Electroplating Theory 2
When electroplating nickel with the salt nickel sulfate, eventually all the nickel is going to end up on the workpiece and you would be left with a weak solution. There are two ways to replenish the supply of the metal, nickel in this case, to the bath. Method one: simply dissolve more nickel sulfate into the bath; method two: the professional way— use an anode composed of nickel that dissolves nickel into the bath at the same rate the workpiece is taking nickel out of bath. Can you see how that would cause the bath to be in balance? One of the ways the rate of corrosion of anodes into the bath is controlled is by the amount of acid or cyanides introduced into the bath. Put too much acid or cyanide into a bath and there will be too much metal dissolved, with a resultant rough plate. Too little acid or cyanide, little corrosion will take place, with the result of a weak bath that is slow to plate the workpiece. Thus, electroplaters check the pH of a bath. What is pH?? It is parts of hydrogen. Remember that all acids contain hydrogen. by checking the pH, we get an indicator of the acidity or alkalinity of a bath and adjust it to a predetermined level to optimize plating efficiency. Fortunately, others have gone before who have determined what the proper pH for a given bath, be it nickel, copper, cadmium, chrome, etc., should be and the formulas supplied in this book conveniently list the bath pH. When you were in high school chemistry, you may have used litmus paper to check pH. That would work, but electroplaters use electric pH meters that take the guess work out. In addition, technology today has brought us equipment that does a very thorough job of pH analysis.
In addition to checking the acidity or alkalinity of a bath we also want to check for metal content. Tests with a hydrometer are performed to measure the amount of metal dissolved in the solution. The hydrometer has a calibrated scale called a Baume’ scale. There are two kinds of hydrometers: 1) the Baume’ hydrometer just mentioned and 2) the specific gravity hydrometer
that measures the density of the liquid relative to water. For example, the specific gravity of sulfuric acid is approximately 1.8, which means it is 1.8 times as dense as water, The second type of hydrometer is commonly seen in auto parts stores where it is used to measure the strength of an auto battery. All formulas in this manual refer to the Baume’ scale.
If electroplating took place with no additions of metal salts or corrosion of the anodes, the process would take longer and longer until plating would cease altogether.
To summarize, we want to check the acidity/alkalinity of a bath and also the metal content of the bath. To do that, we use an electric pH meter and a Baume’ hydrometer.
Electrical nonconductors such as plastics may be plated by first being covered with a conducting material, such as graphite or silver nitrate/sulphide.
To ensure a strong and close bond between the workpiece and the plating metal, the object must be cleaned thoroughly by dipping it into a caustic (soap) solution, often accompanied by mechanical scrubbing. Every molecule of grease must be removed from the workpiece surface, other wise the plating will peel.
After cleaning, the metal must be activated by pickling. Pickling occurs when the workpiece is immersed into an acidic solution where the acid attacks the surface and scours it with a light etch which is conducive to accepting a subsequent metal plate. Obviously, the amount of pickling must be controlled as we do not want to dissolve our workpiece into the pickling solution. The etched surface produced by the pickling operation produces a slightly rough surface into which the plating crystals will interlock and grow. This allows for a very tight bond, equivalent to a weld. Electroplating is sometimes referred to as cold welding. If done properly, it should bond as tightly as a molten weld.
Other factors that affect the quality of the plate are current density (amperes per square foot of cathode surface), pH, temperature (temperature is very important in chromium plating to achieve a good appearance) and purity of the bath.
In electroplating, baths are sometimes referred to in terms of their throwing power. Throwing power refers to the ability of a bath to distribute metal evenly to the workpiece surface. A chrome bath has poor throwing power, thus projections tend to receive heavy deposits, and recesses in the workpiece receive little or no deposits.
This can be counteracted by the use of thieves acid shields. A thief is a conductor placed close to a workpiece projection to attract some of the metal ions to it and prevent the workpiece projection from receiving too heavy of a plate. A shield is a nonconductor placed In front of a projection Lo prevent it from receiving too heavy of a plate.
A larger workpiece will require more amperes of current than will a smaller one. D.C. generators have adjustable current settings. The operator needs to know the approximate square footage of the workpiece and adjust the amperage accordingly. The formulas in the manual give the suggested amperes per square foot or square inch of workpiece surface.
Contaminants in the bath can interfere with the plating process. The anodes can produce sludge and other contaminants during the electroplating process. Consequently they are often bagged (put in porous bags) to trap these impurities. A good filtering system with the appropriate chemicals or activated carbon is important to remove impurities. Baths may also be purified by electroplating dummy pieces before commercial workpieces The electrolyzing process tends to remove impurities.
In decorative chromium plating, a typical sequence of events is degreasing of the workpiece in the soap (caustic) tank, rinsing, pickling, rinsing, nickel acid plating, rinse, acid chromium plating, rinse, and buffing.
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