Cadmium plating

8.1 Properties of cadmium and cadmium coating

(1) Color: off-white

(2) Hardness: 300~500 Mpa

(3) Atomic weight: 112.40

(4) Proportion: 8.65

(5) Melting point: 321C

(6) Atomic value: 2

(7) Standard potential: -0.4029V

(8) Atomic number: 48

(9) Resistance: 6.83m W –cm

(10) Strength: 70Mpa

(11) Boiling point: 767C

(12) Chemical properties: Similar to zinc, but not dissolved in lye, does not change color in room temperature air, and will oxidize in humid air

The protective film can prevent continued oxidation and corrosion. Cadmium oxide is insoluble in water. Cadmium is less soluble in acid than zinc, but has a stronger effect on nitric acid.

Cadmium produces toxic gases when it melts, and its soluble salts are also toxic. Cadmium is a cathodic protection coating on steel substrates, but at high temperatures

Or in the marine climate, it is a sacrificial anode protective coating.

8.2 Cadmium plating

In the marine environment and high temperature (above 70C) hot water, the cadmium coating has strong corrosion resistance and is relatively stable. Good weldability and lubricity

, Strong contact resistance, good resistance to alkaline solution, low hydrogen embrittlement, good gloss and strong adhesion. So aviation, aviation

Many parts in the sea and electronics industry are plated with cadmium. The cadmium coating is easy to polish and can be used as the bottom layer of the paint after phosphate.

Toxic, food utensils cannot be plated with cadmium.

8.3 Types of cadmium electroplating bath

It can be divided into the following four types:

(1) Alkaline cyanide plating bath. (3) Acidic boron fluoride salt plating bath.

(2) Alkaline sulfate plating bath. (4) Neutral sulfate plating bath.

The type of plating bath should be selected according to the shape of the plated part, the requirements of the plating layer, and the uniformity of the plating bath. General simple shape plating

An acid plating bath is used, while an alkaline cyanide plating bath is used for plating parts with complex shapes. Alkaline plating bath has fine crystals and good uniformity.

However, it is toxic, troublesome to dispose of the waste liquid, the electroplating bath is unstable, and the current efficiency of the anode and the anode is different.

Sodium carbonate and hydrocyanic acid. The acidic bath is less hydrogen embrittlement.

8.4 Alkaline cyanide cadmium plating bath formulation

Cadimum oxide 3 oz/gal

Sodium carbonate 4~8 oz/gal

Sodium cyanide (sodium cyanide) 13.5 oz/gal

Sodium hydroxide (sodium hydroxide) 1.9 oz/gal

Current density 5~90 ​​ASF

The bath temperature is 60~100. F

8.4.1 Preparation of alkaline cyanide cadmium plating bath

The steps are as follows:

(1) First check whether the quality of the medicine is in compliance.

(2) Strictly observe the safety rules for the use of highly toxic substances and good ventilation equipment.

(3) Dissolve the required amount of sodium cyanide in a small amount of hot water (50-60°C), and the temperature should not be too high to avoid decomposition.

(4) Dissolve the necessary caustic soda in water in another container.

(5) Pour the caustic soda solution into the sodium cyanide solution.

(6) Mix the required cadmium with water to form a paste, slowly add it to the above-mentioned mixture, stir it fully, and dissolve completely, the temperature can be raised

But not higher than 85°C.

(7) Dissolve other additives in an appropriate amount of water and add them in sequence.

(8) Add 1.5 to 2 pounds of zinc powder per 100 gal, stir fully for 30 minutes, let the solution stand for 4 hours, and filter

Into the plating tank.

(9) Sampling, analysis and calibration of each component.

(10) Air electrolysis with 0.25~0.5V current for 24~48 hours.

(11) Add gloss agent.

(12) The plating bath is normalized with 10ASF plating about two angels.

8.4.2 Additives for alkaline cyanide cadmium plating bath

Additives affect the structure and appearance of the coating in the electroplating bath. Inorganic additives such as nickel, cobalt, molybdenum and selenium compounds are mainly

Improve the appearance and physical properties of the coating, organic additives such as aldehyde, glucose, milk, sugar, gum, molasses

, Sulfonic acid, Turkish red oil, sulfonated castor oil, etc. promote fine-grained coatings, improve uniformity, and hide

Shield metal impurities. When the additive exceeds a certain amount, the coating will become brittle and generate bubbles.

8.4.3 Defects and causes of cyanide cadmium plating

(1) The coating is discolored and stained, the reasons are

1. The bottom metal has holes.

2. The pre-treatment is not clean.

3. Contact with dirty hands.

(2) The coating is burnt black, rough and dark, the reasons are

1. The current density is too large.

2. Too little free cyanide.

3. There are floating impurities.

(3) Poor coating adhesion and surface blistering due to

1. Insufficient cyanide.

2. The alkalinity is too high.

3. Poor pretreatment.

4. The plating part absorbs hydrogen when it is acidic.

(4) A large amount of cyanide gas is generated at the cathode, and the anode is in a crystalline state, and the coating is poor. The reasons are

1. Free cyanide is too high.

2. Insufficient metal salt.

(5) The anode dissolves unevenly and there is slag, the reasons are

1. Insufficient free cyanide.

2. Insufficient hydroxide.

 

8.5 Non-Cyanide Plating Baths

In recent years, due to pollution control and reduction of the formation of hydrogen during electroplating, many non-cyanide cadmium plating baths have been developed, which can be divided into neutral sulfur

Salt plating bath, acid borofluoric acid plating bath and acid sulfate plating bath.

Non-cyanide plating bath has high current efficiency and low hydrogen embrittlement, so high-strength steel should use non-cyanide plating bath. Of which borofluoric acid plating

The bath maintains high current efficiency even at high current density.

8.5.1 Neutral sulfate bath formulation

Ammonium chloride 1.5~3 oz/gal

Ammonium sulfate 10~15 oz/gal

Cadmium (content) 0.5~1.5 oz/gal

Current density 2~15 ASF

The bath temperature is 60~100. F

8.5.2 Formulation of acidic borofluoride cadmium plating bath

Ammonium fluoborate 8 oz/gal

Boric acid 3.6 oz/gal

Cadmium 12.6 oz/gal

Cadmium fluoborate 32.2 oz/gal

Current density 30~60 ASF

The bath temperature is 70~100. F

8.5.3 Formulation of acid sulfate cadmium plating bath

Cadmium oxide (cadmium oxide) 1~1.5 oz/gal

Sulfuric acid (salfuric acid) 4.5~5 oz/gal

Current density 10~60 ASF

The bath temperature is 60~90. F

8.6 Stripping and repair plating of cadmium plating

(1) Steel, copper and copper alloy plated parts: Dissolve and peel off the cadmium coating

1. Dilute hydrochloric acid. spring

2. 100g/l ammonium nitrate.

3. Waste chromic acid solution.

(2) Aluminum and its alloy plating parts: Dissolve and peel off the cadmium coating with 5-10% nitric acid.

(3) Plated parts with poor appearance: Some plated parts can be directly repaired without peeling off the coating. First perform electrolytic cleaning for 1 to 2 minutes,

Remove surface dirt and film, then activate, clean, and stay in the cadmium plating bath for 10 to 30 seconds, and then re-plating.

The time depends on the plating parts.

8.7 Process of cadmium plating

Inspection before plating→solvent degreasing→chemical or electrolytic cleaning→hot water washing→cold water washing

Electrolytic cleaning←cold water cleaning←on the shelf←neutralization←cold water cleaning←acid leaching rust removal

Hot water washing → cold water washing → pickling activation → cold water washing → cadmium plating → cold water washing → drying

Cold water washing ← passivation ← dry ← hot water washing ← cold water washing ← light out ← hydrogen removal

Warm water washing → drying → inspection

Flow Description:

Step 1: Check the surface condition and size of the plated parts.

Step 2: Clean the inside and outside of the parts.

Step 3: Make the surface of the parts wet with water, and clean the cathode first and then the anode after electrolysis. The anode time should be relatively long.

The spring leaf is anodized. Electrolyte composition 30~50g/l NaOH, 20~30g/l Na2CO3, 30~50g/l Na3PO4 and

10~15g/l Na2SiO3.

Step 6: The composition of the solution is 50~100g/l sulfuric acid and 100~150g/l hydrochloric acid.

Step 8: The solution is 50~70g/l sodium carbonate.

Step 11: Same as step 3.

Step 14: Use 30-50g/l sulfuric acid solution to remove the oxide film.

Step 19: Bake for 2~4 hours at 200~250℃.

Step 20: Soak in chromic acid solution for 0.1 to 0.5 minutes.

Step 24: The passivation solution consists of 80~150g/l sodium dichromate and 8~10g/l sulfuric acid. Special spring

8.8 Standards and specifications related to cadmium plating

(1) QQ-A-671 Federal Specification (cadmium anode standard specification)

(2) JIS H8611 Electroplated Coatings of Cadmium on Iron and Steel

(3) ASTM A165 Specification for Electrodeposited Coatings of Cadmium on Steel