A Breakthrough in Steel Drum Pre-Treatment: Phosphorus-Free Conversion Coating
Yang Wenliang
Yang Wenliang delivers a lecture at the fifth steel drum technical training session
Traditional phosphating processes used before coating steel drums involve phosphorus, nickel, and sediment issues, which fail to meet modern environmental standards. The new generation of phosphorus-free conversion coating offers an eco-friendly alternative by forming a nano-scale film on the surface of steel drums. This advanced process improves adhesion and corrosion resistance, making it a superior replacement for conventional phosphating methods.
1. Reaction Mechanism of Phosphorus-Free Conversion Coating
The sol-gel method is one of the key techniques used in this process. It involves using highly reactive compounds as precursors, which are mixed in a liquid phase and undergo hydrolysis and condensation reactions to form a stable sol system. As the sol ages, it gradually polymerizes into a gel with a three-dimensional network structure, filled with a solvent that loses its fluidity. After drying and sintering, this results in nanostructured materials.
Another approach is the transition metal conversion film. This technology uses a transition metal system, where electrochemical reactions in acidic solutions lead to the formation of colloidal oxide particles. These particles coagulate and deposit on the metal surface, creating a nano-scale composite oxide layer. For example, ZrO₂ films can be formed in an acidic H₂ZrF₆ solution. Based on the sol-gel principle, ZrO₂-Me-ZrO₂ sol particles are created, which have strong cohesive properties. As the reaction progresses, a three-dimensional network forms, leading to the deposition of a nano-scale ZrO₂ conversion film.
2. Process of Phosphorus-Free Conversion Film Treatment
The treatment process includes degreasing, rinsing with water (preferably pure water), applying the phosphorus-free conversion agent, followed by additional rinsing and drying. If pure water is not available, tap water can be used, although it may slightly reduce the quality of the conversion film. If the bath turns yellow during use, it is due to iron ions generated from the reaction, but this does not affect the performance of the coating.
3. Types of Phosphorus-Free Conversion Coating
Depending on the equipment, phosphorus-free coatings can be applied through spraying or immersion. Table 1 outlines the process parameters for each method.
Table 1: Process Parameters for Phosphorus-Free Conversion Coating
| Process |
Spray Equipment |
Immersion Equipment |
| Phosphorus-Free Conversion Agent |
2% to 4% |
4% to 6% |
| Temperature |
Normal Temperature |
Normal Temperature |
| Time |
1.5 to 3 minutes |
2 to 5 minutes |
| pH |
2.8 to 4.8 (best at 4.0) |
3.7 to 4.7 (best at 4.0) |
| Film Concentration |
8~45Pt1 |
12~58Pt1 |
1 With 1 mol/L EDTA standard solution, the endpoint is reached when the color changes from magenta to bright yellow. The volume of EDTA consumed (in mL) equals the Pt value of the film concentration. 1mL = 1Pt.
Typically, the bath is replenished regularly to maintain component concentrations. It is recommended to use a circulating filtration system to prevent impurity buildup. If impurities accumulate significantly, the bath should be replaced. Table 2 compares phosphorus-free conversion coatings with traditional phosphate coatings.
Table 2: Comparison Between Phosphorus-Free Conversion Coating and Phosphating Treatment
| Project |
Zinc Phosphating |
Phosphorus-Free Conversion Treatment |
| Membrane Weight |
2 to 4g/m² |
50～200g/m² |
| Membrane Thickness |
3 to 10 μm |
20 to 50 μm |
| Temperature |
35～45°C |
Normal Temperature |
| Time |
Spray: 2 ~ 5min | Soak: 5~10min |
Spray: 1 ~ 3min | Soak: 2 ~ 5min |
| Sediment |
1~2g/m² |
Reduced by more than 90% |
| Table Adjustment Process |
Need |
No need |
| Energy Cost |
100% |
50% |
| Sewage Treatment Cost |
100% |
40% |
1 Energy consumption and sewage treatment costs are compared relative to zinc phosphating at 100%.
4. Advantages of Phosphorus-Free Conversion Films
1. The phosphorus-free process eliminates the use of nickel and chromium, reducing wastewater and environmental impact.
2. The film enhances coating adhesion and provides corrosion protection, replacing traditional iron or zinc phosphating.
3. It is ideal for pre-treatment of steel drums, especially for cleaning barrels.
4. Compatible with existing dip or spray lines, reducing equipment investment.
5. Operates at room temperature, short processing time, no need for table adjustment, saving costs.
6. Minimal sediment, no clogged nozzles, and lower waste disposal costs.
Today, phosphorus-free conversion coatings are widely used in industries like automotive and home appliances. These coatings are becoming a major focus in metal surface treatment, marking a new era in replacing traditional phosphating technologies. For the steel drum industry, phosphorus-free conversion films are set to drive a new wave of technological innovation.
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