Are export tariffs illegal in the United States?

yes, however, it is legal if congress gives consent.

Explanation:

Article I, § 10, clause 2 of the United States Constitution, known as the Import-Export Clause, prevents the states, without the consent of Congress, from imposing tariffs on imports and exports above what is necessary for their inspection laws and secures for the federal government the revenues from all tariffs on imports and exports. Several nineteenth century Supreme Court cases applied this clause to duties and imposts on interstate imports and exports. In 1869, the United States Supreme Court ruled that the Import-Export Clause only applied to imports and exports with foreign nations and did not apply to imports and exports with other states, although this interpretation has been questioned by modern legal scholars.