WHAT TO DO? ELECTROPLATING OR METAL SPRAYING? S. de Vries, J.W. Rauhorst, B. Plasse & F. Kemeling The first step to improve and control the quality of a metal product starts at the desk of the designer. The engineer should understand the limitations imposed by shape and size of components to facilitate quality finishing at an acceptable cost. The designer can exert as much influence on the quality attainable in finishing a part as can the electroplater himself. The metallizing process is an excellent means of protecting iron and steel from corrosion to almost any desired degree, from long life coatings to inexpensive coatings which are competitive with organic coatings such as paint. Heavy coatings of zinc or aluminium can be applied to meet the most severe corrosion conditions and give 15 to 50 years life without any further maintenance. Very thin coatings, particularly of zinc, compete in cost with such other methods such as plating and painting, and usually give much better protection. Metallized coatings are also excellent corrosion resistant undercoatings for organic materials such as paint. Organic finishes on iron and steel usually fail due to corrosion under the coatings and lack of bond of the coatings to the steel. Thin metallized coatings prevent corrosion of the base and offer a strong bond to the organic finish. This article will discusses two coating processes; electroplating and metal spraying. The processes will be explained and the different designing rules of these processes will be compared. Finally these designing rules will be showed with help of some products. ELECTROPLATING is the process of depositing a thin coating of one metal on top of a different metal. This is done for reasons of corrosion resistance, friction reduction, heat tolerance, and decoration. The metal is deposited by passing an electrical (direct) current into a piece of metal (called an anode), through a salt solution of the metal you want to deposit, and is deposited where you want it (called the cathode). Figuur 1 While it sounds simple, it isn't. The process actually involves a lot of fine-tuning and trial and error, as different combinations of salts produce different alloys with different chemical and physical properties. To make matters even more complicated, additional chemicals are added to make the deposit bright and shiny. These chemicals are called brighteners, carriers, and wetters. Without these chemicals, the metal deposit would look dull, gray or black. One of the most common uses for electroplating today is in the auto industry. Numerous car parts are originally steel, and are plated with several layers of nickel for corrosion protection, followed by 1
a layer of chrome for decorative purposes. The electroplating technology makes it possible to coat any object you wish. Every base material that conducts current can be electroplated. Nonconductive materials, like plastics, ceramics or organic material can be made conductive by special treatment. This makes it possible to apply desired surface properties by electroplating There are different methods of the process, the three most important are: 1. Hanging: the products are placed on racks or hooks by contacts of copper wire and placed in a series of baths. 2. Drum or vibration: the articles are treated in a turning drum or barrel. Hanging flexible cathodes make the contact with the cathode. Vibrating: the articles are treated as a vibrating mass in a vibrator. 3. Continuous electroplating or reel-to-reel electroplating the article is a wire or strip or a chain of products cut out of a strip. The contact is made by conductive rolls or conductive strips There are other techniques that can be used but are not so frequently used. METAL SPRAYING Thermal Spray is an industrial coating process consisting of a heat source and a material feedstock - either in powder or wire form - that results in a surface modifying coating. Thermal sprayed metal coatings are depositions of metal, which has been melted immediately prior to projection onto the substrate. The metals used and the application systems used vary but most applications result in thin coatings. Thermal sprayed metallic coatings have been used on such widely divergent projects as motorway and railway bridges, offshore flare booms and valves in pipes for metering gases and oil products. Figuur 2 The wire spraying process offers the facility of being able to deposit coating over large areas with varying thickness. The process is simple and economical to use and can provide solutions to most anti-corrosion problems encountered by experienced engineers. The process for application of Thermal Spray Metal consists of the following stages. 1. Melting the metal at the gun. 2. Spraying the liquid metal onto the prepared substrate by means of compressed air. 3. Molten particles are projected onto the cleaned substrate. There are two main types of wire application available today namely ARC SPRAY and GAS SPRAY. ARC A pair of wires are electrically energized so than an arc is struck across the tips when brought together through a pistol. Compressed air is blown across the arc to atomize and propel the auto fed metal wire particles onto the prepared work piece. 2
GAS In combustion flame spraying the continuously moving wire is passed through a pistol, melted by a conical jet of burning gas (propane or acetylene fuel mixed with oxygen). The molten wire tip enters the cone, atomizes and is propelled onto the substrate. When using either process, operator experience will develop with time but some general guidelines include the development of an optimum distance between the gun and substrate to maintain a satisfactory temperature at which the sprayed metal hits the surface. This distance, combined with the speed of lateral movement of the gun which controls the rate and hence thickness of metal deposited. Thickness checks are made regularly and areas lower than specification can be brought up to thickness immediately. As the spray application produces dust (especially the arc method), it is necessary to progressively clean in front of the gun operator to ensure fresh metal is deposited on clean surfaces. DESIGNING RULES Although the two processes are both very suitable for applying a coating, there are some basic rules, which can help to choose out of these processes 1. Product material: One has to take into account that the product is electrically conductive or not. If it is not, the product has to be coated with a little metal layer first before it can be electroplated. The electrically conductivity does not matter for metal spraying. The metal spraying process has a loss of ±40 %. Most of the drops precipitate on the working piece but yet still ±40 % of the drops fall on the floor or precipitate somewhere else. For this reason it is too expensive to use materials like silver or gold. 2. Location of the process If the product has to be metallized on a different location than the factory the only option is metal spraying. happens in the electroplating factory and can not be done somewhere else. A metal spraying robot can "easily" be moved. 3. Surface If the surface of the product has to be mat than it makes no difference to use the electroplating or the metal spraying process. Though if the surface has to be shining it is wiser to use the electroplating process, there the metal spraying process needs a post processing to make the product shining. 4. Size If the product is small (like nuts, rings, screws) or medium (door handle, handle bar of a bike, washing machine parts) it is cheaper and so preferable to choose electroplating. A large product (aero planes, propellers, bridges) can only be metal sprayed. (The bath of electroplating is limited.) 5. Quantity The choice for the processes can also be made by the quantity of the production. If the production is more than 25 pieces electroplating is much cheaper than metal spraying. 6. Cycle time for the product The product can be metallized between ½ an hour until 12 hours (depends on the complexity of the product) with the metal spraying process. The electroplating process can only realize this in more than 12 hours. 3
7. Coating thickness The thickness of the metal layer can vary between 50 to 500 micron if the metal spraying process is used. The electroplating process can result in a thinner layer that can vary from less than 50 micron until 500 micron. There are also a number of technical properties of the coated product, which can help the designer s decision. The technical properties are: Metal Spraying Impact toughness Better Good Corrosion resistance Good Very good Thermal resistance Not good Good Hardness Good Very good Porosity Less More Bonding More Less Lubrication Good Better Tabel 1 With these rules and properties in mind the designer can look to the next products: CYLINDERS OF THE PLUNGERS OF AN ENGINE 4. Product material: the product is electrically conductive and the layer will not be silver or gold. Both processes can be done 5. Location of process can only be done on location. The process can only be electroplating 6. Surface is mat. Both process. 7. Size: small/medium. Both processes. 8. Quantity: less than 25. is preferable. 9. Cycle time for product: more than 12 hours. Both processes. 10. Coating thickness. Varying from 50 to 500 micron. Most important technical properties: Corrosion resistance Lubrication Impact toughness Thermal resistance Tabel 2 Conclusion: metal spraying Figuur 3 4
BUMPER OF A REMOTE CONTROLLED PLASTIC TOY CAR 11. Product material: the product is not electrically conductive and the layer will not be silver or gold. Both processes can be done. (The product has to be coated with a little metal layer first before it can be electroplated.) 12. Location of process can only be done in factory. Both processes. 13. Surface is shining. Both process. (The metal spraying process needs a post-processing to make the product shining.) 14. Size: small/medium. Both processes. 15. Quantity: more than 25. is preferable. 16. Cycle time for product: more than 12 hours. Both processes. 17. Coating thickness. Varying from 50 to 500 micron. Both processes. Impact toughness Hardness Bonding Tabel 3 Conclusion: electroplating BUMPER OF A CAR 18. Product material: the product is electrically conductive and the layer will not be silver or gold. 19. Location of process can only be done in factory. Both processes. 20. Surface is shining. Both process. (The metal spraying process needs a post-processing to make the product shining.) 21. Size: small/medium. Both processes. 22. Quantity: more than 25. is preferable. 23. Cycle time for product: more than 12 hours. Both processes. 24. Coating thickness. Varying from 50 to 500 micron. Both processes. Impact toughness Corrosion resistance Hardness Bonding Conclusion: electroplating CONCLUSION and metal spraying are both processes applying a coating on a product for several reasons. It cannot be said that one is better than the other is. For different products there are different methods and different demands. It can be very convenient to have some rules and tables at hand that can lead you to the information you need. Taking the right decision will spare a lot of costs or eagerness. Figuur 4 5