What is Chrome Plating?

Chromium is a kind of silvery white metal with blue color. The metal chromium is easily passivated in the air, and a very thin passivation film is formed on the surface, thus showing the properties of precious metals.

Because of its excellent performance, the chrome plating layer is widely used as a protective surface layer and a functional plating layer for a decorative coating system, and has always occupied an important position in the electroplating industry. With the development of science and technology and people's increasing attention to environmental protection, based on traditional chromium plating, microcracks and microporous chromium, black chromium, loose chrome, low concentration chromium plating, high efficiency hard chromium plating, trivalent New processes such as chromium chrome plating and rare earth chrome plating have further expanded the application scope of chrome plating.

According to the composition and performance of the chrome plating liquid, the chrome plating liquid can be divided into the following categories.

Ordinary chrome plating solution Chromium plating solution using sulfate as a catalyst. The plating solution contains only chromic anhydride and sulfuric acid, and its composition is simple and easy to use. It is currently the most widely used chromium plating solution. The ratio of chromic anhydride and sulfuric acid is generally controlled at 100: 1, and the concentration of chromic anhydride varies from 150 to 450 g / L. According to the concentration of chromic anhydride, it can be divided into high concentration (350 ~ 500g / L), medium concentration (150 ~ 250g / L) and low concentration (50 ~ 150g / L) chromium plating solution. Low-concentration chrome plating solution has high current efficiency and high chromium layer hardness, but has poor coverage. It is mainly used for functional plating, such as hard chromium plating and wear-resistant chrome plating. High-concentration plating solution is stable, has good conductivity, and is electrolytic It only needs lower voltage, better coverage than dilute solution, but lower current efficiency. It is mainly used for decorative chrome and complex parts.

Composite chrome plating solution Chromium plating solution using sulfuric acid and fluorosilicic acid as catalysts. The addition of fluorosilicic acid improves the current efficiency, covering ability and brightness range of the plating solution compared with ordinary chromium plating solutions. For example, the current efficiency of the cathode can reach more than 20%. However, fluorosilicic acid has a strong corrosive effect on the anode and cathode parts that are not plated with chromium and the lead lining of the plating tank. Certain protective measures must be taken. The lining and anode are preferably made of lead-tin alloy. This plating solution is mainly used for barrel chromium plating.

Automatically adjust the chromium plating solution. The chromium plating solution uses strontium sulfate and potassium fluorosilicate as catalysts. At a certain temperature and a certain concentration of chromic acid solution, strontium sulfate and potassium fluorosilicate each exist

Compared with the common chromic acid plating solution, compared with other single metal plating solutions, the chromium plating solution has a simple composition, but the chromium plating process is quite complicated and has the following characteristics.

The main component of chrome plating solution is not metal chromium salt, but chromic acid, the oxic acid of chromium, which belongs to strong acid plating solution. During the electroplating process, the cathode process is complicated, and most of the cathode current is consumed by the two side reactions of hydrogen evolution and reduction of hexavalent chromium to trivalent chromium, so the cathode current efficiency of chromium plating is very low (10% to 18%). And there are three anomalies: current efficiency decreases with increasing chromic anhydride concentration, l decreases with increasing temperature, and increases with increasing current density.

In the chromium plating solution, a certain amount of anions must be added, such as SO ₄ ^2- , SiF ₆ ^2- , F- ^, etc., in order to achieve the normal deposition of metallic chromium.

The dispersing ability of the chromium plating solution is very low. For parts with complex shapes, a pictographic anode or

There are many types of chrome plating processes, which can be classified as follows according to their uses.

Protection-decorative chrome-plating Protection-decorative chrome-plating is commonly referred to as decorative chrome. The coating is thin and beautiful. Usually it is used as the outermost layer of multi-layer electroplating. The intermediate layer is then plated with a thin layer of chromium of 0.25-0.5 m on the bright intermediate layer. Common processes are Cu / Ni / Cr, Ni / Cu / Ni / Cr, Cu-Sn / Cr, etc. After the polished surface is plated with decorative chrome, a silver-blue mirror gloss can be obtained. Does not change color in the atmosphere. This type of coating is widely used in the protection and decoration of automobiles, bicycles, sewing machines, clocks, instruments, daily hardware and other components. The polished decorative chrome layer has a high reflectivity to light and can be used as a reflector. Plating micro-holes or micro-crack chromium on multilayer nickel is an important way to reduce the total thickness of the coating and obtain a decorative system with high corrosion protection and protection, and it is also the development direction of modern electroplating technology.

Hard chrome plating (wear-resistant chrome) The coating has extremely high hardness and abrasion resistance, which can prolong the service life of the workpiece, such as cutting and drawing tools, pressing dies and casting dies for various materials, bearings, shafts, gauges, gears, etc. It can also be used to repair the dimensional tolerance of worn parts. The thickness of hard chromium plating is generally 5 to 50 m, and it can also be determined according to needs, and some are as high as 200 to 800 m. Iron and steel parts do not require intermediate plating for hard chromium plating. If special requirements are required for corrosion resistance, different intermediate plating can also be used.

Milk-plated white chromium chromium-plated layer is milky white, low gloss, good toughness, low porosity, soft color, lower hardness than hard chrome and decorative chrome, but high corrosion resistance, so it is often used for measuring tools and instrument panels. In order to increase its hardness, an additional layer of hard chromium can be plated on the surface of the milky white coating, the so-called double-chrome plating, which has both the characteristics of the milky white chromium plating and the hard chromium plating. It is mostly used for plating that requires both wear and corrosion resistance. Components.

Loose chrome plating (porous chrome) is to take advantage of the fine cracks of the chromium layer itself. After hard chrome plating, mechanical, chemical or electrochemical pore release treatment is used to deepen and widen the crack network. The surface of the chromium layer is covered with wide grooves, which not only has the characteristics of wear-resistant chromium, but also can effectively store the lubricating medium, prevent non-lubricating operation, and improve the anti-friction and abrasion capacity of the workpiece surface. It is often used for the plating of the surface of sliding friction parts under heavy pressure, such as the inner cavity of a cylinder bore of an internal combustion engine, a piston ring, and the like.

Black chrome black chrome plating color black with uniform gloss, good decoration, good matting; high hardness (130 ~ 350HV), wear resistance is 2 to 3 times higher than bright nickel under the same thickness; its resistance Corrosion is the same as ordinary chrome plating, mainly depending on the thickness of the intermediate layer. Good heat resistance, no discoloration below 300 ° C. The black chrome layer can be directly plated on the surface of iron, copper, nickel and stainless steel. In order to improve the corrosion resistance and decorative effect, copper, nickel or copper-tin alloy can be used as the bottom layer, and the black chrome layer is plated on the surface. Black chrome plating is commonly used for the protection and decoration of aviation instrument and optical instrument components, solar absorption panels and daily necessities. ^[2]

(1) The role of each component in the bath

1) Chromic anhydride

The aqueous solution of chromic anhydride is chromic acid and is the sole source of chromium plating. Practice has proven that the concentration of chromic anhydride can vary over a wide range. For example, when the temperature is 45 to 50 ° C and the cathode current density is 10 A / dm ² , the chromic anhydride concentration ranges from 50 to 500 g / L, and even as high as 800 g / L, a bright chrome plating layer can be obtained. However, this does not mean that the concentration of chromic anhydride can be arbitrarily changed. The concentration of chromic anhydride used in general production is between 150 and 400 g / L. The concentration of chromic anhydride determines the conductivity of the plating solution. At each temperature, there is a concentration of chromic anhydride corresponding to the highest conductivity. As the temperature of the plating solution increases, the maximum value of the conductivity increases slightly as the concentration of chromic anhydride increases Move in the direction. Therefore, in terms of electrical conductivity alone, a chromium plating solution having a relatively high chromic anhydride concentration should be used. However, when a high-concentration chromic acid electrolyte is used, due to the serious loss brought along with the workpiece, on the one hand, it causes unnecessary consumption of materials, and at the same time, it causes some pollution to the environment. The low concentration plating solution is more sensitive to impurity metal ions and has poor coverage. Too high or too low chromic anhydride concentration will narrow the range of temperature and current density for obtaining bright coatings. Baths with low chromic anhydride concentration have high current efficiency and are mostly used for hard chromium plating. The thicker plating solution is mainly used for decorative plating. Although the performance of the plating solution is related to the chromic anhydride content, the most important thing is the ratio of chromic anhydride to sulfuric acid.

2) Catalyst

In addition to sulfates, fluorides, fluorosilicates, fluoroborates, and mixtures of these anions are often used as catalysts for chromium plating. When the catalyst content is too low, no coating is obtained or very few coatings are obtained, mainly brown oxides. If the catalyst is excessive, it will cause poor covering ability, lower current efficiency, and may result in partial or complete absence of coating. The currently widely used catalyst is sulfuric acid. The content of sulfuric acid depends on the ratio of chromic anhydride to sulfuric acid, which is generally controlled between 80 and 100: 1, and the optimal value is 100: 1. When the sulfate content is too high, the dissolution effect on the colloidal film is strong, the exposed area of the substrate is large, the real current density is small, and the cathode polarization is small. metal. When the above problems occur in production, according to the results of chemical analysis, an appropriate amount of barium carbonate should be added to the plating solution, and then the formed barium sulfate precipitate can be removed by filtration. When the sulfate content is too low, the coating becomes gray and rough, and the gloss is poor. Because the sulfate ion content is too low, only a small part of the film on the surface of the cathode is dissolved, that is, the film formation speed is faster than the dissolution speed, the precipitation of chromium is blocked or the discharge is grown in local areas, so the resulting coating is rough. At this time, an appropriate amount of sulfuric acid can be added to the plating solution. When fluorine-containing anions (F-, SiF ₆ ^2- , BF _4- ) are used as catalysts, the concentration is 1.5% to 4% of the chromic anhydride content. The advantages of this type of bath are: the cathode current efficiency of the bath is high The coating has high hardness and low current density. It is not only suitable for rack plating, but also for barrel plating. In China, fluorosilicate ions are used more. It also has the function of activating the surface of the coating. It can still obtain a bright coating when the current is interrupted or secondary chromium plating, and it can also be used for barrel chromium plating. Generally, H ₂ SiF ₄ or Na ₂ SiF ₆ (or K ₂ SiF ₆ ) is added as the main source of SiF ₆ ^2- . As the temperature of the plating solution containing SiF ^2- ion increases, the working range is wider than that of sulfate ion plating solution. The disadvantages of this plating solution are that it is corrosive to the workpiece, anode, and plating bath, and requires high maintenance, so it is impossible to completely replace the plating solution containing sulfate ion. At present, many manufacturers use sulfate ions and SiF ₆ ^{2- for} better results.

3) Trivalent chromium

The Cr ⁶⁺ ions in the chrome plating solution are reduced at the cathode to produce Cr ³⁺ , and at the same time, they are oxidized again at the anode. Cr ³⁺ ions are the main component of the colloidal film forming the cathode. Only when the plating solution contains a certain amount of Cr ³⁺ can the chromium deposition proceed normally. Therefore, the newly prepared plating solution must take appropriate measures to ensure that it contains a certain amount of Cr ³⁺ .

Use large area cathode for electrolytic treatment.

Add reducing agent to reduce Cr ⁶⁺ to Cr ³⁺ . Alcohol, oxalic acid, and rock sugar can be used as reducing agent. Among them, alcohol (98%) is more commonly used, and the dosage is 0.5mL / L. When adding alcohol, due to the exothermic reaction, it should be added while stirring, otherwise chromic acid will splash out. After adding alcohol, it can be put into use after a little electrolysis.

Add some old tank liquid.

The content of Cr ³⁺ in the ordinary chromium plating solution is about 2 ~ 5g / L. There are also reports that the content of chromic acid is 1% ~ 2%. The allowable content of trivalent chromium and the type, process and impurities of the plating solution Content. When the Cr ³⁺ concentration is low, it is equivalent to the phenomenon that occurs when the sulfate ion content is high. The cathode film is discontinuous, with poor dispersibility, and chromium deposition occurs only at higher current densities; when the Cr ³⁺ concentration is high, it is equivalent to insufficient sulfate ion content, and the thickness of the cathode film is not only significantly reduced. The conductivity of the plating solution increases the tank voltage and reduces the current density range for obtaining bright chrome plating. In severe cases, only a rough, gray plating can be produced. When the content of Cr ³⁺ is too high, a small area cathode and a large area anode are also used to maintain the anode current density of 1 to 1.5 A / dm ^{2 for} electrolytic treatment. The processing time depends on the content of Cr ³⁺ , from several hours. To several days and nights. The effect is better when the bath temperature is 50-60 ° C. ^[2]

There is an interdependent relationship between the cathode current density and temperature during the chromium plating process. When chromium is plated in the same solution, by adjusting the temperature and current density, and controlled within an appropriate range, three different properties of bright chromium, hard chromium, and milky chromium can be obtained, as shown in Figure 4-20. In the low-temperature and high-current-density region, the chromium plating layer is gray or scorched. This coating has network cracks, high hardness, and brittleness. In the high-temperature and low-current density region, the chromium layer is milky white. Cracks, better protection performance, but low hardness, poor wear resistance; when the current density region or the combination of the two is good at medium temperature, a bright chrome plating layer can be obtained. This chrome layer has higher hardness and has fine and dense network cracks. .

When the current density does not change, the current efficiency decreases with increasing temperature; if the temperature is fixed, the current efficiency increases with increasing current density. However, as the ratio of chromic anhydride to sulfate ion decreases, the change becomes smaller accordingly. Therefore, when hard chromium is plated, under the premise of satisfying the performance of the coating, a lower temperature and a higher cathode current density are generally used to obtain a higher deposition rate of the coating. When the temperature is constant, as the current density increases, the dispersion ability of the plating solution improves slightly; on the contrary, the current density does not change, and the dispersion ability of the plating solution decreases to a certain extent as the temperature of the plating solution increases. Generally, medium temperature (45 60 ) and medium current density (30 45A / dm ² ) are used in production to obtain bright and hard chromium plating. Although the process conditions for obtaining bright coatings are quite wide, considering that the dispersion ability of the chromium plating solution is particularly poor, when parts with complex shapes are plated with decorative chromium or hard chromium, it is necessary to strictly control the thickness of the chromium layer in different parts. Temperature and current density. After the chromium plating process conditions are determined, the temperature change of the plating solution is best controlled between soil (1 to 2) ° C. ^[3]

1) Improving the bonding strength of the plating layer Due to the poor dispersion and depth capabilities of the chrome plating electrolyte, some parts with complex shapes will miss plating.

When hard chromium is plated, the peeling of the plating layer often occurs due to the poor bonding force. In the production operation, the following measures can be adopted.

Impulse current for some parts with complex shapes, in addition to using pictographic anodes, protective cathodes and

(1) Protection-decorative chrome plating

Protection-Decorative chrome plating not only requires that the coating has good corrosion resistance in the atmosphere, but also has a beautiful appearance.

This type of coating is also commonly used for the plating of non-metallic materials.

Protection-decorative chrome plating can be divided into general protective decoration chrome and high corrosion-resistant protective decoration chrome. Table 4-28 lists the process specifications for protective decorative chrome plating.

The process conditions for decorative chrome plating also depend on the base metal material to be plated. The working temperature and cathode current density can be adjusted appropriately according to the different substrate materials.

1 General protective decorative chrome

General protective decorative chrome plating uses medium and high concentration of common chrome plating solution, which is suitable for products used in indoor environments. Steel, zinc alloy and aluminum alloy chrome plating must adopt a multilayer system. The main process is as follows.

The process flow of the copper / nickel / chromium system of the steel substrate is:

Degreasing washing etching washing flash cyanide copper or nickel plating washing acid copper washing bright nickel washing chrome plating washing and drying.

The process flow of the multilayer nickel / chromium system is:

Degreasing washing etching washing semi-bright nickel plating washing bright nickel washing chrome plating washing drying.

Weak alkali chemical degreasing of zinc alloy substrate water washing dilute hydrofluoric acid water washing electric degreasing water washing flash cyanide copper plating water washing bright copper plating bright nickel water washing chromium plating water washing drying.

Weak alkaline degreasing of aluminum and aluminum alloy substrate water washing electric degreasing water washing secondary zinc dipping dissolving zinc dipping layer washing once or twice zinc dipping water washing flash cyanide copper (or pre-nickel plating) water washing Bright copper plating washing bright nickel plating washing chrome plating washing drying.

2High corrosion resistance decorative chrome plating

High-corrosion-resistant decorative chrome plating uses a special process to change the structure of the chrome plating layer, thereby improving the corrosion resistance of the plating layer. The plating layer is suitable for occasions with harsh outdoor conditions.

In the protective decorative chrome plating system, the application of multilayer nickel significantly improves the corrosion resistance of the coating. The research found that the corrosion resistance of the nickel and chromium layers is not only related to the nature and thickness of the nickel layer, but also to a large extent. Depends on the structural characteristics of the chromium layer. Although the ordinary protective decorative chrome plating layer obtained from the standard chrome plating solution is only 0.25 to 0.5 m, the internal stress of the plating layer is very large, causing uneven coarse cracks to appear on the plating layer. In the corrosive medium, the chromium plating layer is the cathode, and the bottom layer at the crack is the anode. Therefore, the corrosion is always the bottom layer or the base metal at the crack. Because the area of the underlying metal exposed at the crack is small compared to the area of the chrome layer, the corrosion current density is large, the corrosion rate is fast, and the corrosion has been developing in depth. Since cracks are unavoidable, if the structure of the microcracks is changed to disperse the corrosion, the corrosion can be slowed down. Under this concept, a new process of micro-cracked chromium and micro-porous chromium with high corrosion resistance was developed in the mid-1960s. These two types of chromium are collectively referred to as "micro-discontinuous chromium". Because the formed chromium layer has numerous micropores and micro-cracks, the exposed nickel plating area is increased but dispersed, which greatly reduces the corrosion current density on the surface of the nickel layer. The corrosion rate is also greatly reduced, thereby improving the corrosion resistance of the combined plating layer, and reducing the thickness of the nickel layer by about 5 m.

The micro-cracked chromium is coated with a layer of high-stress nickel of 0.5 to 3 m on the bright nickel-plated layer, and then plated with 0.25 m of ordinary decorative chromium. Because the internal stress of the high-stress nickel layer and the internal stress of the chromium layer are superimposed, 250 to 1500 {uniformly distributed reticular microcracks of chromium were obtained on the centimeter.

The study found that adding a small amount of SeO ₄ ^{2- to the} ordinary chromium plating electrolyte can obtain a chromium plating layer with a large internal stress. The chrome plating obtained in the plating solution containing seO42- is bluish. The higher the SeO ₄ ^2- content, the heavier the blue color of the coating.

Micro-cracked chrome plating can also be obtained by double-layer chrome plating. The process is to first plate a chromium coating with good coverage, and then plate a micro-cracked chromium layer in a chromium-containing chromium plating solution. The disadvantages of the double-layer method are the need to increase equipment, long plating time, and high power consumption. Therefore, single-layer micro-crack chromium has been used to replace it. However, single-layer micro-crack chromium also has the disadvantages of difficult fluoride analysis and uneven micro-crack distribution.

Microporous chromium The most commonly used method for electroplating microporous chromium is to deposit a nickel-based composite coating (nickel-enclosed) with a thickness of less than 0.5 m on bright nickel plating, and then coat the bright chromium layer to obtain a microporous chromium layer.

The uniformly dispersed non-conductive particles in the nickel-based composite coating have a particle size of less than 0.5 m, the suspended amount in the plating solution is 50 to 100 g / L, and the content of the particles in the composite coating is 2% to 3%. Common particles are sulfate, silicate, oxide, nitride and carbide. Because the particles are not conductive, no current passes through the particles during the chrome plating process, and there is no metal chromium deposition on them. As a result, countless tiny pores are formed, with a density of more than 10,000 per square centimeter.

3 precautions for protective decorative plating

Large parts should be rinsed and preheated with hot water before entering the trough. Do not preheat in the plating solution, otherwise it will corrode the surface of the bottom layer with high brightness.

Small parts need to adopt the barrel chrome plating process, and fluorosilicic acid should be added to the barrel chrome plating bath to prevent the surface from being passivated due to the instant non-contact conduction during the barrel plating.

The parts are charged into the slot, and the impulse current is used for complex parts, or the distance between the cathode and the anode is increased.

Every plating layer should be polished to improve the finish, reduce pores and prevent corrosion.

When chromium is plated on nickel, if nickel is passivated, it can be activated by acid leaching, and then chromium plated. The activation method is: immersion in 30% -50% (volume fraction) hydrochloric acid for 30-60s; etch in 20% (volume fraction) sulfuric acid for about 5min; cathodic treatment in 5% (volume fraction) sulfuric acid for 15s Left and right, and then chrome plating, a chrome plated layer with good adhesion can be obtained.

The power supply should adopt full-wave rectification.

When using a high-concentration chromic anhydride plating solution, a recovery tank can be installed to save chromic anhydride, reduce costs, and reduce the amount of wastewater treatment.

(2) Barrel chrome plating

Small parts that require chrome plating, such as ordinary rack plating, are not only inefficient, but fixture marks are often left on the plated parts, and the quality of the coating cannot be guaranteed. Barrel chrome is mostly used for the decoration of small size, large number, and difficult to hang parts

The surface roughness of the workpiece surface after chrome plating is directly related to the following conditions:

1. Surface roughness of the substrate before plating: The smaller the surface roughness of the substrate, the smaller the surface roughness after plating.

2. The temperature of the plating solution: the surface roughness value is large after the temperature is high;

3, the current density: the greater the current density, the greater the surface roughness value after plating;

4. Plating solution concentration: The greater the concentration of the plating solution, the greater the surface roughness value after plating;

5. Plating time: The longer the plating time, the greater the surface roughness value after plating.

There is not much difference in the surface roughness value before decorative chrome plating, and the difference in surface roughness value after hard chrome plating is large. ^[3]

Chromium plating is widely used due to its excellent performance. Especially with the development of machinery manufacturing industry, the amount of chromium plating is getting larger and larger. However, the electrolyte used in the traditional chromium plating process is formulated with highly toxic chromic anhydride. According to reports,. During the chrome plating process, about 2/3 of the chromic anhydride is consumed in the waste water or waste gas, and only about 1/3 of the chromic anhydride is used on the chromium plating. A large amount of waste water and exhaust gas has caused serious pollution to the environment. For many years, although the recovery and treatment of chromium-containing wastewater has been strengthened, it has not been fundamentally solved. In addition, there are still many shortcomings in the traditional chromium plating process. In view of the above problems, the majority of electroplating workers have done a lot of research work on the chrome plating process.

(1) Low concentration chromium anhydride chrome plating process

The low-concentration chromic anhydride chrome plating process refers to a chrome plating process in which the chromic anhydride content of the chromium plating solution is 30 to 60 g / L. The amount of chromic anhydride used is only 1/5 to 1/8 of the ordinary standard chrome plating process, which reduces the environmental impact Pollution, and save a lot of raw materials.

Decorative chromium plating and hard chromium plating can be obtained by using the low chromium anhydride chrome plating process. Its glossiness, hardness, adhesion and cracks can meet the quality requirements. But sometimes a yellow or colored film appears on the surface of the coating, which can be removed in a 5% sulfuric acid solution and then washed in an alkaline solution.

The low chromic anhydride chromium plating solution has better dispersion ability than the conventional chromium plating electrolyte, but the deep plating ability is relatively poor, which brings certain difficulties to parts with complex shapes. At the same time, the conductivity decreases and the tank voltage increases, so the energy consumption is high, and the plating solution heats up quickly. The cathode current efficiency of low chromic anhydride chromium plating reaches 18% to 20%.

Because of the above reasons, the low chromium anhydride chromium plating process is limited to a certain extent. The current research direction is focused on the search for new catalysts to improve the performance of the plating solution and reduce the tank voltage.

(2) Trivalent chromium salt chromium plating

Trivalent chromium electroplating, as the most important and direct and effective substitute for hexavalent chromium electroplating, has been studied for more than 100 years. However, due to the stability of the plating solution and the quality of the chromium coating, it has not been able to communicate with chromium. Compared with acid chromium plating, it has not been able to get large-scale applications.

Most trivalent chromium plating solutions are complex plating solutions, which are composed of a main salt, a complexing agent, a certain amount of conductive salt, a buffering agent, and a small amount of a wetting agent. Table 4-33 lists the composition and process conditions of the trivalent chromium baths studied in China.

1 the role of each component in the bath

The main salt can be trivalent chromium chloride or sulfate, and the chromium content in the electrolyte is preferably 20g / L.

Complexing agents generally use organic acids such as formic acid, acetic acid, and malic acid as complexing agents, preferably formate (potassium formate or amine formate).

The use of formate salt as the auxiliary complexing agent can receive a good effect and act as a stabilizer, so that the bath can be used for a long time without precipitation.

The conductive salts of alkali metal or alkaline earth metal chlorides or sulfates can be used as conductive salts, but nitrates are not suitable. Nitrate discharges on the electrodes and adversely affects the quality of the coating. Commonly used ammonium chloride, Potassium chloride or sodium chloride. Ammonium ions often have special effects, which is conducive to obtaining bright coatings.

The addition of bromide bromide ions can inhibit the formation of hexavalent chromium and the precipitation of chlorine gas. The hexavalent chromium in the electrolyte is extremely harmful.

The buffering agent is used to stabilize the pH value of the plating solution. The best effect is to add boric acid.

Adding sodium lauryl sulfate or sodium lauryl iodide to the wetting agent can reduce the pinholes of the coating and improve the quality of the coating.

The plating solution is more sensitive to metal impurities, such as Cu ²⁺ , Pb ²⁺ , Ni ²⁺ , Fe ²⁺ , and Zn ²⁺ . Avoid the introduction of impurities during operation, and pay attention to the electric man tank. If the plating solution contains a small amount of impurities, it can be electrolytically treated with a small current (DK 1 2A / dm2). If the content is too high, it can be treated with a corresponding cleaning agent.

2 The main characteristics and problems of trivalent chromium salt plating

The biggest feature of the trivalent chromium salt chrome plating electrolyte is that it can be operated at room temperature and the cathode current density is also low. Generally it is controlled at about 10A / dm2, which saves energy and reduces investment in equipment.

Trivalent chromium has low toxicity, eliminates or reduces environmental pollution, is beneficial to environmental protection, and has a large cathodic polarization effect of the plating solution, and the coating has fine crystals. The dispersing ability and deep plating ability of the plating solution are better than chromic chromium plating; the cathode The current efficiency is around 20%.

The chrome-plated layer obtained from the trivalent chromium electrolyte is slightly yellowish, which is not as beautiful as chromic chrome plating, the coating has good bonding strength, high internal stress, and has micro-crack properties. The maximum thickness of the coating can only reach about 3tim, and the hardness is low, and it cannot be used for hard chromium plating.

Trivalent chromium plating is not suitable for thick chromium. The main reasons are as follows:

The pH value of the plating solution, especially the increase in the pH value of the layer near the cathode surface, leads to the formation of Cr (OH) ₂ colloid, which hinders the continued thickening of the trivalent chromium coating;

Hydrolyzate of Cr ³⁺ undergoes hydroxyl bridge and polymerization reaction, forming polymer chain aggregates to be adsorbed on the cathode, hindering the reduction of Cr ³⁺ ;

The enrichment of Cr ²⁺ reduced intermediate product Cr ²⁺ can initiate and promote the Cr ³⁺ hydroxyl bridge reaction;

The active complex of Cr ³⁺ gradually decreases and disappears during continuous electrolysis.

Sharif et al. In the aminoacetic acid system used methods such as increasing the plating solution circulation speed, lowering the pH value, and increasing the concentration of active complexes to achieve a thick plating of trivalent chromium at a speed of 100-300 m / h; In the trivalent chromium plating system using urea as a complexing agent, trivalent chromium can be plated at a speed of 50 to 100 m / h by adding methanol and formic acid; Hon9 and the like use a dual-tank electroplating process by adding three carboxylic acids The mixture is plated with a trivalent chromium coating with a thickness of 50 to 450 m and a good performance; the US Commerce Bureau and Atotech have also plated a trivalent chromium coating with a thickness of 100 to 450 m. Compared with hexavalent chromium, trivalent chromium plating is easy to operate, safe to use, and has no environmental problems. However, there are the disadvantages of large equipment investment and high cost. And users are used to the color of hexavalent chromium, and there is an adaptation process in chromaticity. The performance comparison of trivalent chromium plating and hexavalent chromium plating is shown in Table 4-34.

Trivalent chromium electroplating technology has been developed so far, and foreign research on decorative electroplating technology has entered a period of gradual perfection and maturity, and its production and application have continued to expand. However, due to the high prices of imported products and the instability of the process, it is still difficult to popularize and apply them on a large scale in China. As for the hard chromium plating of trivalent chromium, it has been reported that more than 30% of factories in North America have started to use trivalent chromium instead of hexavalent chromium for electroplating.

(3) Rare earth chromium plating

In the mid-1980s, rare earth chromium plating additives were developed. The main component was rare earth compounds, which have been widely used in China. Adding a small amount (1 to 4 g / L) of rare earth compounds to the chrome plating electrolyte can reduce the chromic anhydride content in the electrolyte to 150 g / L, and obtain high gloss at a lower temperature (30 to 40 ° C). The bright chrome plating layer has a cathode current efficiency of 22% to 26%, which is significantly higher than that of conventional chrome plating electrolyte. The use of rare earth chromium plating additives can save a lot of energy and raw materials, and also greatly reduce the environmental pollution of chromic anhydride. Although the mechanism of action of rare earth metal cations can not be perfectly explained, it is known from experimental phenomena that the addition of rare earth elements can produce characteristic adsorption on the cathode, which changes the properties of the cathode membrane and makes the critical precipitation potential of chromium smaller. Increased hydrogen evolution overpotential to maximize current efficiency; X-ray diffraction patterns also confirmed that the crystal structure of the coating changed after the addition of rare earth cations, causing the surface grains to be preferentially oriented, grain refinement, and brightness increased. The hardness of the coating has improved.

Despite the many advantages of rare earth chrome plating, there are also some issues that need to be addressed:

After some rare earth additives are chromium-plated for more than 5 minutes, the coating is white and not bright. Sometimes a yellow film on the coating is difficult to remove, the hardness is unstable, and the appearance is difficult to meet the requirements;

F must be introduced into the rare earth additive, too much fluoride, and the low current density region of the plated parts is prone to electrochemical corrosion;

Rare earth additives are mostly physical mixed systems with complex components and unreliable and unstable plating solutions;

The maintenance of the plating solution is difficult.

(4) Chrome plating with organic additives

The chromium plating solution using the organic additive and halogen release agent is called "the third-generation chromium plating solution". Their common characteristics are: the cathode current efficiency is as high as 22% to 27%, it does not contain F, does not corrode the substrate, and has strong covering ability. , HV is also up to more than 1000, can be used for hard chromium plating, also can be used for micro-crack chromium plating, with double or triple-layer nickel process, used for electroplating of automobile or motorcycle shock absorbers, has begun to get applications in the machinery industry .

Organic additives include organic carboxylic acids, organic sulfonic acids and their salts. Halogen release agents refer to potassium iodate, potassium bromate, potassium iodide, potassium bromide, and the like. The mechanism of organic additives in the plating solution has yet to be determined. It is generally believed that the addition of organic substances activates the base metal and improves the covering ability of the plating solution; increases the hydrogen evolution overpotential and improves the current efficiency; and due to the entrainment of organic substances, formation Chromium carbide increases the hardness of the coating. Table 435 lists the process specifications of organic additive chromium plating solution.

In alkylsulfonic acid, s / c1, current efficiency can reach 27%, and HV> 1i00.

Nitrogen-containing organic compounds: nicotinic acid, glycine, isonicotinic acid, pyridine, 2-aminopyridine, 3-chlorochloropyridine, and picolinic acid.

HEEF (High Efficiency Etch Free) has high efficiency, no corrosion in low current area, cathode current efficiency reaches 25%, 900 1000HV. The composition of the cylinder is: HEEF25550mL / L, sulfuric acid 2.7g / L, temperature 55 60 , electrolysis 4 6h, (voltage> 6V, Yin-yang area ratio 15: 1). ^{[4] [3]}