We continue to review some of the most important materials in heat treatment and metallurgy.
Phosphorus (chemical symbol: P)
Phosphorus exists in three different forms, or allotropes, known simply as red, white and black phosphorus. Due to their reactivity properties, the red and white allotropes both have commercial uses. Red phosphorus is used in safety matches, fireworks, smoke bombs and pesticides, and it is formed by heating white phosphorus to 250°C (482°F) or by illuminating it with sunlight. White phosphorus emits a faint glow when exposed to oxygen, a process called chemiluminescence (Fig. 1) and can spontaneously ignite when exposed to air. As a result, it is commonly stored under water. Primary uses for white phosphorous are for incendiaries, smoke, tracer, illumination and other munitions.
Phosphorus was discovered in 1669. It was the first element to be discovered that had not been already known since ancient times. In perhaps the most unorthodox discovery of an element, German physician and alchemist Hennig Brand obtained it from human urine, which contains considerable amounts of dissolved phosphates resulting from normal metabolism. Brand was actually attempting to obtain gold from the urine. He let it evaporate for days until it gave off an intense stench, then concentrated it by boiling until it formed a paste. He heated the paste to a higher temperature until it gave off fumes, which he condensed in water. Instead of creating gold, he obtained a white, waxy substance that, to his surprise, glowed in the dark.
Brand had discovered phosphorus, in the form of sodium hydrogen phosphate. His determination was commendable because it takes about 1,100 liters (290 U.S. gallons) of urine to make 60 grams (0.13 pounds) of phosphorus. It was named phosphorus from the Greek word "phosphoros," meaning "bringer of light."
Today, phosphorus is not derived from urine but is found in the minerals apatite and fluorapatite, which in the U.S. is mined mostly in Florida. Other mining operations exist in Kazakhstan, China, Morocco and Tunisia. Phosphorus is also found in other minerals, such as phosphophyllite, turquoise (Fig. 2) and vivianite.
Phosphorus is used as an additive in steel and is a very effective solid-solution strengthener of ferrite. The addition of only 0.17% phosphorus increases both the yield and tensile strength of low-carbon sheet steel while simultaneously improving the bake hardening response and deep drawability. Because of these properties, rephosphorized high-strength steels are widely used for cold-forming applications. Phosphorus is also used as an additive in steels to improve machining characteristics and atmospheric corrosion resistance.
Here are some Interesting facts about phosphorus.
- Atomic number: 15
- Atomic weight: 30.973761998
- Melting point: 317.30 K (44.15°C or 111.47°F)
- Boiling point: 553.65 K (280.5°C or 536.9°F)
- Density: 1.82 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Non-metal