In general the Shia jurisprudential ruling is that a Muslim can marry any other Muslim as long as they are Muslims (except a Naasibi who shows enmity towards Ahlul Bait a.s).

This means that a Shia(male or female) can marry a Sunni, however the condition is that the Shia is able to preserve their Shia belief and be free to practice their Shia belief and not be negatively influenced by their partner. As for a Salafi in general, they have a strong tendency against the Shia in which case things could come to surface during the relationship which could make it difficult for the Shia spouse. 

In a nutshell, due to extreme possibility of perversion from the right path of Ahlul Bait a.s. in such a case for the Shia woman, the well known scholars won't permit such a marriage unless she finds herself so firm in beliefs and knows for sure that she won't change.