rubbish vs scrap metal

rubbish and scrap metal both are nouns.

Word	Noun	Adjective	Verb	Adverb
rubbish	Yes	No	Yes	No
scrap metal	Yes	No	No	No

As nouns, scrap metal is a hyponym of rubbish; that is, scrap metal is a word with a more specific, narrower meaning than rubbish:

Other hyponyms of rubbish include debris, detritus, dust, junk, rubble, litter.

rubbish (noun)	scrap metal (noun)
nonsensical talk or writing	discarded metal suitable for reprocessing
worthless material that is to be disposed of

rubbish (verb)	scrap metal (verb)
attack strongly