go bad vs decay

go bad is not a noun while decay is a noun.

Word	Noun	Adjective	Verb	Adverb
go bad	No	No	Yes	No
decay	Yes	No	Yes	No

As verbs, decay is a hypernym of go bad; that is, decay is a word with a broader meaning than go bad:

go bad (noun)	decay (noun)
	the organic phenomenon of rotting
	a gradual decrease; as of stored charge or current
	the process of gradually becoming inferior
	the spontaneous disintegration of a radioactive substance along with the emission of ionizing radiation
	an inferior state resulting from the process of decaying

go bad (verb)	decay (verb)
become unfit for consumption or use	fall into decay or ruin
stop operating or functioning	lose a stored charge, magnetic flux, or current
	undergo decay or decomposition