplating methods
[Edit this paragraph]
Electroplating is divided into rack plating, barrel plating, continuous plating and brush plating, which are mainly related to the parts to be plated. The size is related to the batch size. Rack plating is suitable for products of general size, such as car bumpers, bicycle handlebars, etc. Barrel plating is suitable for small parts such as fasteners, washers, pins, etc. Continuous plating is suitable for batch production of wire and strip. Brush plating is suitable for local plating or repair. Electroplating solutions include acidic, alkaline, acidic and neutral solutions with added chromium compounds. No matter what plating method is used, the plating tank, hanging equipment, etc. that come into contact with the products to be plated and the plating solution should have a certain degree of safety. Versatility.
Coating classification
Coatings are divided into two categories: decorative protective coatings and functional coatings.
Decorative protective coatings are mainly chromium plating layers on ferrous metals, non-ferrous metals and plastics, especially the copper-nickel-chromium layer on steel, and the nickel-chromium layer on zinc and steel. In order to save nickel, people have been able to plate copper-nickel/iron-high-sulfur nickel-nickel/iron-low solid nickel-chromium layer on steel. Tin/nickel coatings, similar to chrome plating, can be used on analytical balances, chemical pumps, valves and flow measuring instruments.
There are many types of functional coatings, such as: ① Sliding bearing cover coatings to improve compatibility and embedding with the journal, lead-tin, lead-copper-tin, lead-indium, etc. Composite coating; ② Used for hard chromium coating on wear-resistant medium and high-speed diesel engine piston rings. This coating can also be used on plastic molds, with the characteristics of non-stick mold and long service life; ③ In the sliding of large herringbone gears Copper plating on the surface can prevent early roughening of the sliding surface; ④ Zinc plating used to prevent the steel matrix from atmospheric corrosion; ⑤ Copper-tin coating to prevent nitriding; ⑥ Used for brazing in the manufacture of radios and televisions and to prevent steel and aluminum Corroded tin-zinc coating between galvanic cells. Engineering coatings suitable for repair and manufacturing include chromium, silver, copper, etc. Their thicknesses are relatively large, and the hard chromium layer can be as thick as 300 microns.
The electroplating process is basically as follows
[Edit this paragraph]
1 Connect the plated metal to the positive electrode
2 To be The electroplated object is connected to the negative electrode
3 The positive and negative electrodes are connected with an electrolyte solution composed of positive ions of the plated metal
4 After the DC power supply is supplied, the metal of the positive electrode will undergo Oxidation (losing electrons), the positive ions in the solution are reduced (gaining electrons) into atoms at the negative electrode and accumulate on the surface of the negative electrode.
The aesthetics of the electroplated object after electroplating is related to the size of the current. The smaller the current, the more beautiful the electroplated object will be; otherwise, some uneven shapes will appear.
The main uses of electroplating include preventing metal oxidation (such as rust) and decoration. The outer layer of many coins is also electroplated.
Sewage produced by electroplating (such as electrolyte that has lost its effectiveness) is an important source of water pollution.
Electroplating terminology
[Edit this paragraph]
3 Plating method terminology
3.1 Chemical passivation
The process of placing the workpiece in a solution containing an oxidizing agent to form a thin passive protective film on the surface.
3.2 Chemical oxidation
The process of forming an oxide film on the metal surface through chemical treatment.
3.3 Electrochemical oxidation
In a certain electrolyte, metal parts are used as anodes. After electrolysis, a protective, decorative or other functional oxidation layer is formed on the surface of the parts. membrane process.
3.4 Electroplating
The process of using the principle of electrolysis to deposit metal or alloy on the surface of the workpiece to form a uniform, dense, and well-bonded metal layer.
3.5 Conversion film
A surface film layer containing a compound of the metal formed by chemical or electrochemical treatment of the metal.
3.6 Steel bluing (chemical oxidation of steel)
Heating steel parts in the air or immersing them in an oxidizing solution causes them to form a usually blue (black) color on the surface. The process of forming a colored thin oxide film.
3.7 Inrush current
The instantaneous large current that passes through the current process.
3.8 Bright plating
Under appropriate conditions, electroplating with a glossy coating can be obtained directly from the plating tank.
3.9 Alloy electroplating
The process of depositing two or more metals (including non-metallic elements) under the action of electric current.
3.10 Multi-layer electroplating
Electroplating in which several metal layers with different properties or materials are deposited successively on the same substrate.
3.11 Impact plating
A method of electrodepositing a thin metal layer in a specific solution with high current density in a short time to improve the bonding force between the subsequently deposited coating and the substrate.
3.12 Phosphating
The process of forming an insoluble phosphate protective film on the surface of steel parts.
3.13 Thermal Resistance
The process of heating plated parts to diffuse the base metal and the deposited metal (one or more) to form an alloy.
4 Terminology of pre-plating and post-plating treatment
4.1 Chemical degreasing
In alkaline solution, saponification and emulsification are used to remove oil stains on the surface of the part. process.
4.2 Electrodegreasing
In an alkali-containing solution, the workpiece is used as an anode or cathode, and the oil stain on the surface of the workpiece is removed under the action of electric current.
4.3 Light Emission
The process of soaking in a solution for a short time to form a bright surface on the metal.
4.4 Mechanical polishing
A mechanical processing process that uses a high-speed rotating polishing wheel coated with polishing paste to improve the surface brightness of metal parts.
4.5 Organic solvent degreasing
The process of using organic solvents to remove oil stains on the surface of parts.
4.6 Hydrogen Removal
Heat metal parts at a certain temperature or use other methods to eliminate the process of hydrogen absorption inside the metal during the electroplating production process.
4.7 Stripping
The process of removing the coating from the surface of the part.
4.8 Weak etching
Before electroplating, the process of removing the extremely thin oxide film on the surface of metal parts in a solution of a certain composition and activating the surface.
4.9 Strong etching
The process of immersing metal parts in an etching solution with a higher concentration and a certain temperature to remove oxides and rust on the surface of metal parts .
4.10 Pre-plating treatment
In order to expose the real surface of the part material, eliminate internal stress and other special purposes, various pre-treatments such as oil stains, oxides and internal stress must be removed. Technical processing.
4.11 Post-plating treatment
Carried out to enhance the protective properties of plated parts, improve decorative properties and other special purposes (such as passivation, hot melting, sealing and hydrogen removal, etc.) deal with.
5 Materials and Equipment Terms
5.1 Anode Bag
It is made of cotton or chemical fiber fabric and placed on the anode to prevent anode sludge from entering the solution. bag.
5.2 Brightener
It is an additive used in the electrolyte to obtain a bright coating.
5.3 Resistants
Substances that can slow down the speed of chemical or electrochemical reactions.
5.4 Surfactants
Substances that can significantly reduce interfacial tension even when added in very low amounts.
5.5 Emulsifier
A substance that can reduce the interfacial tension between immiscible liquids to form an emulsion.
5.6 Complexing agent
A substance that can combine with metal ions or compounds containing metal ions to form complexes.
5.7 Insulating layer
A layer of material applied to a certain part of the electrode or rack to make the surface of that part non-conductive.
5.8 Hanger (clamp)
A tool used to hang parts so that they can be placed in a tank for electroplating or other processing.
5.9 Wetting agent
A substance that can reduce the interfacial tension between the part and the solution and make the surface of the part easily wetted.
5.10 Additives
A small amount of additives contained in the solution that can improve the electrochemical performance of the solution or improve the quality of the coating.
5.11 Buffering agent
A substance that can maintain the pH value of a solution essentially constant within a certain range.
5.12 ??Moving cathode
A cathode that uses a mechanical device to make the plated part and the pole perform periodic reciprocating motion together.
6 Terminology related to testing and inspection
6.1 Discontinuous water film
Usually used for uneven wettability caused by surface contamination, making the surface The water film becomes discontinuous.
6.2 Porosity
The number of holes per unit area.
6.3 Pin Kong
The tiny pores from the coating surface to the underlying covering layer or base metal. It is due to the obstruction of the electrodeposition process at certain points on the cathode surface, causing the It is caused by the fact that the coating cannot be deposited here, but the surrounding coating continues to thicken.
6.4 Discoloration
Changes in the color of metal or coating surfaces (such as darkening, loss of color, etc.) caused by corrosion.
6.5 Bonding strength
The strength of the bond between the coating and the base material.
6.6 Peeling
The phenomenon that the coating separates from the base material in flakes.
6.7 Peeling
The surface coating is broken or peeled off due to certain reasons (such as uneven thermal expansion or contraction).
6.8 Orange Peel
A surface treatment layer similar to the corrugated shape of orange peel.
6.9 Sponge plating
A loose and porous deposit formed during the electroplating process that is not firmly combined with the base material.
6.10 Burnt coating
A dark, rough, loose and poor-quality deposit formed under excessive current, which often contains oxides or other impurities.
6.11 Pitting
In plating or corrosion, small pits or holes formed on the metal surface.
6.12 Roughness
During the electroplating process, the coating is rough and not smooth due to various reasons.
6.13 Coating solderability
The ability of the coating surface to be wetted by molten solder.
Electro-zinc plating
[Edit this paragraph]
Electro-zinc plating: It uses electrolysis to form a uniform, dense and well-bonded metal or alloy on the surface of the workpiece. The process of depositing layers.
Compared with other metals, zinc is a metal that is relatively cheap and easy to plate. It is a low-value anti-corrosion plating layer. It is widely used to protect steel parts, especially against 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).
At present, electroplating solutions in China can be divided into four major categories:
1. Cyanide zinc plating:
Because (CN) is highly toxic, environmental protection has put forward strict restrictions on the use of cyanide in electro-galvanizing, and continues to promote the reduction of cyanide and the replacement of cyanide electro-zinc plating bath systems. Development requires the use of low cyanide (micro cyanide) plating solutions.
After electroplating using this process, the product quality is good, especially color plating, and the color remains good after passivation.
2． Zincate zinc plating:
This process evolved from cyanide zinc plating. 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 Institute of Radio, Film and Television. 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.
Note: after the product comes out of the tank—gt; washed with water—gt; light (nitrate acid)—gt; washed with water—gt; passivation—gt; washed with water—gt; washed with water—gt; scalded to dry—gt; Drying—gt; aging treatment (80~90oC in the oven.
3. Chloride galvanizing
This process is widely used in the electroplating industry, accounting for as high as 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 zinc plating
This process is suitable for continuous plating (wire, strip, Simple, thick and large parts and components) and low cost.