Cadmium coatings are applied to iron, steel, brass and aluminium and give excellent resistance to corrosion in most conditions and especially in marine and alkaline environments. Cadmium, like zinc, also provides sacrificial protection to a substrate such as steel by being preferentially corroded when the coating is damaged and small areas of the substrate are exposed. In addition to corrosion protection, cadmium coatings provide a low coefficient of friction and therefore good lubricity, predictable torque characteristics, good electrical conductivity, protection from galvanic corrosion (in particular when in contact with aluminium), easy solderability, low volume corrosion products and reduced risks of operating mechanisms being jammed by corrosion debris for many components in a wide range of engineering applications throughout industry.
Cadmium coatings are particularly useful in the electrical, electronic, aerospace, mining, offshore, automotive and defence industries where they are applied to bolts and other fasteners, chassis, connectors and other components.
Electroplating accounts for over 90 per cent of all cadmium used in coatings but mechanical plating and vacuum or ion deposition have some commercial significance. The coating is normally specified in thickness' between 5 and 25 µm depending on the severity of the atmosphere. Chromate post-treatment of the coating can increase coating life.
Cadmium is electrodeposited on the metal article from an electrolyte solution of cadmium salts in barrels or vats. These electrolyte solutions are nearly always based on the alkaline cyanide system. Other solution types are used, such as those based on fluoroborates, but these have not proved popular as they lack the excellent combination of brightness, covering power, throwing power and wide operating parameters of the alkaline cyanide system. When a current is passed through the electrolyte, cadmium is electrodeposited on the metal article at the cathode and cadmium from the anode goes into solution. Large or delicate articles are attached to racks and vat-plated whilst small components, such as bolts, washers, nuts, springs and clips can be vat-plated in drum cages or plated in a rotating barrel.
(b) Mechanical plating
This process uses mechanical energy to deposit metal coatings on small components by the impact of glass beads. Either cadmium or mixed-metal coatings of cadmium-tin or cadmium-zinc can be applied when glass beads, proprietary chemicals, water and metal powder are tumbled with the components in a rotating barrel. The process is suited to components such as fasteners and clips which are small enough to be plated in a barrel.
(c) Vacuum and ion deposition
Conventional thermal vapour deposition involves heating of cadmium in a vacuum until it vaporises. Cadmium atoms then condense on the substrate to form a thin high quality coating of cadmium. Ion deposition in argon atmospheres adds more energy to this coating process and uses 'sputter cleaning' to clean the substrate surface. As a result, ion deposition is said to give improved coating adhesion, density and uniformity. Components such as undercarriage legs of transport aircraft, helicopter rotor parts (as shown in Fig. 1) and other high strength steel components have been successfully coated using this method.
Fig.1 Helicopter 'rotary rudder head spindle' coated with cadmium by ion deposition. Approx. length 20 cm.