nickel electroplating

28
RANDHIR KUMAR SINGH ASST PROFESSOR OPJIT

Upload: rahul-pandey

Post on 29-Dec-2015

227 views

Category:

Documents


23 download

DESCRIPTION

corrosion prevention technique

TRANSCRIPT

Page 1: Nickel Electroplating

RANDHIR KUMAR SINGH

ASST PROFESSOR

OPJIT

Page 2: Nickel Electroplating

Nickel is chosen as the material to make the valve because of several reasons. Electrodeposited nickel can be strong, tough and resistant to corrosion, erosion and wear. Its mechanical properties can be varied at will between wide limits by changing plating conditions, by alloying with other elements, and by incorporating particles and fibers within the electrodeposited nickel matrix.

The scheme of nickel-electroplating cell is shown in Fig.1 where the cathode is the wafer to plate with a conducting layer and the photo resist structure on it. When the power supply is turned on, the positive ions in the solution are attracted to the negatively biased cathode. The nickel ions that reach the cathode, gain electrons and are deposited on the surface of the cathode forming a layer. Simultaneously, another reaction that depends on the nickel solution used to plate, occurs at the anode, to produce ions and electrons for the power supply.

Page 3: Nickel Electroplating

Fig 1: Scheme of the electrochemical plating of Ni

Page 4: Nickel Electroplating

Properties of nickel coatings

Basics of nickel electroplating

Nickel coating thickness

Problems and troubleshooting

Watts nickel plating solutions

Nickel sulfamate solutions

All-Chloride solutions

Sulfate-Chloride solutions

Fluoborate solutions

Hard nickel solutions

Page 5: Nickel Electroplating

Decorative appearance. Lustrous bright, satin semi-bright or black nickel coatings may be obtained by different plating methods.

Corrosion protection. Wear resistance. Nickel deposited on a part made of a

softer metal protects the part from wear. Hardness of nickel plating may be controlled by the plating process parameters.

Low coefficient of friction. Ferromagnetism. Ferromagnetic parts (steel) may be

plated by nickel without changing their magnetic properties.

Controllable internal mechanical stresses. Low stress coatings are important in electroforming and applications, in which Fatigue strength is critical.

Page 6: Nickel Electroplating

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: Watts nickel plating solutions Nickel sulfamate solutions All-Chloride solutions Sulfate-Chloride solutions All-Sulfate solutions Hard nickel solutions

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

Page 7: Nickel Electroplating

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.

Page 8: Nickel Electroplating

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.

Page 9: Nickel Electroplating

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.

Page 10: Nickel Electroplating

Thickness of electroplated nickel coating may be calculated from the Faraday’s law. Nickel coating thickness in US units: h = 0.000869(c.J.t) where: h - coating thickness, μ inch; c - coefficient of cathode efficiency (about 0.95); J - electric current density, A/ft²; t - time, min. Nickel coating thickness in metric units: 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.

Page 11: Nickel Electroplating

Roughness

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

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.

Page 12: Nickel Electroplating

Poor adhesion

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.

High stress and low ductility

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.

Page 13: Nickel Electroplating

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)

Page 14: Nickel Electroplating

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)

Page 15: Nickel Electroplating

Brighteners:

Carrier brighteners (e.g. paratoluene sulfonamide, benzene sulphonic

acid) in concentration 0.1-3 oz/gal (0.75-23 g/l). Carrier brighteners

contain sulfur providing uniform fine Grain structure of the nickel plating.

Levelers, second class brighteners (e.g. allyl sulfonic acid, formaldehyde

chloral hydrate) in concentration 0.0006-0.02 oz/gal (0.0045-0.15 g/l)

produce (in combination with carrier brighteners) brilliant deposit.

Auxiliary brighteners (e.g. sodium allyl sulfonate, pyridinum propyl

sulfonate)in concentration 0.01-0.5 oz/gal (0.075-3.8 g/l).

Inorganic brighteners (e.g. cobalt, zinc) in concentration 0.01-0.5 oz/gal

(0.075-3.8 g/l). Inorganic brighteners impart additional luster to the

coating.

Type of the added brighteners and their concentrations determine the

deposit appearance: brilliant, bright, semi-bright, satin.

Page 16: Nickel Electroplating

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)

Page 17: Nickel Electroplating

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)

Page 18: Nickel Electroplating

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)

Page 19: Nickel Electroplating

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)

Page 20: Nickel Electroplating

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)

Page 21: Nickel Electroplating

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)

Page 22: Nickel Electroplating

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)

Page 23: Nickel Electroplating

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)

Page 24: Nickel Electroplating

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)

Page 25: Nickel Electroplating

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)

Page 26: Nickel Electroplating

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)

Page 27: Nickel Electroplating

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)

Page 28: Nickel Electroplating