Electroplating



This post details the procedure for the following:



1. COPPER PLATING ON LOW CARBON STEEL

2. NICKEL-CHROMIUM PLATING ON ALUMINUM AND ITS ALLOY

3. GOLD PLATING ON ZINC-BASED ALLOYS (INCLUDING DIE CASTINGS)

4. SILVER PLATING ON COPPER, BRASS AND OTHER COPPER-BASED ALLOYS



ELECTROPLATING is a process of coating an object, usually metallic,with one or more relatively thin, tightly adherent layers of some other metal by means of electrochemical process. Electrochemical process involves electrical and chemical energy.

Principles of Electroplating

In electroplating, the object to be plated is immersed in a solution containing dissolved salts of the metal to be deposited. The set up is made up of a cathode and an anode with the object to be plated usually the cathode connectedto the negative (-) terminal of a direct current source. To complete the electrical circuit, another metal is connected to the positive (+) terminal and both are immersed in the solution. This metal is made up of the same material as the metal to be deposited and is called the anode. When current is applied, the electrical energy carried is converted to chemical energy by decomposition, a reaction in which the elements are divided into positive and negative charged ions. The movement of positively charged ions towards the cathode surface results to metal deposition.

Preparation of the work surface

The most important step in the entire plating operation is the surface preparation of metals or the cleaning process. This is because the appearance and acceptance of the article depend primarily on a sound finish achieved with a clean and active substrate. Likewise, improper cleaning process leads to rejects and decreased profitability.

Before the desired coating can be applied to the component, the surface to be plated should be clean and free of all “foreign” matters like heavy scale of oxide films, rust, workshop soils & oils, grease, dirt, and any other material. All these should be removed to ensure strong adherence of electrodeposited metal to the piece. This is achieved through the following methods:

1. Descaling - this is done by polishing, tumbling and blasting with sand, grit or vapor. This is then followed by pickling process where the component is immersed in acid to remove rust, scale, tarnish, light oxides and dirt.

2. Polishing - this is done after the surface has been subjected to grinding or wire brushing. This method alters the surface of the metal through the use of a variety of abrasives: coarse, medium, fine and so on. This operation smoothens the surface of the metal.

3. Buffing - this method further smoothens the metal surface and improves its appearance by the application of very fine abrasives to produce different types of finishes (e. g., satin finish, brushed or butler finish, mirror brightness finish, etc.)

4. Alkaline Soak cleaning - the component is dipped in hot alkaline solution to remove oil and wax.

5. Electrocleaning - this process is performed on the metal surface prior to electroplating. It is more efficient than soak cleaning and considerably minimizes chemical attack on the metal surface. The component becomes the cathode (direct cleaning) or anode (reverse cleaning) or alternately the cathode and anode in an alkaline solution.

6. Ultrasoniccleaning- the use of high-frequency waves called ultrasonic waves enhance cleaning efficiency of components with complex shapes like jewelry, electronic and other precision parts.

EFFECTS OF IMPROPER CLEANING

1. Deposits on substrate poorly adhere
2. Non-uniform appearance of deposit
3. Pitting of deposit
4. Unplated areas
5. Poor corrosion resistance
6. Micro-roughness of deposit
7. Contaminated baths

EQUIPMENT & ACCESSORIES NEEDED

1. Rectifier
2. Plating Tank
3. Anode: e.g., Nickel, Copper, Stainless Steel, Silver
4. Heater
5. Filter
6. Air Blower/Agitator
7. Thermostat
8. Thermometer
9. Racks and Jigs
10. Copper Bus Bar
11. Bench Grinder
12. Wire Wheel Brush
13. Polishing Wheel
14. Buffing Wheel
15. Measuring Glassware (Beaker, Graduated Cylinder)


PLATING PROCESSES

A. COPPER PLATING ON LOW CARBON STEEL


MATERIALS AND SOLUTIONS NEEDED

I. Copper Cyanide (CuCN) Plating Solution

Copper Cyanide 30 g/l
Sodium Cyanide 48 g/l
Sodium Carbonate 15 g/l
Sodium Hydroxides 3.75 g/l
Rochelle Salt 30 g/l
Current Density 0.5 - 4 A/dm2
Temperature 24 - 66 oC
Voltage 6 V
pH 12 - 12.6
Dipping Time 30 sec. - 5 min.
Anode - Oxygen-free Copper

II. Acid Copper Plating Solution

Copper Sulfate 225 g/l
Sulfuric Acid 56 g/l
Current density 2 - 10 A/dm2
Temperature 21 - 48 oC (RT)
Voltage 6 V
Dipping Time 15 min.
Anode Phosphorized copper

III. Acid Dip Solution

Sulfuric acid 50 ml
Water 1 liter

SOLUTION PREPARATION:

COPPER CYANIDE PLATING SOLUTION

1.Fill 2/3 of the storage tank with demineralized water and dissolve the required amount of sodium cyanide (NaCN) with continuous stirring.
2. In a separate container, mix copper cyanide (CuCN) with water to form a thin slurry. Pour this mixture gradually into the storage tank.
3. Add the rest of the required materials after dissolving the copper cyanide.
4. Carbon treat the solution if necessary then filter into the plating tank. Dilute to volume with water.
5. Adjust to the required pH with sodium hydroxide (NaOH).
6. Analyze solution for free cyanide (CN).

ACID COPPER PLATING SOLUTION

1. Fill 2/3 of the storage tank with demineralized water, then heat to 60 C.
2. Dissolve copper sulfate (CuSO4•5H2O) with continuous stirring.
3. Treat bath with activated carbon. Stir the solution.
4. Agitate for at least one hour, then allow activated carbon to settle for eight hours. Filter into the plating tank.
5. Add sulfuric acid (H2SO4) and agitate through mixing and dilute to volume.
6. Add the necessary amount of brighteners.

ACID DIP SOLUTION

1. Pour the water in a clean container.
2. Gradually add the sulfuric acid to the container. Mix thoroughly.

CAUTION: To avoid accident always add ACID to WATER

PLATING PROCEDURE:

1. Degrease the low-carbon steel by dipping in alkaline solution for 5 min.
2. Electro-clean the steel by applying current of 8 Amp/dm for 30-60 sec.
3. Rinse with water for 5-10 sec.
4. Dip for 10-15 sec in of 50ml/l sulfuric acid solution.
5. Rinse with water for 5-10 sec.
6. Plate with copper in copper cyanide solution for 2-5 min. Using current density of 4.0 Amp/dm .
7. Rinse with water for 5-10 sec.
8. Dip for 5 sec. in 50 ml/l sulfuric acid solution.
9. Rinse with water for 5-10 sec.
10. Plate in acid copper at room temperature for 15 min. using current density of 4.0 Amp/dm .
11. Rinse with water and dry.

B. NICKEL-CHROMIUM PLATING ON ALUMINUM AND ITS ALLOY

MATERIALS AND SOLUTIONS NEEDED

I. Nickel Plating Solution

Nickel Sulfate 225 g/l
Nickel Chloride 30 g/l
Boric Acid 40 g/l
Brightener I (Vegastar I) 30 ml/l
Brightener II (VegastarII) 2 ml/l
Anti-pitting agent 0.5 - 1 ml/l
Current density 2 - 4 A/dm2
Temperature 45 - 70 oC
Voltage 6 - 12 V
pH 4 - 4.5
Dipping Time 15 min.
Anode Nickel

II. Chrome Plating Solution


Chromic Acid 250 - 300 g/l
Sulfuric Acid 2.5 - 3 g/l
CrO3 : H2SO4 100 : 1 ratio
Trivalent Chromium (Cr+3) 2 - 3 g/l
Non-mist pellets 0.5 g/l
Current density 11 - 23 g/l
Temperature 52 oC +/- 3oC
Voltage 6 - 12 V
Dipping Time 1 - 2 min.
Anode Lead (Pb) with 5-7% Tin or Antimony

III. Copper Cyanide Solution (see above)

SOLUTION PREPARATION:

NICKEL SOLUTION

1. Fill 2/3 of the storage tank with demineralized water and heat up to 65 C.
2. Add boric acid (H3BO3) in specified quantity and agitate to dissolve.
3. Add nickel sulfate (NiSO4), then the nickel chloride (NiCl2). Dissolve.
4.Treat with activated carbon (2-5 g/l) and agitate for about 2 hours.
5. Filter the solution into the plating tank.
6. Add the brighteners in specified quantities and adjust the volume of the solution with water.
7. Adjust the pH to working condition.
8. Perform weak electrolysis if necessary (1-2 A/dm2 for 20 min. or until completely covered with nickel, then reduce to 0.1-0.2 A/dm2).

CHROME SOLUTION

1. Fill tank with water to about 3/4 of the final volume.
2. Heat until temperature is about 50 oC.
3. Add chromic acid (CrO3) very slowly with continuous and vigorous stirring.
4. Add sulfuric acid in specified quantity.
5. Stir up the plating solution for about 2 to 3 hours for perfect dissolution.
6. Heat the solution until the temperature reaches 50 o C. (Do not exceed this temperature and maintain it for 2 hours then make an electrolysis without plating the article. This procedure is for obtaining a trivalent chromium ions (Cr+3) in the plating).

PLATING PROCEDURE:

1. Degrease the aluminum by dipping in hot alkaline solution for 5 seconds.
2. Rinse with water for 5-10 seconds.
3. Dip in 50% by volume nitric acid for 10 sec.
4. Rinse with water for 5-10 sec.
5. Dip in sodium zincate solution for 1-3 min. This solution contains 525 gms of caustic soda, 100 gms zinc oxide, 10 gms rochelle salt and 1 gm ferric chloride per liter solution. The solution should be used at room temperature and the articles should be kept moving during immersion.
6. Rinse with water thoroughly.
7. Strip the zincating by dipping in 50% by volume Nitric acid.
8. Rinse with water.
9. Repeat step no. 5
10. Rinse with water thoroughly.
11. Plate with copper in copper cyanide solution (see section 3).
12. Rinse with water.
13. Plate with nickel for 15 min. at 4.0 Amp/dm .
14. Rinse with water.
15. After nickel plating, the article can be plated with chromium at 10 Amp/dm for 1-2 min.
16. Wash the article in hot water and dry.


C. GOLD PLATING ON ZINC-BASED ALLOYS (INCLUDING DIE CASTINGS)

MATERIALS AND SOLUTIONS NEEDED

a. Strike Gold Solution

Potassium Gold Cyanide 1.25 - 2 g/l
Potassium Cyanide 7.5 g/l
Dipotassium Phosphate 15 g/l
Temperature 60 - 70oC
Current Density 1 - 4 A/dm2
Anode Stainless Steel/Gold/
Platinized titanium

b. Acid Gold Solution

Potassium Gold Cyanide 4 - 12 g/l
Citric Acid 10 g/l
Potassium Citrate 50 g/l
Hydrogen Sulfate 6 g/l
Nickel Citrate 60 g/l
pH 3 - 6
Current Density 10 A/dm2
Temperature room temperature
Anode Carbon and/or Platinized titanium

c. Acid Dip Solution

Sulfuric acid 65 ml
Water 3 liters

d. Cyanide Dip Solution

Sodium Cyanide 30 grams
Water 1 liter

e. Acid Copper Solution (see above).
f. Copper Cyanide Solution (see above).
g. Nickel Solution (see above).

SOLUTION PREPARATION

GOLD PLATING SOLUTION

1. Fill 2/3 of the storage tank with distilled water.
2. Dissolve the required amount of potassium cyanide.
3. Add the potassium gold cyanide in specified amount and stir to dissolve.
4. Add the rest of the required chemicals and dilute to volume with distilled water.

ACID DIP SOLUTION

1. Pour each 1 liter of water into three containers.
2. Measure 5, 10 and 50 ml of sulfuric acid. Then pour to the containers. Mix then set aside.

CAUTION: To avoid fatal accident, NEVER mix ACID &

CYANIDE SOLUTION

CYANIDE DIP SOLUTION

1. Place 1 liter of water in a container
2. Pour gradually 30 grams of sodium cyanide. Mix, then set aside.

STRIKE GOLD PLATING SOLUTION (see above)
COPPER CYANIDE SOLUTION (see above)
ACID COPPER SOLUTION (see above)
NICKEL SOLUTION (see above)

PLATING PROCEDURE:

1. In separate clean plating tanks, prepare the solutions of strike gold, acid gold, copper cyanide, acid copper and nickel copper.
2. Degrease the zinc-die cast by dipping in alkaline solution for 5 min.
3. Electroclean the zinc-die cast by applying current of 5 Amp/dm for 30-60 sec.
4. Rinse with water for 5-10 sec.
5. Dip in 5 ml/l Sulfuric acid solution for 30-60 sec.
6. Rinse with water thoroughly.
7. Immerse the zinc-die cast in copper cyanide solution with current on. Initially, use a current density of 2.5 Amp/dm for 2 min, then reduce the current density to 1.2 Amp/dm and continue to plate for 5 min.
8. Rinse with water for 5-10 sec.
9. Dip in 10 ml/l Sulfuric acid solution.
10. Rinse with water.
11. Plate in Acid Copper at room temperature for 15 min. using current density of 4.0 Amp/dm .
12. Rinse with water.
13. Dip in acid of 50 ml/l Sulfuric acid for 5 sec.
14. Plate in Nickel solution for 15 sec. using current density of 4.0 Amp/dm
15. Rinse with water
16. Dip in 30 g/l Cyanide dip solution
17. Rinse with water
18. Plate with strike gold solution
19. Plate with gold in Acid Gold solution.
20. Rinse with water and dry.

D. SILVER PLATING ON COPPER, BRASS AND OTHER COPPER-BASED ALLOYS

MATERIALS AND SOLUTIONS NEEDED:

a. 1st Strike Solution

Silver Cyanide 2 g/l
Copper Cyanide 10 g/l
Potassium Cyanide 60 g/l
Temperature 21 - 30 oC
Current density 1.6 - 2.5 A/dm2
Voltage 4 - 6 V

b. 2nd Strike Solution

Silver Cyanide 3 - 4.5 g/l
Potassium Cyanide 62 - 75 g/l
Temperature 21 - 30 oC
Current Density 1.6 - 2.5 A/dm2
Voltage 4 - 6 V

c. Silver Plating

Silver Cyanide 30 g/l
Potassium Cyanide 50 g/l
Potassium Carbonate 13 g/l
Temperature 21 - 27 oC
Current density 0.5 - 1.6 A/dm2

d. Acid Dip Solution (see above)
e. Cyanide Dip Solution(see above)

SOLUTION PREPARATION:

SILVER PLATING SOLUTION

1. Fill 2/3 of the storage tank with distilled water.
2. Dissolve the required amount of potassium cyanide.
3. Add and stir to dissolve the silver cyanide in specified quantities.
4. Add the rest of the required chemicals and dilute to volume with distilled water.

1ST AND SECOND STRIKE SOLUTION (see above)
ACID DIP SOLUTION (see above)
CYANIDE DIP SOLUTION (see above)

PLATING PROCEDURE:

1. Degrease the copper by dipping in alkaline solution for 5 min.
2. Electroclean the copper by applying current of 8 Amp/dm for 30-60 sec.
3. Rinse with water for 5-10 sec.
4. Acid Dip for 5 sec. in 50ml/l sulfuric acid.
5. Plate in Nickel solution for 15 sec. using current density of 4.0 Amp/dm .
6. Rinse with water thoroughly for 10 sec.
7. Dip in 30 g/l sodium cyanide solution.
8. Rinse with water for 5-10 sec.
9. Plate in 2nd Silver Strike solution for 8-12 sec. at room temperature at
2.0 Amp/dm .
10. Rinse with water for 5-10 sec.
11. Plate in Silver plating solution for 5-10 sec. with applied current of 0.5-
1.5 Amp/dm .
12. Rinse with water.
13. Apply the silver plated article with discoloration prevention solution.


Source: Metal Industry Research and Development Center, DOST; photo courtesy of www.ppc1904.com

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