Colourless chromatising on aluminium is a surface treatment process that enhances corrosion resistance and provides a good base for subsequent painting or adhesive bonding, without imparting any visible colour. It is typically based on trivalent chromium (Cr³⁺) compounds as a safer alternative to hexavalent chromium (Cr⁶⁺).
Key Features:
- Appearance: Clear or slightly iridescent, virtually colourless.
- Corrosion Resistance: Moderate on its own, often used before painting.
- Adhesion Promotion: Improves paint or adhesive bonding.
- Environmentally Safer: Trivalent chromium or non-chrome formulations are RoHS compliant.
Typical Formulation (Trivalent Chrome-Based):
- Chromium(III) nitrate or sulphate: 3–5 g/L
- Complexing agents (e.g., formic acid): 0.5–2 g/L
- Fluorides (e.g., ammonium bifluoride): 0.2–1 g/L
- pH: ~1.5–2.5
- Temperature: Room temperature (25–35 °C)
- Time: 30–120 seconds immersion
Application Steps:
- Cleaning: Alkaline cleaner to remove grease.
- Deoxidizing/Desmutting: Acidic solution to remove oxides and smut.
- Chromatising: Dip or spray application of the colourless chromate solution.
- Rinse: Cold water rinse to remove residues.
- Drying: Air dry or low-temperature oven drying.
Alternatives:
- Non-chrome treatments (e.g., zirconium/titanium-based).
- Sol-gel coatings (organically modified silanes).
Formulation -1
Colourless Trivalent Chromium Chromatising – Concentrate Formula (RoHS-Compliant)
| Sl. No. | Raw Material | Function | Concentration in Concentrate (%) |
|---|---|---|---|
| 1 | Chromium(III) nitrate nonahydrate | Source of Cr³⁺ | 15.0 |
| 2 | Formic acid (85%) | Complexing agent, pH control | 5.0 |
| 3 | Ammonium bifluoride | Activator, improves adhesion | 3.0 |
| 4 | Boric acid | Buffering agent | 1.0 |
| 5 | Non-ionic wetting agent (e.g., Triton X) | Surface wetting, uniform film | 0.5 |
| 6 | Deionized Water | Solvent | q.s. to 100 |
Dilution for Use (Bath Make-Up):
- Concentrate addition: 50 mL/L (5% v/v)
- Bath pH: 1.8–2.2 (adjust with dilute nitric or formic acid)
- Temperature: Room temp (25–35 °C)
- Time: 60–120 seconds immersion
- Appearance: Clear or faint iridescent film
SOP – Process Sequence:
- Degreasing: Alkaline cleaner at 50–60 °C for 5–10 minutes
- Rinse: Tap water
- Desmut: Nitric + HF or HF-free acid cleaner (for alloyed aluminium)
- Rinse: DI water
- Chromatising: Immerse in working bath (60–120 sec)
- Rinse: DI water
- Drying: Hot air or low-temp oven (<60 °C)
Formulation – 2
Colourless Trivalent Chromium Chromatising – Powder Concentrate Formula
RoHS-compliant, suitable for aluminium conversion coating before painting or sealing
| Sl. No. | Raw Material | Function | % w/w in Powder Concentrate |
|---|---|---|---|
| 1 | Chromium(III) nitrate nonahydrate | Cr³⁺ source | 40.0 |
| 2 | Ammonium bifluoride (NH₄HF₂) | Film activator, oxide removal | 10.0 |
| 3 | Boric acid | pH buffer | 3.0 |
| 4 | Sodium gluconate or citric acid | Complexing/stabilizing agent | 5.0 |
| 5 | Non-ionic surfactant (powder type) | Wetting/dispersing agent | 1.0 |
| 6 | Silica-based flow agent (e.g., Aerosil) | Anti-caking | 0.5 |
| 7 | Sodium sulphate (anhydrous) | Filler/carrier for bulk and dilution | 40.5 |
Bath Make-Up (Working Solution):
- Powder concentrate addition: 10 g/L
- Bath pH: 1.8–2.2 (adjust with dilute nitric/formic acid if needed)
- Temperature: 25–35 °C
- Treatment Time: 60–120 seconds (immersion or spray)
SOP Summary:
- Alkaline Clean: Degrease at 50–60 °C, 5–10 minutes
- Rinse: Tap water
- Desmut: Acidic desmutting for alloyed aluminium (e.g., nitric + HF or HF-free)
- Rinse: DI water
- Chromatising: Immerse in prepared bath (10 g/L) for 1–2 min
- Rinse: DI water
- Dry: Oven or air dry (max 60 °C)
Formulation – 3
Colourless Trivalent Chromium Chromatising – Two-Pack Powder Concentrate Formula
Designed for better stability and shelf life; mix just before use.
Part A – Trivalent Chromium Base (Powder)
| Sl. No. | Raw Material | Function | % w/w in Part A |
|---|---|---|---|
| 1 | Chromium(III) nitrate nonahydrate | Cr³⁺ source | 65.0 |
| 2 | Boric acid | pH buffer | 4.0 |
| 3 | Sodium sulphate (anhydrous) | Bulk filler/carrier | 30.5 |
| 4 | Silica flow agent (Aerosil) | Anti-caking | 0.5 |
Part B – Activator Pack (Powder)
| Sl. No. | Raw Material | Function | % w/w in Part B |
|---|---|---|---|
| 1 | Ammonium bifluoride (NH₄HF₂) | Fluoride activator | 60.0 |
| 2 | Sodium gluconate | Complexing/stabilizing agent | 10.0 |
| 3 | Sodium sulphate (anhydrous) | Diluent/bulk carrier | 29.5 |
| 4 | Silica flow agent (Aerosil) | Anti-caking | 0.5 |
Working Bath Preparation:
- Part A: 8 g/L
- Part B: 2 g/L
- Total: 10 g/L in deionized water
- pH Adjustment: 1.8–2.2 using dilute formic or nitric acid if required
- Temperature: 25–35 °C
- Treatment Time: 60–120 seconds
Process Sequence:
- Alkaline Cleaning → 2. Water Rinse →
- Desmutting → 4. Water Rinse →
- Chromatising (Mixed Bath) → 6. DI Water Rinse →
- Drying (Air or oven below 60 °C)
Formulation – 4
Colourless Trivalent Chromium Chromatising – Liquid Concentrate (Spray Application Variant)
RoHS-compliant, designed for aluminium surface passivation before paint. Optimized for spray booths or hand spray systems.
Liquid Concentrate Formula (Single-Pack)
| Sl. No. | Raw Material | Function | % w/w in Liquid Concentrate |
|---|---|---|---|
| 1 | Chromium(III) nitrate solution (20%) | Cr³⁺ source | 35.0 |
| 2 | Formic acid (85%) | pH adjuster, complexing agent | 8.0 |
| 3 | Ammonium bifluoride (40% solution) | Film activator, oxide etching | 6.0 |
| 4 | Citric acid (20% solution) | Complexant, bath stabilizer | 5.0 |
| 5 | Non-ionic surfactant (e.g., Triton X-100) | Surface wetting, film uniformity | 0.5 |
| 6 | DI Water | Solvent | q.s. to 100 |
Working Solution (Spray Bath Make-Up)
- Concentrate dosage: 50 mL/L
- Working pH: 1.8–2.2
- Spray pressure: 1.0–1.5 bar
- Temperature: Ambient (25–35 °C)
- Spray time: 60–90 seconds
- DI Water rinse: Recommended
- Drying: Hot air or oven <60 °C
SOP for Spray Application
1. Pre-Cleaning
- Use an alkaline degreaser (e.g., NaOH/surfactant blend) at 50–60 °C for 5–10 min.
- Rinse with tap water.
2. Desmutting (if alloyed Al)
- Acidic desmut: e.g., nitric + ammonium bifluoride, or HF-free acidic desmut solution.
- Rinse with deionized water.
3. Chromatising by Spray
- Use low-pressure spray system.
- Spray prepared bath (50 mL/L concentrate in DI water).
- Ensure uniform wetting of surface for 60–90 seconds.
4. Final Rinse
- Rinse with DI water to remove residual acids and salts.
5. Drying
- Dry at ambient or forced air (below 60 °C).
- Avoid wiping to prevent film disruption.
Bath Maintenance Tips
- Monitor pH daily, adjust with dilute formic/nitric acid.
- Replenish with concentrate to maintain performance.
- Use stainless steel or plastic tanks and piping.
- Filter regularly to remove aluminium fines or precipitates.
Compatible Paints on Chromatised Aluminium
Compatible Paint Systems for Colourless Trivalent Chromatised Aluminium
Trivalent chromium coatings provide good adhesion and moderate corrosion resistance, making them ideal primers for the following paint systems:
1. Acrylic-Based Paints
- Type: Thermoplastic (TPA) or Thermosetting (TSA)
- Application: Aerosol touch-up, façade panels, appliance bodies
- Advantages: Fast drying, UV-resistant
- Adhesion: Excellent over trivalent conversion coatings
2. Polyester Powder Coatings
- Type: TGIC or HAA crosslinked polyesters
- Application: Outdoor furniture, architecture, electronics
- Advantages: Durable, weather-resistant
- Pretreatment Compatibility: Trivalent chromate improves bonding and corrosion resistance
3. Epoxy Primers
- Type: Two-pack amine-cured epoxy primers
- Application: Aerospace, automotive undercoats, defence
- Advantages: Excellent adhesion and chemical resistance
- Notes: Often overcoated with PU or alkyd topcoats
4. Polyurethane Paints
- Type: Aliphatic or aromatic polyisocyanate-cured
- Application: Industrial equipment, aircraft, commercial vehicles
- Advantages: High gloss, excellent durability
- Trivalent Compatibility: Strong adhesion when applied over trivalent-treated aluminium
5. Water-Based Paints
- Type: Acrylic, polyurethane dispersions
- Application: Consumer goods, electronics, architectural panels
- Advantages: Eco-friendly, low VOC
- Requirement: Surface must be fully dry after trivalent treatment
6. Silicone-Based Coatings
- Type: High-temperature or hydrophobic coatings
- Application: Cookware, chimneys, solar panels
- Notes: Requires a tightly bound oxide/chromate layer for anchoring
Paint Performance Enhancement Recommendations
- Use within 24–72 hours after chromatising for best adhesion.
- Perform a cross-hatch adhesion test (ASTM D3359) post-curing.
- Avoid fingerprints/oil contamination after coating and before painting.
Suitability of Other Metals for Chromatising
The colourless trivalent chromium chromatising formula is primarily designed for aluminium and its alloys, but with appropriate surface conditioning and mild adjustments, it can also be applied to the following metals:
✅ Metals That Can Be Chromatised Using This Formula or Slight Variants
| Metal / Alloy | Compatibility | Notes / Requirements |
|---|---|---|
| Aluminium (pure & alloys) | ✔️ Native target | Optimal performance; excellent adhesion base for paint |
| Zinc (galvanized steel) | ⚠️ Partial | Requires acid cleaning/desmutting, bath pH may need tightening |
| Magnesium alloys | ⚠️ Conditional | Needs acidic fluoride pre-cleaning, may require bath modification |
| Tin-plated surfaces | ⚠️ Moderate | Mild chromatising effect; verify adhesion and corrosion resistance |
| Cadmium (plated) | ✔️ Good | Works well; used in aerospace applications |
| Zinc-aluminium alloys (e.g. ZA) | ⚠️ Limited | Etching control required; check for white salt formation |
| Steel (mild/carbon) | ❌ Not suitable | No passivation; trivalent chromium does not form protective film |
| Copper and brass | ❌ Not suitable | No chromate film formation; may oxidize or stain |
| Stainless steel | ❌ Not suitable | Very low reactivity; requires different passivation (e.g., nitric) |
⚠️ Modifications Needed for Other Metals
- Zinc and Magnesium: Adjust fluoride content slightly (increase NH₄HF₂ to 8–10 g/L in working bath)
- Bath pH: Lower end (1.5–1.8) works better for zinc/magnesium
- Desmutting: Essential for alloys with silicon, copper, or magnesium
- Spray vs. Dip: Spray is better for reactive metals to reduce pitting or streaking
NOTE:
This trivalent chromatising system is best suited for aluminium. It may be adapted with caution for zinc, cadmium, and magnesium with appropriate pre-treatment and pH control. It is not effective for ferrous, copper, or stainless substrates.
Raw Material Suppliers in India
Indian Suppliers for Raw Materials Used in Colourless Trivalent Chromatising
1. Chromium(III) Nitrate Nonahydrate
- SD Fine-Chem Ltd. (SDFCL) – Mumbai
- Loba Chemie Pvt. Ltd. – Mumbai
- SRL (Sisco Research Laboratories) – Mumbai
- HIMEDIA Laboratories – Mumbai
2. Ammonium Bifluoride (NH₄HF₂)
- Tata Chemicals Ltd. – Gujarat
- Indian Fluorides Ltd. – Gujarat
- Navin Fluorine International Ltd. – Mumbai
- Triveni Chemicals – Vapi, Gujarat
3. Formic Acid (85%)
- GNFC (Gujarat Narmada Valley Fertilizers & Chemicals) – Bharuch
- Balaji Amines Ltd. – Maharashtra
- Aarti Industries – Mumbai
- RCPL (Rashtriya Chemicals & Petrochemicals Ltd.) – Mumbai
4. Boric Acid
- Gujarat Boron Derivatives Pvt. Ltd. – Gujarat
- Orris Fine Chem Pvt. Ltd. – Ahmedabad
- Shivam Chemicals – Vapi
- IG Petrochemicals Ltd. – Maharashtra
5. Citric Acid
- Godavari Biorefineries Ltd. – Karnataka
- Anil Bioplus Ltd. – Ahmedabad
- Foodchem International (via Indian agents) – Mumbai
- Adani Wilmar (bulk) – Ahmedabad
6. Sodium Gluconate
- Sodium Gluconate India (SGI) – Ahmedabad
- Roquette India Pvt. Ltd. – Gujarat
- Sujata Chemicals – Pune
7. Non-ionic Surfactants (e.g., Triton X-100 / Equivalent)
- Galaxy Surfactants Ltd. – Navi Mumbai
- Godrej Industries – Mumbai
- Aarti Surfactants – Vapi
- KLJ Group – Delhi NCR
8. Deionized Water / DI Water Systems
- Ion Exchange India Ltd. – Mumbai
- Thermax Ltd. – Pune
- Pentair Water India Pvt. Ltd. – Goa
