PetroStreet

Explain me what role Mo plays in steels used in petrochemicals?

Cr gives stainless steel its basic passivity; Mo makes that passive layer much more resistant to chloride attack, especially pitting and crevice corrosion.
In stainless steels, there is a commonly used measure:

PREN=%Cr+3.3(%Mo)+16(%N)

Pitting resistance depends heavily on Mo presence.

Pitting resistance gets improved with Mo.
There are stainless steels without Mo as well having even higher percentages of Cr & Ni utilized for high temperature application having no issues of getting pitting corrosion in service.