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Electroplating is the most widely used method of nickel plating (the alternative method is electroless nickel plating).
The following solutions are used for nickel electroplating:
Nickel electroplating is a process of nickel deposition over a part immersed into an electrolyte solution and used as a cathode, when the nickel anode is being dissolved into the electrolyte in form of the nickel ions traveling through the solution and depositing on the cathode surface.
Surface preparation
Prior to plating operation the cathode (work piece) surface should be cleaned from mineral oils, Rust protection oils, Cutting fluids (coolants), greases, paints, animal lubricants and vegetable lubricants, fingerprints, miscellaneous solid particles, oxides, scale, smut, rust.
Anodes
Small parts of high purity primary nickel (nickel rounds or nickel squares) loaded into titanium baskets are used as anodes for nickel electroplating. Dimensions of nickel rounds: 1” (25 mm) diameter and up to 0.5” (12 mm) thick. Dimensions of nickel squares: 1”x1” (25×25 mm) and up to 0.5” (12 mm) thick.
Sometimes nickel bars and rods are used as anodes.
Current efficiency
Current efficiency is a ratio of the current producing nickel deposit to the total passing current.
Anode current efficiency in nickel electroplating is about 100%. It may decrease at high PH when nickel dissolution is accompanied by discharging hydroxyl ions (OH-).
Cathode efficiency of nickel electroplating is 90-97%. 3-10% of the electric current is consumed by discharging hydrogen ions (H+), which form bubbles of gaseous Hydrogen (H2) on the cathode surface.
Anti-pitting additives
Hydrogen bubbles formed on the cathode surface and adhered to it may cause pitting of the deposit. In order to enhance removal of the bubbles wetting agents are added to the electrolyte. Wetting (anti-pitting) agents (e.g. sodium lauryl sulphate) decrease the surface tension of the cathode and force the hydrogen bubbles out of the surface.
Filtration
Continuous filtration of nickel plating baths with active carbon filters permits to control both presence of foreign particles and organic contaminations (products of brightener decomposition etc). The filtration pumps should turn over the solution a minimum 1-2 times tank volume per hour.
Air agitation
Air agitation by low pressure blowers is used in nickel electroplating to enhance removal of the hydrogen bubbles discharged at the cathode.
Temperature
Nickel electroplating processes are conducted at increased temperature, which results in lower electrolyte resistance and therefore permits to decrease the voltage. Additionally higher temperatures aid dissolution and prevent precipitation of boric acid and other components.
Thickness of electroplated nickel coating may be calculated from the Faraday’s law.
Nickel coating thickness in US unites:
h = 0.000869*c*J*t
where:
h - coating thickness, μinch;
c - coefficient of cathode efficiency (about 0.95);
J - electric current density, A/ft²;
t - time, min.
Nickel coating thickness in metric unites:
h = 0.205*c*J*t
where:
h - coating thickness, μm;
c - coefficient of cathode efficiency (about 0.95);
J - electric current density, A/dm²;
t - time, min.
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Roughness of nickel coating is generally caused by foreign particles suspended in the electrolyte solution: air dust, torn anode bags, dropped parts, precipitates of boric acid, metallic impurities or drag-in of incompatible solutions, particles of filter carbon powder, parts of filter paper. Roughness may be also a result of deposition in low brightener solutions at high current density.
Corrective actions: proper filtering, preventing drag-in, temperature control.
Pitting is a result of hydrogen bubbles adhered to the cathode surface. It usually occurs at low concentrations of wetting agent, low air agitation, high current densities, low boric acid concentrations.
Corrective actions: check the concentrations of ant-pitting (wetting) agent and boric acid, increase air agitation, decrease the current density.
Poor adhesion (peeling, blisters, low adhesion strength) of nickel coatings may be generally caused either by poor pretreatment cleaning or poor acid activation of the part surface. Activation acid contaminated with copper or chromium or improper activation acid cause adhesion problems. For example: lead containing alloys are activated by methane sulfonic acid or fluorides.
Corrective actions: check cleaning operations, check the activation acid.
Different nickel electroplating solutions produce coatings with different levels of internal mechanical stress and ductility. The lowest stress and maximum ductility are provided by nickel sulfamate solutions. Brittle coatings are caused by excessive concentrations of organic agents (levelers, brighteners), decomposition products of brighteners, nickel chloride and metallic contaminants.
Corrective actions: active carbon treatment, control of nickel chloride.
Brighteners
In order to achieve bright and lustrous appearance of nickel plating organic and inorganic agents (brighteners) are added to the electrolyte.
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Watts solution was developed by Oliver P. Watts in 1916. Now it is most popular nickel electroplating solution. Plating operation in Watts solutions is low cost and simple.
Bath composition:
Nickel sulphate, NiSO46H2O: 32-40 oz/gal (240-300 g/l)
Nickel chloride, NiCl26H2O: 4-12 oz/gal (30-90 g/l)
Boric acid, H3BO3: 4-6 oz/gal (30-45 g/l)
Operating conditions:
Temperature: 105-150°F (40-65°C)
Cathode current density: 20-100 A/ft² (2-10 A/dm²) PH: 3.0-4.5
Mechanical properties:
Tensile strength: 50000-70000 psi (345-485 MPa)
Elongation: 10-30%
Hardness: 130-200 HV
Internal stress: 18000-27000 psi (125-185 MPa)
Brighteners:
Type of the added brighteners and their concentrations determine the deposit appearance: brilliant, bright, semi-bright, satin.
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Nickel sulfamate solution is used for electroforming and for producing functional nickel coating. Nickel coatings deposited in nickel sulfamate baths possess lowest internal stress. High nickel concentrations of sulfamate electrolytes permit to conduct electroplating at high current densities (high rates of deposition).
Bath composition:
Nickel sulphamate, Ni(SO3N2)2: 40-60 oz/gal (300-450 g/l)
Nickel chloride, NiCl26H2O: 0-4 oz/gal (0-30 g/l)
Boric acid, H3BO3: 4-6 oz/gal (30-45 g/l)
Operating conditions:
Temperature: 105-140°F (40-60°C)
Cathode current density: 20-250 A/ft² (2-25 A/dm²)
PH: 3.5-4.5
Mechanical properties:
Tensile strength: 60000-88500 psi (415-610 MPa)
Elongation: 5-30%
Hardness: 170-230 HV
Internal stress: 0-8000 psi (0-55 MPa)
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All-Chloride solutions operate at low voltage and permit deposition of thick coatings. The main disadvantage of all-chloride baths is high internal stress of the coatings.
Bath composition:
Nickel chloride, NiCl26H2O: 30-40 oz/gal (225-300 g/l)
Boric acid, H3BO3: 4-4.7 oz/gal (30-35 g/l)
Operating conditions:
Temperature: 110-150°F (43-65°C)
Cathode current density: 25-100 A/ft² (2.5-10 A/dm²) PH: 1-3
Mechanical properties:
Tensile strength: 90000-14000 psi (620-930 MPa)
Elongation: 4-20%
Hardness: 230-260 HV
Internal stress: 40000-50000 psi (275-340 MPa)
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Sulphate-Chloride solutions produce depositions with internal stress lower than that in All-Chloride solutions. Sulphate-Chloride bath operate at voltages lower than Watts baths. This type of electrolyte permit deposition at high rates (high electric current) as compared to Watts bath.
Bath composition:
Nickel sulphate, NiSO46H2O: 20-30 oz/gal (150-225 g/l)
Nickel chloride, NiCl26H2O: 20-30 oz/gal (150-225 g/l)
Boric acid, H3BO3: 4-6 oz/gal (30-45 g/l)
Operating conditions:
Temperature: 110-125°F (43-52°C)
Cathode current density: 25-150 A/ft² (2.5-15 A/dm²) PH: 1.5-2.5
Mechanical properties:
Tensile strength: 70000-105000 psi (480-725 MPa)
Elongation: 5-25%
Hardness: 130-200 HV
Internal stress: 30000-40000 psi (200-275 MPa)
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Fluoborate solutions permit high rate depositions due to higher (than in Watts solution) nickel concentration. Fluoborate solutions are mainly used for electroforming and for deposition of thick coatings.
Bath composition:
Nickel fluoborate, Ni(BF4)2: 30-40 oz/gal (225-300 g/l)
Nickel chloride, NiCl26H2O: 0-2 oz/gal (0-15 g/l)
Boric acid, H3BO3: 2-4 oz/gal (15-30 g/l)
Operating conditions:
Temperature: 100-160°F (38-70°C)
Cathode current density: 30-250 A/ft² (3-25 A/dm²) PH: 2.5-4.0
Mechanical properties:
Tensile strength: 55000-87000 psi (380-600 MPa)
Elongation: 5-30%
Hardness: 125-300 HV
Internal stress: 13000-29000 psi (90-200 MPa)
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All-Sulfate solution are used mainly in applications where insoluble anodes are required (plating tubes and small fittings).
Bath composition:
Nickel sulphate, NiSO46H2O: 30-53 oz/gal (225-400 g/l)
Boric acid, H3BO3: 4-6 oz/gal (30-45 g/l)
Operating conditions:
Temperature: 100-160°F (38-70°C)
Cathode current density: 10-100 A/ft² (1-10 A/dm²) PH: 1.5-4.0
Mechanical properties:
Tensile strength: 60000-70000 psi (415-485 MPa)
Elongation: 10-30%
Hardness: 200-390 HV
Internal stress: 30000-45000 psi (200-300 MPa)
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Hard nickel solution are used in applications where high tensile strength and hardness are required.
Bath composition:
Nickel sulphate, NiSO46H2O: 24 oz/gal (180 g/l)
Ammonium chloride, NH4Cl3: 3.3 oz/gal (25 g/l)
Boric acid, H3BO3: 4 oz/gal (30 g/l)
Operating conditions:
Temperature: 110-140°F (43-60°C)
Cathode current density: 25-50 A/ft² (2.5-5 A/dm²) PH: 5.6-5.9
Mechanical properties:
Tensile strength: 60000-88500 psi (415-610 MPa)
Elongation: 5-30%
Hardness: 170-230 HV
Internal stress: 0-8000 psi (0-55 MPa)
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