Is sex chat considered adultery, does it require expiation, and what is the ruling of religion on this?

The relationship between a man and a non-mahram woman via chat is forbidden. It is a type of zina (fornication/adultery) that does not necessitate the prescribed punishment (hadd). The zina of the eyes is looking, the zina of the ears is listening, the zina of the tongue is speaking, the zina of the hand is striking/touching, and the zina of the feet is walking (towards sin). There is no expiation for this act other than sincere repentance.