(2) Stainless steel is an alloy and belongs to metal material.
(3) Through the analysis of four kinds of metal products, it can be known that the iron pot uses the thermal conductivity of metal.
(4) The recycling of scrap steel aims at saving metal resources.
(5) The use of iron pots is beneficial to human health, mainly because it can provide iron for human body. Keeping the surface of the iron pan clean and dry can prevent rust.
(6) Because aluminum reacts with oxygen to form a dense oxide film, the aluminum inside is protected, and metallic aluminum is not easy to be corroded when exposed to air.
So the answer is: (1) oxygen; Water; Pig iron; Steel (2)B(3)B(4)A(5) can provide iron for human body; Keep the surface of the iron pan clean and dry. (6) Aluminum reacts with oxygen to form a dense oxide film, which protects the aluminum inside.