100 Questions on the Basics of Electroplating

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1. Why can an electrolyte conduct electricity? Answer: The way in which an electrolyte conducts electricity is different from that of metal conductors. In metal conductors, current is carried by the movement of free electrons, while in electrolytes, it is carried by charged ions. In an electrolyte, the charges of positive and negative ions are equal, so it does not exhibit electrical properties; this is what we call electrical neutrality. When we apply a voltage to the electrolyte, due to the attractive force of the strong electric field, the ions move toward the electrode with the opposite polarity to their own. Cations move toward the cathode, while anions move toward the anode. Their movement allows current to pass through, and this is how the electrolyte conducts electricity. 2. During the electroplating process, the hangers become hot to the touch – is this due to too high a temperature of the plating solution? Answer: Although the heating of the hangers is related to the temperature of the solution, the main reasons are: (1) The current passing through the hangers is too high. (2) Poor contact on the hanger leads to an increased resistance, causing the hanger to heat up. 3. What are the main factors that control the thickness of the electroplated layer? Answer: The main factors controlling the thickness of the electroplated layer are current density, current efficiency, and electroplating time. 4. Are brass plating and bronze plating the same type of alloy plating? Answer: No, brass plating is an alloy plating made of copper and zinc, while bronze plating is an alloy plating made of copper and tin. 5. What relationship does Faraday’s law describe? Please explain Faraday’s first and second laws. Answer: Faraday’s law describes the relationship between the amount of electric charge that passes through an electrode and the weight of the reactants at that electrode; it is also known as the electrolysis law. Faraday’s first law: The weight of metal deposited during electrolysis is proportional to the current passing through the electrolyte and the time elapsed. W = KIt. Where W is the weight of the deposited substance in grams; K is a constant of proportionality (the electrochemical equivalent); I is the current intensity in amperes; and t is the time of electrolysis in hours. Faraday’s second law states that when the same amount of current passes through different electrolytes, the weight of the metal deposited is proportional to the chemical equivalent of each electrolyte. K=CE, where C is the proportionality constant. 　E – Chemical equivalent. 6. Why must the plated parts be washed with clean water after chemical degreasing before undergoing mild acid etching? Answer: Since typical chemical degreasing solutions are alkaline, if these solutions are introduced directly into the acid etching solution, a neutralization reaction between acid and alkali occurs, reducing the concentration and effectiveness of the acid. The products of the neutralization reaction adhere to the workpiece, affecting the quality of the coating. Therefore, after chemical degreasing, the workpiece must be thoroughly rinsed with clean water before it can be placed in the acid etching solution. 7. Burring and coarse grains appear on the electroplated layer; what usually causes this, and how can it be resolved? Answer: The appearance of burrs and coarse grains on the plating layer is mainly caused by contamination of the plating solution with suspended impurities. Its sources are: dust in the air, sludge from the anode, and hydrolysis products of metal impurities. In addition, there are issues such as abnormal plating solution composition and suboptimal operating conditions. The solution is to adjust the composition of the plating solution and the operating conditions. If it is caused by suspended impurities, the plating solution should be filtered. 8. What is the basic procedure for preparing an electroplating solution? Answer: The basic procedure for preparing an electroplating solution is as follows: (1) First, place the required electroplating chemicals in a measuring container (a small tank), then add an appropriate amount of water to dissolve them; be careful not to pour the chemicals directly into the plating tank. (2) The impurities present in the solution can first be removed using various chemical methods, and then treated with activated carbon. (3) The treated and settled solution is filtered into a clean plating tank, and water is added to the standard volume. (4) Adjust the plating bath process parameters (pH value, temperature, additives, etc.). (5) Finally, electrolytic deposition is carried out at a low current density to remove other metal ion impurities until the solution is suitable for use. 9. Why should the hangers be coated with insulating material? Answer: In the manufacture of hangers, aside from the hooks and the conductive parts that come into contact with the product, all other parts should be coated with insulating material. This helps to reduce current loss and metal loss, ensures that the effective area of the product is plated, increases the effective current, and enhances the durability of the hangers. 10. What is the rust-removing effect of sulfuric acid and hydrochloric acid? Can nitric acid remove rust? Answer: For rust removal, concentrated hydrochloric acid generally yields the best results, offering high efficiency; even if used for an extended period, it does not cause excessive corrosion or damage to the base metal. Sulfuric acid is effective at removing surface rust, but the rust removal process is slow; moreover, prolonged use can lead to further corrosion, causing significant damage to the product’s substrate. Nitric acid cannot be used for rust removal, as it is highly oxidizing; upon contact with metals, it causes oxidation and generates large amounts of the highly toxic gas nitrogen oxide. 11. What is the impact of pre-plating treatment on the quality of the electroplated layer? Answer: Long-term production experience has shown that most quality issues that occur in electroplating processes are not caused by the electroplating process itself. It is mostly due to improper pre-plating treatment of metal products. The quality of properties such as the flatness, adhesion, and corrosion resistance of the coating is closely related to the quality of the pre-coating treatment. The surface condition of metal products before electroplating and the degree of its cleanliness are crucial factors in achieving a high-quality plating layer. On rough metal surfaces, it is difficult to obtain a smooth and shiny coating, and the coating also has many pores, which reduces its corrosion resistance. If there is some kind of oil residue on the metal surface, a proper coating cannot be obtained. 12. What is the definition of free cyanide in a cyanide plating bath? Answer: In a cyanide plating bath, the excess cyanide that is not bound within complex ions is called free cyanide. For example, the free cyanide in copper plating baths is the excess cyanide that does not form = coordination ions. 13. In cyanide-based copper plating, passivation occurs at the anode, resulting in poor dissolution; why does the concentration of free cyanide increase? Answer: During cyanide-based copper plating, the anode does not dissolve properly. Although some cyanide ions are oxidized and consumed at the anode, more free cyanide ions are generated at the cathode due to the discharge of copper-cyanide complexes, which leads to an increase in the concentration of free cyanide in the plating solution. 14. What is the impact of the anode material used in acidic bright copper plating on the quality of the coating? Answer: In the acidic bright copper plating process, if an electrolytic copper anode is used, copper powder is likely to be generated, resulting in a rough coating; moreover, the amount of brightener consumed increases rapidly. Therefore, a copper anode containing a small amount of phosphorus (0.1–0.3%) should be used, as this can significantly reduce the formation of copper powder. However, if a copper anode with too high a phosphorus content is used, it will reduce the anode’s dissolving capacity, resulting in a decrease in the copper content in the plating solution. 15. In a nickel plating solution, as the anode area decreases and the anode current density increases, does the pH value of the solution rise or fall? Answer: The pH value of the solution falls. This is because the anode decreases, the current density increases, the anode becomes passivated and does not dissolve; after passivation of the anode, oxygen is released, the H+ concentration in the solution increases, thereby increasing acidity and lowering the pH. 16. What should be added to the nickel plating solution to promote anodic dissolution? Is it possible to add a large amount of boric acid? Answer: To promote the dissolution of the nickel anode, an appropriate amount of chloride ions should be added. Boric acid has no effect in promoting the dissolution of nickel anodes. 17. What harmful impurities need to be taken into account regarding the interference in bright nickel plating solutions? Answer: For bright nickel plating, the following needs to be considered: (1) Impure industrial raw materials. If nickel sulfate contains copper, zinc, and nitrate ions, the anode nickel plate contains impurities such as iron ； (2) Pollution during the production process. If cleaning is not thorough, copper and chromium brought in from the product or hangers. Decomposition products of organic additives. These are all harmful impurities in the bright nickel plating, and it is necessary to remove them. 18. Is peeling and flaking of the coating that occurs after nickel plating followed by chromium plating mainly caused by poor pre-plating treatment? Answer: Poor pre-plating treatment is one factor contributing to the peeling of the coating after nickel plating followed by chromium plating, but it is not necessarily the only cause; it is also related to the condition of the plating solution as well as the phenomenon of double-layer nickel formation. 19. Why cannot metallic chromium be used as a chromium plating anode? Answer: Soluble metallic chromium is not used as an anode for chromium plating, mainly because it dissolves very easily during the plating process. The current efficiency for the dissolution of chromium metal at the anode is **much higher** than that for the deposition of chromium metal at the cathode. As a result, as the plating process progresses, the chromium content in the plating solution inevitably increases, making it impossible to carry out normal plating. Moreover, when metal chromium is used as the anode, it dissolves into the solution mainly in the form of trivalent chromium ions, resulting in a significant accumulation of trivalent chromium ions in the plating solution. At the same time, since metallic chromium is very brittle and difficult to shape into various forms, it cannot be used as an anode; instead, lead or lead alloys are generally used as the anode in the chromium plating process. 20. Why is a partial brown film formed in the chromium plating layer? Answer: The formation of a partial brown film in the chromium plating layer is mainly caused by a deficiency of sulfate ions. Furthermore, too low a bath temperature or interference from impurities such as Cl- can also cause a brown film to form in the chromium plating. 21. What is electrolysis? Answer: Electrolysis is the process in which, when an electric current passes through an electrolyte, redox reactions occur at the electrodes, causing the electrolyte to be decomposed under the influence of the current. When electricity is applied, the cations of the electrolyte move toward the cathode, where they gain electrons and are reduced to new substances ； Anions move toward the anode, where they lose electrons and are oxidized to form new substances. Sometimes oxidation of the electrode material also occurs at the anode. For example, electrolyzing molten sodium chloride. NaCl → Na+ + Cl- (cathode); Na+ + e → Na (anode); 2Cl- – 2e → Cl2↑. The electrolysis industry plays a vital role in the national economy; the smelting of many non-ferrous and rare metals, the production of chemicals, as well as processes such as electroplating, electropolishing, and anodizing are all carried out through electrolysis. 22. What is electroplating? Answer: Electroplating is a method in which a thin layer of another metal is deposited on the surface of a metal part through electrolysis. Electroplating includes processes such as pre-plating treatment (oil removal, rust removal), applying the metal layer, and post-plating treatment (passivation, hydrogen removal). Used to prevent metal products from corroding, repair worn areas, and improve durability, reflectivity, conductivity, and aesthetics. During electroplating, the metal part is used as the cathode, while the metal plate or rod to be plated serves as the anode; both are attached to copper electrodes and immersed in an electrolyte containing the components of the plating layer, after which direct current is applied. In some cases, insoluble anodes are also used, such as lead or lead-antimony alloys for chromium plating. 23. What is current intensity? Answer: Current intensity, commonly referred to as current, is the amount of electric charge that passes through the cross-section of a conductor per unit of time. The unit is ampere, abbreviated as A. 24. What is current density? How is it calculated? Answer: Current density refers to the intensity of electric current per unit area of the electrode. In electroplating, one square decimeter is used as the basic unit of measurement; therefore, the current intensity passing through an electrode with an area of one square decimeter is referred to as the current density of that electrode. The cathode current density is denoted by DK, while the anode current density is denoted by DA; the unit is amperes per square decimeter, or A/d㎡. (Amperes per square inch is also used abroad.) For example, if the total area of the plated surface is 50 square decimeters and the current used is 100 amperes, then the current density is 100 amperes ÷ 50 square decimeters = 2 amperes per square decimeter. The cathode current density has a significant impact on the quality of the coating; both too high and too low values result in coatings of poor quality. The current density also directly determines the coating deposition rate, affecting production efficiency. 25. What is current efficiency? Answer: The weight of metal deposited at the cathode as current passes through the plating solution does not necessarily correspond to the weight calculated according to Faraday’s law (which states that the weight of material deposited or dissolved on the electrode during electrolysis is proportional to the amount of electricity passed). Generally, it is less than the theoretical value. This is because during electrolysis, not only are metal ions reduced to metal through discharge, but other side reactions also occur. For example, the deposition of hydrogen consumes a certain amount of electrical energy. Therefore, to precipitate a certain amount of metal, the actual current required is greater than the theoretically calculated value. Therefore, the ratio of the current value required by theoretical calculation to the actual current value required is called current efficiency. The higher the current efficiency, the less electrical energy is wasted. 26. Given the current density and plating time, how can the thickness of the plated layer be determined? Answer: First, determine the current efficiency for this process based on the type of metal being plated; then look up the electrochemical equivalent and density (specific gravity) of that metal in a table. After that, use the following formulas for calculation:
■ Formula for calculating the thickness of the plated layer d (d: micrometers):
d = (C × Dk × t × ηk × 100) / (60 × r)
■ Formula for calculating the plating time t (t: minutes):
t = (60 × r × d) / (C × Dk × ηk × 100)
■ Formula for calculating the cathode current density Dk (Dk: A/dm²):
Dk = (60 × r × d) / (C × t × ηk × 100)
■ Formula for calculating the cathode current efficiency:
ηk = (60 × r × d) / (C × t × Dk × 100)
Where:
C = electrochemical equivalent (grams/ampere-hour)
Dk = cathode current density (amperes/square decimeter)
t = plating time (minutes)
ηk = cathode current efficiency (%)
r = density of the metal in the plated layer (grams/cubic centimeter)
Example: If the current efficiency of the nickel plating solution is 95%, the cathode current density is 2.5 A/dm², what is the thickness of the plated layer after 20 minutes of plating? From the table, the electrochemical equivalent of nickel is 1.095 and its density is 8.8 g/cm³.
d = (1.095 × 2.5 × 20 × 95% × 100) / (60 × 8.8) = 9.85 μm

27. What are anodic coatings and cathodic coatings? For iron substrates, which types of coatings such as zinc, copper, nickel, chromium, and copper-tin alloys belong to which category? Answer: Coatings can be classified as either anodic or cathodic coatings, depending on the electrochemical relationship between the coating metal and the substrate metal. Under normal conditions, when the electrode potential of the coating metal is more negative than that of the substrate metal, it is called anodic coating; otherwise, it is called a cathodic coating. The electrode potential of the zinc coating is more negative than that of the iron substrate, hence the zinc coating is a positive-type coating. The potential of copper, nickel, and copper-tin alloy coatings is positive relative to that of the iron substrate. Therefore, it is a negative-type coating. According to the standard potential, the chromium coating has a more negative value than iron; however, since the chromium coating is easy to purify, its potential tends to be positive, which classifies it as a cathodic coating as well. Since the potential of metals changes depending on the conditions, it is also possible for whether a coating is an anodic coating or a cathodic coating to change. For example, under normal conditions, a tin coating acts as a cathodic coating for iron, but in organic acids it becomes an anodic coating. 28. What is the protective effect of anodic coatings and cathodic coatings on the base metal? Answer: The principle of protection provided by anodic coatings relies on the fact that their potential is more negative than that of the base metal; their electrochemical potential is higher, which makes them act as the anode in a corrosion cell, thereby delaying the corrosion of the base metal. Even when there is slight exposure of the base metal, the coating still provides protection; therefore, the number of pores in the anodic coating has little impact on its protective properties. In terms of thickness, the greater the thickness of the coating, the stronger its protective effect. The cathodic coating serves only as a purely mechanical barrier for the base metal; it does not provide the electrochemical protection that an anodic coating does. Therefore, protective effects can only be achieved when the porosity of the coating is very low. Otherwise, at the pores or defects in the coating, the base metal will act as the anode in a corrosion cell, accelerating the corrosion of the base metal. Generally, the porosity of a coating decreases as its thickness increases; therefore, the greater the thickness, the stronger the protective properties of the cathodic coating. 29. What are the general quality requirements for parts before electroplating? Answer: Before electroplating, the parts must be free of oxide scales, rust, stains, and oil; their surfaces must be fully wettable by water, with no water droplets remaining on them. 30. Why is it necessary to rinse the plated parts with clean water after electroplating? Answer: Because after the parts come out of the plating tank, their surfaces and pores are covered with a large amount of plating solution, and such solutions are usually somewhat corrosive. If not cleaned thoroughly, it will cause corrosion to the coating and the substrate, affecting the product’s appearance and protective properties. Therefore, after the plated parts come out of the tank, they should be immediately rinsed clean with water and then dried. 31. Why are cast iron parts more difficult to electroplate than other steel parts? Answer: The surfaces of cast iron parts are often uneven and porous, and a rough and porous coating can only be obtained on such surfaces. Furthermore, free graphite is present on the surface of pig iron; it not only affects the adhesion between the coating and the base metal, but also acts as the cathode in a corrosion cell when pores exist in the coating, leading to rapid degradation of the coated metal. The graphite in pig iron can also reduce the hydrogen overpotential, facilitating the deposition of hydrogen at that location and thereby hindering metal deposition. As a result, cast pig iron parts are more difficult to electroplate than other steel parts. 32. In copper plating using cyanide, what should be added when there is a shortage of free cyanide? Can copper cyanide be added? Answer: In copper plating using cyanide, when free cyanide is insufficient, sodium (or potassium) cyanide should be added. The addition of cuprous cyanide further reduces free cyanide, causing the copper coating to become coarse and granular in structure. CuCN + 2NaCN = Na2Cu(CN)3. 33. When preparing a copper plating solution using cyanides, is it correct to dissolve copper cyanide in powder form in warm water and then add it to the plating tank? Answer: No, it is not correct. Because cuprous cyanide is insoluble in water. Cuprous cyanide should be dissolved in a solution of sodium (or potassium) cyanide, with the amount of sodium cyanide being 1.15 times that of cuprous cyanide. 34. Is it appropriate to use air stirring in a cyanide-based copper plating solution? Answer: Air stirring is not suitable in a cyanide-based copper plating solution. Because carbon dioxide in the air reacts with the base in the solution to form carbonates. Excessive carbonate can have an adverse effect on the copper coating. CO2 + 2NaOH = Na2CO3 + H2O. 25. What is the role of sulfuric acid in the copper plating solution? How does its concentration affect the quality of the coating? Answer: Sulfuric acid in the copper plating solution serves the following functions: (1) It prevents copper from hydrolyzing, thereby avoiding the formation of cuprous oxide or other basic salt precipitates. (2) Reducing the effective concentration of copper ions to make the crystal structure of the copper coating finer. (3) Reduce the resistance of the solution, increase its conductivity, and decrease energy consumption. (4) Prevent the formation of rough or dendritic plating layers at high current densities. The sulfuric acid content in the plating solution is generally 60–80 grams per liter; levels that are too high or too low are not suitable. Excess sulfuric acid can make the coating brittle and reduce the solubility of copper sulfate. Too low a level can cause problems such as rough plating, reduced anodic passivation, and diminished dispersion capacity of the solution. 36. What is the role of boric acid in nickel plating solutions, and what happens if there is not enough of it? Answer: Boric acid acts as a buffer in nickel plating solutions, helping to maintain the pH level of the plating solution. A significant effect is observed only when the concentration exceeds 31 grams per liter, but it should not be too high, as the solubility of boric acid at room temperature is around 40 grams per liter. Thanks to the buffering effect of boric acid, the pH value of the solution in the cathode region does not change sharply, which allows for the use of a higher cathode current density without the precipitation of hydroxides at the cathode. It also has the effect of increasing cathode polarization and improving the properties of the coating. However, an excessive amount of it will reduce the cathode current efficiency. When it is below 20 grams per liter, the buffering effect is weak, which exacerbates changes in the pH value of the plating solution, affects the quality of the plated layer, and may even prevent the plating process from proceeding. 37. During nickel plating, which has a higher pH value, the electrolyte near the anode or the electrolyte near the cathode? Why? Answer: The electrolyte near the cathode has a higher pH value. This is because hydrogen gas is deposited on the cathode during the electrolysis process, causing the pH value to rise. 2H++3→H2↑ 38. Among the passivation film layers formed after galvanizing, which one – the white passivation layer or the colored passivation layer – has better corrosion resistance? Answer: Among the passivation film layers formed after galvanizing, the colored passivation layer has better corrosion resistance than the white passivation layer. This is because during the dulling treatment, a layer of the already formed colored passivation film is dissolved in the bleaching solution, turning the colored passivation film white. Since the white passivation film is thinner than the colored passivation film, its corrosion resistance is reduced. 39. Barium carbonate is used to remove excess sulfuric acid from chromium plating solutions. How many grams of barium carbonate are needed for 1 gram of sulfuric acid? Answer: Theoretically, about 2 grams of barium carbonate are required to remove 1 gram of sulfuric acid. 40. Why cannot chromium be plated using a newly prepared pure chromic acid solution? Why is it necessary to carry out an electrolytic treatment or add some used plating solution in order to obtain a coating of good quality? Answer: In a newly prepared pure chromic acid solution, without trivalent chromium and catalytic anions, it is difficult to facilitate cathodic electrolytic reduction; the hexavalent chromium ions in chromic acid do not get reduced to trivalent chromium ions, nor do they turn into metallic chromium. The result of electrolysis is equivalent to the decomposition of water: hydrogen is produced at the cathode, 2H++2e→H2↑, while oxygen is produced at the anode, 4OH--4e→2H2O+O2↑. So, chromium cannot be plated. When sulfuric acid is added to the solution, it reacts with the trivalent chromium in the solution to form complex positively charged chromium sulfate 2+. Due to the small size and high charge of the sulfate ions, they are easily adsorbed by the positively charged chromium sulfate, allowing the Cr ions to pass through the colloidal membrane of basic chromium acid and enabling the deposition of metallic chromium at the cathode. In the newly prepared chromium plating solution, since there is no trivalent chromium, chromium does not adhere easily to the surface; therefore, an electrolytic treatment is required to generate trivalent chromium. Adding some old plating solution is also intended to ensure that the solution contains a certain amount of trivalent chromium, which facilitates the deposition of metal. · Answer: Polishing is a machining process that uses the abrasives on a polishing wheel to cut the surface of a part. The tiny particles of abrasive bonded to the working surface of the grinding wheel possess sharp edges; when the wheel rotates at high speed, these particles cut away small metal shavings from the surface of the workpiece, thereby improving its smoothness and finish. They also remove various macroscopic defects on the surface, such as corrosion marks, scratches, burrs, welds, pores, and rust. 82. How are grinding wheels made? Answer: Grinding wheels are circular discs made of cotton cloth and other fibrous materials such as special paper, leather, woolen fabric, felt, etc., with cowhide covering the outside; they are created through pressing, bonding, or sewing methods. Grinding wheels made of different materials have varying degrees of softness and elasticity. Method of bonding the abrasive on the grinding wheel: Generally, the particles of adhesive (such as bone glue and leather glue) are crushed, soaked in clean cold water for 6–12 hours to allow the adhesive to absorb water, water is added in an appropriate ratio to the adhesive, and then the mixture is heated in a water bath at 65–70°C until it reaches a molten state. When heating the glue, direct heating should be avoided, as too high a temperature can cause the glue to decompose and lose its adhesive properties. After applying the adhesive to the grinding wheel, it is immediately rolled slowly under pressure in a basin or trough containing emery powder, so that the emery is firmly bonded to the surface of the grinding wheel by the adhesive. It can be applied once it is completely dry. The quality of sand adhesion on the grinding wheel has a significant impact on production efficiency, service life, and product quality; the key factors lie in the preparation of the adhesive and the bonding process. The concentration of the glue depends on the grit size of the emery; the coarser the sand grains, the higher the concentration of glue required. The finer the sand particles, the lower the glue concentration. For example, the cowhide glue used to bind sand grades 100–180 has a concentration of around 30%. 83. How to choose a grinding wheel? Answer: When selecting a grinding wheel, the hardness and shape of the metal should be taken into consideration; for parts with hard materials and simple shapes, a harder wheel (such as a felt wheel) should be used. For materials that are soft (such as copper, aluminum, and their alloys) and parts with relatively complex shapes, grinding wheels with high elasticity and softness (such as cloth wheels) should be used. During polishing, it is very important to choose the appropriate rotational speed of the polishing wheel. Production experience shows that the rotational speed of the polishing wheel should generally be kept between 1200 and 2800 revolutions per minute. When the part shape is simple or the steel components are being rough-ground, a higher rotational speed can be used; whereas for parts with complex shapes or when polishing non-ferrous metals (copper, zinc, aluminum) and their alloys, a lower speed should be employed. 84. What are the types of polishing materials, and how should they be selected? Answer: Abrasives are the main materials used in the cutting and smoothing of metal surfaces. In actual production, common abrasives include synthetic emery, corundum, emery, diatomite, quartz sand, and pumice. Synthetic emery – possessing very high hardness and low toughness (compared to other abrasives) – is mainly used for rough grinding and polishing of low-strength metals such as pig iron, brass, bronze, etc. Corundum – there are both synthetic and natural types. The higher the alumina Al2O3 content in corundum, the greater its hardness. Corundum is a good material for rough grinding using grinding wheels, as it possesses greater toughness than synthetic emery. The particles of corundum are polyhedral, and their sharp edges are less acute than those of synthetic emery. Suitable for polishing more tough metals with high fracture resistance, such as quenched steel, malleable cast iron, and manganese bronze. Emery – with moderate hardness and excellent toughness, suitable for polishing all metals. Diatomaceous earth – having less sharp cutting edges and excellent toughness, it can smooth out defects such as scratches and dents on metal surfaces. It is a universal material for grinding and polishing. Quartz sand – particles with moderate hardness; it does not have sharp cutting edges and also possesses good toughness, making it a versatile material for grinding and polishing. Pumice – it has low hardness and is quite brittle; it is suitable for grinding and polishing wood, leather, rubber, plastics, and glass, among other materials. Based on the properties of these various abrasives, it can be concluded that when grinding hard metal products, synthetic emery, corundum, or emery is suitable; subsequent fine grinding can then be carried out using fine-grained emery, pumice, quartz powder, etc. When polishing softer metal products, diatomaceous earth and pumice are generally used. 85. What is polishing? Answer: Polishing is a mechanical processing process in which a polishing wheel made of a softer material (such as layers of cloth, linen, silk, etc.) along with polishing paste is used to smooth the surface of a part. Polishing is used for the pre-processing of products prior to plating, aiming to further improve the minor irregularities on the product surface and thereby enhancing its surface finish. It can also be used for the fine finishing of the coating after plating, giving the coating surface a decorative appearance and improving the corrosion resistance of the parts. During the polishing process, the metal surface is smoothed out, but there is no significant loss of material; for example, when parts are polished after plating, the loss is only 5–20% of the weight of the plating layer. Polishing wheels include cloth wheels and felt wheels, among others. The size of the wheel depends on the characteristics and requirements of the parts being produced. The circumferential speed of the polishing wheel is generally 20 to 35 meters per second. 86. What are the different types of polishing pastes, and how to choose the right one? Answer: As the polishing wheel rotates, a thin layer of polishing paste should be applied evenly over its surface in order to remove any minor irregularities on the surface of the product. Polishing paste is a paste form made by combining polishing materials such as diatomaceous earth, quartz sand, pumice, iron oxide red, Vienna lime, and chromium oxide with certain binders such as stearic acid and paraffin. Polishing pastes are generally available in three colors: white, red, and green. Depending on the material of the item to be polished, a different polishing paste should be used. White polishing paste is composed of anhydrous and highly pure calcium oxide, a small amount of magnesium oxide, and some binders. It is suitable for polishing softer metals such as aluminum, copper, and their alloys, as well as for polishing plastics. Red polishing paste is made up of iron oxide red (Fe2O3) powder along with some binders. Fe2O3 has moderate hardness, which makes it suitable for polishing steel products and giving them a polished finish after grinding. Green polishing paste – its abrasive is chromium oxide, a hard and sharp powder, suitable for polishing cemented carbide steel, chrome plating, and stainless steel. 87. What precautions should be taken when using grinding and polishing machines? Answer: The following points should be noted when using grinding and polishing machines: (1) Before turning the machine on, it is necessary to secure the grinding or polishing wheel in place first, and turn on the vacuum cleaner as well. (2) After turning it on, check the rotation direction of the axle; it must not rotate in the opposite direction, to prevent the grinding wheel or polishing wheel from falling off the axle and causing injury. (3) When grinding or polishing the product, hold the product or its fixture firmly, and gently press the grinding wheel or polishing wheel in the correct position (the product or fixture should be pressed down so that the grinding/polishing wheel is at the same level as the wheel axis) to prevent the product or fixture from slipping from one’s hands, which could damage the machine or cause injury. (4) After polishing for a period of time, the surface of the polishing or grinding wheel should be leveled to ensure a uniform and smooth surface on the polished product. When using manually operated grinding and polishing machines, it is essential to pay attention to safety. Special care must be taken when grinding thin items to avoid edge curling that could cause finger cuts. The operator must tie up the edges of their clothing and sleeves tightly and wear a safety helmet while operating, in order to prevent any accidents involving personal injury. 88. What is rolling? Answer: Rolling is a process in which friction between the part and the abrasive, as the drum rotates, is used for grinding, smoothing, and removing burrs and rust from the part. Rolling can be done in dry or wet methods: (1) In dry rolling, abrasives such as sand, emery, broken glass, and leather are used. (2) During wet barrel rolling, steel balls, crushed stones, sawdust, alkaline solutions, tea seed powder, etc. are used as abrasives. The rotation speed during rolling is determined by the characteristics of the part and the structure of the roller, and is generally between 15 and 50 revolutions per minute. When the rotation speed is too high, the high centrifugal force prevents the parts from rubbing against each other as the drum rotates, thus preventing the smoothing effect; if the speed is too low, the efficiency is low. 89. What is electrical polishing of metals? Answer: Electrical polishing is a process that uses electrochemical principles to make the surface of metal parts smooth and shiny. It can improve the corrosion resistance and light reflectivity of materials. Compared with mechanical polishing, it also has advantages such as higher productivity, easier operation, and reduced labor intensity. During electropolishing, anodic dissolution occurs, causing the concentration of metal salts near the anode to increase continuously, and a viscous film is formed (as shown in the figure). This film has poor electrical conductivity and induces anodic polarization, resulting in an increase in the anode potential ； At the same time, on the uneven surface of the part, the thickness distribution of the film is uneven; the film thickness is smaller in the raised areas, resulting in lower resistance and a concentration of electric current. This accelerates the dissolution of those raised areas, causing the size of the rough protrusions on the part’s surface to decrease rapidly, thereby achieving the goal of smoothing out the part. This film also hinders the dissolution of the anode, thereby increasing the polarization of the anode. Therefore, as the anodic dissolution occurs, an oxide film is formed on the surface of the part; this film has a certain degree of stability, which makes the part less susceptible to the effects of chemicals and puts it in a mild passivated state. This is the main reason why the surface of the part gains a glossy appearance. 90. Why are many electropolishing solutions based on phosphoric acid? Answer: This is because phosphoric acid possesses excellent properties such as high viscosity, low chemical solubility for metals, ease in forming a \"film\", and a low limiting current density for polishing. Therefore, phosphoric acid is used as the electropolishing solution in most cases. 91. What is the definition of an alloy coating? Answer: An alloy coating is defined as any coating formed by the co-deposition of two or more metals, regardless of their structural form. As long as their grains are fine enough that we cannot distinguish between them with the naked eye, and the amount of the less abundant metal in its composition is at least 1%, such a coating can be considered an alloy coating. In some single-metal plating processes, a small amount of metal brightener is used. For example, in cyanide copper plating, a small amount of lead is used as a luminizing agent, and lead will also deposit at the cathode. However, we are *accustomed not to call it an alloy, as its content in the coating is far less than 1%. 92. What are the common metal impurities in nickel plating solutions? Answer: Nickel plating electrolytes are very sensitive to impurities; the presence of certain metal impurities, even in trace amounts, can degrade the quality of the nickel coating. Metal ions such as copper, iron, and zinc are the most common harmful metal impurities in nickel plating baths. Iron deposits on the cathode preferentially to nickel in the electrolyte, making the coating brittle and prone to cracking. The maximum content of iron in the nickel plating solution shall not exceed 0.1 grams per liter. Copper has a low deposition potential, so it deposits preferentially at the cathode, giving the nickel layer a brownish-black or grayish-black color. The maximum allowable content of copper is 0.02 grams per liter. The allowable content of zinc in the nickel plating solution is not more than 0.01 grams per liter; when there is a small amount of zinc in the electrolyte, a bright plating layer forms at the edges and corners of the plated parts. At slightly higher levels, it causes black streaks, spots, and scale-like plating on the entire surface of the part. 93. How to remove metal impurities from the nickel plating solution? Answer: Method for removing iron ions: Adjust the pH of the nickel plating solution to 4. Add an appropriate amount of hydrogen peroxide (about 1–2 ml/l), stir the plating solution, and heat it to around 70°C. Use a dilute sodium hydroxide solution to raise the pH of the solution to 6; stir for 2–3 hours, then measure the pH value again. If the pH decreases due to the precipitation of iron, raise the pH back to 6. Let it stand overnight, filter it, then add sulfuric acid and adjust the pH to the normal range before attempting plating. Methods for removing copper ions: Since the pH value at which copper ions precipitate through hydrolysis is similar to that of nickel, chemical methods for removing copper are not effective, resulting in significant loss of nickel. Removing copper impurities from nickel-plating solutions by electrolysis using a low current density is a cost-effective and practical method. That is, with a current density of 0.05–0.1 amperes per square decimeter, using corrugated iron plates as the cathode, electrolysis is carried out under stirring until normal production can be achieved. Method for removing zinc ions: Raise the pH of the plating solution to 6.2 using a dilute sodium hydroxide solution or calcium carbonate, heat it to 70°C, and stir continuously for 1–2 hours. Then measure the pH again; if it is below 6.2, continue raising it to 6.2. Allow the solution to stand for more than 4 hours, then filter it. Finally, adjust the pH to the appropriate range for plating. It can also be treated by electrolysis using a low current density (0.2–0.4 amperes/dm2). 94. What is the effect of organic impurities in the nickel plating solution on the plated layer, and how can they be removed? Answer: If organic impurities are present in the nickel plating solution, it will severely affect the quality of the plated layer. It makes the nickel layer brittle and cause it to flake off, as well as creating pitting on the coating. Organic impurities are often removed by adsorption using activated carbon, at a dosage of about 2–5 grams per liter. Stir vigorously for about an hour, then let it settle and filter. Additionally, an electrical treatment can also be employed: the plating solution is heated to 50–60°C and stirred vigorously, with used parts used as the cathode, and the treatment is carried out at a low current density for 4–6 hours. This method is simpler, but its effectiveness is not as good as that of activated carbon adsorption. 95. How to maintain the nickel plating electrolyte? Answer: The nickel plating electrolyte is sensitive to impurities, so it should be kept pure and clean at all times. During use, in addition to preventing iron, copper, and other parts from falling into the tank, the plating tank should be covered immediately after use. The conductive copper rod holding the anode should be covered with a plastic plate or film. It was found that there was dirt floating on the surface of the solution, which was carefully removed using a gauze mesh. To prevent the nickel sludge and dirt that have settled at the bottom of the tank from rising again, strong stirring should be avoided during operation unless there is equipment for continuous filtration, so as not to disturb the sediment and affect the quality of the coating. 96. Why does double nickel plating improve the protective properties of the nickel layer? Answer: Double nickel plating involves first depositing a layer of semi-brilliant nickel with a columnar structure and low sulfur content (or low sulfur levels), followed by another layer of fully brilliant nickel with a layered structure and higher sulfur content. The thickness of the first layer of nickel usually accounts for about 2/3 of the total thickness. The mechanical properties of the coating are primarily determined by the semi-bright nickel (or ordinary dull nickel) layer, which has good toughness. Between the two nickel layers, their electrode potentials differ due to the varying sulfur content. Generally speaking, the potential of the first layer of semi-bright nickel is more positive than that of a fully bright nickel layer. If semi-brilliant nickel contains no sulfur at all, then there is a relatively large potential difference (about 155 MV) between brilliant nickel and semi-brilliant nickel. In this way, when the fully bright nickel layer at the cracks or pores in the chromium plating is corroded through to the underlying nickel layer, a galvanic cell can be formed due to the potential difference between the two nickel layers. The nickel layer with a more positive potential is the cathode in the galvanic cell, while the fully bright nickel layer with a more negative potential is the anode in the galvanic cell. This causes the direction of corrosion to shift from vertical to horizontal, thereby improving the protective properties of the nickel layer. 97. What is the effect of chloride ions in bright acidic copper plating solutions on the coating? Answer: A small amount of chloride ions is necessary in bright acidic copper plating solutions in order to obtain a fully bright coating; their concentration is generally kept between 10 and 80 milligrams per liter. If this concentration is too low, both the leveling ability of the plating solution and the brightness of the coating are reduced, and bright dendritic patterns may appear. In severe cases, the coating becomes rough with pores, or it may even burn. When it is too high, the brightness of the coating decreases and the areas with low current density remain dark. Chloride ions can be added in the form of hydrochloric acid or copper chloride solution. Since copper sulfate and activated carbon used in general industry contain small amounts of chloride ions, it is not necessary to add additional chloride ions when preparing a new solution. When there is an excess of chloride ions in the plating solution, silver carbonate can be used to convert them into silver chloride precipitate, which can then be removed by filtration. 98. How should a bright copper plating solution be purified? Answer: After prolonged use, if the plating solution becomes cloudy and adding brighteners does not restore it to its original condition, the solution needs to be purified. The method is as follows: Heat the solution to above 50°C, then add 1–2 ml/l of 30% hydrogen peroxide while stirring; stir thoroughly for about 1 hour. Next, add 3–5 g/l of activated carbon and continue stirring for another hour. After allowing it to stand overnight, filter the solution. The purified solution should then have all the brighteners added back in accordance with the original formulation levels. If copper sulfate needs to be added when adjusting the plating solution, it should first be dissolved in water, treated with hydrogen peroxide and activated carbon, and then added to the plating solution; this helps to reduce the accumulation of iron and organic impurities. 99. What is the role of free sodium cyanide and free sodium hydroxide in zinc plating solutions containing cyanide? Answer: Sodium cyanide in such solutions serves as the main complexing agent, while sodium hydroxide acts as a secondary complexing agent and helps to improve electrical conductivity. Since the polarizing effect of zinc tetracyanide ions on zinc deposition is greater than that of zincate ions, increasing the concentration of sodium cyanide enhances cathodic polarization, resulting in finer crystal structure of the coating and a more uniform coating thickness; however, the cathodic current efficiency decreases. Conversely, increasing the sodium hydroxide concentration reduces cathode polarization because the equilibrium of the zinc complex shifts toward zincate, which slightly improves the cathode current efficiency; however, the crystals in the coating become coarser. In a cyanide plating bath without sodium hydroxide, its current efficiency is very low. In an alkaline solution without cyanide, if no other additives are added, the coating becomes rough, dull, or even spongy. 100. Why is preheating necessary before hard chromium plating? Answer: Regardless of the type of steel part to be plated with hard chromium, it must be \"preheated\" in the plating tank before the plating process begins. The so-called “preheating” process involves placing the parts in the chromium plating tank before applying electricity, heating them without any current being passed through, so as to raise the temperature of the metal parts to a level close to or equal to that of the plating solution; only after that can electricity be applied for plating. This is because when hard chromium plating is applied, the coating is usually thick, with high internal stress and high hardness. Moreover, the expansion coefficients of the substrate and chromium often differ significantly; if plating is carried out without preheating, the substrate part gradually heats up and expands during the plating process, which can cause the coating to crack or even separate from the substrate. The larger the parts to be plated (the greater the diameter of shaft-like components), the longer the “preheating” time should be; this is a prerequisite for achieving a coating with good adhesion.