Can we chant mantras during Surya Grahan?

Yes — mantra chanting is not only permitted but strongly encouraged throughout the entire Grahan period. The merit of mantra repetition during a Grahan is described in the Puranas as equal to months of regular practice. Recommended mantras include the Surya Mantra (Om Hraam Hreem Hraum Sah Suryaya Namah), the Gayatri Mantra, the Mahamrityunjaya Mantra, and the Hanuman Chalisa.