(1. Changzhou Karl Mayer Textile Machinery Co., Ltd., Changzhou 213161, Jiangsu, China; 2. High-tech Research Institute, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China)
Abstract : The current technological process, solution formulation and operating conditions of hard chrome plating on aluminum and its alloys are studied. At the same time, the problems that should be paid attention to in the process flow and the detection parameters of the chrome plating layer are also introduced. The six pre-plating processes have similar technological processes and their own characteristics. The plating obtained by several processes has good bonding strength, micro-hardness, wear resistance and impact resistance.
Key words: Aluminum; Aluminum alloy; Electroplating; Hard chromium. CLC number: TQ153.4 Document code: A Article ID: 1000 4742 (2008) 04 0013 03
0 Overview Aluminum and its alloys are widely used in transportation, aerospace, electronics, construction, decoration, textile and other industries due to their good electrical and thermal conductivity, high ductility, low density, high strength, and ease of molding. However, its low surface hardness, poor wear resistance, and easy corrosion also affect its application range and life. Surface modification of aluminum and its alloys can improve these disadvantages. One of the more effective methods is to plate hard chrome [12].
There are many aluminum alloy parts in the textile industry that the author is engaged in that require hard chromium plating on the surface to improve its wear resistance and corrosion resistance. In view of the urgent requirements of our company for the localization of such parts, we have cooperated with Nanjing University of Aeronautics and Astronautics in the hard chromium plating of aluminum alloys. The Vickers hardness of the chrome plating layer reaches 1000, which is nearly 10 times higher than that of the alloy matrix. At the same time, the wear resistance and corrosion resistance of the hard chrome layer all meet the design requirements.
1 Principle Aluminum is an active metal with high chemical activity. Its electrode potential is very low (Φ = -1.67V), and it is highly oxidative. At the same time, it is a kind of amphoteric metal, and it is easy to passivate in the air, which brings difficulties to the aluminum alloy plating. Due to defects such as trachoma and blistering, cast aluminum alloys are prone to trap residual liquids and gases during electroplating, which can cause hydrogen embrittlement and coating shedding.
The key to the electroplating of aluminum and its alloys is the bonding force between the coating and the base metal. The key to the bond strength is whether the preplating is reasonable. The commonly used processes include two methods of zinc leaching [35], electroless nickel-platinum [67], nickel plating after immersion zinc [3], galvanizing after immersion zinc [8], and anodic oxidation of phosphoric acid [4]. Hydrochloric acid etching method [9] and so on. The processes of these processes are roughly the same. They are first to remove the oxide film on the surface, and then to obtain a stable intermediate layer through different methods, and to perform electroplating later. The stable intermediate layer can prevent the regeneration of the natural oxide film and protect the bare aluminum surface before plating. At the same time, it forms an ultra-microscopic, uniform concave-convex structure with a large pore volume and a small resistance; it ensures that the metal deposits quickly during plating. The nucleus is formed more and adhered well, and the high hardness chromium layer can be prevented from directly contacting with the soft aluminum matrix, which may cause cracking and depression.
2 Aluminum and aluminum alloy plating hard chrome 2.1 Process sandblasting → alkali etching → water washing → acid etching → water washing → pre-plating → water washing → chrome plating → water washing → blow drying → hydrogen removal 2.2 Main process description 2.2.1 Sandblasting treatment The surface of the plated part can be pre-treated by sandblasting, which can not only make the surface of the part uniform, but also increase the microhardness of the surface of the aluminum alloy, increase the surface area of ​​the plating, and improve the binding force of the coating. Sand blasting can use dry or water spray, use different mesh glass sand, adjust the parameters of sandblasting to get even surface with different roughness. After sandblasting, the glass sand remaining on the surface should be removed in time to avoid impact on subsequent processes.
2.2.2 Alkaline Erosion Degreasing Alkaline Etching Solution Formula and Process Conditions:
Sodium hydroxide 50 to 100 g/L, trisodium phosphate 30 to 45 g/L, sodium carbonate 20 to 30 g/L, 60 to 80°C, 0.5 to 1.0 minutes.
This process can be repeated, but the time must be short to prevent over-corrosion. Use hot and cold water after degreasing. Degreasing with organic solvents is also sometimes possible.
2.2.3 Acid etching light and acid etching recipe and process conditions:
φ (nitric acid) = 75%, φ (hydrofluoric acid) = 25%, room temperature, 3 ~ 5s. The volume fraction of nitric acid and hydrofluoric acid can be appropriately adjusted according to the content of silicon in the plating part. After etching, the residues of copper, magnesium, silicon, zinc, manganese and their compounds remaining on the surface of the workpiece after alkali etching can be removed, and the metal crystallized structure is exposed to obtain a uniform, clean, shiny or off-white surface, and the surface of the plated part is activated. .
2.2.4 Pre-plating (1) Two leaching zinc methods One zinc immersion liquid composition and process conditions:
Zinc oxide 100g/L, sodium hydroxide 500g/L, potassium sodium tartrate 50g/L, ferric chloride 1g/L, room temperature, 30~60s.
Dezincification fluid composition and process conditions:
Nitric acid 200mL/L, room temperature, 3 ~ 5s.
Secondary zinc bath composition and process conditions:
Zinc oxide 20g/L, sodium hydroxide 120g/L, potassium sodium tartrate 50g/L, ferric chloride 2g/L, room temperature, 20-40s.
The zinc dipping process is related to the binding force and quality of the plating. Zinc immersion should be thin and detailed and good adhesion zinc layer, the surface is uniform, and has a micro-gloss. Sometimes the surface of the aluminum part is immersed in the heavy metal prior to zinc immersion, so that the potential is shifted and the binding force of the coating is increased.
(2) Galvanizing after galvanizing Zinc plating solution formulation and process conditions:
Zinc chloride 60 to 70 g/L, potassium chloride 180 to 220 g/L, metaborate 25 to 35 g/L, ZL21 brightener 14 to 18 mL/L, pH 5 to 6, room temperature, 1 to 4 A/dm2.
The plated part may be galvanized after one or two zinc dippings. When galvanizing, if the workpiece is more complex, it can be flashed with the inrush current (2 times the normal current) in the first 1 to 2 minutes to deposit the coating in the deep recesses and improve the deep-deposition ability. Zinc plating after zinc immersion provides a more dense and more adherent zinc layer. It can not only improve the binding force of the coating, but also improve the yield, especially for parts with complex shapes and higher requirements.
(3) Nickel plating after zinc plating Chemical nickel plating liquid formulation and process conditions:
Nickel sulphate 35g/L, boric acid 15g/L, sodium hypophosphite 15~20g/L, sodium acetate 15g/L, sodium citrate 10g/L, pH 4.8-5.5, 80-90C, 15-20min.
Nickel plating liquid formula and process conditions:
Nickel sulphate 200 to 250 g/L, sodium citrate 200 to 250 g/L, boric acid 30 g/L, ammonium chloride 5 g/L, pH 6.6 to 6.8, 55 to 65°C, 0.5 to 1.0 A/dm2.
When nickel plating is carried out, it must be charged into the tank, first electroplated with inrush current, and then transferred to normal current plating. Electroless nickel plating has complex control, high cost, and defects such as roughness on the surface of the plating layer; and the chromium plating using a nickel plating process has high quality, high yield, and low cost. After the zinc immersion, the nickel plating can obtain dense and better adhesion of the nickel plating, which can improve the binding force and the yield of the plating, and the cost is relatively high.
(4) Electroless Nickel Plating Phosphorus Alkaline Electroless Nickel Phosphorus Liquid Formulation and Process Conditions:
Nickel sulfate 20 to 30 g/L, sodium hypophosphite 20 to 30 g/L, complexing agent (sodium citrate and triethanolamine) 50 to 80 g/L, ammonia 22 to 25 mg/L, pH 9.5 to 10.5, 25 to 35 °C, 3 ~ 5min.
Alkaline electroless plating has a low working temperature and can effectively inhibit the dissolution of the zinc film, thereby obtaining a thin and fine, uniform nickel-phosphorus alloy layer to protect the zinc film.
Acidic electroless nickel plating phosphorus liquid formula and technological conditions:
Nickel sulfate 25-30 g/L, sodium hypophosphite 25-30 g/L, 2-hydroxypropionic acid 27-30 mL/L, pH 4.2-4.8, 88-92°C, loading 0.5-1.0 dm2/L, deposition rate 20 μm /h.
When the nickel-phosphorus alloy layer reaches 15 to 20 μm, the underlying layer can be ensured to be uniform and free of pores, so that the alloy layer peeling off can be avoided during chrome plating. When the alloy layer reaches 25 μm, the binding force is excellent.
(5) Phosphoric acid anodizing Phosphoric acid anodizing liquid formula and process conditions:
Phosphoric acid 250 ~ 350g / L, 20 ~ 20 °C, 3 ~ 15min, 1 ~ 2A/dm2.
Anodic oxidation of aluminum and its alloys produces a porous oxide film with a certain thickness and special structure. The oxide film has an ultra-microscopic, uniform concave-convex structure, a large pore volume, and a small electrical resistance. The electroplating on the oxide film layer has many crystal nuclei, the deposited layer can quickly cover the surface, and firmly adheres to the film pores, so that a qualified plating layer with smooth, uniform, fine crystalline and good adhesion can be obtained.
(6) Hydrochloric acid etching Composition and process conditions of hydrochloric acid etching solution:
Hydrochloric acid 120～160g/L, corrosion inhibitor 7～15g/L, room temperature, 2～6min.
The concentration of hydrochloric acid not only affects the appearance of the coating but also affects the adhesion of the coating to the substrate. At the same time to control the erosion time to prevent excessive corrosion. Hydrochloric acid etching is a simpler and more convenient method.
2.2.5 Chromium plating and hydrogen removal (1) Chrome plating Chromium plating liquid formulation and process conditions:
Chromium anhydride 130-150g/L, sulfuric acid 0.5-0.8g/L, rare earth additive 1.5-2.0g/L, trivalent chromium 1-2g/L, 50-55°C, 40-45A/dm2, deposition rate 40μm/h .
Chromium plating formulations due to the use of different pre-plating process, need to make the appropriate adjustments. And because of the different additives, the appearance of the coating will also have a big difference.
(2) Dehydrogenation In order to eliminate the internal stress formed between the plating layer and the substrate, the binding force between the plating layer and the substrate is increased, and the hydrogen removal treatment is performed after plating. Baking temperature 160 °C, baking time 1h.
3 coating performance test Chrome plating layer in addition to a good anti-corrosion properties and smooth and bright appearance, the more important is the combination of strength, hardness and wear and other properties.
The test of the adhesion of the chrome plating layer has been tested with heating, bending and impact tests [10]. After the test, there was no peeling or peeling of the coating, indicating that the bonding force was good.
The hardness test is based on the part size, substrate material, coating thickness, indentation diameter, load size, etc. using different hardness tester. In measuring the hardness of the plating layer, a Vickers microhardness tester is often used. According to the thickness, a load of 5g to 200g may be added so that the indentation depth is 1/7 to 1/10 of the thickness of the plating layer. The Rockwell hardness tester can be used when the thick chromium plating layer is larger than 100μm.
The abrasion resistance is usually measured by a thickness reduction method, a mass loss method, a bulk wear method, a grinding medium consumption method, a cut thickness time method, a radioactive isotope method, and the like. The test shows that the chromium plating layer has a greater wear resistance when the Vickers hardness is 7355 MPa to 7845 MPa. The thickness of the chrome plating layer also has a certain relationship with the wear resistance, and at the same time it has an influence on the service life.
4 Discussion (1) Six commonly used pre-plating processes for hard chrome plating of aluminum and its alloys have similar pretreatment and subsequent hard chrome plating specifications.
(2) Different pre-plating processes can form an intermediate layer with an ultra-microscopic, uniform concave-convex structure, and a large pore volume and a small resistance, which can prevent the regeneration of the natural oxide film; and avoid the high hardness of the chromium layer and the comparison. The cracks and dents may be caused by the direct contact of the soft aluminum matrix. A certain thickness of the coating obtained through several electroplating processes has good bonding strength, microhardness, wear resistance and impact resistance.
(3) The two zinc leaching processes are stable, but the process is complicated and the operating conditions are harsh. Since zinc immersion is performed in a strong alkali solution, it requires a large amount of water to flush. The anodic oxidation process is relatively simple, but the operating temperature, voltage, etc. must be strictly controlled, and the temperature of the electrolyte is liable to rise, affecting the pre-treatment effect and affecting the quality of the plating layer. Zinc-plating and nickel plating processes are stable after immersion zinc, and the yield is high. It is particularly suitable for parts with complex structures and higher requirements. Electroless nickel-phosphorus plating is required to control the thickness of the nickel-phosphorus alloy layer to ensure good adhesion. Hydrochloric acid etching is a simple and convenient method.
(4) In the implementation process, suitable electroplating process can be selected according to its own conditions, environmental requirements and workpiece requirements.
references:
[1] Hu Runan, Chen Songyu. Practical Chrome Plating Technology [M]. Beijing: National Defense Industry Press, 2005: 297 303.
[2] Xu Zhenming, Xu Xiaobiao. Surface treatment of aluminum and magnesium [M]. Shanghai: Shanghai Scientific and Technical Literature Press, 2005:240.
[3] Yang Limin, Jin Wujun. The Selection of Special Pretreatment Processes for Chromium Plating of Aluminum Alloys[J]. Plating and Finishing, 1999,21(4):20 21.
[4] Ren Libin, Zhang Weiguo, Yao Suwei.Apparent and sectional analysis of hard chrome plated on aluminum alloy[J].Plating & Finishing,1999,21(2):36.
[5] Ruan Bing. Practice of hard chrome plating on aluminum alloy[J]. Plating & Finishing, 2006,25(1):49 50.
[6] Dong Chaoyang. Hard chrome plating of aluminum and its alloys[J]. Plating and Finishing, 2004,26(2):24.
[7] Wang Hailin, He Qinglin, Guo Qingchun, et al.Process of electroplating hard chrome plating on aluminum-based electroless nickel-phosphorous plating[J].Journal of Materials Protection,1999,32(3):7 8.
[8] Shi Lei, Shi Yong, Dong Xinmin. Aluminum and aluminum alloy electroplating [J], Surface Technology, 2007, 36 (4): 87 88.
[9] Yao Suwei, Ren Libin, Zhang Weiguo, et al. Direct chrome plating on aluminum alloys. [J]. Electroplating and Environmental Protection, 1999, 19(1):16.
[10] Liu Yubo. Encyclopedia of surface treatment technology [M]. Beijing: China Measurement Press, 1996:57 58.
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