A soft, heavy, bluish-gray metal (symbol Pb), lead is obtained chiefly from the mineral galena. It surface-oxidizes easily, but is then very resistant to corrosion. It is soluble in nitric acid but not in sulfuric or hydrochloric, and is one of the most stable of the metals. Its crystal structure is face-centered cubic. It is very malleable, but it becomes hard and brittle on repeated melting because of the formation of oxides. The specific gravity of the cast metal is 11.34, and that of the rolled is 11.37. The melting point is 327°C, and boiling point is 1750°C. The tensile strength is low, that of the rolled metal being about 25 MPa with elongation of 52% at normal temperatures, but at low temperatures the strength is greatly increased. At -40°C it is about 89 MPa, with elongation of 30%. The coefficient of expansion is 0.0000183, and the thermal conductivity is 8.2% that of silver. The electric conductivity is only 7.8% that of copper. When used in storage batteries, the metal is largely returned as scrap after a period and is remelted and marketed as secondary lead, as is also that from pipes and cable coverings. Lead is highly toxic and, thus, poses a health hazard. Inhalation of dust and fumes should be avoided and it should not be used in contact with food or drink products.
Not only is lead the most impervious of all common metals to x-rays and gamma radiation, it also resists attack by many corrosive chemicals, most types of soil, and marine and industrial environments. Although lead is one of the heaviest metals, only a few applications are based primarily on its high density. The main reasons for using lead often include low melting temperature, ease of casting and forming, good sound and vibration absorption, and ease of salvaging from scrap.
With its high internal damping characteristics, lead is one of the most efficient sound attenuators for industrial, commercial, and residential applications. Sheet lead, lead-loaded vinyls, lead composites, and lead-containing laminates are used to reduce machinery noise. Lead sheet with asbestos or rubber sandwich pads are commonly used in vibration control.
The natural lubricity and wear resistance of lead make the metal suitable, in alloys, for heavy-duty bearing applications such as railroad-car journal bearings and piston-engine crank bearings. Lead is also widely used as a constituent in solders. Most common solders are the lead-tin alloys; melting temperature can be as low as 183°C.
Forms
Sheet and Foil
Because of its malleability, lead and its alloys are readily rolled to any desired thickness down to 0.01 mm. Sheets are easily fabricated by burning or soldering. Standard widths run to 2.4 m or more for sheet and sheets may be cut to any desired size. Blanks for impact extrusion, gaskets, washers, or other purposes may be stamped out. Tin-coated lead can be produced by rolling lead and tin together.
Extrusions
Lead is easily extruded in the form of pipe, rod, wire, or any desired cross section like window cames (H-shaped), rounds, hollow stars, rectangular duct. Commercially available extrusions range in size from 612 mm pipe down to solder wire 0.25 mm in diameter. Lead is extruded over paper, rubber, or plastic in making electrical cable and around steel bars. Common flux-cored solder is a lead extrusion; toothpaste tubes are impact extrusions.
Castings
One of the simplest metals to cast, lead is used in tiny die castings and massive cast counterweights. Type metal, renowned for its ability to reproduce minute detail, is a lead alloy. Lead grids for most batteries are die-cast. Casting temperature (usually about 316°C) is moderate. Arsenic, antimony, or tin are frequently alloyed to impart strength or special properties. Small die castings can have wall thicknesses as low as 1.3 mm and "as-cast" dimensions are reproducible to 0.03 mm.
Coatings
Protection of underlying iron and steel is the main objective in most lead coating. In the purely protective class one finds terne-plate for roofing, fireproof frames and doors, automotive parts, and containers for paint and oil. Lubricity imparted by the coating cases drawing and stamping operations and produces an excellent surface for soldering — hence, television chassis and automotive gas tanks are made of terne. Other hot-dip processes as well as electroplating and flame spraying are also used for outdoor hardware automotive mufflers, bearings, bushings, nuts, bolts, and for maintenance as well.
Laminations
Developed originally for x-ray protection, a large family of laminated lead materials now exists. In addition to their original niche these are finding increasing use in sound isolation and noise control. Typical examples include lead-plywood, lead-gypsum board, lead-cinder block, leaded plastic-fabric laminates, leaded plastic, and glass-fiber combinations.
Cladding
Metallic lead in thicknesses from 3.2 to 305 mm or more may be bonded to other metals. Thus, for example, lead and steel may be combined for corrosion resistance and strength or lead and copper for gamma shielding and heat transfer. In many instances a product such as a tank or chemical reactor is completely or partially fabricated in steel and then clad with bonded lead as a unit.
Powder
Spheres, irregular grains, and flakes of lead from 4 |im diameter up find use in special greases, as a constituent of bearings, brake, and clutch facings, in filling plastics and rubber, and in paints and pile-joint compounds. Wire rope is usually treated with such a powder to lubricate it and to fill any nicks in the filter, thus renewing it with self-lubricating surfaces.
Shot
This form of lead is produced in abundance — about 30,360 x 103 kg go into shotgun ammunition each year. Ammunition sizes range from 1 to 11.3 mm; small shot is made for other uses. Easily handled, it is a preferred form when mass or shielding is required inside an irregular enclosure. It is also used in making free-machining steels.
Wool
By passing molten lead through a fine sieve and allowing it to solidify in the air, a loose rope of filters is produced. Under pressure, usually by being driven into a crevice with a calking iron and hammer, the fibers weld into a homogeneous mass. This permits the forming of a solid metal seal where temperature or explosion hazards prohibit jointing procedures requiring heat. Continuous lead fiber is also produced by being spun on textile machines.
Alloys
In its unalloyed form as 99.85% minimum, lead is soft and weak; it requires support for mechanical applications. This "chemical lead" is used primarily in corrosive chemical-handling applications such as tank linings.
"Hard lead" — lead alloyed with 1 to 13% antimony — has sufficient tensile strength, fatigue resistance, and hardness for many mechanical applications. These alloys can be cast, rolled, or extruded and are especially suited for castings requiring good detail and moderate strength. Rolled antimonial alloys are harder and stronger than the cast alloys. Battery-plate lead contains 7 to 12% antimony.
Calcium (0.03 to 0.12%) forms another series of mechanically suitable alloys with lead. These alloys age-harden naturally at room temperature — usually for 30 to 60 days — after being cast or worked. Properties of wrought Pb-Ca alloys are somewhat directional, being greater in the longitudinal direction. Uses include cable sheathing and grids in storage batteries.
Tin, added to Pb-Ca alloys in amounts to about 1.5%, raises tensile strength and stress-rupture resistance but increases aging time to 180 days. Tin is also used to reduce the coefficient of friction for bearing applications. Higher-tin-bearing alloys are primarily used in solders, which normally contain from 40 to 60% tin.
Lead alloys may exhibit greatly improved mechanical or chemical properties as compared to pure lead. The major alloying additions to lead are antimony and tin. The solubilities of most other elements in lead are small, but even fractional weight percent additions of some of these elements, notably copper and arsenic, can alter properties appreciably.
Cable-Sheathing Alloys
Lead is used as a sheath over the electrical components to protect power and telephone cable from moisture. Alloys containing 1% antimony are used for telephone cable, and lead-arsenical alloys, containing 0.15% arsenic, 0.1% tin, and 0.1% bismuth, for example, are used for power cable. Aluminum and plastic cable sheathing have replaced lead alloy sheathing in many applications.
Battery-Grid Alloys
Lead alloy grids are used in the lead-acid storage battery (the type used in automobiles) to support the active material composing the plates. Lead grid alloys contain 6 to 12% antimony for strength, small amounts of tin to improve castability, and one or more other minor additions to retard dimensional change in service. No lead alloys capable of replacing the lead-antimony alloys in automobile batteries have been developed. An alloy containing 0.03% calcium for use in large stationary batteries has had success.
Chemical-Resistant Alloys
Lead alloys are used extensively in many applications requiring resistance to water, atmosphere, or chemical corrosion. They are noted for their resistance to attack by sulfuric acid. Alloys most commonly used contain 0.06% copper, or 1 to 12% antimony, where greater strength is needed. The presence of antimony lowers corrosion resistance to some degree.
Type Metals
Type metals contain 2.5 to 12% tin and 2.5 to 25% antimony. Antimony increases hardness and reduces shrinkage during solidification. Tin improves fluidity and reproduction of detail. Both elements lower the melting temperature of the alloy. Common type metals melt at 238 to 246°C.
Bearing Metals
Lead bearing metals (babbitt metals) contain 10 to 15% antimony, 5 to 10% tin, and for some applications small amounts of arsenic or copper. Tin and antimony combine to form a compound that provides wear resistance. These alloys find frequent application in cast sleeve bearings, and are used extensively in freight-car journal bearings. In some cast bearing bronzes, the lead content may exceed 25%.
Solders
A large number of lead-base solder compositions have been developed. Most contain large amounts of tin with selected minor additions to provide specific benefits, such as improved wetting characteristics.
Free-Machining Brasses, Bronzes, Steels
Lead is added in amounts from 1 to 25% to brasses and bronzes to improve machining characteristics. Lead remains as discrete particles in these alloys. It is also added to some construction steel products to increase machin-ability. Only about 0.1% is needed, but the tonnage involved is so large that this forms an important use for lead.
Uses
Lead wool is lead in a shredded form used for calking. Sheet lead is produced by cold-rolling, and is used as a sound barrier in building construction.
Lead has a high capacity for the capture of neutrons and gamma rays and is used for radiation shielding in the form of sheet lead or as metal powder in ceramic mortars and blocks, paints, and in plastic composite structures. DS Lead is a dispersion-strengthened lead containing up to 1.5% lead monoxide evenly distributed through the structure. The oxide combines chemically with the lead, doubling the strength and stiffness of the metal, but increasing its brittleness. It is used for chemical piping and fittings. A neoprene-lead fabric is a neoprene fabric impregnated with lead powder. It has a radiation shielding capacity one-third that of solid lead sheet. It comes in thicknesses of 0.08 to 0.64 cm, and its flexibility makes it suitable for protective clothing and curtains. Shielding cements for x-ray and nuclear installation shielding are metallic mortars containing a high percentage of lead powder with ceramic oxides as binders and other elements for selective shielding. They are mixed with water to form plasters or for casting into sections and blocks. The formulation varies with the intended use for capture, attenuation, or dissipation of neutrons, gamma rays, and other radiation. Shielding paints are blended in the same manner.
Battery-plate lead for the grid plates of storage batteries and Silvium alloy for positive-plate grids are additional applications. Lead-coated copper used for roofing and for acid-resistant tanks as well as frangible bullets, which shatter on striking a target surface and are used for aerial gunner practice, are two other uses.
Antimonial lead is an alloy containing up to 25% antimony with the balance lead, used for storage-battery plates, type metal, bullets, tank linings, pipes, cable coverings, bearing metals, roofing, collapsible tubes, toys, and small cast articles. The alloy is also known as hard lead.
One hard lead has 10% antimony and 90% lead and melts at 252°C. Called cable lead, or sheathing lead, it is used to cover telephone and power cables to protect against moisture and mechanical injury. Terne-plate, as the coated steel is called, is widely used for automobile gasoline tanks and also has been used for roofing on buildings.
