Chromium plating encompasses two fundamentally different processes serving distinct engineering functions. Hard chrome provides wear resistance and dimensional restoration for industrial components, while decorative chrome delivers aesthetic appeal with mild corrosion protection. Understanding the differences is critical for specification and regulatory compliance.
Hard Chromium Plating
Hard chrome (functional chrome) deposits thick chromium layers (typically 20–500 µm) directly onto the substrate for engineering purposes. Key characteristics:
- **Hardness**: 850–1050 HV (Vickers), among the hardest electrodeposited coatings available\n- **Wear resistance**: Excellent abrasion and erosion resistance. Coefficient of friction 0.12–0.16 against steel\n- **Temperature resistance**: Maintains hardness up to approximately 425°C\n- **Applications**: Hydraulic cylinder rods, piston rings, plastic injection mold surfaces, industrial rollers, aerospace landing gear
The process uses hexavalent chromium (CrO₃) baths at 50–60°C with high current densities (30–60 A/dm²). Bath chemistry is typically 250 g/L CrO₃ with catalyst systems (fluoride or mixed catalyst). Deposition rates are relatively slow at 25–50 µm/hour.
Decorative Chromium Plating
Decorative chrome deposits a very thin chromium layer (0.2–0.8 µm) over copper and nickel undercoats. The total system thickness including undercoats is typically 30–50 µm. The chromium layer provides tarnish resistance and a distinctive bright appearance.
The typical layer structure is: substrate → copper strike (5–10 µm) → semi-bright nickel (15–25 µm) → bright nickel (5–10 µm) → microporous or microcracked chromium (0.2–0.5 µm). This multi-layer system provides corrosion protection through the electrochemical relationship between nickel layers.
REACH Regulation and Hexavalent Chromium
Hexavalent chromium compounds (Cr6+) are classified as carcinogenic, mutagenic, and toxic to reproduction under EU REACH Regulation. Since September 2017, their use requires specific Authorization under REACH Annex XIV.
**Current status**: Many hard chrome operations in the EU operate under time-limited REACH Authorizations. Companies must demonstrate that socioeconomic benefits outweigh risks and that suitable alternatives are not yet available for their specific application.
Trivalent Chromium Alternatives
Trivalent chromium (Cr3+) processes are the primary replacement technology:
- **Decorative Cr3+**: Mature technology, widely adopted. Achieves comparable appearance to Cr6+ processes. Slightly different color tone (slightly darker/bluer) that some markets prefer.\n- **Functional Cr3+**: Still under development for full replacement of hard chrome. Current Cr3+ functional coatings achieve 700–900 HV — approaching but not yet matching Cr6+ hard chrome performance.\n- **HVOF thermal spray**: Tungsten carbide-cobalt (WC-Co) coatings applied by High Velocity Oxy-Fuel spraying achieve 1100–1400 HV and are replacing hard chrome for many applications including landing gear and hydraulic rods.
Specification and Testing
Hard chrome specifications typically reference ISO 6158 (engineering chromium coatings) or industry-specific standards (AMS 2406 for aerospace). Testing includes: thickness measurement (EN ISO 2177 by coulometric method), hardness (EN ISO 4516 micro-indentation), adhesion (thermal cycling or bend testing), and surface finish (EN ISO 4287 profilometry).