ELECTROPLATING
©Copyright 1999, 2001 Mario S Pennisi Email:mariopen@coatfab.com

WHAT IS ELECTROPLATING?
Electroplating was first reported almost 200 years ago. It may be defined as the process wherein an electric current is carried across an electrolyte and in which a substance is deposited at one of the electrodes.

Electrolysis is possible because solvents, water in particular, have the ability to ionise substances dissolved in them, that is they split them into components that carry positive and negative charges. These ions are electrically charged and are attracted to oppositely charged electrodes where they are neutralised by the charges on these electrodes. The products of electrolysis appear on the electrodes. The cathodic product is the deposition of metal and the anodic product most often is the dissolution of metal.

WHY ELECTROPLATE?
Electroplating is a means available to the surface finisher to apply a metal coating to a metallic or plastic component.

Electroplating is used to deposit a very wide range of pure metals and alloys for use in decorative, functional and jewellery applications.

Nickel/chromium composites (usually called "chrome" plating), copper, brass (an alloy), bronze (an alloy) and zinc are used for decorative applications.

For functional (engineering) applications zinc, tin, heavy coatings of nickel, "hard" chromium, silver and gold are used. Various alloys are deposited for bearing surfaces. Functional uses include building up worn parts and changing the surface characteristics of a metal such as electrical inductance, or conductivity, or corrosion resistance.

For jewellery applications, gold, silver and rhodium are plated. Gold is often plated on 18-carat and 9-carat solid gold jewellery to obtain colour matches.

PREPARATION
The basis of any good coating is preparation.

For electroplating, oils and greases are removed in weak to strong alkali solutions (depending on the metal being cleaned and the soils being removed). The surface oxides are removed in acids (strong pickles are used to remove heavy oxides and rust from steel). After rinsing the parts are presented for electroplating.

HOW IS IT DONE?
Electroplating is carried out in an electrolytic cell. The part to be plated is made the cathode and the metal to be deposited is often made the anode. The electrolyte contains salts of the metal being deposited as well as other compounds.

The parts to be plated are placed on racks (for still plating) or barrels (for barrel plating of small components such as fasteners) and the racks or barrels immersed in the electrolyte. A DC (direct current) electric current is passed between the items (negative terminal), the electrolyte and the anode. The composition of the electrolyte, the temperature and current are all controlled within close limits to give consistent results. In most cases, the electrolyte is filtered continuously to remove solids, which might otherwise stick to the part being plated and so produce rough deposits.

When the current is applied, metal simultaneously deposits from the electrolyte onto the part and dissolves from the anode to replenish the electrolyte. When an insoluble anode is used such as in chromium plating or gold plating, the metal ions in the electrolyte are replenished by adding chemicals.

The process for decorative nickel plating after the preparation treatment is similar to the following.

After plating the items are rinsed, dried, inspected and packed.

When zinc is plated, the surface of the zinc reacts with the air and becomes stained. To overcome this tendency to stain and to corrode quickly, the zinc coated parts are dipped in a chromate solution for a few seconds. This chromate treatment (called a chromate conversion coating) can be clear or blue, or iridescent gold, or iridescent gold/green, or black in colour. The corrosion resistance is least for the clear and blue chromates and highest for the iridescent chromates.

INSTALLATIONS AND MAINTENANCE
During installations the electroplated coating should be protected from damage from abrasion and materials of construction such as mortar and brick cleaning chemicals.

Once installed, maintaining the initial appearance of the electrodeposit is a simple matter. The soot and grime, which builds up on surfaces from time to time, contains moisture and salts which will adversely affect the electroplated coating and must be removed. Metallic coatings should be washed down regularly (at least once each 6 months in less severe applications and more often in marine and industrial environments). The coating should be washed down with soapy water - use a neutral detergent - and rinsed off with clean water.

When electroplated parts are installed without damage to the coating and the coating is maintained regularly, it should be relatively permanent.

Although the film is not metallurgically bonded to the metal, it should not crack or peel when applied correctly.