Yes, EROSION is a Scrabble word!

Letter values in Scrabble US: E 1, R 1, O 1, S 1, I 1, O 1, N 1.

Is it a valid word?

EROSION is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

(geology) the mechanical process of wearing or grinding something down (as by particles washing over it)
condition in which the earth's surface is worn away by the action of water and wind
a gradual decline of something
erosion by chemical action

Also: eating away, wearing away.