What Is Zinc Plating?
Zinc plating (also called zinc electroplating) deposits a thin layer of zinc onto steel parts through an electrochemical bath. The zinc serves as a sacrificial barrier — it corrodes before the steel does, protecting the base metal even if the coating is scratched or damaged.
It’s the most common corrosion protection for steel fasteners, brackets, stampings, and machined parts. If you’ve ever bought a bolt from a hardware store that has a shiny silver or gold finish, it’s zinc plated.
How It Works
Parts are cleaned (alkaline soak, acid pickle), then immersed in an electrolyte bath containing dissolved zinc salts. Electric current causes zinc to deposit on the part (cathode). After plating, a chromate conversion coating is applied for additional protection and color.
Chromate Conversion Coatings
The chromate step after plating is critical — it dramatically increases corrosion resistance and determines the final color. Think of zinc as the armor and chromate as the sealant.
| Chromate Type | Color | Salt Spray (ASTM B117) | RoHS | Notes |
|---|---|---|---|---|
| Clear / Blue | Bright silver-blue | 8–24 hours to white rust | Yes (trivalent) | Most common. Decorative. |
| Yellow / Gold (Hex) | Iridescent gold | 96–200 hours | No (hexavalent) | Best protection. Being phased out. |
| Yellow (Trivalent) | Light gold | 72–120 hours | Yes | RoHS replacement for hex yellow. |
| Black | Black | 24–72 hours | Yes (trivalent) | Cosmetic. Lower protection. |
| Olive Drab | Military green | 96–200 hours | No (hexavalent) | Mil-spec applications. |
Zinc Plating vs. Other Zinc Coatings
| Process | Thickness | Method | Best For |
|---|---|---|---|
| Zinc Electroplating | 0.2–1.0 mil | Electrolytic bath | Small parts, fasteners, precision |
| Hot-Dip Galvanizing | 3–5 mil | Dip in molten zinc | Structural steel, outdoor |
| Mechanical Galvanizing | 0.5–3.0 mil | Tumble with zinc dust | Fasteners, avoids hydrogen embrittlement |
| Zinc-Nickel Plating | 0.3–0.8 mil | Electrolytic (Zn + Ni alloy) | Automotive, high-temp, superior corrosion |
| Sherardizing | 0.5–2.0 mil | Tumble in zinc dust at ~700°F | Small parts, uniform coating |
Hydrogen Embrittlement
This is the hidden danger of zinc plating. During the acid pickling and electroplating process, atomic hydrogen can diffuse into high-strength steel and cause catastrophic brittle failure — sometimes hours or weeks after plating.
Rule of thumb: Any steel part with hardness above HRC 39 (approximately 180 ksi tensile) requires a hydrogen embrittlement relief bake within 4 hours of plating — typically 375°F for 4–24 hours per ASTM B850.
This is why high-strength bolts (Grade 8, Class 12.9) are often mechanically galvanized instead of electroplated — the process doesn’t generate hydrogen.
Specifications
- ASTM B633 — Standard specification for electrodeposited zinc coatings on iron and steel
- ASTM F1941 — Zinc-coated fastener spec (replaces older IFI specs)
- ASTM B850 — Post-plating hydrogen embrittlement bake
- QQ-Z-325 — Federal spec (older, still referenced)
- AMS 2402 — Aerospace zinc plating
When to Specify Zinc Plating
- Mild steel parts needing basic corrosion protection
- Fasteners and hardware (most common application)
- Indoor or mild outdoor environments
- When a clean, bright appearance is desired
- Budget-conscious — zinc plating is one of the cheapest metal finishes
When to Choose Something Else
- Marine/harsh outdoor — use hot-dip galvanizing or zinc-nickel
- Aluminum parts — zinc plating doesn’t work on aluminum (use anodize or chromate conversion)
- High-strength steel (>HRC 39) — consider mechanical plating or alternative coatings to avoid embrittlement risk
- High-temperature — zinc melts at 787°F. Use nickel or cadmium for high-temp applications.
Need parts zinc plated? Get a quote — we work with multiple plating shops in the DFW area.