Zinc-Nickel vs. Cadmium Plating: Which Should You Specify?
How does corrosion protection compare?
Both coatings protect steel sacrificially — they corrode preferentially, so the substrate is protected even at scratches and cut edges. Chromated cadmium per AMS-QQ-P-416 Type II must show no white corrosion after 96 hours of neutral salt spray, with red-rust protection scaling with thickness class. High-alloy zinc-nickel (12–16% Ni) per AMS 2417 with a trivalent passivate is typically specified at 500 hours without red rust, and good deposits test out to 720–1,000 hours. On raw salt-spray numbers, zinc-nickel wins, and it also holds up better at elevated temperatures, where cadmium’s protection falls off above roughly 450°F.
What about hydrogen embrittlement?
Cadmium plating is an embrittling process. AMS-QQ-P-416 requires steels at HRC 40 and above to be baked at 375°F for 23 hours, starting within 4 hours of plating — a hard constraint on lead time and a permanent risk item on landing gear-class hardware. Alkaline zinc-nickel is a low-hydrogen-embrittlement (LHE) process, one of the main reasons aerospace qualified it as the cadmium replacement. High-strength parts still get a relief bake per ASTM B850, but the intrinsic hydrogen load is lower.
Where do RoHS and environmental rules come down?
Cadmium is a toxic heavy metal, restricted under RoHS, ELV, and REACH; it survives only under aerospace/defense and certain electrical exemptions. Zinc-nickel with trivalent passivate is fully RoHS compliant. If your product has any commercial or export exposure, zinc-nickel is the safe long-term callout; cadmium locks you into exemption-dependent supply.
Is zinc-nickel a true drop-in replacement for cadmium?
Mostly, with known gaps. Zinc-nickel matches or beats cadmium on corrosion, temperature, and embrittlement risk. Where it falls short: cadmium’s natural lubricity gives more consistent torque-tension behavior on threaded fasteners (zinc-nickel usually needs a supplementary lubricant or topcoat to match), and cadmium’s galvanic compatibility with aluminum is slightly better, though high-alloy zinc-nickel is close. Solderability also favors cadmium. “Drop-in” is true for most brackets, housings, and structural hardware; fasteners and electrical contacts deserve an engineering look rather than a global find-and-replace.
When is cadmium still the right call?
Three cases. First, legacy drawings: if the part is qualified with cadmium and requalification is impractical, you run cadmium. Second, threaded fasteners where torque-tension consistency is flight-critical and the joint was engineered around cadmium’s friction behavior. Third, connectors and electrical hardware where cadmium’s low, stable contact resistance and solderability are baked into the design. Outside those, new designs should specify zinc-nickel.
How do costs compare?
Per-part plating costs are broadly comparable; zinc-nickel chemistry costs more to operate, while cadmium carries heavier waste-treatment, compliance, and (for HRC 40+ steel) 23-hour bake overhead. The bigger cost driver is availability: cadmium capacity keeps shrinking as shops exit the chemistry, which shows up as longer lead times and freight to distant suppliers. That scarcity, not the rack price, is what usually hurts.
