It is the most commonly used industrial gas. Nitrogen is used to treat the melt in the manufacture of steel and other metals and as a shield gas as a heat of iron, steel and other metals. It is used as a process gas, together with other gases for reduction of carbonization and nitriding. Refineries, petrochemical plants and marine tankers use to purge equipment, tanks and pipelines of dangerous vapors and gases and to maintain an inert and protective atmosphere in tanks storing flammable liquids. Nitrogen is used an inert gas to push liquids though lies, to clear pipelines and sweep out material/s before using the line to transport another material. Nitrogen food grade is use to freeze and store food items. It is used to extend the shelf life of packaged foods by preventing oxidation, mold, insect infestation and moisture migration. Also, nitrogen is used as a shield gas in the packing of some medicines to prevent degradation by oxidation or moisture absorption. Nitrogen can also be used as coolant for severe environmental testing, or as a refrigeration source for chilling circulating dry air.