Beyond A4/316: Why Bimetal Screws Are the New Standard for PV and Coastal Projects

When Can A4/316 Stainless Fall Short?

Let’s be clear: A4/316 is a good material. But “good” isn’t always “good enough.” In a true coastal or offshore environment, you’re dealing with more than just moisture; you’re dealing with a constant mist of chlorides. This salt attacks the protective layer on stainless steel. Combine that with the constant, high tension a pre-loaded bolt is under in a PV racking system, and you create the perfect conditions for stress corrosion cracking (SCC). SCC often shows little surface rust before it causes a sudden, brittle fracture. Relying solely on A4/316 in these spots is a gamble on your structure’s long-term health.

How Do Bimetal Screws Address These Issues?

Think of a bimetal screw as getting the best of both worlds by design. It’s not a coating or a plating. It’s a carbon steel core—the same high-strength, predictable material used in critical structural bolts—permanently clad in a sleeve of stainless steel.

This does two critical things:

• Reliable strength: The core is typically Grade 8.8 or 10.8 steel. It provides the solid, elastic stretch needed to maintain clamp force, something that lower-strength solid stainless bolts can struggle with.

• Corrosion resistance where it matters: The stainless cladding forms a continuous barrier on all exposed surfaces, protecting the core from pitting and crevice corrosion.

You’re not using expensive stainless for the whole bolt, just where it counts. This makes it a cost-smart solution for high-risk applications.

When and Where Should You Use Bimetal Screws?

You don’t need bimetal for every job, but it’s strongly recommended for:

• Solar installations within 5km of a coastline, especially single-axis trackers under constant mechanical stress

• Any permanent structure in the splash zone—piers, boardwalks, seaside railings

• Industrial equipment in chemical or fertilizer processing plants with aggressive atmospheres

If a project’s manual mentions “C5-M” corrosivity categories or requires a 25+ year service life with minimal maintenance, that’s your cue.

How Should Buyers Vet a Bimetal Supplier?

The term “bimetal” isn’t tightly regulated. To verify quality, demand:

• Core’s Mill Certificate: Must check tensile strength, yield strength, and grade.

• Proof of Bonding: Ask how the stainless is bonded to the core; co-extrusion is reliable.

• Traceability: Heat numbers on the MTC should match packaging and documentation.

• Sample Testing: Conduct a salt spray test alongside an A4/316 screw to verify performance.

Conclusion

Choosing fasteners for harsh environments is no longer about picking a material from a catalogue. It’s about matching engineered solutions to real-world risks. Bimetal screws aren’t a magic fix for everything, but for high-stakes intersections of strength and severe corrosion, they are a targeted answer. As a buyer, demand proof of core strength, cladding integrity, and traceability. That’s how you move from hoping a connection lasts to knowing it will.

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