In Greek mythology, it was believed that the river Styx would corrode everything but what?

Styx, in Greek mythology, is one of the rivers of the underworld. The word styx literally means 'shuddering' and expresses loathing of death. In Homer’s 'Iliad' and 'Odyssey', the gods swear by the water of the Styx as their most binding oath. According to Hesiod’s 'Theogony', if a god perjured himself, he was rendered insensible for a year and then banished from the divine society for nine years.

Hesiod personified Styx as the daughter of Oceanus and the mother of Emulation, Victory, Power, and Might. Perhaps because of its similarity to Hesiod’s description in 'Theogony', the Styx later was identified with the stream now called Mavronéri (Greek: 'Black Water') near Nonacris in the Aroania Mountains (near modern Sólos) in Arcadia.

The ancients believed that the river’s water was poisonous and would dissolve any vessel containing it except one made of the hoof of a horse or an ass. There is a legend that Alexander the Great was poisoned by Styx water. In another legend, mentioned by the Roman poet Statius (1st century AD), Thetis dipped her son Achilles into the Styx to render him invulnerable; because she held him by his heel, he remained vulnerable there.