PV racking runs outdoors for decades; bolt corrosion grade and installation torque directly decide structural safety and service life. Selection follows the site corrosion environment, profile type and module fixing method. This guide covers common corrosion systems, torque and locking points—final values per racking maker specs and project design.
Standardize fastener specs and corrosion grade within a project to cut mixing and field errors, easing quality control.
Outdoor racking bolts use hot-dip galvanizing, dacromet and stainless. HDG serves general outdoor use with life tied to coating thickness per standard; dacromet is hydrogen-safe with high grades for high-strength and coastal projects; SUS304/316 stainless suits coastal and aggressive sites, SUS316 resisting chlorides best.
Coastal projects should prefer SUS316 or high-grade dacromet over plain zinc. Watch galvanic corrosion between dissimilar metals—use insulating washers where needed.
Racking bolts are mainly M8, M10 and M12 at grade 8.8. Reference torque ranges roughly M8 about 20-25 N·m, M10 about 40-50 N·m, M12 about 70-80 N·m; friction varies by coating and lubrication, so correct torque accordingly per maker specs.
Use calibrated torque wrenches, tighten in the specified sequence and steps to keep preload even, and sample-check critical joints with records for acceptance.
Clamp bolts fix module frames; low torque loosens clamps and slides modules, high torque damages frames or profiles. Match clamp size to frame height; adjust torque compensation with rubber pads per maker advice.
Bifacial modules and windy sites need clamp force checks—more or wider clamps as needed—with post-install contact checks.
Wind and thermal cycling put cyclic loads on bolts, so locking matters. Options include spring washers, serrated lock washers, nylon lock nuts and pre-applied locker; combine on critical joints. Vibration-test methods support validation.
During O&M inspection, check rust and loosening; retighten to spec, replace corroded fasteners promptly to keep the system safe.
Project fasteners come with material certificates, coating/salt-spray reports and torque references per batch for EPC and owner acceptance. Critical projects may add third-party checks on coating thickness, hardness and salt spray.
Specify corrosion grade, strength grade and torque when buying; suppliers produce and inspect per standards for batch consistency and long-term service.