Functional Materials and surfaces
Group Assignment A
GALVANIC DEPOSITION OF NICKEL
Nickel plating was first developed by J Bird 1837. The nickel plating process is extensively used for decorative, engineering and electroforming purposes. The appearance and other properties of electrodeposited nickel can be varied over wide ranges by controlling the composition and the operating parameters of the plating solution.
1. The Basic Process
Nickel plating is similar to other electroplating processes that employ soluble anodes.
• It requires the passage of direct current (power supply) between two electrodes that are immersed in a conductive, aqueous solution of nickel salts (electrolyte).
• The flow of direct current causes one of the electrodes (the anode) to dissolve and the other electrode (the cathode) to become covered with nickel.
The nickel in solution is present in the form of divalent positively charge ions (Ni+). When current flows, the positive ions react with two electrons (2e-) and are converted to metallic nickel (Ni) at the cathode surface. At the metallic nickel is dissolved to form divalent positively charged ions which enter the solution. The nickel ions discharged are replenished by those at the anode.
Fig 1: Basic Process Fig 2: Nickel plating in the industry
1. The discharge of nickel ions is not the only reaction that can occur at the cathode. A small percentage of current is consumed in the discharge of hydrogen ions from water which reduces the cathode efficiency for nickel deposition to 92-97 percent.
2. Under normal operating conditions, the nickel ions concentration and the pH of the solution will slowly increase as plating proceeds.
2. Chemical Reactions
3. The Bath Constituents
The nickel plating solution described by watts( 1916) eventually replaced all others in use up to that time. It remains the basis of most decorative nickel plating processes. The composition of the modern watt bath is included in Table 1. The bath also consist of complexing agents, supporting electrolyte and additives.
Table 1: Nickel electroplating
Plating Rate at 100% Ni
Nickel Sulfate NiSO4.6H2O Contains 22.3% nickel.
Nickel Chloride NiCl2.6H2O Contains 24.7% nickel
Nickel Sulfamate Ni(NH2SO3)2 Contains 23.2% nickel
Nickel Carbonate NiCO3 Contains 46.0% nickel
• Nickel Sulfate is used in when concentration when high current densities are required. It is relatively inexpensive and is major source of the nickel ions.
• Nickel Chloride is used to improve anode corrosion, but also increases conductivity and uniformity of coating thickness distribution.
• Boric acid is used in nickel plating solution for buffering purposes.
• Wetting agents or surfactants are almost always added to control pitting. Their function is to lower the surface tension of the plating solution so that air and hydrogen bubbles do not cling to the parts being plated.
4. Deposition Parameters
The Table 2 and Table 3 below shows some of the Deposition Parameters during nickel Plating. The table 3 has been calculated on the assumption that cathode efficiency is 95.5%. From the table, one can estimate the time required to deposit a specified thickness of nickel at a specified current density. Other Deposition quality control includes:
• Control solution composition: The composition of the solution must be controlled within specified limits, and contamination by metallic and organic substances must be prevented.
• Controlling pH, Temperature, Current Density and Water Quality: The pH and the temperature must be maintained within the prescribed limits.
• The nickel plating process should be operated at specified current density by estimating the surface area of the parts and calculating the total current required.
Table 2: Nickel electroplating Deposition Parameters
Table 3: Nickel electrodeposition data based on 95.5% cathode efficiency
5. Advantages and Disadvantages of the Process and Coatings
Coating •Reduces Friction : It reduce the build up of friction in certain materials such as electrical connectors which improves performance and reduces premature wear and tear.
•Attractive looking , Decorative layer
•Often a good under layer for other surface coatings •Discolouring of nickel in air necessitates the use of a top layer( Chromium).
•Nickel may cause allergic reactions
Process •Different bath compositions can be used for very differing applications
•Can be applied on a wide variety of substrate coatings •Layer thickness and structure depend on the current density, as in the case in most electrochemical processes. This limits the shape of objects to be coated.
• Engineering applications exist in the chemical, nuclear, telecommunications: Because of nickel’s ability to cover imperfections of base metal it is widely used to coat decorative items. A thin layer of chromium on nickel deposits gives helps in maintaining the coating even in the severe condition
• Consumer electronics and computer industries: Nickel is used as barrier layer in circuit boards
• Automotive and Aerospace Industry: Satin nickel is used as coating for wheel plates and bumpers.
ELECTROLESS DEPOSITION OF THE NICKEL
Electro less deposition of Ni is an auto-catalytic chemical technique which is continuously depositing a layer of Nickel on a surface. This method is characterized by the reduction of metal ions at the surface of a catalytic substrate immersed into an aqueous solution of metal ions. It works contrary to a galvanic cell, which uses a redox reaction to obtain current. In addition, power supplies are not needed.
1. Chemicals reactions
Ni2+ + 2e- ? Ni0 Reduction E0=-25mV
H2PO2- + H2O ? H2PO3- + 2H+ +2e- Oxidation E0=+50mv
Ni2+ + H2PO2- + H2O ? Ni0 +H2PO3- + 2H+ E0=+25mV
2. Bath constituent
Nickel source: the perfect source of nickel cations is nickel sulfate and Ni(H2PO2)2 would be an excellent acid to eliminate the sulfate anions that won’t be necessary in the reaction.
Reducing agent: it reduces metallic ions to the metal state. Hypophosphite is the most used agent but also we can find other agents as Sodium Borohydride, Dimethylamine Borane or Hydrazine.
Additives: They are responsible for performing, mainly, three functions:
1. Preventing the pH of the solution from decreasing too fast.
2. Preventing the precipitation of nickel salts.
3. Reducing the concentration of free nickel ions.
Also, the deposition can be affected by the additives and hence it can change the result.
Stabilizers: The main function is to stabilize the solution and prevent it from decomposition or, at least, retard the precipitation. The particles that precipitate are nickel phosphide or nickel boride, depending on the reducing agent used. It is possible to find four types of stabilizers: Group IV elements, unsaturated organic acids, heavy metal cations (Sn, Pb…) or oxygen-containing compounds.
3. Deposition parameters
The main parameter in the electroless deposition of nickel is the deposition rate, which is dependent on:
• The temperature: the higher the temperature the faster the deposition rate is.
Figure 1: Effect of solution temperature on the deposition rate Fields, 1984
• The solution PH: by increasing the PH of the solution the coating reduces its content of P and speeds the deposition rate.
• Bath age: the nickel bath has to be replaced in other to maintain the efficiency. A bath usually has 6g/l Ni ion and the bath should be replaced when the Ni ions replaced hit 30-80g/l.
Other parameters like the concentration of nickel in the solution, concentration of the reducing agent, the bath load and the Agitation factor also influence the deposition rate.
4. Final properties
The properties of electroless nickel deposition are strongly related to the content of phosphorus (P) of the deposit. And most of the mechanical properties are related to the material in which the nickel is coated. The most important final properties are:
• Ferromagnetic properties for coating with less than 4% P. Over 10% P, the depositions are nonmagnetic.
• Electroless nickel deposition can have an electrical resistivity between 7.8×10-6 ?-cm and ten times more due to the co-deposition of P.
• Thermal expansion coefficient between 11.1 µm/m/°C (11%P) and 22.3 µm/m/°C (3% of P)
• Low porosity for thicknesses above 25 µm.
• Hardness between 710 HV (4% of P) and 530 HV (10% of P)
• Good corrosion resistance against nitric acid, hydrochloric acid on low and medium P content coting.
The use of electroless deposition of nickel is chemistry process widely extended in the industrial industry. The main applications are:
Industry Application Reason of use
Chemical Process Industry Control valves
Cooling tower water pumps
Nuts and bolts High abrasion resistance
Good wear performance
Food Industry Packaging equipment
Molds and rolls Resistance to stress corrosion cracking
Good resistance to fatigue failure
Oil and Gas Industry Ball valves
Flow control devices
Khuff gas wells High resistance to corrosive and abrasive environments, specially in hydrochloric environments
Automotive Industry Fuel injection systems
Differential pinion shafts Good resistance to wear
Aerospace Industry Compressor section
Manual locking systems
Missile guidance system Good corrosion resistance and erosion resistance
Electronics Industry Memory discs
Transistor chips Good solvability, corrosion resistance and appearance
6. Advantages and limitation of the process and the coating
Process Electrical power not need it.
Flexibility in thickness (1-100 µm)
Ability to coat irregular shapes High cost of waste treatment of bath chemicals.
Coating Homogeneous plating
Excellent corrosion protection Porosity of the plating, especially for thicknesses above 100 µm
7. Based on the article Nickel plating by George Di Bari in the ASM handbook, volume 5, surface engineering, published by ASM international, Materials Park, OH 44073, 1994, page 201, and reproduced with the permission of the publisher.
• Pratheek Ullal,
• Antony Varghese,
• Ana Dolado Pineda
• Isabel Maria Medina Agudo,