Copper alloys transfer heat well but pure copper is a soft metal, prone to dings and dents. For this reason, other trace metals are added in, making it harder. The type of copper alloy used is also dependent on the fluid being used. For example, for sea water, an alloy called CuSn7Zn2Ni5 is used. This means the metal is predominately copper, 7 percent tin (Sn), 2 percent zinc (Zn) and 5 percent nickel (Ni). The downside of copper is it is not corrosion resistant, making it unsuitable for high corrosive fluids like highly acidic water.
Steel is cheap, relatively easy to form and conducts heat well. The downside is it very rust prone, and is unsuitable for even mildly corrosive fluids like water with high mineral content. High carbon, or hard steel, is recommended by engineers for crude oil applications.
Aluminum is plentiful, lightweight and easily formed, but it is very soft, so other metals --- such as nickel, iron and bronze ---are added to the molten aluminum to make an alloy.
Stainless steel is steel with chrome and nickel added in to make it rust proof. A special type of stainless steel, called 410, is recommended for heavy gases and oils containing gases.