Electrodeposition or electrocrystallization is a branch of applied electrochemistry. From the point of view of engineering application, it is through research and development and product design to use this technology to obtain the coating layer or improve the surface of the base material so that it has more excellent engineering properties. After the 1980s, the term "surface engineering" was adopted and popular in the world to summarize the various surface-optimized technological fields in current engineering, hoping to integrate these different engineering methods into a complete design, manufacture, and operation process. In the unified production sequence. This generalization actually shows the undoubted importance of surface problems in the entire process of engineering design and product use and maintenance, reflecting the summary of production and operation experience . Numerous cases have repeatedly proved that the performance, quality and safety are closely related to the state of the material surface. Ignoring superficial problems will have irreparable consequences. Moreover, the quality requirements of the surface and the behavior at work need to be carefully considered from the very beginning of product design.
In fact, the optimization of the surface cannot be simply regarded as a later addition of surface modification. Because in , any processed product has different material body and surface properties. The processing process generally makes the surface in a non-equilibrium state or forms a metastable structure. Surface residual stress or microcracks are common phenomena. These conditions will change the working conditions of the parts, shorten the service life, and even cause sudden accidents under certain conditions. The overall consideration starting from product design can ensure the production of good products. This includes determining the expected environment , the requirements for materials and the possible damage mechanism when the workpiece is working, determining the surface characteristics that should be possessed and the surface treatment methods that can be selected.
The impact of surface engineering on a product is not simply used to add its value or be regarded as a simple means to make up for the lack . Because the reasonable combination of surface treatment method and base material can not only complement each other , but also have the effect of bringing out the best in each other. The combination of the surface layer and the base material can provide internal and external characteristics that a single base material is difficult to provide, thus forming a useful composite material.
Although electroplating is an important means of surface engineering, however, due to its own special conditions, chemical or electrochemical deposition methods have also been used as a way to develop and prepare new materials. The traditional methods are the preparation of pure metals, alloys and direct electroforming to produce parts. Newer methods are used to prepare different types of composites, including powder, short- fiber , long-fiber winding, and whisker-reinforced materials. Preparation of semiconductor materials, including thin-film semiconductor elements, flat-panel display devices, large-area cheap solar cells, photoresistors, capacitors, photoconductors, conductance, magnetic conductance and memory elements, as well as some special components, such as light-emitting and fluorescent components, etc. etc. have been gradually listed on the schedule. From these aspects, electroplating is different from surface engineering in the pure sense. This book is a sub-volume of the complete book on corrosion and protection, so it will focus on the engineering treatment of surfaces, and is more limited to the application of corrosion and protection.
(1) Electroplating method
The electroplating processes that are usually widely used in corrosion and protection include: electroplating methods with external currents, deposition methods without external currents or using internal currents or electroless plating, and direct chemical conversion on the surface of substrate materials.
1. Electroplating method with applied current
Electrodes are embedded in the electrolyte and current is passed through. At this time, an electrochemical reaction occurs . This electrochemical reaction involves reduction of ions on the cathode surface and oxidation on the anode surface, both processes utilized in the electroplating process. And not only small ions can be discharged to form a coating, but larger particles that can be charged, such as polymer paint or rubber particles, can also be deposited on the electrode by this method.
2. Electroplating method without external current
Materials with different potentials are used to contact the plated parts, and the internal current generated can also be deposited. The substitution reaction or self-catalytic reduction on the interface between the base material and the solution makes the ions deposit as a coating, which can save the trouble of applying external current and does not need to configure power equipment. However, such methods are inevitably subject to various limitations of chemical reaction conditions.
3. Surface transformation
Oxidation or reduction processes that occur when electrons are lost or captured are also commonly used to form protective films on surfaces. This surface treatment method is to generate reaction products through surface transformation, thereby providing many functions of the surface film layer. Such as corrosion protection, anti-friction and anti-wear, improving the adhesion of coatings and adhesives, providing colorless, black or colored decorative film layers etc.
All the above-mentioned methods must be completed by the conversion of ions and the exchange of electrons on the interface. When a charged particle migrates in a conductive medium, it actually transfers both charge and mass. The particles after discharge accumulate in an orderly or disorderly manner to form the deposition layer we need. In this way, the relationship of the amount of deposition can be characterized by Faraday's law. The numerical interaction relationship essentially reflects the measurement of the number of reacting particles passing through the two sides of the interface. Tight particle counts make electroplating a process that can be easily monitored with more stringent parameters and processes. Because of this, coupled with flexible use, low cost and easy operation, it has been widely used in industrial production.
(2) Coating application
Electroplating is used as a surface treatment, usually for two main purposes, namely: to give or improve the surface state of the product or to make the surface structure have some useful function. According to industry habits, it is generally divided into decorative, protective or functional electroplating.
1. Protective decorative plating
Many physical or engineering characteristics of materials are the main basis for selecting materials when designing products. Once a part has the functional requirements it was designed to meet, it is often difficult to coordinate the chemical properties of its surface with it. In the actual use environment, corrosion of products is common and one of the problems that has a particularly large impact on product life. Rust not only damages the appearance of the product, but also destroys the function of the product in most cases. For example: moving parts stuck, poor electrical contact, contamination or damage to adjacent materials, including metals, especially textile materials, deformation or fracture of parts, etc. In such cases, the appearance of rust on the surface is not merely a matter of aesthetics.
The so-called protective coating is often concerned with its protective performance, including chemical protection, namely anti-rust, and mechanical protection, that is, anti-wear or anti-friction. However, in a commodity economy, the marketing of many products is influenced by appearance even if the function is the same. Whether the appearance of the product is beautiful or not can often affect the customer's impression of the internal quality of the product, that is to say, the appearance of the product can enhance the value of the product and promote marketing. Therefore, even the protective coating cannot ignore the decorative function of its appearance. Conversely, the coating used to decorate the exterior also needs to be able to protect. This coating cannot be used alone if it is only beautiful but cannot protect or even cause accelerated damage to the base material or damage to the parts or materials combined or in contact with it. Therefore, the functions of protection and decoration need to be considered at the same time in actual use, and it is difficult to distinguish them absolutely .
2. Functional electroplating
Functional electroplating mainly refers to those coatings that can make the surface of materials have certain special functions. At present, with the continuous emergence of various new coatings, in fact, the surface of the material can be endowed with various functional characteristics through electroplating. Coatings with force, magnetism, electricity, sound, light, and expected chemical properties have all been greatly developed. Improve surface hardness, improve friction or reduce wear, improve electrical conductivity or reduce contact resistance, enhance magnetism or increase reflection, resist high temperature welding and bonding, prevent diffusion and penetration, help lubrication, use in grinding, increase size or Repairing worn parts, etc., are common occasions where coatings are used.
Traditional electroplating is mainly used in decoration and antirust. In recent years, the development of high technology has accelerated the development of functional coatings , thus greatly expanding the scope of application of electroplating. The inclusion-type coating makes the electroplating layer change from a pure metal structure to a composite structure, forming a composite coating of metal and non-metal. This kind of coating can provide various additional functional properties such as reinforcement, anti-wear, self-lubrication, fluorescence, color, electromagnetic, disinfection and grinding ability, so it has broad development and application prospects. The application of this coating comes from lubrication, producing fluorescence, and making abrasive tools, etc., are examples of the application of this coating.
After more than half a century of development, electroplating has been applied to almost all fields of production and life. Among them , the research and development of new coatings with unique functions is one of the key points of development. In order to meet higher requirements, new varieties and new processes are needed. But in any case, the needs of technological development and engineering design, market demand, quality , cost and environmental protection have always influenced the application and development of surface technology. The smooth development of the market often becomes a symbol of the success of new technology research and development and promotion.
Due to the increasing variety and functions of coatings, it is also easy to adapt to the consumption trend of product personalization, diversification and rapid replacement . More flexible production and the use of computers and networks to adapt will also become an inevitable trend.
