If the powder coating isn’t sticking well or isn’t holding up against corrosion, the problem might lie in your pretreatment process.
Phosphating is a chemical process that creates an insoluble metal phosphate coating on the surface of metal. This layer is essential for enhancing the adhesion of the powder coating and improving the corrosion resistance of the final finish. It serves as a protective base layer, ensuring that the coating adheres properly and lasts longer.
Curious about how it works and why it’s so crucial? Let’s dive into the details of phosphate pretreatment and its impact on your coating process.
Phosphate Coatings and Treatment Conditions for Different Metals
Different metals require specific phosphate coatings and treatment conditions:
- Steel is treated with zinc phosphate, iron phosphate, or calcium phosphate.
- Zinc materials are treated using phosphoric acid or chromic acid.
- Aluminum is treated with chromate salts or a mixed acid of chromic acid and phosphoric acid.
Phosphate Coating Types and Treatment Conditions for Different Metal Materials
| Metal Material | Type of Phosphate Coating | Coating Thickness | Treatment Method | Concentration (%) | Treatment Temperature (°C) | Treatment Time (min) |
| Steel | Zinc Phosphate | Thick | Immersion | 3-6 (Immersion) | 20-90 | 3-15 |
| Steel | Zinc Phosphate | Medium | Immersion/Spray | 3-6 (Spray) | 35-70 | 1-5 |
| Steel | Zinc Phosphate | Thin | Spray | 3-6 | 20-60 | 1-3 |
| Steel | Iron Phosphate | Very Thin | Immersion/Spray | 1-10 | 40-60 | 1-3 |
| Zinc | Zinc Phosphate | Thin | Immersion | 1-5 | 50-70 | 0.1-3 |
| Zinc | Zinc Phosphate | Medium | Spray | 1-5 | 50-70 | 0.1-3 |
| Zinc | Chromate Salt Coating | Thin | Immersion/Spray | 1-5 (Immersion /Spray) | 25-70 | 0.5-3 (Immersion) 0.5-2 (Spray) |
| Aluminum | Chromate Oxide Coating | Colored | Immersion/Spray | 1-5 (Immersion /Spray) | 20-50 (Immersion) 20-40 (Spray) | 0.5-3 (Immersion) 0.5-2 (Spray) |
| Aluminum | Chromate Oxide Coating | Non-colored | Immersion/Spray | 0.5-1 (Immersion /Spray) | 20-70 (Immersion) 20-40 (Spray) | 0.5-3 (Immersion) 0.1-3 (Spray) |
| Aluminum | Chromic Phosphoric Acid | Colored | Immersion/Spray | 1-5 (Immersion /Spray) | 40-50 | 0.5-3 (Immersion) 0.1-3 (Spray) |
Spray Phosphating Process Steps
The spray phosphating process involves a series of steps that ensure the metal surface is properly prepared for coating. Below is an overview of the pretreatment process:
- Degreasing: The metal surface is first degreased to remove any oils or contaminants that could affect the phosphating process.
- First Rinse: A water rinse is applied to remove any remaining degreasing agents.
- Second Rinse: A second water rinse is performed to ensure the surface is completely clean.
- Phosphating: The phosphating solution is sprayed onto the metal surface, creating a protective phosphate layer.
- Third Rinse: This rinse helps to remove any remaining phosphating solution from the surface
- Final Rinse: A final rinse ensures the surface is clean.
- Deionized Water Rinse: The final step involves rinsing with deionized water to prevent any mineral deposits on the metal surface.
Spray Phosphating Process
| Process Step | Temperature (°C) | Time (min) | Spray Nozzles (per unit) | Tank Capacity (L) |
| Degreasing | 40-60 | 3.0 | 380 | 7000 |
| First Rinse | Room Temperature | 0.5-1.0 | 114 | 2000 |
| Second Rinse | Room Temperature | 0.5-1.0 | 114 | 2000 |
| Phosphating | 40-60 | 1.0-3.0 | 304 | 7000 |
| Third Rinse | Room Temperature | 0.5-1.0 | 114 | 2000 |
| Final Rinse | Room Temperature | 0.5-1.0 | 114 | 2000 |
| Deionized Water Rinse | Room Temperature | Up-and-Down Rinse | 38 | — |
The Principle of Phosphating
The main component of phosphating materials is acidic phosphate, with the molecular formula M(H₂PO₄)₂, where M represents metals such as zinc, iron, or calcium. These salts dissolve in water and decompose, releasing free acid. The free acid reacts with the metal surface, releasing hydrogen gas. Meanwhile, the hydrogen phosphate further decomposes into phosphate and phosphoric acid. Specifically, dihydrogen phosphate (M(H₂PO₄)₂) breaks down into phosphate (MPO₄) and phosphoric acid (H₃PO₄).
3M(H₂PO₄) ⇌ 3MHPO₄ + 3H₃PO₄
3MHPO₄ ⇌ M₃(PO₄)₂ + H₃PO₄
2Fe + 3M(H₂PO₄)₂ ⇌ M₃(PO₄)₂↓ + 2FeHPO₄↓ + 2H₃PO₄ + 2H₂↑
As the surface of steel continuously interacts with the phosphating solution, the free acid decreases, causing the pH at the interface between the workpiece and the solution to rise. This pH increase drives further ionization reactions, leading to a gradual increase in the concentration of monohydrogen phosphate and phosphate ions. Once these compounds reach saturation, they begin to crystallize and precipitate onto the metal surface. These crystalline particles continue to grow until an insoluble phosphate coating forms. The phosphate coating is primarily composed of crystals of phosphate M₃(PO₄)₂ and monohydrogen phosphate MHPO₄.
Compared to sandblasting, phosphating is a simpler process with easier equipment setup, lower operational costs, and straightforward automation potential. However, its effectiveness in enhancing coating adhesion is generally not as high as that of sandblasting. The main methods of phosphating include immersion, spraying, or a combination of both.
For workpieces with mild oil contamination or rust, the typical process involves separate steps for degreasing, derusting, phosphating, and passivation. In special cases, a multi-functional treatment that combines degreasing, derusting, phosphating, and passivation into a single step may be used. Phosphating can be performed in solutions where the main components are dihydrogen phosphate salts of zinc, manganese, zinc-calcium, alkali metals, other metals, or ammonia.
Equipment in contact with the phosphating solution, such as pipes, nozzles, pumps, and tanks, should be made from materials resistant to phosphoric acid corrosion to ensure they do not affect the performance of the phosphating solution or the quality of the phosphate coating.
Types of Phosphating Processes
Phosphating can be categorized into five different types:
1. By Type of Treatment Solution:
When phosphating processes are classified by the composition of the treatment solution, they can be divided into zinc phosphate, zinc-calcium phosphate, manganese phosphate, zinc-manganese phosphate, and iron phosphate systems. Below is a table showing the composition and properties of the phosphate coatings according to the type of treatment solution used:
| Classification | Main Components in Phosphating Solution | Main Components of Phosphate Coating | Coating Appearance | Coating Weight (g/m²) |
| Zinc Phosphate | Zn(H₂PO₄)₂ | Zinc Phosphate [Zn₃(PO₄)₂·4H₂O]; Zinc-Iron Phosphate [Zn₂Fe(PO₄)₂·4H₂O] | Light to Dark Gray Crystalline | 1-60 |
| Zinc-Calcium Phosphate | Zn(H₂PO₄)₂ and Ca(H₂PO₄)₂ | Zinc-Calcium Phosphate [Zn₂Ca(PO₄)₂·2H₂O]; Zinc-Iron Phosphate [Zn₂Fe(PO₄)₂·4H₂O] | Light to Dark Gray Crystalline | 1-15 |
| Manganese Phosphate | Mn(H₂PO₄)₂ and Fe(H₂PO₄)₂ | Manganese-Iron Phosphate [Mn₂Fe(PO₄)₂·4H₂O] | Gray to Dark Gray Crystalline | 1-60 |
| Zinc-Manganese Phosphate | Mn(H₂PO₄)₂ and Zn(H₂PO₄)₂ | Zinc-Manganese-Iron Phosphate [ZnFeMn(PO₄)₂·4H₂O] | Gray to Dark Gray Crystalline | 1-60 |
| Iron Phosphate | Fe(H₂PO₄)₂ | Iron Phosphate [Fe₃(PO₄)₂·8H₂O] | Dark Gray Crystalline | 5-10 |
2. By Coating Weight:
Phosphating processes can be categorized based on the phosphate coating weight into three types: heavyweight (>10g/m²), medium weight (1-10g/m²), and lightweight (<1g/m²). Below is the table that outlines the classification, formulation, and process conditions for zinc phosphate treatment solutions.
| Phosphating Solution Composition & Process Conditions | Heavyweight | Medium Weight | Lightweight |
| Zinc Dihydrogen Phosphate (g/L) | 28-36 | 30-40 | 50-70 |
| Nitric Acid (g/L) | 42-56 | 80-100 | 80-100 |
| Phosphoric Acid (g/L) | 9.5-13.5 | — | — |
| Sodium Nitrate (g/L) | — | — | 0.2-1 |
| Sodium Acetate (g/L) | — | — | 1-1.5 |
| 601 Detergent (g/L) | — | — | 30 |
| Ethanol (g/L) | — | — | 5 |
| Total Acidity (points) | 60-80 | 60-80 | 75-95 |
| Free Acidity (points) | 10-14 | 5-7 | 4-6 |
| Tank Temperature (°C) | 92-98 | 60-70 | 15-35 |
| Processing Time (min) | 10-15 | 10-15 | 20-40 |
3. By treatment Temperature
Phosphating processes can be classified based on the treatment temperature into three categories: high-temperature (80-98°C), medium-temperature (50-70°C), and low-temperature (room temperature). The following tables detail the formulations and process conditions for each temperature category.
High-Temperature Phosphating (80-98°C)
| Treatment Solution & Process Conditions | Formula 1 | Formula 2 | Formula 3 |
| Zinc Manganese Phosphate (g/L) | 30-40 | 30-40 | 30-35 |
| Zinc Dihydrogen Phosphate Zn(H₂PO₄)₂·2H₂O (g/L) | — | 30-40 | — |
| Zinc Nitrate Zn(NO₃)₂·6H₂O (g/L) | — | 55-65 | 55-65 |
| Manganese Nitrate Mn(NO₃)₂·6H₂O (g/L) | 15-25 | — | — |
| Free Acidity (points) | 3.5-5.0 | 6-9 | 5-8 |
| Total Acidity (points) | 35-50 | 40-58 | 40-60 |
| Tank Temperature (°C) | 94-98 | 90-95 | 90-98 |
| Time (min) | 15-20 | 8-15 | 15-20 |
Medium-Temperature Phosphating (50-70°C)
| Treatment Solution & Process Conditions | Formula 1 | Formula 2 | Formula 3 | Formula 4 |
| Zinc Manganese Phosphate (g/L) | 30-35 | 30-40 | 150-200 | 170-210 |
| Zinc Dihydrogen Phosphate Zn(H₂PO₄)₂·2H₂O (g/L) | — | — | — | — |
| Zinc Nitrate Zn(NO₃)₂·6H₂O (g/L) | 80-100 | 80-100 | — | — |
| HT Phosphate Concentrate (mL/L) | — | — | — | — |
| Y836 Phosphate Concentrate (mL/L) | — | — | — | — |
| Free Acidity (points) | 5-7 | 5.7-7.5 | 3-5 | 4-6 |
| Total Acidity (points) | 50-80 | 60-80 | 40-60 | 50-55 |
| Tank Temperature (°C) | 50-70 | 60-70 | 50-70 | 65-70 |
| Time (min) | 10-15 | 10-15 | 3-8 | 4-6 |
Low-Temperature Phosphating (Room Temperature)
| Treatment Solution & Process Conditions | Formula 1 | Formula 2 | Formula 3 | Formula 4 |
| Zinc Dihydrogen Phosphate Zn(H₂PO₄)₂·2H₂O (g/L) | 60-70 | 50-70 | — | — |
| Zinc Nitrate Zn(NO₃)₂·6H₂O (g/L) | 60-80 | 80-100 | — | — |
| Sodium Nitrite (NaNO₂) (g/L) | — | 0.2-1.0 | — | — |
| Sodium Fluoride (NaF) (g/L) | 3-4.5 | — | — | — |
| Zinc Oxide (ZnO) (g/L) | 4-8 | — | — | — |
| BONDERITE339 Phosphate Concentrate (mL/L) | — | — | — | (Na₂CO₃) 2.8 |
| BONDERITE399S Starter (mL/L) | — | — | — | 55 |
| 842A Phosphate Concentrate (mL/L) | — | — | 50 | 55 |
| 842B Phosphate Concentrate (mL/L) | — | — | 20 | — |
| Free Acidity (points) | 3-4 | 4-6 | 1.5-3 | 0.7-1.0 |
| Total Acidity (points) | 70-90 | 75-95 | 25-35 | 25 |
| Tank Temperature (°C) | 50-70 | 60-70 | 15-25 | 25-35 |
| Time (min) | 30-40 | 20-40 | 10-20 | 1.5-2 |
4. By Treatment Process
Phosphating processes can be classified based on the method of application into immersion, spraying, and brushing. The following table provides the formulation and process conditions for immersion and spray applications using zinc phosphate-based phosphating solutions.
| Formulation & Process Conditions | Immersion Zinc Phosphating Solution | Spray Zinc Phosphating Solution |
| Zinc Nitrate (g/L) | 60-80 | 7 |
| Zinc Dihydrogen Phosphate Zn(H₂PO₄)₂ (g/L) | 60-70 | 10 |
| Zinc Oxide (g/L) | 4-8 | — |
| Sodium Fluoride (NaF) (g/L) | 3-4.5 | — |
| Sodium Nitrite (NaNO₂) (g/L) | — | 0.3 |
| Sodium Nitrate (NaNO₃) (g/L) | — | 7 |
| Total Acidity (points) | 70-90 | 10-12 |
| Free Acidity (points) | 3-4 | — |
| Tank Temperature (°C) | 20-30 | 55-65 |
| Treatment Time (min) | 3-5 | 2-3 |
5. By Treatment Method
Phosphating processes can be classified into chemical phosphating and electrochemical phosphating. The requirements for phosphating solutions vary depending on the coating method:
- Electrostatic Powder Coating Systems: Zinc phosphate is the preferred choice for these systems.
- Fluidized Bed Coating Systems: Zinc-calcium phosphate is often selected, particularly considering the preheating of the workpiece.
To ensure the phosphate coating is fine and dense, enhancing its rust and corrosion resistance, surface adjustment is performed before phosphating. This adjustment can be done using a weak acidic oxalic acid solution or, more commonly, a weak alkaline titanium phosphate solution, applied through either immersion or spray.
Additionally, post-treatment is carried out to further improve the corrosion resistance of the phosphate coating. Below is the table detailing the formulation and process conditions for post-treatment of phosphate coatings.
| Formulation & Process Conditions | Formula 1 | Formula 2 | Formula 3 | Formula 4 | Formula 5 |
| Sodium Dichromate (g/L) | 60-80 | 50-80 | — | — | — |
| Acetic Acid (g/L) | — | — | 1-3 | 30-35 | 100 |
| Sodium Phosphate (g/L) | 4-6 | — | — | — | — |
| Soap (g/L) | — | — | — | — | — |
| Lanolin or Rust Preventive Oil (%) | — | — | — | — | — |
| Tank Temperature (°C) | 80-85 | 70-80 | 70-95 | 80-90 | 105-110 |
| Time (min) | 5-10 | 8-12 | 3-5 | 3-5 | 5-10 |
Phosphating Process Optimization and Considerations
1. Variability in Phosphate Coatings:
- Material Differences: The crystalline structure, corrosion resistance, and color of phosphate coatings can vary due to the types and amounts of trace elements present in the metal being treated.
- Accelerators: To speed up the phosphating process, accelerators like sodium nitrite are commonly used. However, it’s essential to balance the amount carefully, as too little slows down the process, while too much can lead to excessive sludge formation. Sodium nitrite decomposes in acidic solutions, releasing NO, so replenishment is often necessary during the process. Other accelerators such as hydrogen peroxide, chlorates, and nitrates can also be used.
2. Special Considerations for Aluminum Alloys:
- Aluminum Ion Interference: When treating aluminum alloys, it’s important to monitor aluminum ion concentration in zinc phosphate solutions. At concentrations above 0.3g/L, aluminum ions can inhibit the formation of zinc phosphate coatings. The addition of fluorides can help, as they form insoluble sodium fluoroaluminate, allowing the phosphating process to proceed smoothly.
3. Controlling Acidity:
- Total Acidity: This parameter reflects the concentration of the phosphating solution. Maintaining the correct total acidity ensures that the concentration of film-forming ions remains within the required range. If total acidity is too low, the phosphating effect diminishes.
- Free Acidity: The concentration of free acid greatly influences the quality of the phosphate coating. High free acidity accelerates corrosion on the steel surface, producing excessive gas bubbles that disrupt the coating formation, resulting in coarse, loose, and yellowish coatings with poor corrosion resistance. Conversely, low free acidity slows down the reaction, leading to poor coating formation and increased sediment in the solution. Proper control of free acidity is crucial for maintaining coating quality.
4. The Importance of the “Acid Ratio”:
- Definition: The “acid ratio” refers to the ratio of total acidity to free acidity, typically controlled within a range of 5 to 30.
- Effects of Acid Ratio: A low acid ratio indicates high free acidity, leading to slow film formation, longer phosphating times, and higher required temperatures. A high acid ratio results in faster film formation, shorter phosphating times, and lower required temperatures. Maintaining the correct acid ratio is key to ensuring consistent phosphating quality.
5. Temperature’s Impact on Phosphating:
- High Temperatures: High phosphating temperatures speed up the film formation process, resulting in thicker coatings with enhanced corrosion resistance. However, excessively high temperatures can reduce coating quality, leading to dust and particle adhesion, which affects paint adhesion.
- Low Temperatures: Low temperatures slow down the reaction, causing incomplete film formation with large crystal grains, reducing corrosion resistance.
6. Phosphate Coating Appearance:
- Ideal Appearance: Post-phosphating, the coating should appear light gray to dark gray or colored, with a dense, continuous, and uniform crystalline structure.
- Permissible Defects: Minor water marks, passivation marks, wiping stains, and slight gray streaking are acceptable. Uneven color and crystallization due to localized heat treatment, welding, and surface conditions are also allowed, as is the absence of phosphate coating on weld areas.
- Unacceptable Defects: Loose phosphate layers, rust spots, green stains, missing coatings (except on weld seams), and severe surface gray streaks are not permitted.
7. Simplified Processes for Less Contaminated Workpieces:
- “Three-in-One” Solutions: For workpieces with minor rust or oil contamination, a “three-in-one” surface treatment solution combining degreasing, derusting, and phosphating can be used. This solution includes surfactants for degreasing, phosphoric acid for derusting, and phosphates as film-forming agents, along with a few additives.
- Advantages and Limitations: The “three-in-one” solution is easy to prepare, manage, reduces pollution, and improves working conditions. However, its use is limited, with longer processing times and lower-quality phosphate coatings. A detailed formula and process for this “three-in-one” treatment can be found in the following table.
| Composition | Usage and Conditions |
| Phosphoric Acid (g/L) | 150-300 |
| Zinc Dihydrogen Phosphate (g/L) | 40-50 |
| Sodium Nitrate (g/L) | 3-5 |
| OP Emulsifier (g/L) | 3-5 |
| Tank Temperature (°C) | 50-70 |
| Treatment Time (min) | 5-10 |
8. Phosphating of Zinc and Galvanized Parts:
- Mechanism: The phosphating process for zinc and galvanized parts is similar to that for steel. Zinc phosphate treatment is generally preferred.
- Importance: Zinc’s high reactivity can lead to unwanted reactions with fatty acids in paint, forming stearates. Phosphating prevents these reactions, making it an essential pre-paint treatment for zinc and galvanized surfaces.
Conclusion
Phosphating remains a widely used and effective method for preparing metal surfaces for coating, offering reliable corrosion resistance and improved adhesion. However, as environmental regulations become stricter, more advanced methods like ceramic conversion coating are gradually replacing traditional phosphating processes. These newer methods not only meet higher environmental standards but also provide enhanced performance.
If you’re looking to upgrade your coating process or need high-quality powder coating equipment, contact us today! We’re here to help you achieve the best results with the latest technology.
