Copper
Among industrial metals, it is the best conductor of electricity.
The corrosion of copper due to air exposure remains superficial, as it is coated by a protective layer of basic carbonate, called verdigris. When cold, it can oxidize only in the presence of acids; vinegar and acidic foods can thus lead to toxic salts, that is why cooking utensils made of copper are usually tinned. It can be attacked by concentrated sulphuric acid, and it dissolves readily in nitric acid, also diluted.
The main properties of pure copper, to which its industrial uses are due, are:
- The highest electrical conductivity among industrial metals (95% of the conductivity of silver, the most conductive metal). Therefore, it is used for cables and wires, electrical equipment (parts of motors, switches, contactors, etc.).
- Excellent thermal conductivity. Great for the construction of boilers, stills, heat exchangers, etc..;
- Sufficient resistance to atmospheric corrosion. It is used for piping, cladding of buildings (the formation of verdigris protects them against further oxidation). In order to improve certain characteristics (tensile strength and fatigue resistance, hardness, machinability, increase of recrystallization temperature, which may lead to better preserve the strain hardening, better corrosion resistance), it is added with certain elements in small amounts: 0.5% arsenic, or 0.1% silver, or 0.8% cadmium, or finally 0.5% tellurium, selenium or sulphur.
The various types of copper employed industrially are the following: common copper, for all normal applications which do not require high conductivity; deoxidized copper, generally with phosphorus, for applications that require welding or brazing (the presence of phosphorus gives copper good deoxidation qualities, but reduces its conductivity); copper electrolyte, used for its conductivity and purity (processing of high purity alloys); flame-refined copper (treatment of impure copper in a reverberatory furnace), currently used in mechanical and electrical construction for its good conductivity; OFHC (oxygen free high conductivity) copper, of American origin, oxygen-free and therefore valued for applications requiring very high conductivity, malleability, and weldability with glass (electronics, radio valves, etc..).
Copper alloys
Several elements are soluble in liquid or solid copper, and this has allowed the development of alloys with varied properties. The addition of other elements to copper reduces the electrical and thermal conductivity of some alloys; on the other hand, it allows better mechanical properties, higher castability in jets, given the lower melting point, and greater corrosion resistance.
The main copper alloys for industrial use are common and special bronzes, common and special brasses, other copper alloys (copper-beryllium, copper-chromium, copper-phosphorus, copper-magnesium, copper-manganese, copper-nickel, copper-lead, copper-silicon, copper-titanium, copper-vanadium), argentane. White copper is an alloy formed by copper, zinc, and arsenic.
Copper alloys are used in the most varied forms, which facilitates their use as foundry castings (finished or raw), or as hot or cold-worked products for forging, lamination, drawing, extrusion, stamping, etc.. (sheets, plates, tubes, rods, wires), or, finally, as finished pieces obtained by powder metallurgy processes.
Brass is a copper-zinc alloy. First we must distinguish between binary brass, consisting only of copper and zinc, and ternary brass, where a third element characterizes the alloy. As regards binary brass, there is an α phase when the Zn content is less than about 36%; the crystalline structure of the alloy is similar to the one of copper, that is, cubic with centred faces. This kinds of brass has excellent cold workability (stamping and moulding) and good hot workability. α-β brass (where the β phase is cubic with a centred body) has a zinc percentage between 36 and 45%; it is suited for hot working. The alloys with Zn percentages above 45% have no practical interest.
The machinability of brass is good, but its toughness causes the formation of very long chips; this is why lead is added to it, because it is insoluble and foreign to the crystalline structure, and then it is dispersed to the edges of the grains: that way, the chips become very short or even dusty and tools undergo minor wear and heating, with consequent improvement of the quality and speed of processing. Leaded brass is also called free machining brass.
Consonni srl makes brass chromium plating.