Stainless‑steel fasteners are widely applied in automation, chemical‑processing, coastal equipment, solar and medical devices. SUS304, SUS316L and SUS317L are mainstream grades. Many buyers simply select SUS304 for all scenarios, yet pitting and crevice‑corrosion failures frequently occur under chloride‑ion, acid‑base and industrial‑exhaust conditions. Understanding material boundary and matching service environment prevents premature failure.
SUS304 (06Cr19Ni10) is general‑purpose austenitic stainless steel without molybdenum. It performs well under dry‑indoor, normal atmospheric and clean‑water conditions with cost advantage. Its weakness lies in poor chloride resistance. Chloride ions break passivation film and trigger pitting under salt‑spray or sea‑water splash. Surface rust stains may appear under acid‑rain and heavy‑dust conditions. Suitable for inland equipment and non‑coastal outdoor projects. Not recommended within 10 km from coastline.
SUS316L (022Cr17Ni12Mo2) contains 2‑3% molybdenum with low‑carbon formula. Molybdenum greatly improves chloride‑pitting resistance and organic‑acid tolerance. It becomes mainstream for coastal and mild‑chemical scenarios such as seaside machinery, coastal PV and fishery equipment. SUS316L is not rust‑proof forever; heavy chloride or continuous seawater immersion will still cause corrosion. Low‑carbon “L” grade reduces inter‑granular corrosion risk for welding and high‑temperature service.
SUS317L (022Cr19Ni13Mo3) owns higher molybdenum, nickel and chromium content than SUS316L. It delivers superior performance under heavy salt‑spray and mild‑chemical media, widely used for marine hardware and seawater‑desalination equipment. Higher cost restricts its usage; ordinary coastal projects do not need over‑specification to avoid unnecessary expense.
Two typical corrosion modes: Pitting forms local micro‑holes induced by chloride accumulation; the component may look intact externally. Crevice‑corrosion happens inside narrow gaps between bolt, washer and profile where moisture and salt get trapped. Even SUS316L suffers crevice‑corrosion without proper sealing.
Common engineering pitfalls: Using SUS304 for seaside projects leads to early pitting; misunderstanding SUS316L for continuous seawater immersion; mixing SUS316L bolts with SUS304 nuts/washers brings galvanic corrosion; counterfeit SUS201 masquerades as SUS304 with poor anti‑corrosion performance.
Installation notes: Bolts, nuts and washers shall adopt identical material grade. EPDM gaskets are suggested for gap sealing. Control assembly torque to avoid stainless‑steel thread galling; anti‑seize compound is recommended. PMI spectrometer inspection is suggested for incoming lots to prevent material mixing‑up.
Quick reference for grade selection: Dry‑indoor equipment → SUS304; inland non‑coastal outdoor → SUS304; within 10 km from coastline → SUS316L; intermittent seawater contact & mild‑chemical environment → SUS317L or higher‑grade alloy. Continuous seawater immersion requires duplex‑steel or special‑alloy evaluation instead of ordinary austenitic stainless‑steel.
Stainless‑steel does not mean zero‑rust. Corrosion‑resistance varies by grade. Material certificate and proper grade selection based on chloride concentration, temperature and gap condition guarantee long‑term service life.