RANDHIR KUMAR SINGH

ASST PROFESSOR

OPJIT

Electroplating is a process of coating deposition on a part, immersed into an electrolyte solution and used as a cathode, when the anode is made of the depositing material, which is dissolved into the solution in form of the metal ions, traveling through the solution and depositing on the cathode surface.

A scheme of the electroplating of copper in the aqueous solution of copper sulfate (CuSO4) is shown in the picture:

Positively charged copper ion moves towards the negative cathode and when it reaches the cathode surface it accepts two electrons, converts to the copper atom and deposits on the cathodes surface.

The amount of the deposited material is directly proportional to the amount of electric charge, passed through the circuit.

Since amount of electric charge Q=I*t (I electric current, t time), then the Faradays law may be expressed by the formula:

W = It/(nF), Where

W weight of the deposited material; weight of one mole of the metal; n - number of electrons transferred by the ion into solution (n=2 in the above example); F Faradays constant, F = 96485 Coulombs.

Cathode efficiency is the ratio of the actual amount of the deposited material to the theoretical amount that should be deposited.

Current efficiency is the percentage of current, which is actually used for the deposition at the cathode or for the anode dissolving (not including the current used for the side reactions).

Throwing power uniformity of the thickness of a coating deposited on irregularly shaped part.

Leveling is ability of electroplating process to deposit smooth uniform coating on the rough surface.

Leveling is achieved by addition of leveling agents into the electrolyte solution.

Brightening is an ability of electroplating process to deposit bright fine-structure coating.

Brightening is achieved by addition of brighteners into the electrolyte solution.

Rack plating

Large and complex parts are coated on racks

copper wire structures, holding the parts.

Barrel plating

Small parts in a batch are plated in rotating

barrels.

Electroless plating is a process in which

plating is achieved entirely by chemical

reaction occurring at the part surface in the

solution with no electric current involved.

In electroless plating the part surface serves

as a catalyst for the reaction.

The source of the electrons, required for

reducing metal ions, is a reducing agent

(reductant).

Nickel, copper, gold can be electroless

plated.

Electroforming is a process of making metal

articles by electroplating of the metal onto a

pattern (mold, mandrel), followed by

removal of the deposited layer.

Fine molds for compact disks, screen-printing

cylinders, metal bellows and other high

accuracy micro parts are manufactured by

the electroforming method

Surface preparation is a series of cleaning treatments of

the substrate surface prior to the coating operation intended

for ensuring strong and uniform adhesion of the coating to

the substrate.

Adhesion is a state of bonding a coating to the substrate

surface.

Bonding strength (adhesive strength) is a tensile stress

(tensile load applied to the unit bonding area) required to

peel the coating from the substrate surface.

Contaminants to be removed from the part surface

Technologies of surface preparation

mineral oils (Rust protection oils, Cutting fluids

(coolants), greases, etc.);

miscellaneous organic soils (paints, animal lubricants

and vegetable lubricants, fingerprints);

polishing and buffing compounds;

miscellaneous solid particles (dust, abrasive grits,

chips);

oxides, scale, smut, rust.

1. Mechanical cleaning

Mechanical removal of solid particles, burrs, scales and oxides from the part surface by abrasion, Abrasive blast cleaning, vibratory finishing or Shot peening.

Ultrasonic cleaning scrubbing action produced by numerous small vacuum cavities forming as a result of high frequency (20 45 kHz) sound waves traveling in a fluid, in which the part is submerged.

As a cleaning fluid aqueous solution of alkaline detergents at room temperature is used.

Ultrasonic cleaning is highly effective in removing solid particles, dirt and smut.

Spray cleaning cleaning by aqueous solutions (alkaline or mild acids) delivered to the part surface through spray nozzles at high pressure.

2. Soak cleaning

Solvent cleaning removal of organic soils (mineral oils,

fingerprints etc.) by dissolving them in a solvent.

The solvent may be used in the liquid state, when the part is

either immersed into the solvent or sprayed by it.

The solvent may also be used in the gaseous state (vapor

degreasing) when the soils are dissolved by the solvent

vapors.

Use of some solvents like trichloroethylene and

trichlorathane (chlorinated hydrocarbons) is forbidden or

restricted in most of the world due to their adverse effect on

the human health.

Emulsion cleaning removal of soils by organic solvents dispersed in aqueous solution, containing emulsifiers (surface active substance preventing coalescence of the dispersed component).

Emulsion cleaning is followed by alkaline cleaning to remove the organic components of the emulsion from the part surface prior to coating.

Alkaline cleaning removal of organic soils (mineral oils, fingerprints, wax etc.) and some solid particles by hot alkaline solutions.

The application methods are either immersion or spraying, followed by water rinse.

The work temperature of alkaline solutions is 130-190F (5084C).

3. Electrocleaning

Electrocleaning is an electrolytic process conducted in an alkaline electrolyte, through which a direct current is passed.

The cleaned part is connected as either the anode or the cathode.

Electrocleaning combines effects of the soak alkaline cleaning and mechanical cleaning provided by gas bubbles, which form on the part surface as a result of the electrochemical reaction.

Electrocleaning is able to remove organic soils impregnated into the part surface, solid particles adhered to the surface and oxides.

Electrocleaning is followed by water rinse and then acid activation.

The following electrocleaning methods are used:

Anodic electrocleaning (reverse electrocleaning).

In this method the part is connected to the positive side of the rectifier.

The following reaction takes place at the anode surface:

4[OH] =2H2O + O2 + 2e

Oxygen bubbles are liberated at the part surface assisting cleaning effect.

Cathodic electrocleaning (direct electrocleaning).

In this method the part is connected to the negative side of the rectifier.

The following reaction takes place at the anode surface:

4H2O + 4e = 4[OH] +2H2

Hydrogen bubbles (in a quantity twice as much as oxygen quantity)are liberated at the part surface.

Cathodic electrocleaning is more effective than anodic electrocleaning due to the more intensive gas liberation.

Disadvantages of the cathodic electrocleaning are:

possible impurities deposition on the part surface

and hydrogen embrittlement of the part, caused by

diffusion of hydrogen.

Periodic-reverse electrocleaning

In this method the part is alternatively connected

anodically and cathodically at controlled time

intervals.

The method combines the advantages of the both:

anodic and cathodic electrocleaning.

4. Acid activation.

Acid treatment is used to remove oxides, scales and activate

the metal surface of the part prior to coating.

Mild acids (citric, phosphoric) are used for activation of

aluminum, zinc castings and other sensitive alloys.

Strong acids (hydrochloric, sulfuric and nitric) are used for

activation of steels and stainless steels.

Acid salts are used for more controllable and safe activation

process.