Question

why most of the metals lose their lusture when exposed to air and moisture for a lond time

Answer 1

the top layer of metal is removed
when exposed to air because they get corroded. the moisture in the air reacts with metal to form oxide or sulphides or carbonates.

Answer 2

When metals exposed to air and moisture for longer times then their outer surface gets oxidised and forms oxide layer ion the surface of metals. Because of this oxide layer on the surface metal appears with dull or less shiny.  When cleaned these metal surfaces with tamarind or mild acidic chemicals then this oxide layer get removed thus metal surface again appears as shiny.                                                           #Hope it helps you