How to control hardness in hard chrome plating?

Controlling hardness in hard chrome plating involves optimizing process parameters, bath composition, and post-treatment to achieve the desired hardness (typically 800–1000 HV). Here are the key methods:

 

Author: Anna

 

Controlling hardness in hard chrome plating involves optimizing process parameters, bath composition, and post-treatment to achieve the desired hardness (typically 800–1000 HV). Here are the key methods:

---

 

1. Bath Composition & Control

• Chromium Concentration:

  • Maintain CrO₃ (chromic acid) at 200–400 g/L for standard hard chrome.

  • Higher CrO₃ (e.g., 400 g/L) increases hardness but reduces deposition efficiency.

• Catalyst (Sulfate, SiF₆²⁻):

  • Sulfate (SO₄²⁻) ratio: CrO₃:SO₄²⁻ = 80:1 to 120:1 (critical for hardness).

  • Fluorosilicates (SiF₆²⁻) improve microhardness (up to 1200 HV) but increase brittleness.

• Trivalent Chromium (Cr³⁺):

  • Keep Cr³⁺ < 5 g/L (excess reduces hardness and adhesion).

---

 

2. Current Density & Temperature

• Current Density:

  • 30–60 A/dm² (higher current density → finer grain structure → higher hardness).

  • Too high (>60 A/dm²) causes roughness and hydrogen embrittlement.

• Temperature:

  • 50–60°C (lower temps increase hardness but reduce ductility).

  • >65°C softens deposits (due to larger grain formation).

---

 

3. Post-Plating Treatments

• Heat Treatment (Baking):

  • 200–250°C for 2–4 hrs relieves hydrogen embrittlement while retaining hardness.

  • Higher temps (>300°C) may soften chrome.

• Shot Peening:

  • Improves surface compressive stress, enhancing fatigue resistance without reducing hardness.

---

 

4. Impurity Control

• Metal Contaminants (Fe, Cu, Ni):

  • Remove via dummy plating or ion exchange (impurities reduce hardness).

• Organic Contaminants:

  • Use activated carbon filtration (organics cause porosity and lower hardness).

---

 

5. Alternative Hard Chrome Processes

• Hexavalent vs. Trivalent Chrome:

  • Traditional Cr⁶⁺ baths yield harder deposits (~1000 HV) than Cr³⁺ baths (~800–900 HV).

• Composite Chrome Plating:

  • Add SiC, Al₂O₃, or diamond particles to increase hardness (up to 1400 HV).

---

 

Troubleshooting Low Hardness

Issue	Solution
Soft deposits	Increase current density; lower bath temp; check sulfate ratio.
Brittle coating	Reduce SiF₆²⁻; bake for hydrogen relief.
Poor adhesion	Improve pre-cleaning; optimize Cr³⁺ levels.

---

 

Best Practices Summary

1. Strict bath control (CrO₃, sulfate, Cr³⁺).

2. Optimize current & temperature (50–60°C, 30–60 A/dm²).

3. Post-plate baking (200–250°C).

4. Avoid contaminants (metals/organics).

 

Note: Hardness testing (Vickers/Knoop) should be performed post-baking for accurate results.