decay vs crumble

decay is a noun but crumble is not a noun.

Word	Noun	Adjective	Verb	Adverb
decay	Yes	No	Yes	No
crumble	No	No	Yes	No

As verbs, decay and crumble are synonyms defined as:

Other synonyms of decay include dilapidate.

decay (noun)	crumble (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

decay (verb)	crumble (verb)
fall into decay or ruin	break or fall apart into fragments
lose a stored charge, magnetic flux, or current	fall into decay or ruin
undergo decay or decomposition	fall apart