Electroplating

Electroplating is another technique used to protect iron from corrosion. The most common plating metal used for this purpose is chromium. 

Electrolysis is used to electroplate an object by making the object the cathode in an electrolytic cell. 

Electroplating is a useful process to produce thick metal structures on a substrate. Many metals such as copper (Cu), Au, and Ni can be electroplated. The substrate to be electroplated is immersed in an electroplating solution that contains a reducible form of the ion of the desired metal. The substrate is maintained at a negative potential (cathode) relative to an inert positive counter electrode (anode e.g., platinum). During the electroplating, electrons are supplied to the surface of the exposed metallic regions on the substrate, and the metal ions are reduced to their atomic form and subsequently deposited onto the substrate surface. 

The surface of the substrate to be electroplated must first be metalized to allow currents to flow. This is usually done by sputtering a thin, submicron layer of metal (seed layer) onto the surface. Multiple thin layers of metal may 

Electroplating process, (a) Molds are first formed on the substrate to define regions to be electroplated. (b) Electroplating setup. 

Electroplating is achieved by passing an electrical current through a solution containing dissolved metal ions and the metal object to be plated. The metal object serves as the cathode in an electrochemical cell, attracting metal ions from the solution. Ferrous and nonferrous 

FiauRE 14.11 Shows how a holiday (hole) in a coating cathodic to a base may cause a pit in the base metal. (A) electrolyte (B) cathodic layer and (C) base metal. The indicated disbonding may or may not take place. 

For small, intricate shapes such as bolts and screws, plating metal may be applied by barrel plating. A rotating barrel containing the parts to be plated is turned slowly in the electrolyte, while current is discharged to the tumbling parts inside. 

Brush plating is also possible, particularly for touch-up of small surfaces. In this case, an absorbent pad holding the electrolyte is wrapped around a metal anode and the brush is moved over the cathodic surface. 

Chrome plate, which was extensively used on automotive parts until the introduction of plastics, is actually a three-ply coating. A flash of copper is first laid down, followed by a nickel coating which comprises most of the finished thickness. A thin, hard coating of bright chromium is then applied as the topcoat The total thickness is normally 25 to 50 //m. 

Electropolishing is the reverse of electroplating (see below) and the material to be polished is made the anode. The process involves the selective removal of metal from the uneven anode surface. The frictional resistance of electropolished parts is lower than with mechanically polished parts moreover, it remains lower during subsequent wear. 

Electropolishing is especially suited to stainless steel and irregular shapes such as forks and spoons. It can also be used for Ni and Al, and for the latter it is capable of results better than those obtained with chemical polishing. 

Results depend upon pre-cleaning of the surface, bath composition, current density and operating temperature. For steel, the electrolyte can be 75% H3PO4 at 65°C, although mixtures with sulphuric and/or chromic acids are also used. 

Aluminium can be electropolished with a mixture of 60% H3PO4 and 40% H2SO4 with 0.1% glycerol. There are also alkaline compositions, based on trisodium phosphate with NaOH or Na2C03 which are used for electropolishing. 

More drastic electropolishing procedures lead to electrolytic stripping. These stripping procedures employ various acid mixtures in which phosphoric and sulphuric acids are the main components. 

After special pretreatments, specific grades of plastics can be put through electroplating processes similar to those used in the plating of metals. Electroplated plastic products are very durable and provide 

In these processes, specialized equipment actually deposits a fine spray of molten metal on the plastic surface. The relatively thick, rough surface is generally used in non-appearance internal surfaces for electromagnetic and radio frequency shielding, as well as static electricity dissipation. 

Not all surface finishing is carried out using electrochemical methods, but electroplating, anodizing and related processes together with electrophoretic painting represent a large portion of the industry. 

Electroplating is the process of electrochemically depositing a layer of metal onto a surface. The object to be plated is made the cathode in an electrolyte bath containing the metal ion, M , so that the reaction at the cathode is 

Where possible the anode reaction is the dissolution of the same metal M-ne -------------------- M -  

The objective of an electroplating process is to prepare a deposit which adheres well to the substrate and which has the required mechanical, chemical and physical properties. Moreover it is of overriding importance that the deposit properties meet their specification on all occasions, i.e. the process is reproducible. On the other hand, many metals may, by modification of the bath and plating conditions, be deposited with different properties. It is for this reason that it is not possible to define a single set of conditions for electroplating each metal the bath, current density, temperature, etc., will depend to some extent on the deposit properties required. 

Because of the importance of reproducibility in the deposit, it is important that the plating bath is stable for a long period of time. It is also necessary that the quality of deposit is maintained over a range of operating conditions since some variation in concentrations and current density are, periodically, bound to occur particularly when different objects are to be plated. Tolerance of the bath to mishandling during operation on the factory floor is an additional advantage. 

How many electrons does a copper ion in copper sulfate solution take from a cathode in electroplating  

1-cm x 10-cm copper strip for use as anode detergent solution steel wool 5-cm 20-22 bare copper wire tweezers 

Read the entire laboratory activity. Using the above equations to guide you, form a hypothesis about how many copper atoms you expect to lose from the copper anode for each copper atom deposited on the cathode. How many electrons do you expect to pass through the circuit for each copper atom deposited at the cathode Record your hypothesis on page 166. 

These instructions will assume that you are plating a key. Clean the surfaces of the key and copper anode with steel wool. 

A method used to ensure the complete filling of vias is electroless plating of nickel. Electroless plating is a process to plate metal without electrical current involved. As a result, it eliminates the seed layer needed in electroplating. To plate nickel on copper, the copper surface must first be activated in a palladium chloride solution. This activation treatment allows palladium to bond to copper at certain sites so that the subsequent nickel plating can nucleate. 

A typical electroless plating solution is composed of a cation provider such as nickel sulfate, a reducing agent such as ammonium hypophosphite, and additional additives tiiat help prevent the bath from decomposition, i.e., plating spontaneously. When an activated substrate is immersed in the plating bath at a temperature of 80 C and a pH around 6, nickel cation in the bath are reduced by hypophosphorous acid, and the nucleation of nickel deposition starts at the activated locations. Because nickel readily plates to itself (self-catalysis), the deposition continues and eventually fills the via locations in the dielectric with nickel metal. The reduction reaction can be expressed by the following equations  

Apply and pattern photo resist mask using image reversal technique. 

The electricity is carried through the electrolyte by ions. In the molten state and in solution the ions are free to move to the appropriate electrodes due to weakened forces of attraction between them. 

The electric current enters and leaves the electrolyte through electrodes, which are usually made of unreactive metals such as platinum or of the non-metal carbon (graphite). These are said to be inert electrodes because they do not react with the products of electrolysis. The names given to the two electrodes are cathode, the negative electrode which attracts cations (positively charged ions), and anode, the positive electrode which attracts anions (negatively charged ions). 

Note that the conduction which takes place in the electrodes is due to the movement of delocalised electrons (pp. 51 and 55) whereas in the electrolyte, as stated earlier, the charge carriers are ions. 

Electrolysis is very important in industry. To help you to understand what is happening in the process shown in the photographs, we will first consider the electrolysis of lead(u) bromide. 

The break-up (decomposition) of lead(n) bromide into its constituent elements by the passage of an electric current is called electrolysis. 

The electrolytic application of thin metal coatings to other metals evolved during the middle of the last century. Today, it is one of the highest-volume chemical technologies literally every metal-working plant has its own plating sechon. 

During recent decades, demands regarding the quality and properties of metal coatings have increased sharply. This is due, on one hand, to advances in microelectronics, and on the other hand, to increasing uses of metal parts in corrosive environments. 

Landau, U., E. Yeager, and D. Kortan, Eds., Electrochemistry in Industry New Directions, Plenum Press, New York, 1982. 

Lapicque, E., and A. Strock, Electrochemical Engineering and Energy Storage, Plenum Press, New York, 1994. 

Pletcher, D., Industrial Electrochemistry, Chapman Hall, London, 1982. 

Two moles of electrons must be supplied to deposit 1 mole of nickel. At the anode, the chloride ions are 

Chlorine gas is evolved, and the electrons released oxidise the nickel anode, which dissolves in the process  

The overall result is the transfer of nickel from the anode to the cathode  

It is this same process that causes the corrosion of anodes in all batteries. As mentioned above, the real reactions that take place during electroplating are far more complex than this. As a first approximation these can be written as follows. 

The tensile strengths of electroplated Ni continuous carbon fibers have been determined and examined by Weibull analysis [46]. 

Copper can be coated on carbon fibers [47] and Zhu and workers have used a three-step electrodeposition process for the fabrication of Cu composites [48]. 

Cheng and co-workers have coated carbon fiber by chemical silver plating [49]. 

One of the most common uses of electrochemistry is the plating of a thin layer of an expensive or attractive metal onto a base of cheaper metal. The shiny layer of chromium metal that used to be very common on automobile bumpers and other trim before it became too expensive for this application is plated onto steel with an electrochemical process. A thin layer of silver is commonly plated onto silverware to make eating utensils look like the real thing using an electrochemical process. The use of electrochemistry for plating metals onto surfaces is called dectroplating. 

The silver ion is replaced in solution by oxidation of silver metal at the silver metal anode  

Electroplating is commonly used to prevent corrosion (rusting) of metal. Zinc metal electroplated onto steel prevents rust The corrosion of tin cans is inhibited by a very thin layer of tin plated onto rolled steel. 

Many factors are controlled for plating a thin film on a particular solid sirrface. Thus, the following shoirld be considered [2] 

The most common industrial applications of electroplating can be summarized as follows  

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HOCH2C = CCH2OH. White solid, m.p. 58 C, b.p. 238- C prepared by the high pressure reaction between ethyne and methanol and also from BrMgCCMgBr and methanal. Used in electroplating (Ni), as a corrosion inhibitor, and in paint and varnish removal. 

Ni(NH4)2(S04)2,6H20. Blue-green crystalline material formed from a solution of the components. Used in electroplating. 

An important group of polymers used as moulding resins and in extruded forms (e.g. film). Can be electroplated. Useful polymerization is by Ziegler catalysis and gives an isotactic material. U.S. production 1983 1 -7 megatonnes. 

Roughness has important implications in wetting applications. While the eutectic solder, SnPb, normally forms a contact angle of 15-20° with copper, it completely wets the surface of rough electroplated copper and forms a fractal spreading front [69]. 

Sonoelectrochemistry has been employed in a number of fields such as in electroplating for the achievement of deposits and films of higher density and superior quality, in the deposition of conducting polymers, in the generation of highly active metal particles and in electroanalysis. Furtlienuore, the sonolysis of water to produce hydroxyl radicals can be exploited to initiate radical reactions in aqueous solutions coupled to electrode reactions. 

Type 2 tlie inliibiting species takes part in tlie redox reaction, i.e. it is able to react at eitlier catliodic or anodic surface sites to electroplate, precipitate or electropolymerize. Depending on its activation potential, tlie inliibitor affects tlie polarization curve by lowering tlie anodic or catliodic Tafel slope. 

Then there are other places such as chemical waste exchanges, pool supply companies, electroplating companies, photography supply shops, agriculture companies, specialty gas canister companies and just about any place where a chemical can be sold. 

The concentration of cyanide, CN, in a copper electroplating bath can be determined by a complexometric titration with Ag+, forming the soluble Ag(CN)2 complex. In a typical analysis a 5.00-mL sample from an electroplating bath is transferred to a 250-mL Erlenmeyer flask, and treated with 100 mL of H2O, 5 mL of 20% w/v NaOH, and 5 mL of 10% w/v Kl. The sample is titrated with 0.1012 M AgN03, requiring 27.36 mL to reach the end point as signaled by the formation of a yellow precipitate of Agl. Report the concentration of cyanide as parts per million of NaCN. 

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