It is estimated that the number of metal materials and equipment scrapped due to corrosion in the world every year is about one quarter to one third of the annual metal production. It can be seen that it is very important to study the corrosion and protection of metals. Among them, electroplating and hot dip plating are widely used. However, these two processes have some disadvantages in application, such as high energy consumption and serious pollution. Especially for the electroplating of high-strength workpieces, the effect is not ideal. Due to hydrogen embrittlement in electroplating, the mechanical strength of the workpiece is greatly affected; However, in hot plating, the high temperature annealing of steel is harmful because the temperature is too high (≥450℃). The existence of these problems urges people to constantly explore new anti-corrosion technologies.
Mechanical electroplating technology is a new surface protection technology that developed countries such as Europe, America and Japan have begun to apply in industry in the past twenty or thirty years. Zinc layer, tin layer, cadmium layer, aluminum layer and mixed layers of these metals can all be obtained by mechanical electroplating. In the mixed layer, zinc and cadmium, zinc and aluminum, zinc and tin, cadmium and aluminum can be deposited in various proportions, thus providing excellent corrosion protection. Each metal deposit has many advantages of corrosion resistance. Other soft ductile metal powders, such as copper, brass, indium, gold, silver and lead, can also be deposited mechanically. This high-performance coating can provide sacrificial anode protection in outdoor, industrial and marine environments, and can protect 10-30 years or longer. In recent years, mechanical electroplating has been paid more and more attention by the metal parts industry because of its advantages of low energy consumption, low cost, simple process, diverse formulas, convenient operation, high production efficiency, no hydrogen embrittlement and less waiting time for environmental pollution. In recent ten years, the author has devoted himself to the research of mechanical electroplating technology, and made good progress in the surface brightness of coating, the selection of composite coating and the extension of neutral salt spray test time.
1 mechanical electroplating process
Mechanical electroplating process is a process of mixing activator, metal powder, impact medium and a certain amount of water into slurry, which is put into the drum together with the workpiece. With the help of the mechanical energy generated by the rotation of the dry roll, zinc coating is gradually formed on the iron-based surface under the simultaneous action of mechanical collision between activator and impact medium. Obviously, the principle of this process is different from hot dip plating and electroplating. When it is carried out at room temperature, there is no metallurgical reaction at high temperature, dendrite structure and metal compound formed by hot plating, which avoids the influence of high temperature annealing on the strength and properties of the workpiece. In this process, no electric field directly acts on the surface of the workpiece, so there is no reduction reaction in the electroplating process, which fundamentally avoids the generation and harm of hydrogen embrittlement.
The typical mechanical electroplating process can be divided into four stages: (1) surface pretreatment: this stage is mainly to remove oil stains and oxides on the surface of the workpiece to expose the metal matrix for electroplating. (2) Flash plating: In order to prevent iron-based oxidation and promote the close combination between the coating and the substrate, a thin metal layer, usually a copper layer, is often formed on the pretreated workpiece surface before plating. This process takes only 30-90s, which is customarily called "flash copper". (3) Electroplating: after flash plating, the electroplating stage is entered. The amount of metal powder and activator needed in electroplating mainly depends on the surface area of the workpiece and the thickness of the coating. For example, on a workpiece with a total surface area of 1m2, 25? M zinc layer, about 200g zinc powder is needed. (4) Post-treatment: Post-plating separation-rinsing-drying-passivation-sealing all belong to this stage. After electroplating, the workpiece is usually separated from the medium by vibrating screen and magnetic separator. The separated medium can be returned to the drum for reuse, while the workpiece is washed, dried and boxed. If necessary, the workpiece can be further passivated or sealed with organic matter to improve corrosion resistance.
The process of mechanical zinc plating can be divided into degreasing → rinsing → pickling (or shot blasting) → rinsing → flash plating → mechanical plating → separation → rinsing → drying → passivation, etc.
2 mechanical plating equipment and raw materials
The typical working machine of mechanical plating equipment is a polygonal drum with one end open or half open. Its main function is to provide mechanical impact force, so that the metal powder, activator and water in the drum can quickly form uniform mixed slurry, thus ensuring that the plated parts turn and rotate in the drum, and the required coating can be plated under the action of impact medium. Most plating barrels are octagonal, and the ratio of diameter to axial length is not more than1:3; The working position is 20-30 degrees from the horizontal position. The impact medium used in electroplating process should not only provide impact energy, but also play a buffering role to reduce the impact between heavy workpieces and the damage of sharp chips or edges to the coating. Therefore, in addition to a certain strength and good wear resistance, the surface should be smooth and angular. At present, the most commonly used glass beads are 0.5-4mm in size and have various specifications. The mixing ratio depends on the shape, size, weight and coating material of the workpiece. There are too many media with large particle size, and the coating surface is uneven, so it is not easy to form a coating in cracks and depressions; However, there are too many medium with small particle size, and the impact force is not enough, so the adhesion of the coating decreases.
All kinds of chemical additives added in the process of mechanical electroplating are always called activation punishment. Its main function is to help metal powder disperse in water, stabilize the pH value of the plating solution and improve the performance and quality of the coating. Therefore, activators are usually composed of a variety of chemicals. In order to ensure the quality of the coating and improve the uniformity and thickness of the coating, the above metal powder and the corresponding activator are generally added in batches with an interval of 3-5 minutes. After feeding, the impact was strengthened for 5- 10 min to make the coating structure more uniform and compact, and finally the required coating was formed. The proportion of activator depends on the requirements of salt spray resistance time, coating thickness, workpiece structure and surface properties. At present, the improvement and development direction of mechanical electroplating process is mainly alloying metal powder, looking for efficient dispersant, improving compactness, brightness and electroplating efficiency, reducing cost and basically prolonging corrosion resistance time.
3 coating performance characteristics
Mechanical coating is a kind of coating composed of uniform and flat metal particles, as shown in Figure 2. According to the coating thickness, it can be divided into two categories: one is 25.4-88.9? M, called MG (mechanical zinc plating), can replace hot dip plating products; The other is less than 25.4? M, called MP (mechanical electroplating), can replace electroplating products. The properties of these two coatings are basically the same, but the thickness and selection are different. The characteristics of mechanical coating are: the appearance of coating is uniform silvery white, but the color is not as good as electroplating, and there are tiny concave and convex points; The coating has good uniformity, adhesion and coating ability. This is especially important for some workpieces with deep holes, grooves and threads; The corrosion resistance of the coating is good, which is usually measured by neutral salt spray test. Fig. 2 is the surface morphology of the traditional mechanical zinc plating layer I prepared with 325 mesh electric furnace zinc powder. A few zinc powder particles in the coating change from spherical to ellipsoid, and the larger zinc powder particles have a greater tendency of plastic deformation. While smaller particles are filled in the gaps between large particles or sandwiched between deformed particles. Fig. 3 shows the surface morphology of mechanical zinc plating layer II prepared by the author with flaky zinc powder. It is found that the activity of flaky zinc powder is obviously enhanced and the electroplating efficiency is improved. The smoothness of the coating surface is obviously improved, and flaky zinc powder is superimposed layer by layer to form a dense coating. The brightness is also obviously improved, and the passivation effect is better than that of coating I. 5% sodium chloride solution was prepared with deionized water, and neutral salt spray test was conducted at 35℃. Coating II has no red rust when it reaches 1000 hours, which is close to Dacromet coating with long corrosion resistance by mechanical zinc plating reported in domestic literature.
The two pictures show that there is fine wool connection between the particles, which is the bonding effect of inert metals, welding with zinc powder particles or mixing with other inclusions to fill the gaps and form a mechanical coating. Because metal ion M2+ with more correct potential than metal zinc is added to mechanical zinc plating, chemical reaction will occur in acidic bath environment: M2++Fe→Fe2+ 10m, and the generated M will appear on the surface of zinc powder particles in the form of fine brush. The generation of m will lead to the change of charge on the surface of zinc powder particles and coating, and promote the mutual adsorption between them. In addition, M, as a new phase, is easy to attach to zinc powder particles for nucleation and growth, which will promote the agglomeration and adsorption of zinc powder particles.
Fig. 2 SEM(2000×) morphology of mechanically galvanized surface I Fig. 3 SEM(2000×) morphology II of mechanically galvanized surface.
Development, present situation and trend of mechanical electroplating technology
The research of mechanical electroplating began in 1950s. 1953 American Peenplate Company obtained the first patented technology. In 1960s, mechanical electroplating began to be used in industrial production. However, at that time, it was limited to zinc plating of small workpieces such as washers, gaskets and springs, which took a long time and was inefficient. The utilization rate of zinc powder is only 20%-30%. From the late 1970s to 1980s, with the improvement of technology; With the improvement of activator performance, mechanical zinc plating is widely used for zinc plating of various metal parts, such as bolts, nuts, nails, cement nails, malleable cast iron pipe fittings and so on. At the same time, the electroplating time is shortened and the efficiency is significantly improved. With modern mechanical electroplating technology, the electroplating time is further shortened, the utilization rate of metal powder can reach 90-95%, the whole process can be completed in 30-4 min, and the thickness can be adjusted at will between10-100μ m. Foreign countries have specially formulated corresponding standards for mechanical electroplating, such as American ASTM B635- Standard for Mechanical Deposition of Cadmium and Tin Alloy Coatings on Steel Surfaces. B695- Standard for Mechanical Deposition of Zinc Coating on Steel Surface; B696- Standard for Mechanical Deposition of Cadmium Coatings on Steel and Other Surfaces. At present, foreign mechanical electroplating technology has developed from simple mechanical zinc plating to plating metals such as cadmium, tin, copper, silver, lead, bismuth, indium and alloys of brass, cadmium, tin, tin, zinc and cadmium. Thereby further improving the performance of the coating and having a wider application range.
The research on mechanical electroplating in China began in the middle and late 1980s. However, it is mainly limited to mechanical zinc plating, and its application scope is very small. Compared with foreign countries, there is a big gap. The main reason is that we don't know the dull coating formed by mechanical electroplating, and we don't know the excellent hydrogen embrittlement produced by electroless plating and the high temperature annealing produced by hot dip galvanizing correctly. Although China promulgated the ministerial standard of mechanical zinc plating for iron and steel products in 1999, the mechanical zinc plating process is still rare in China's mechanical parts processing enterprises, mainly distributed in coastal export processing enterprises such as Zhejiang, Shandong, Fujian, Shanghai and Jiangsu, and the coating is only a one-component zinc plating layer, and the thickness of the processed coating is usually less than 20-60 microns. The composition, surface smoothness, passivation and corrosion resistance of ultra-thin coatings with coating thickness less than 10 micron and ultra-thick coatings with coating thickness of 60-10 micron are the key research contents in the future. It is believed that with the increasing internationalization of China's machinery processing industry, the demand for infrastructure construction and the strengthening of environmental awareness, mechanical electroplating products will gradually replace some electroplating and hot dip galvanized products. Mechanical zinc plating has the characteristics of less pollution, low energy consumption, uniform coating, easy control of thickness, no brittle effect, simple process, convenient operation, good mechanical properties and corrosion resistance, and has broad development prospects.