Guest Editorial -For Plateworld.com                                                 

 Don Baudrand, Don Baudrand Consulting,   e-mail:donwb@tscnet.com

 

Doís and Doníts of Bright Zinc Plating

Part II Acid Zinc and Zinc Alloys

 

The development of acid chloride zinc plating solutions was pursued to provide plating solutions that would meet the demand for new zinc deposits to meet many applications

for the automotive and aerospace industries. Items that were previously plated with cadmium now required something else to substitute for highly toxic cadmium. Fasteners had to meet torque/tension requirements coupled with higher demands for better corrosion resistance.

There are numerous formulas for the acid zinc group. Some of the typical plating solution make up formulas are listed below. Among them are acid chloride formulas with full ammonium chloride, low ammonia, and no ammonia. Others use potassium chloride or sodium chloride. Some contain boric acid. There are a number of alloys that can be plated from these formulas, such as zinc-cobalt, zinc-iron, Zinc-manganese,

From the acid chloride group of plating solutions, also came zinc solutions that are based on sulfate and sulfamate instead of chlorides. The sulfamate formulations produce very hard deposits due to the inclusion of zinc hydroxide in the deposit.

Typical plating solutions make up formulas

Acid chloride                                         Zinc-Nickel

Zinc (metal) 2.6 oz/gal. (19.5 g/L)

Potassium chloride 25 oz/gal. (187.5 g/L)

Boric acid 4 oz/gal. (30 g/L)

pH 5

Brighteners as required. (Proprietary)

Temperature 90F (32C)

Zinc (metal) 5.3 oz/gal. (39.75g/L)

Nickel 3.3 oz/gal. (24.75g/L)

Ammonium chloride 37 oz/gal. (277.5 g/L)

PH 5.7

Temperature 100F (38C)

Zinc-cobalt

Zinc (metal) 6 oz/gal (45 G/L)

Cobalt 1.3 " (9.75 g/L)

Boric acid 3 oz/gal (22.5 g/L)

Potassium chloride 25oz/gal. (287.5 g/L)

PH 5

Temperature 100F (38C)

Brighteners as required (proprietary)

Zinc-iron

Zinc sulfate.7 H2O 1.2 oz/gal. (9 g/L)

Iron sufate.7H20 33.3 oz/gal. (250g/L)

Sodium sulfate 4 oz/gal. (30g/L)

Sodium citrate.3H20 2.7 oz/gal. (20g/L)

Potassium chloride 1.33 oz/gal. (10 g/L)

pH 1.7

Brighteners as required

Zinc-Magnesium

Zinc sulfate 9.33 oz/gal. (70 g/L)

Magnesium sulfate 6.7 oz/gal. (50 g/L)

Trisodium citrate 26.7 oz/gal 200 g/L)

pH 3

Temperature 104F (40C)

Note: Zinc chloride (ZnCl2) contains 48% zinc and 52% chloride by wt. To raise the zinc content by 1 oz/gal. (7.5 g/L) add 2.08 oz/gal (15.6 g/L) of dry zinc chloride. The total chloride will be raised by 1.08 oz/gal. (8.1 g/L)

ADVANTAGES AND DISADVANTAGES OF SOME OF THE ZINC PLATING SOLUTIONS

Cyanide Zinc Advantages

Tolerant to impurities
Good throwing power
Good covering power
Requires less cleaning Minimal equipment solution

Disadvantages of Cyanide Zinc

High cost of waste treatment
Minimal leveling
Difficult to plate cast iron and high
High carbon steels
Highly toxic solution

Zinc Chloride and Alloys Advantages

Reduced waste costs
High cathode efficiency
Very bright deposits
Plates cast iron, malleable iron, and
Carbonitrided steels
Leveling
Reduced cooling
Wide range of current density

Disadvantages of Zinc Chloride

Poor throwing power
Acid resistant equipment
Limited thickness
Requires better cleaning
Solution is corrosive to surroundings

Alkaline Non-Cyanide Zinc Advantages

Good throwing power
Good covering power
Ease of waste-treatment Low cost
Good corrosion resistance
Solution not corrosive to tanks and equipment

Disadvantages of Non-Cyanide Zinc

Lower cathode efficiency
Requires good cleaning
Potential for delayed blistering
Less bright
Cannot plate cast iron or high carbon steels

 

Doís and Doníts of Acid Zinc Plating

Doís

Clean thoroughly prior to plating
Add brightener by amp-hour meter
Rinse thoroughly
Requires periodic peroxide treatment to remove iron contamination
Cover copper buss bars.
Mild air agitation OK
Filter through a carbon pack continuously
Maintain the correct temperature.
Limits are narrow
Anode current density should be about 20 A/sq ft. (2.15 A/sq Decimeter)
Bag anodes Zinc balls in titanium baskets or bagged slab anodes. Use 99.9% zinc

Doníts

Allow high chloride. More than 20 oz/gal. (150 g/L) will cause surfactant oil out.
Short cut final rinsing. Loss of brightness and poor chromate reception.
Allow copper contamination to prevent dark deposit
Allow pH to rise above limit, dull deposits result.
Use iron or lead heating and cooling coils.

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