(1) protection-decorative chromium plating

Protective decorative chromium plating requires not only good corrosion resistance in the atmosphere, but also beautiful appearance.

This coating is also commonly used for electroplating nonmetallic materials.

Protective decoration chromium plating can be divided into general protective decoration chromium plating and high corrosion resistance protective decoration chromium plating. Table 4-28 lists the process specifications of chromium plating for protective decoration.

The technological conditions of decorative chromium plating also depend on the base metal material to be plated. The working temperature and cathode current density can be properly adjusted according to different matrix materials.

1, chromium plating for general protective decoration

Generally, chromium plating for protective decoration adopts medium and high concentration of common chromium plating solution, which is suitable for products used in indoor environment. The chromium plating of steel, zinc alloy and aluminum alloy must adopt multi-layer system, and the main process flow is as follows.

① The technological process of copper/nickel/chromium system of steel matrix is as follows:

Degreasing → water washing → etching → water washing → flash plating copper cyanide or nickel → water washing → acid copper → water washing → bright nickel → water washing → chromium plating → water washing and drying.

The technological process of multilayer nickel/chromium system is as follows:

Degreasing → water washing → etching → water washing → semi-bright nickel plating → water washing → bright nickel → water washing → chromium plating → water washing → drying.

② Weak alkali chemical degreasing of zinc alloy matrix → water washing → dilute hydrofluoric acid leaching → water washing → electrolytic degreasing → water washing → flash copper plating → water washing → bright copper plating → bright nickel → water washing → chromium plating → water washing → drying.

③ Weak alkali degreasing of aluminum and aluminum alloy matrix → water washing → electrolytic degreasing → water washing → secondary zinc dipping → dissolving zinc dipping layer → water washing → secondary zinc dipping → water washing → flash plating of cyanogen copper (or pre-nickel plating) → water washing → bright copper plating → water washing → bright nickel plating → water washing → chromium plating → water washing → drying.

2. Decorative chromium plating with high corrosion resistance

High corrosion resistance decorative chromium plating is to change the structure of chromium plating layer through special process, thus improving the corrosion resistance of the coating. The coating is suitable for outdoor conditions.

In the protective and decorative chromium plating system, the application of multi-layer nickel significantly improves the corrosion resistance of the coating. It is found that the corrosion resistance of nickel-chromium layer is not only related to the nature and thickness of nickel layer, but also depends on the structural characteristics of chromium layer to a great extent. Although the common protective decorative chromium plating layer obtained from standard chromium plating solution is only 0.25 ~ 0.5 micron, the internal stress of the coating is very large, which leads to uneven coarse cracks in the coating. In corrosive medium, the chromium coating is the cathode, and the bottom layer at the crack is the anode. Therefore, the bottom layer or base metal at the crack is always corroded. Because the exposed bottom metal area at the crack is very small compared with the chromium plating layer, the corrosion current density is very high and the corrosion speed is very fast, and the corrosion has developed in depth. Because cracks are inevitable, if the structure of micro-cracks is changed to disperse corrosion, then corrosion can be slowed down. Under this idea, a new process of micro-crack chromium and micro-pore chromium with high corrosion resistance was developed in the mid-1960s. These two kinds of chromium are collectively called "micro-discontinuous chromium". Because the chromium layer formed has many micropores and microcracks, the exposed nickel plating area is enlarged but dispersed, which greatly reduces the corrosion current density and corrosion speed of the nickel layer surface, thus improving the corrosion resistance of the composite coating and reducing the thickness of the nickel layer by about 5 microns. ..

① Micro-crack chromium Plating a layer of high-stress nickel of 0.5 ~ 3 microns on the bright nickel plating layer, and then plating ordinary decorative chromium of 0.25 μ m.. Because the internal stress of the high-stress nickel layer and the internal stress of the chromium layer are superimposed, 250 ~ 1500 microcracks of chromium can be obtained in one square centimeter.

It is found that the chromium plating layer with high internal stress can be obtained by adding a small amount of SeO42- into the common chromium plating electrolyte. The chromium coating obtained in the plating solution containing seO42- is blue. The higher the SeO42- content, the heavier the blue color of the coating.

Micro-crack chromium plating can also be obtained by double-layer chromium plating. The process is as follows: firstly, a chromium coating with good coverage is plated, and then a microcrack chromium layer is plated in a chromium plating solution containing fluoride. The disadvantages of the double-layer method are the need for additional equipment, long electroplating time and high power consumption. Therefore, it has been replaced by single-layer microcrack chromium at present, but single-layer microcrack chromium also has some shortcomings such as difficult fluoride analysis and uneven microcrack distribution.

② Microporous chromium. At present, the most widely used method for electroplating microporous chromium is to plate a nickel-based composite coating (nickel seal) with a thickness not exceeding 0.5μm on the bright nickel layer, and then plate a bright chromium layer to obtain a microporous chromium layer.

The particle size of uniformly dispersed non-conductive particles in the nickel-based composite coating is below 0.5μm, the suspension in the plating solution is 50 ~ 100 g/L, and the particle content in the composite coating is 2% ~ 3%. Commonly used particles are sulfate, silicate, oxide, nitride and carbide. Because the particles are non-conductive, no current flows on the particles during chromium plating, and there is no metal chromium deposition on the particles. As a result, countless tiny pores are formed, and the density can reach more than 10 thousand per square centimeter.

3. Precautions for protective decorative electroplating

(1) Before putting large pieces into the bath tank, they should be washed and preheated with hot water, otherwise the bottom surface with high brightness will be corroded.

(2) Small parts should be plated with chromium by barrel plating, and fluosilicic acid should be added to the plating solution to prevent the parts from being passivated due to instantaneous non-contact conduction during barrel plating.

(3) The parts are charged into the tank, and pulse current is used for complex parts, or the distance between cathode and anode is increased.

④ Each electroplating layer should be polished to improve the smoothness, reduce porosity and prevent corrosion.

⑤ When plating chromium on nickel, if nickel is passivated, it can be activated by acid leaching and then plated with chromium. The activation method is: soak 30 ~ 60sEtch5ming in 30% ~ 50% (volume fraction) hydrochloric acid and 20% (volume fraction) sulfuric acid for about 5 minutes; Cathodic treatment in 5% (volume fraction) sulfuric acid for about 15s, and then chromium plating, a chromium plating layer with good adhesion can be obtained.

⑥ Full-wave rectification should be adopted for power supply.

⑦ When using high-concentration chromic anhydride plating solution, a recovery tank can be installed to save chromic anhydride, reduce cost and reduce wastewater treatment capacity.

(2) Rolling chromium plating

Small parts that need chromium plating, such as hanging plating at ordinary times, are not only inefficient, but also often leave traces of fixtures on the plated parts, which can not guarantee the quality of the coating. Rolling chromium plating is mostly used for decorative multilayer electroplating of parts with small volume, large quantity and difficult hanging, such as copper/bright nickel/chromium or bright low tin bronze/chromium. This method can improve the production efficiency and reduce the cost. But it is only suitable for electroplated parts with simple shape and certain self-weight; It is not suitable for parts with flat plating, low self-weight and high appearance requirements.

Matters needing attention when rolling chromium plating are as follows:

(1) The chromium plating solution for barrel plating is prepared with distilled water or deionized water, and it should be cleaned to prevent impurities from being brought in, especially Cl-;

(2) Sulfate radical should be controlled properly, not too high, so as to avoid yellowing or chromium plating on the surface of parts, and excess sulfuric acid can be removed by barium carbonate; '

(3) fluosilicic acid can activate the coating and expand the bright range, which is indispensable and should not be excessive;

(4) charging into the tank, and starting to use impulse current for about 65438±0 ~ 2min；;

⑤ Before the parts are loaded into the drum, the chromic acid solution in the drum must be cleaned to prevent the parts from being corroded by chromic acid;

⑥ After the roller is used for a period of time, it is treated with hydrochloric acid to remove the chromium layer on the roller net;

⑦ The parts are small and the temperature can be slightly lower. In order to avoid the temperature rise of electroplating solution, it is best to use a cooling device.

(3) hard chromium plating

Hard chromium is also called wear-resistant chromium. Hard chromium coating should not only have a certain luster, but also require high hardness, good wear resistance and firm combination with substrate.

The thickness of the coating should vary according to the application. Under light mechanical load and general protection, the thickness is10 ~ 20μ m; The thickness under sliding load is 20 ~ 25 microns, and the pressure is not too great; When the mechanical stress is large and the corrosion resistance is strong, the thickness is as high as 150 ~ 300 microns; The size and thickness of the repaired parts can reach 800 ~1000μ m.

Wear-resistant chromium plating solution with low chromic anhydride concentration is generally used, and standard chromium plating solution is also used in some factories. The process conditions should adopt lower temperature and higher cathode current density, which depends on the use conditions of parts and the requirements for chromium layer. Generally, the cathode current density is 50 ~ 60℃ (generally 55℃) and 25 ~ 75a/dm2 (mostly 50A/dm2). Once the process conditions are determined, the process conditions should be kept as constant as possible during the whole electrodeposition process, especially the temperature, and the change should not exceed 65438 0℃.

The following problems should be paid attention to when plating hard chromium.

(1) No matter what the material of the plated part is, as long as the workpiece is large, it needs preheating treatment, because the hard chromium plating takes a long time, the coating is thick, the internal stress is large, the hardness is high, and the thermal expansion coefficient of the base material and chromium is quite different. If electroplating is carried out without preheating, the base material will easily expand when heated, resulting in "skin exposure", and the preheating time depends on the size of the workpiece.

(2) The materials used for hanging tools must be insoluble in hot chromic acid solution and will not have other chemical effects. The fixing device should also have sufficient cross-sectional area and good contact with the conductive parts. Otherwise, due to the large current, the cell voltage will rise and local overheating will occur.

The cross-sectional area of the fixture should be selected according to the conductivity of various materials. The allowable current of several common materials is: copper -3a/mm2, brass -2.53a/mm2 and steel -2a/mm2. The fixture structure should be welded as far as possible; The non-working part of the fixture shall be insulated with PVC plastic cloth or coated with acid-resistant glue.

③ When installing the suspension, it should be considered to facilitate the escape of gas and prevent the formation of "air pockets", resulting in local non-coating or uneven coating thickness.

④ Pictographic anode should be used for chromium plating of complex parts, and cathodic protection should be added at both ends of cylindrical parts to avoid burning at both ends and Bo Tu in the middle; Parts with sharp edges and corners can be shielded with metal wires.

⑤ In order to improve the adhesion of the coating, reverse current, large current pulse and stepping power supply can be used. The commutation time is 0.5~3min, and the cathode current density is 30 ~ 40a/dm2. The high current impulse is 80 ~ 120a/dm2, and the duration is 1 ~ 3min.

⑥ For steel parts easy to evolve hydrogen, hydrogen removal treatment should be carried out after plating.

(4) plating porous chromium.

Porous chromium coating is a kind of hard chromium coating with a certain density and depth of reticular grooves, which has good oil storage capacity. When working, the lubricating oil stored in the groove is squeezed out and overflows to the surface of the workpiece. Because of capillary action, lubricating oil can also penetrate the whole surface of the workpiece along the groove, thus improving the lubrication performance of the whole surface of the workpiece, reducing the friction coefficient and improving the wear resistance.

There are mechanical, chemical or electrochemical methods to obtain porous chromium.

(1) On the surface of the parts to be chrome plated, mechanically press the surface of the substrate into conical or pyramidal pits or turn it into grooves, then chrome plated and polished. This method is simple and easy to control, but its adsorption performance for lubricating oil is not ideal.

(2) The chemical method uses the high activity of the original crack edge of the chromium plating layer to corrode in dilute hydrochloric acid or hot dilute sulfuric acid, so that the chromium at the crack edge is preferentially dissolved, thereby deepening and widening the crack and achieving the purpose of loosening the hole. This method has high chromium loss, uneven dissolution and difficult quality control.

(3) After hard chromium plating by electrochemical method, the anode is loosened in alkaline solution, chromic acid, hydrochloric acid or sulfuric acid after hydrogen removal and grinding. Because the potential at the crack of chromium layer is lower than that at the plane, the chromium at the crack is preferentially dissolved, thus deepening and widening the crack. The pore depth after treatment is generally 0.02 ~ 0.05 μ m.

In the process of anodic etching, the speed of crack deepening and widening is controlled by electric quantity (etching intensity). Any anode current density can be selected within a suitable corrosion intensity range, and the corrosion intensity can still remain unchanged as long as the time is changed accordingly. The etching strength is determined according to the original thickness of the chromium plating layer.

The etching strength of the chromium coating with the thickness below 100μm is 320 A min/dm2, and the etching strength of the chromium coating with the thickness of 100 ~ 150μm and the chromium coating with the etching strength above 400 A min/dm2 and150μ m is 480 A min. For chromium-plated parts with strict size requirements, in order to control the size, it is best to use low current density for anode porosity; When dense mesh is required, a slightly higher anode current density can be used; Loosen the anode after polishing the chrome-plated parts, and the etching strength should be lower than the above value 1/2 ~ 1/3.

The mesh crack density of porous chromium layer depends on the original crack density of hard chromium coating. Therefore, the chromium plating process has a great influence on the chromium plating of loose holes and must be strictly controlled.

(5) Black chromium coating

Black chromium coating is superior to the black coating obtained by electrochemical method, and is widely used in decorative coatings and solar absorption layers that need extinction in aviation, automobiles, instruments and meters. The blackness of black chromium coating is caused by the physical structure of the coating. It is not pure metal chromium, but is composed of chromium hydrate and chromium trioxide, showing a dendritic structure. Metal chromium is dispersed in chromium oxide in the form of particles, forming absorption centers and blackening the coating. Generally, the higher the oxide content of chromium in the coating, the darker the black color. The corrosion resistance of black chromium coating is better than that of ordinary chromium coating. Although the hardness of black chromium coating is only 130 ~ 350 HV, its wear resistance is equivalent to that of ordinary chromium coating. Black chromium coating has high thermal stability. When heated to 480℃, the appearance has no obvious change and the adhesion with the bottom layer is good. Chromic anhydride is the main component in the plating solution, and the black chromium coating can be obtained when its content is in the range of150 ~ 400 g/L. The concentration of chromic anhydride is low and the dispersion ability of plating solution is poor; At high concentration, although the dispersion ability of the plating solution is improved, the wear resistance of the coating is reduced. Generally, it is selected between 200-350g/L..

Sodium nitrate and acetic acid are blackening agents. When the content is too low, the coating is not black, the conductivity of electroplating solution is low and the cell voltage is high. If the concentration is too high, the deep plating ability and dispersion ability of the plating solution are poor. Generally, sodium nitrate is controlled at 7 ~ 12g/L, and acetic acid is controlled at 6 ~ 7g/L. In the plating solution with sodium nitrate as blackening agent, the coating is easy to "float" without boric acid, especially at high current density. Adding boric acid can reduce "floating ash". When boric acid reaches 30g/L, "floating ash" can be completely eliminated. The addition of boric acid can also improve the deep plating ability of the plating solution and make the coating uniform.

The bath temperature and cathode current density have great influence on the color and bath performance of black chromium coating. The optimum conditions are that the temperature is lower than 25℃ and the current density is higher than 40A/dm2. The cathode current density is too small, and the coating is grayish black or even rainbow color; But not too big. When it is more than 80A/dm2, the coating is easy to burn and the temperature of the plating solution rises seriously. When the temperature is higher than 40℃, gray-green floating ash is produced on the surface of the coating, and the deep plating ability of the plating solution is reduced. Therefore, in the process of electroplating black chromium, cooling measures must be taken. SO42- and Cl- are harmful impurities in black chromium plating solution. SO42- makes the coating light yellow instead of black, which can be removed by precipitation of BaCO3 or Ba(OH)2. Cl- makes the coating appear yellow-brown floating ash, deionized water should be used when preparing the solution, and harmful impurities should be strictly controlled in the production process; Hangers and anode copper hooks shall be protected by tin plating.

Black chromium coating can be directly plated on iron, copper, nickel and stainless steel, or copper, nickel or copper-tin alloy can be used as the bottom layer to improve corrosion resistance and wear resistance. Auxiliary anodes should be used for parts with complex shapes, and the anode material should be lead-tin alloy containing 7% tin or high-density graphite.

Parts coated with black chromium can be painted or dipped in oil after drying, which can improve luster and corrosion resistance.

(6) plating milky white chromium

Milky chromium is generally 30 ~ 60μ m thick, with good corrosion resistance, but low hardness and poor luster. The process, pre-plating preparation and post-plating treatment of opalescent chromium plating are basically the same as those of hard chromium plating. The main difference is that the required temperature is higher (65 ~ 75℃) and the cathode current density is lower (25 ~ 30a/dm2).