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Dura®, Durachrome®, are trade names of Plating Resources, Inc. Copyright and all other World Rights Reserved,, 2014.

 

 

 

III. PROCESSING CYCLES

After the article has been fabricated and has the desired surface finish it is ready for the plating operation. This operation comprises a number of steps that take the part through several sequences. These sequences and the solutions used will vary depending upon the finish to be applied. A typical sequence for zinc plating may be as follows.

Cycle Operation
1. Load parts onto racks or barrels
2. Alkaline soak clean
3. Electroclean
4. Water rinse
5. Acid pickle activate
6. Water rinse
7. Zinc electroplate
8. Water rinse
9. Dry parts
10. Unload parts

In order to better understand the entire process each sequence is discussed individually.

A. Cleaning
Cleaning is used to remove unwanted soils, greases, and oils from the metal surface. These come from the fabrication and prefinishing operations. The object is to achieve a chemically clean surface so that the plating deposit will adhere. Any residual surface will detract from plating adhesion and are a frequent cause of blistered and peeled deposits.

Among the many types of cleaning used today are solvent cleaning, vapor, degreasing, emulsion cleaning, alkaline soak, and electrocleaning. The first three, solvent, vapor, and emulsion types are considered pre-cleaners and are always followed by alkaline soak electrocleaning in the plating operation. A surface that is efficiently cleaned will water break free. The absence of water breaks will assure that all organic films have removed and that the part is ready for further processing.

Alkaline soak cleaning is used to remove most soils. The solution typically contains alkalis, phosphates, silicates, carbonates, soaps, and surfactants. Cleaning occurs by actions known as emulsification, saponification, and dispersion. Most soak cleaners are operated at a temperature of 150-200°F with the parts being immersed from 5 to 20 min.

After the bulk of the soil has been removed in the soak cleaner the parts are transferred to an electrocleaner. The electrocleaner serves the same purpose as the soak cleaner but to a much greater degree. In electrocleaning a charge is applied to the part much as in the plating operation. The charge, anodic for reverse cleaning and cathodic for direct cleaning, causes either oxygen or hydrogen to be released from the solution. The gas has a micro scrubbing action. The chemical composition, operating times, and temperatures are similar to those used in soak cleaning. Typical current densities for electrocleaning are 40-70 A/ft2 of surface processes.

In alkaline soak or direct or reverse electrocleaning, care should be exercised in selecting the cleaner formulation as well as the operating parameters. This is necessary so as to provide a chemically clean part while not degrading or eroding the base metal. During operation the cleaning solutions become saturated with oils and soils. This contamination necessitates either replenishment with fresh cleaner or replacement from time to time of the entire cleaning solution. Numerous plating defects have been traced to a cleaner that has not been maintained at the concentrations and/or parameters that were specified.

B. Activation
After the part has been thoroughly cleaned it must be activated prior to plating. This activation is accomplished by various acid solutions and is frequently termed "pickling." The purpose of activation is to remove oxides and scale from the metallic surface. These oxides are formed during fabrication, heat treating, handling, and storage of the parts. Steel and other metals will form a thin oxide layer during such handling and storage. Heat treating will cause a more severe form of oxide known as scale.

The acid solutions commonly used are sulfuric, hydrochloric, nitric, and hydrofluoric or mixtures of these acids. The selection of the proper acid, dipping time, and temperature is dependent upon the metal being processed and the thickness of the oxide layer. Typical pickling acid formulations for a mild oxide condition on steel parts are as follows.

  1. Hydrochloric acid, 20° Baume, at a concentration of 50% by volume. A temperature of approximately 70°F is used for 2-15 min.
  2. Sulfuric acid, 66° Baume, at a concentration of 15% by volume. A temperature of approximately l20°F is used for 2-15 min.

When activating metals with acids it is desirable to set the operating parameters so as to remove all oxide while not overpickling, which, will remove excessive metal and can frequently cause smutting of the surface. Parts that are heavily scaled will require the use of an inhibitor (typically, an organic amine) in the acid solution. The purpose of an inhibitor is to provide a better wetting action to the acid, which helps dissolve and remove scale while slowing the acid attack on the fresh and exposed metal surface. Such inhibitors can effectively permit removal of scale while preventing the formation of smut or excessively removing metal.

C. Electroplating
After the parts are cleaned and acid activated they are ready for electroplating. The surface is rinsed in water and the part placed immediately into the plating solution. A fast transfer time is required here so that re-formation of an oxide layer is prevented.

Direct current electricity is then applied to the electrolyte, as shown in Fig. 25.2. The current is continued for a predetermined period of time until the desired amount of metal is deposited upon the cathodic workpiece.

The current is measured in amperes, coulombs, or faradays. A deposit of 1 g equivalent the metal will be deposited for each faraday. The equivalent weight equals 1 g weight divided by the valence of the metal. The valence is determined by the number of charges that the metal ion has. A faraday is a unit of electricity and is equal to 96,500 C (or ampere-seconds). Table 25.1 shows the amount of metal deposited at 100% efficiency. In order to determine the deposit thickness that will be achieved it is also important to consider the current distribution and the cathode efficiency.

A proper distribution is achieved when the anodes and cathodes are positioned in such a way as to have the current flow be uniform across the entire cathode surface. Area of a part, such as a protruding corner, that are closer to the anode than other areas will receive a higher current density and consequently a thicker deposit. Proper positioning and masking in the plating solution will alleviate this problem so that a deposit of uniform thickness is achieved. In most cases a current efficiency of 100% is not achieved. The cathode efficiency is the percentage of current that deposits metal. The balance of the current is consumed in parasitic actions, such as evolving hydrogen at the cathode surface. When the cathode efficiency is known the plating rate (grams of metal per unit area per unit time) can be calculated as follows:

Plating rate =
gram equivalents metal
Area X time
=
faraday X %efficiency
Area X time

The plating engineer is also concerned with other features of the deposit. Metallurgical factors, such as hardness, residual stress, hydrogen embrittlement, and brightness, considered. Frequently agents are added to the plating solution in order to modify the deposit that would normally be obtained. By controlling the bath composition, the choice and concentration of addition agents, current density, and temperature, the plater is usually able to obtain deposits with the desired properties.

Metal
Valence
Atomic weight
Grams deposited per faraday
Cadmium
2
112.41
56.21
Chromium
6
52.01
8.67
Copper
1
63.57
63.57
Copper
2
63.57
31.78
Gold
1
197.20
197.20
Nickel
2
58.71
29.36
Silver
1
107.88
107.88
Tin
4
118.70
29.67
Zinc
2
65.38
32.69

D. Rinsing and Drying
After each step in the processing cycle the chemical film that remains on the workpiece surface must be rinsed off with suitably clean water. Certain parts may have areas that trap solution; these areas must be thoroughly rinsed. It is important to remove all chemicals from the surface prior to the next sequence in processing. This is to avoid unwanted reactions on the surface as well as to avoid solution contamination.

Water rinsing is normally done in a tank in the processing line. At least one rinse tank is used following each process. Frequently several rinse tanks are used for double or triple rinsing. Multiple rinses are used where a high work flow tends to contaminate the first rinse quickly. In multiple rinsing each rinse dilutes the surface film further until contaminants are removed to a tolerable level.

There are several types of rinse tanks, but they generally fall into two categories: running and still. A running rinse has a continuous supply of water to the tank and over- flows. This is used in high-production lines to keep the rinse water at a level of purity sufficient to avoid contamination of the next tank. Still rinsing has no water flow and is used only for small production runs.

To economize on water costs a conductivity controller is commonly used to control the contamination level in the rinse tank. With this device inlet water flows only when required to keep the tank water at the desired purity. For further water economy a cascade rinse is used in a mu1tibay tank. In a cascade rinse the water flows over and under baffles separating each bay so that it is used several times. Flowing water is used in the last bay with an overflow in the first bay. The work flow is opposite to the water flow so that the emerging work is rinsed by the purest water in the last bay.

Several methods of improving rinsing efficiency are used, such as heated water and air agitation. Merely submersing a part is not sufficient to maximize the rinsing operation unless contact is maintained for a long period of time. Warm water tends to dilute the surface film much quicker. By agitating the water with air the films are more quickly diluted. Spray rinsing directly over the process tank is frequently employed as a method of water conservation. A spray rinse will remove approximately 80% of the surface film and return the chemicals directly back into the process tank. Spray rinsing should be followed by normal tank rinsing to remove the balance of the surface film.

Today the electroplater is concerned with regulations for pollution control. These regulations limit the amount of certain chemicals that may be discharged. A high-production plating line is capable of discharging substantial amounts of these controlled chemicals from the rinsing operations. This, coupled with high water costs, frequently dictates that a controlled water volume be used.

At the completion of the entire process and the final water rinse, the parts should be thoroughly dried. Complete drying is necessary if staining and rusting are to be prevented. All moisture should be removed from the surface as well as from recesses and crevices. Heated as well as forced air from a fan or blower is commonly used to effect complete drying. Other techniques include drying by absorption with ground corncobs. This is used frequently on small parts. Centrifuges and infrared equipment are also employed for drying certain parts.