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Symbol: Cr
Atomic number: 24
Atomic weight: 51.996
Classification: Metal
Crystal structure: Body Centered Cubic (bcc)
Characteristic: Hard, brittle, silvery gray metal.
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Hard chromium (chrome) is a functional relatively thick coating deposited on a metallic substrate (commonly steel) in order to impart particular surface properties:
Hard chromium is also used for restoring undersized (worn, eroded. mis-machined) surfaces.
Thickness of hard chromium depositions may vary in the range 0.1-10 mil (2.5-250 μm).
Hard chromium is thicker than Decorative chromium. Additionally hard chromium is normally crack-free in contrast to decorative chromium depositions, which have micro-discontinuity (micro-porous, micro-crack) structure.
Typical applications of hard chromium Electroplating:
Hexavalent chromium (Cr6+) baths are used for hard chromium deposition.
The main component of all hard chromium plating solutions is chromium trioxide (CrO3) referred also as chromic acid. The second component is a catalyst, which is either sulfate (SO42-) or fluoride.
By-product of the electroplating process in hexavalent chromium solutions is trivalent chromium (Cr3+). Ions of trivalent chromium continuously reoxidize to the hexavalent state at the anode. Normal level of the trivalent chromium is about 1-2% of the chromic acid concentration. Higher contents of trivalent chromium may cause reduction of throwing power and plating rate, pitting and treeing of the deposit. If the trivalent chromium is too high (more than 2%) reoxidation operation should be carried out at high anode area/cathode area ratio (30) at cathode current density 20 A/ft² (2 A/dm²).
Cathode current efficiency of hard chromium electroplating is low: about 10-20%.
80-90% of the electric current passing between the anode and the cathode is used for gaseous Hydrogen formation.
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In this process chromic acid is catalyzed by sulfate ions (SO42-).
Chromic acid/sulfate ratio is one of the most important process parameters. It varies within the range 125 - 200.
Low ratio solutions are characterized by high plating rate but low throwing power. High chromic acid/sulfate ratio may cause gray or even “burnt” deposition on high current density areas. Normally plating solutions with chromic acid/sulfate ratio 155 are used.
Bath formulation:
Chromic acid (CrO3): 20-35 oz/gal (150-263 g/l);
Sulfate (SO42-): 0.13-0.23 oz/gal (1-1.73 g/l). Source of sulfate ions is sulfuric acid.
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In this process chromic acid is catalyzed by a mixture of sulfate and fluoride ions.
Fluoride bath have higher than sulfate baths current efficiency. Additionally fluoride baths may operate at higher current density not causing burning and treeing. As a result plating rate in fluoride baths may be 50% higher than in conventional sulfate catalyzed baths. Fluoride ions are chemically active and may attack the unplated surfaces. In order to prevent etching of the areas, which are not to be plated, they should be masked.
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High temperatures are used for deposition of crack-free chromium coating. Increase of the bath temperature causes reduction of the plating rate. Excessive bath temperature may result in formation of soft dull deposit.
Too low current density decreases economical effectiveness of the process. Too high current density may cause “burning”. Optimal current density is determined by the bath temperature: higher temperatures require higher current densities.
Properly operating anodes are coated with dark brown lead peroxide. If an anode has a lighter color (yellow-orange) it should be cleaned. Cleaning operation is immediately followed by immersion of the anode connected to the operating power supply into the bath. This operation results in formation of conductive lead peroxide coating on the anode surface.
High ripple exceeding 5% and current interruptions may cause dull or even laminated deposit.
Agitation helps to homogenize the bath temperature.
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| Problem | Cause | Corrective action |
|---|---|---|
| Dull deposites | 1. High chromic acid/sulfate ratio 2. Metallic impurities 3. Improper current density/temperature relationship 4. Intermittent electric contact 5. Rough substrate surface | 1. Ad sulfuric acid 2. Remove the impurities by cation exchange and check the source of contamination 3. Determine and adjust the relationship 4. Check electric connections 5. Smooth the substrate surface |
| Low plating rate | 1. High chromic acid/sulfate ratio 2. Low current density 3. Scaled anodes 4. Excessive current leakage 5. High temperature | 1. Ad sulfuric acid 2. Increase voltage 3. Clean anodes 4.Check the electric circuit and redesign the thief 5. Check the temperature control system |
| Bright areas on crack-free deposits | 1. Low temperature 2. High current density | 1. Increase temperature 2. Reduce current |
| Pitting | 1. Poor substrate surface 2. Suspended solid particles in solution | 1. Improve surface finish operation 2. Filter bath |
| Poor throwing power | 1. Low chromic acid 2. Low chromic acid/sulfate ratio 3. Low current density 4. Poor electric contact 5. Scaled anodes 6. Passive anodes 7. Poorly spaced anodes 8. Thieves too large or too close 9. Non-uniform distribution of plated parts 10. High temperature | 1. Ad chromic acid 2. Precipitate excessive sulfate with barium carbonate 3. Raise current density 4.Check the electric connection 5. Clean anodes 6. Clean and reactivate anodes 7. Relocate anodes to provide uniform current distribution 8. Redesign thieves 9. Rearrange the parts on rack 10. Decrease temperature |
| Nodular deposits | 1. High chromic acid/sulfate ratio 2. Rough substrate surface 3. Contaminants on surface 4. Magnetized substrate 5. Low temperature 6. High current density | 1. Ad sulfuric acid 2. Improve substrate surface finish 3. Improve surface preparation (cleaning) 4. De-magnitize substrate 5. Increase the temperature 6. Reduce current density |
| Poor adhesion | 1. Contaminants on surface 2. Poor etching 3. Intermittent electric contact | 1. Improve surface preparation (cleaning) 2. Check and improve etching operation (bath composition, etching current, time, etc.) 3. Check electric connections |
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