As environmental regulations become more stringent, traditional phosphate pretreatment methods face increasing scrutiny due to the pollution they generate and the need for complex wastewater treatment systems. Naturally, this raises the question: Is there a more environmentally friendly alternative to phosphating? The answer is yes—ceramic conversion coating, also known as nano-ceramic treatment, offers a more sustainable option.
Nano-ceramic conversion treatment utilizes fluorozirconic acid (or salts) as the primary raw material. Through methods like coating, immersion, or spraying, it forms a nano-ceramic ZrO₂ conversion film on surfaces such as steel, galvanized, aluminum, magnesium alloys, and aluminum alloys. This film offers excellent rust protection while being eco-friendly, energy-efficient, and low in emissions. In addition to its high-quality conversion coating, the process is known for its simplicity, low cost, and ease of operation.
In this article, we’ll dive deeper into the ceramic conversion coating process and explore its benefits, making it a highly appealing alternative to traditional phosphate treatments.
Composition of Nano-Ceramic Conversion Coating Agent
The nano-ceramic conversion coating agent primarily uses fluorozirconic acid (or its salts) as the core film-forming substance. To enhance the coating process and improve performance, the agent also contains various additives such as corrosion inhibitors, film-forming promoters, pH buffers, stabilizers, and wetting agents. These components work together to create a high-quality, durable protective coating on metal surfaces, while ensuring the process remains stable and efficient.
Reference Formula for Nano-Ceramic Conversion Coating (Film)
| Composition | Dosage (g/L) | Composition | Dosage (g/L) |
| Fluorozirconic Acid | 20-25 | Zirconium Fluoride | 6-6.5 |
| Silica | 10-15 | Tartaric Acid | 5-10 |
| Fluotitanic Acid | 32-35 | Sodium Fluoride | 5-10 |
| Potassium Fluoride | 10-20 | Polyethylene Glycol Methacrylate | 3-5 |
| KH550 (Silane Coupling Agent) | 2-5 | Water | Remaining Amount |
Treatment Process
- Nano-Ceramic Treatment Process: Alkaline degreasing → Tap water rinse → Deionized water rinse → Nano-ceramic coating treatment → Drying → Coating.
- Process Flow: Pre-degreasing → Main degreasing → Water rinse → Deionized water rinse → Nano-ceramic treatment → Deionized water rinse → Drying → Coating.
- Related Parameters:
① Treatment can be performed via immersion or spraying.
② Equipment Materials: Treatment tanks should be made of stainless steel, thick-walled plastic, or carbon steel with corrosion-resistant lining; heat exchangers and nozzles should be stainless steel or nylon; piping and pumps should also be stainless steel.
③ Tank Solution Concentration: 30-40 g/L.
④ Ceramic Conversion Point: 3-8.
⑤ Solution pH: 3.8-5.5.
⑥ Solution Temperature: 10-40°C.
⑦ Immersion or Spray Time: 30 seconds to 2 minutes.
Comparison Between Nano-Ceramic Treatment and Phosphating Treatment
Comparison of Advantages and Disadvantages of Nano-Ceramic Coating Treatment vs. Phosphating Treatment
| Comparison Point | Nano-Ceramic Coating Treatment | Phosphating Treatment |
| Thickness | The coating is thin, about 50nm (0.05μm), and the coating quality is 0.05~0.2g/㎡. The molecular bonding structure makes the adhesion of the film stronger than the phosphating film. | The thickness of the phosphating film is generally 23μm, several times that of the nano-ceramic coating, with a film weight of 23g/㎡, and the film tends to be brittle when thicker. |
| Processing Time | Short processing time, only 30s~2min | Processing time is 4~10min |
| Processing Temperature | Low processing temperature, no heating required, can be done at room temperature | Usually requires heating, 35~55℃ |
| Process Simplicity | The process is simple, no need for surface conditioning or passivation treatment | Requires surface conditioning process and equipment, and also requires phosphating process and passivation treatment equipment |
| Cost | Low cost, direct costs are only half that of phosphating | Higher cost, about double that of nano-ceramic treatment agents |
| Environmental Impact | Excellent environmental performance, free of chromium, nickel, and other toxic heavy metals, no heavy metal discharge; no phosphate, no COD (chemical oxygen demand) discharge, no BOD (biochemical oxygen demand) pollution, no wastewater treatment costs, and no need for waste disposal or transportation | Environmental treatment issues exist, requires wastewater treatment, residue discharge, with pollutants like phosphate, nitrite, and heavy metals |
| Management | Easy management of the treatment liquid, checking parameters such as pH value and calcium concentration in the treatment liquid, and the conductivity of the rinse water | More complex management of treatment liquid, including checking total acid concentration, free acid concentration, additives, metal content, and temperature control |
| Substrate Compatibility | Wide range of applicable metal substrates, suitable for cold-rolled steel, hot-rolled steel, galvanized steel, magnesium-aluminum parts, etc. | Except for zinc phosphating, other phosphating treatments have a limited range of metal substrates they can treat |
| Salt Spray Resistance | The salt spray resistance of the treatment film is not as good as that of zinc phosphating. It is comparable to the performance of zinc phosphating with a non-chromium passivation treatment. Some believe it is equivalent to zinc phosphating, and better than iron phosphating. | Zinc phosphating with non-chromium passivation has better salt spray resistance than nano-ceramic treatment. Opinions on the effectiveness vary, and there is no consensus. |
Mechanism of Nano-Ceramic Treatment on Steel
During the nano-ceramic treatment process, which uses fluoro-zirconic acid (or its salts) as the primary material, various chemical reactions may occur when forming the ceramic coating. It is believed that the ceramic ZrO₂ film formed on steel is part of an alkaline cathodic film formation process. The resulting ceramic coating may consist of complex compounds such as FeO₂OH, ZrF, ZrOF₂, FeOF, ZrO₂, and H₂O.
During the film formation process, it is crucial to control the concentration of fluoro-zirconic acid and the pH value of the zirconium solution because, as indicated in the “pattern” reaction ZrF62−
Some theories suggest that the formation of zirconium oxide conversion films on steel surfaces occurs in five stages:
- Activation of the substrate,
- Rapid film growth,
- Slower film growth,
- Dynamic steady-state film growth,
- Film dissolution.
Additionally, it is believed that ZrO₂ films form in micro-cathode regions, which reduces the active surface area of these cathodic regions, thereby inhibiting further film formation. This suggests that the zirconium salt film formation process is a self-restricting one, which limits the development of a high-quality conversion film.
Effect of Different Substrates and Different Treatment Agents on Neutral Salt Spray Resistance
Effect of Different Substrate Treatments on the Corrosion Resistance in Neutral Salt Spray Tests
| Treatment Process | Cold-rolled Steel (mm) | Cast Iron (mm) | Electro-galvanized Steel (mm) | Hot-dip Galvanized Steel (mm) | Aluminum Plate (mm) |
| Nano Ceramic Coating | 1.8 | 1.5 | 1.5 | 1.7 | 0 |
| Iron Phosphate | 16.2 | 16.8 | 12.1 | 14 | |
| Iron Phosphate (without Sealing) | 5.2 | 6.3 | 10.2 | 13.2 | 0 |
| Zinc Phosphate | 2.1 | 4.3 | 8.3 | 9.5 | |
| Zinc Phosphate (without Sealing) | 1.2 | 2.1 | 5.6 | 7.4 | 0 |
Comparison of Different Substrate Treatments and Their Resistance to Neutral Salt Spray (750h)
| Processing Technology | Substrate | 750h Neutral Salt Spray/mm |
| Nano Ceramic Coating | Cold Rolled Steel | 4.2 |
| Nano Ceramic Coating without Passivation | Cold Rolled Steel | 7.7 |
| Iron Phosphate + Chrome-free Treatment | Cold Rolled Steel | 4.2 |
| Zinc Phosphate + Chrome-free Treatment | Cold Rolled Steel | 1.7 |
| Nano Ceramic Coating | Galvanized Steel | 9.5 |
| Iron Phosphate + Chrome-free Treatment | Galvanized Steel | 11.4 |
| Zinc Phosphate + Chrome-free Treatment | Galvanized Steel | 7.3 |
| Nano Ceramic Coating | Aluminum Plate | 0.0 |
| Iron Phosphate + Chrome-free Treatment | Aluminum Plate | 0.0 |
| Zinc Phosphate + Chrome-free Treatment | Aluminum Plate | 0.0 |
The results from the table “Effect of Different Substrate Treatments on the Corrosion Resistance in Neutral Salt Spray Tests” clearly demonstrate that different substrate materials and pretreatment methods significantly impact their resistance to neutral salt spray. Additionally, the type of passivation agent used after phosphating plays a critical role in performance. Therefore, choosing the most suitable pretreatment process based on the user’s specific conditions is advisable.
Similarly, from the table “Comparison of Different Substrate Treatments and Their Resistance to Neutral Salt Spray (750h),” it is evident that for cold-rolled steel, nano-ceramic treatment provides corrosion resistance equivalent to iron phosphate with chromium-free passivation but falls short of zinc phosphate with chromium-free passivation. For galvanized steel, nano-ceramic treatment performs between zinc and iron phosphating, while for aluminum, all three treatments show excellent resistance.
Measurement Methods for pH and Ceramic Coating Points of Nano-Ceramic Treatment Agents
- Method for Measuring the pH of Nano-Ceramic Treatment Agents:
The pH value can be measured directly using pH test paper or a pH meter. - Method for Measuring the Ceramic Coating Points of Nano-Ceramic Treatment Agents:
Take 10 mL of ceramic coating solution and place it into a 250 mL conical flask. Add 20 mL of reagent A (buffer solution), followed by reagent B (complexing agent), and 3-5 drops of reagent C (indicator). Heat the mixture on an electric furnace to 80-90°C. While hot, titrate it with EDTA standard solution until the solution changes from purple-red to bright yellow. The volume of EDTA standard solution used, divided by 10, is the ceramic coating point.
Conclusion
Nano-ceramic treatment agents provide an environmentally friendly, efficient, and cost-effective alternative to traditional phosphating processes. They deliver high-quality ceramic conversion coatings that improve corrosion resistance and adhesion for powder coating applications. Accurate measurement of pH and ceramic coating points ensures optimal performance in treatment processes.
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