We continue to review some of the most important materials in heat treatment and metallurgy.
Cadmium (chemical symbol: Cd)
Cadmium is a soft, silvery-grey metal with a bluish hue (Fig. 1). This ductile element is similar to zinc, its northern neighbor on the periodic table, although cadmium has a tendency to form complex compounds, which zinc tends not to do. Due to being corrosion-resistant, Cadmium is often used as a coating or plating in thicknesses from 5-25 µm (0.0002 to 0.001 inches).
Like zinc, it serves as a sacrificial coating when applied to other metals, such as steel and iron. It is preferentially corroded when the coating is damaged and small areas of the substrate are exposed, providing a self-healing action and retarding further corrosion.
Cadmium’s use as a plating material for fasteners, chassis and connectors extends to a variety of industries, such as: automotive, defense, aerospace, electrical, electronic, mining, offshore and more. Much like its southern neighbor on the periodic table, mercury, cadmium has a relatively low melting point for a metal – 321°C (610°F).
Cadmium was discovered in 1817 by German chemist Friedrich Stromeyer. He made his discovery while employed by the government to inspect the practices of pharmacies in the town of Hildesheim, Germany. Hildesheim’s pharmacies were selling zinc carbonate rather than the more effective zinc oxide to treat skin ailments, a common use for zinc oxide today. He took a sample of the medicine in question and heated it, knowing that zinc carbonate would burn a bright white color. Instead, it burned with a yellow/orange tint that could not be readily explained. After further analysis he established that the yellow color was caused by “the presence of a peculiar metallic oxide, the existence of which had not hitherto been suspected.” After isolating this new metal oxide from the zinc oxide, he further reduced it to examine the world’s first raw cadmium. Stromeyer then named his discovery after “cadmia,” the Latin name for calamine, a longstanding name for zinc ore.
In addition to electroplating, another widespread use for cadmium is in rechargeable nickel-cadmium batteries and the less common silver-cadmium battery. Compounds of cadmium are used to make pigments for paint. Some of the most brilliant yellow (Fig. 2), red and orange paint varieties available incorporate cadmium pigments. Approximately 1,000 tons of cadmium per year is used for this purpose.
Here are some important facts about cadmium.
- Atomic number: 48
- Atomic weight: 112.414
- Melting point: 594.22 K (321.07°C or 609.93°F)
- Boiling point: 1040 K (767°C or 1413°F)
- Density: 8.69 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
- Period number: 5
- Group number: 12
- Group name: none