PV mounting systems are exposed outdoors for decades, so fasteners must meet strength, corrosion and anti-loosening requirements at the same time.
In materials, carbon steel class 8.8+ with hot-dip galvanizing, or stainless steel A2/A4, are common choices; for coastal and highly corrosive environments, 316 stainless steel or Dacromet is recommended.
Selection should consider local wind loads, mounting structure and design life, and be confirmed with the mounting manufacturer.