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DESCRIPTIONcorrosion prevention technique
RANDHIR KUMAR SINGH
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.
Fig 1: Scheme of the electrochemical plating of Ni
Properties of nickel coatings
Basics of nickel electroplating
Nickel coating thickness
Problems and troubleshooting
Watts nickel plating solutions
Nickel sulfamate solutions
Hard nickel solutions
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.
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
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: 1x1 (2525 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 Faradays law. Nickel coating thickness in US units: h = 0.000869(c.J.t) where: h - coating thickness, inch; c - coefficient of cathode efficiency (about 0.95); J - electric current density, A/ft; t - time, min. Nickel coating thickness in metric units: h = 0.205(c.J.t) where: h - coating thickness, m; c - coefficient of cathode efficiency (about 0.95); J - electric current density, A/dm; t - time, min.
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.
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.
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.
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 : 105-150F (40-65C) Cathode current density : 20-100 A/ft (2-10 A/dm)
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)
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
Type of the added brighteners and their concentrations determine the
deposit appearance: brilliant, bright, semi-bright, satin.
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).
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)