Why High-Strength Fasteners Must Upgrade from "Zinc Plating" to "Zinc-Nickel"

The most traditional fastener corrosion protection is electrogalvanizing: electrodeposit a layer of pure zinc onto the steel, relying on zinc being more active than iron—the "sacrificial anode" effect—to protect the steel substrate. But a pure zinc layer is thin with loose corrosion products; under harsh in-vehicle, outdoor and marine-atmosphere conditions, red rust often appears within tens to a hundred-plus hours. As NEV requirements for chassis, battery tray and electric-drive housing joint life rise to match the whole vehicle's life, pure zinc is increasingly insufficient.

Zinc-nickel alloy plating (Zn-Ni alloy plating) introduces nickel into the zinc bath so the deposit becomes a zinc-nickel intermetallic compound. Adding nickel brings two essential changes: first, the coating's electrode potential is more negative than pure zinc and more resistant to self-corrosion; second, the corrosion products are denser, forming a barrier layer that keeps corrosive media out. YF Zhichengjia's surface-treatment system already covers electrogalvanizing, hot-dip galvanizing, PVD, Dacromet and high-end LC coating across all categories; zinc-nickel plating is one of the core processes aimed at automotive-grade corrosion needs, with no heavy-metal additives in the raw materials to meet RoHS/REACH export compliance.

Corrosion Mechanism and Salt-Spray Performance of Zinc-Nickel Coatings

The most universal method for evaluating coating corrosion resistance is the neutral salt spray test (NSS), run per GB/T 10125-2012 Corrosion Tests in Artificial Atmospheres - Salt Spray Tests. This standard defines three methods: neutral salt spray (NSS), acetic acid salt spray (ASS) and copper-accelerated acetic acid salt spray (CASS), continuously spraying a 5% NaCl solution and periodically observing the time to red rust on the specimen surface to rate protection. When engineers say "how many salt-spray hours", they default to the time to first red rust in NSS.

At the same film thickness, zinc-nickel corrosion resistance is far superior to pure zinc: pure blue-white zinc plating usually shows red rust at 96–120 h, while a zinc-nickel coating after passivation and sealing typically reaches 720–1000+ hours without red rust—several times that of pure zinc. It must be emphasized that this gap comes mainly from nickel content and post-treatment (passivation, sealing): a nickel content controlled at 12%–15% by weight gives the best corrosion resistance; if the coating is too thin or sealing is poor, salt-spray time drops markedly.

Hydrogen Embrittlement: The Most Dangerous Side Effect of Plating High-Strength Bolts

Electroplating is an electrochemical process. During acid pickling/degreasing and plating, hydrogen ions are reduced on the steel surface to atomic hydrogen that diffuses into the metal. High-strength steel (tensile strength ≥1000 MPa, i.e. 10.9 grade and above) is extremely sensitive to hydrogen—diffused hydrogen collects at stress-concentration locations such as thread roots and, under sustained preload, triggers delayed fracture (hydrogen-induced cracking), often breaking suddenly hours to days after assembly with no prior warning. This is why plating high-strength bolts is called the industry's "most dangerous process side effect".

For this reason, in situations extremely sensitive to hydrogen embrittlement, engineers turn instead to hydrogen-embrittlement-free dry coating routes—for example PVD physical vapor deposition, Dacromet (zinc-aluminum coating) or DLC diamond-like carbon coating. YF Zhichengjia's LC coating technical literature explicitly marks PVD/DLC as "eco, hydrogen-embrittlement-free" processes, precisely as an alternative to the plating route for high-strength, high-reliability service.

How to Choose Among Zinc-Nickel, Pure Zinc and Dacromet

Fastener corrosion protection is not "the thicker the better"; it is a trade-off among service condition, strength grade, embrittlement sensitivity and cost. The table below puts the three routes most often compared in engineering side by side:

A key difference is embrittlement sensitivity: zinc-nickel and pure-zinc plating are both electrolytic processes and both face the high-strength-steel de-embrittlement problem, whereas Dacromet/zinc-aluminum coating is a dip-coating-plus-baking non-electrolytic process that introduces essentially no hydrogen, making it friendlier to 12.9-grade and above ultra-high-strength bolts—at the cost of a relatively thick coating that has some effect on thread go/no-go gaging.

Selection Recommendations and Process-Control Key Points

• 8.8 grade and below, general indoor service: pure zinc + passivation is enough; lowest cost.
• 10.9-grade automotive high-strength bolts, high in-vehicle/outdoor corrosion demands: prefer zinc-nickel plating + chrome-free passivation sealing, and strictly run post-plating de-embrittlement baking.
• 12.9 grade and above, extremely embrittlement-sensitive: evaluate switching to zinc-aluminum coating (Dacromet) or PVD/DLC hydrogen-embrittlement-free routes.
• Corrosion acceptance: run neutral salt spray per GB/T 10125-2012, using time to first red rust as the criterion, and require batch reports on salt spray, film thickness and adhesion.
• Friction matching: the coating changes thread friction coefficient; confirm the tightening process with a torque-clamp-force test to avoid preload spread within the same bolt batch.