Electroplating of metal: types, methods, process description
Electroplating is the application of a metal film to protect products and to give them additional characteristics: resistance to corrosion, hardness, wear resistance, decorativeness, etc. Any metal product needs additional protection, even aluminum parts are coated with galvanic insulation.
The scheme by which the plating of the metal is realized is quite simple. It includes a product to which a protective coating is applied, a container with an electrolyte solution where the product is placed. The third participant in the process is a metal plate on which a positive current is applied, it acts as an anode, the product placed into the solution becomes the cathode where the negative charge is applied.
When the electrical circuit is closed, the metal of the anode (plate) dissolves in the electrolyte and under the action of the current rushes towards the negatively charged product (cathode), thereby creating a durable coating.The electrolyte is a conductive solution for moving metals from the anode to the cathode. The size of the tanks (baths) with electrolyte is different, depending on the production tasks.
Products of large sizes are placed on suspensions through which a negative charge is passed, the design is kept on weight in the volume of the bathroom. Small products receive electroplated coating in the drum-type baths, where a large number of products are galvanized at the same time. In this case, a negative charge is applied to the drum, rotating in the tank with electrolyte, where the anode is wound.
There are bell cast baths, where the electroplated coating is simultaneously applied to a large number of very small parts, such as hardware. The products are poured into the tank, the electrolyte composition is poured and the anode is installed. Baths are given a slow rotation, during which the products are evenly covered with protective metal.
The galvanic method of coating products allows you to create a durable protective coating on metals, isolating parts from the corrosive effects of working environments.Insulation can be made of various metals, the application is carried out by anodic and cathodic spraying.
The cathode coating is characterized by the fact that at the slightest violation of the integrity of the deposited layer, the metal under it is destroyed more intensively, which is facilitated by the coating technology itself. An example of rapid erosion are products made of tinned metal, where the insulating layer is tin.
Anodic electroplating has other characteristics. When conditions of a threat of corrosion arise, galvanic isolation is exposed to destruction, the metal remains intact for a long time. Anodized products are well protected from corrosive environments, mechanical damage. The most common type of insulation is galvanizing. The method allows you to save all the characteristics of the processed product, its appearance, shape and size.
Electroplating is divided into several types depending on the purpose of the product:
- Protective and decorative. The purpose of the application is to obtain high aesthetic characteristics and protect products from damaging factors.
- Protective.Insulate metal parts from the action of corrosive media, mechanical damage.
- Special purpose. Electroplating is applied to obtain new properties - increased wear resistance, increased hardness characteristics, and magnetic, electrical insulating properties of the finished product. In some cases, galvanization is used to restore the original appearance of the product or after prolonged use.
Types of coatings
Galvanic coating method is implemented by applying various metals to the product, each of them has its own characteristics and goals in the further operation of the part or object:
- Silver - increases aesthetic value, protects against corrosion, improves reflective, conductive characteristics. The type of application is in demand in the production of static relays, contactors, electromagnetic relays, electromagnetic starters, microchips and other electronic products.
- Nickel plating is the most demanded plating of steel, copper and aluminum products. The nickel layer reliably protects products or machine parts from rust,formed under the influence of the environment, as well as from types of corrosion arising as a result of contamination with aggressive environments of the working environment - alkalis, acids, salts. Nickel-plated products demonstrate high resistance to strong mechanical damage, abrasion.
- Chrome plating - increases wear resistance, hardness of anodized surfaces, allows you to improve the appearance, restore damaged parts to the original parameters. Depending on changes in the technological mode, they receive electroplated coating with various parameters and properties - gray matte (increase in hardness, but low wear resistance), brilliant (high wear resistance, hardness), milky plastic (aesthetics, high degree of anticorrosion protection, low hardness), galvanizing - Anticorrosion treatment of solid steel sheets, car parts, building and finishing materials.
- Electroplated gold plating - used in jewelry, electronics and other industries. The layer of gold gives the details high reflective properties, aesthetics, protection against corrosion, increases the conductive qualities.
- Copper plating is often used to coat metal in order to protect against corrosion, copper improves its conductive properties, metal with such a coating is often used to produce electrical conductors that are used outdoors.
- Brass - used to protect against corrosion damage to steel, aluminum and alloys. The brass layer provides the necessary adhesion of metal parts with rubber.
- Rhodium is a special coating applied to impart high resistance to parts in chemical aggressive environments, to provide additional mechanical wear resistance. Also, rhodium plating gives products decorativeness, protects silver objects from oxidation, dullness.
Regulation of quality and technological processes of electroplating is carried out using GOST 9.301-78.
Electroplating is a multi-level process that is implemented in three main steps (preparation, coating, final processing of the finished product).
Surface preparation for further electroplating is the most time-consuming and crucial stage of the whole process. The quality of the obtained protective coating depends on the accuracy and adequacy of its implementation.If there is the slightest trace of fat and oxide film on the metal surface, it will be impossible to obtain a homogeneous continuous protective film - the coating will not be able to penetrate into the layers of the base metal, bubbles may form, breaks, etc.
Defects can occur in places where there are burrs, surface irregularities, in places of poorly polished junctions, places not sufficiently dust-free. Electroplating requires low surface roughness, thorough cleaning after grinding and mandatory treatment with degreasing agents.
Part Processing Types
Machining and achieving the ideal smoothness of metal parts is achieved at home by grinding the surface with emery paper and other abrasives; sandblasting, chemical, and automated methods of achieving results are used on an industrial scale. At the preparatory stage, isolation of parts or individual places that are not subject to galvanization is carried out.
Depending on the type of the applied metal, various preparations are carried out. Before galvanizing or curing, the surface of the protected part is degreased andpickled. Chrome plating and nickel plating is preceded by mechanical grinding, degreasing, and removal of the oxide film. Degreasing is carried out in two stages - starting work and complete degreasing.
Pre-parts are washed with solvents - white spirit, gasoline, special organic mixtures, etc. Final processing is carried out using alkaline solutions or by electrochemical method. After that, the parts are washed with hot water, activation and easy etching of the metal is carried out to remove the smallest oxide films, which improves the adhesion of the surface of the part with electroplated coating of the metal.
How the process is implemented
The deposition of a protective layer of metal on the products is carried out using special equipment. Differences in the application of electroplating types are reflected in the recipe of the electrolyte used.
The galvanic method of coating metals and other materials is as follows:
- Electroplating baths are filled with electrolytic solution. They place the anodes and processed products. The size and appearance of the bath depends on the size of the parts that require coating.
- The heating device brings the temperature of the electrolytic composition to the desired technologically reasonable value.
- Current is supplied to the structure from a source equipped with a voltage regulator.
- The process of electroplating takes some time, its value is determined by the size of the part, the achievement of the required thickness of the protective layer.
In some cases, with the electroplating method of coating, the workpieces are hung on the cathode rod located in the bathroom, and metal plates are placed on the anode rod that will cover the products. To obtain certain characteristics of the coating, metal salts, organic compounds, bright agents, etc. can be introduced into the electrolyte.
To accelerate the process of transferring metals, the electrolyte is stirred, which makes it possible to use a high current density. Reversing the direction of current provides a smooth surface.
The exact time of the galvanic coating process is established empirically - by applying a protective layer on the part, measuring the thickness of the resulting layer for a certain period of time under the given conditions of the technological process.At the stage of fitting, special attention is paid to the layer thickness in the cavities and cavities of the treated experimental part.
The thickness of the electroplated coating is determined according to the data on the average thickness of the applied layer, depending on the conditions in which the part will be used. They are divided into groups:
- Light Conditions (LS) - parts are used in closed heated rooms with a relatively dry atmosphere, or the product will be used for a short period in an environment where there are no active corrosive agents. The thickness of the single-layer coating is about 7 microns, multi-layer - 15 microns.
- Medium Conditions (CC) - the parts will be used in an environment with medium humidity, pollution, small amounts of fuel, industrial emissions or evaporation of sea water. The thickness of the single-layer coating is 15 microns, multi-layer - 30 microns.
- Harsh conditions (JS) - provide for the operation of parts in conditions of high humidity, high levels of pollution with industrial gases, fuel waste, solids, dust. The thickness of the single-layer coating - 30 microns, multilayer - 45.
Data on the thickness of electroplated coating of parts in one layer contains GOST 2249-43. These include zinc coatings. Controls the multi-layer plating of GOST 3002-45 (nickel coatings). The thickness of the layer can be changed according to the design requirements or in those cases when the workpiece is designed for a short service life. Service life of galvanizing is up to 5 years, for other types of coatings up to 3 years.
Finished product processing
Electroplating of parts ends with a stage of additional processing. In this process, the following operations are implemented:
- Coloring by paint and varnish compositions.
- Oiling or polishing.
- Silvering the compositions against dullness.
Lightening and passivation increase the anti-corrosion properties of galvanized products and cadmium coatings. The process of passivation is the immersion of products in a special solution that forms a protective film on the surface of the part with a thickness of up to 1 micron.
Products from steel, copper with electroplated coating are additionally treated with oils - oil is oiled.This is done in order to improve the protective qualities of metallic insulation and contributes to the improvement of corrosion resistance.
The requirements for the quality of electroplated coatings depend on the operating conditions of the processed product. To assess the application using these types of controls:
- Evaluation of the appearance of the part by visual inspection, comparison with reference samples (surface finish, color, the presence or absence of gloss).
- Determination of electroplating thickness and porosity is performed under laboratory conditions (measurement).
- Corrosion resistance according to TU or GOST (test).
- Mechanical, physical stability (reflective properties, plasticity, wear resistance, electrical and temperature resistance, hardness, etc.)
The advantages of this method of protection of metal products include:
- High anti-corrosion properties.
- Resistance to mechanical and physical damage.
- Resistance to aggressive environments of natural and industrial origin.
- Low porosity of the coating.
- Hardness, wear resistance.
- The ability to adjust the thickness of the applied coating during application.
The disadvantages of the method include high energy consumption, environmental threats, the high cost of cleaning measures.