What Is Phosphating?
Phosphating is a surface treatment in which a material comes into contact with a phosphating solution, forming a phosphate conversion coating on the metal surface through chemical and electrochemical reactions. The phosphate coating consists of a series of water-insoluble crystals of different sizes. Fine porous structures are formed at the junctions between these crystals. This porous crystalline structure improves the surface adsorption capability, corrosion resistance, friction reduction performance, anti-galling properties, and wear resistance of the workpiece.
Phosphating can apply to materials such as steel, cast iron, aluminum, zinc, magnesium, and their alloys.
Phosphating of metal surfaces mainly serves two purposes.
- The first is to act as a base layer for painting or coating, thereby improving coating adhesion and surface finish quality.
- The second is to improve and enhance the functional properties of the metal surface, such as increasing electrical insulation, lubricity, friction reduction, corrosion resistance, and wear resistance. For many mechanical components and fasteners, the purpose of phosphating is to improve surface functionality, especially corrosion resistance and anti-seizure performance.
Types of Phosphating
In most cases, a surface treatment process produces only one characteristic appearance or color. However, phosphating treatments can produce different colors depending on the specific phosphating chemicals used and the intended application. As a result, phosphated surfaces are commonly seen in gray, iridescent, or black finishes.
Iron Phosphating
After treatment, iron-based phosphating produces a rainbow-like iridescent or bluish appearance, which is why it is often referred to as color phosphating.
The phosphating solution is primarily based on molybdate compounds and forms an iridescent phosphate coating on the steel surface. This type of phosphating is mainly used as a pretreatment layer for painting or coating applications, improving corrosion resistance and enhancing the adhesion strength between the substrate and subsequent coatings.
Zinc Phosphating
Zinc phosphating generally produces a gray-colored coating and is therefore commonly referred to as gray phosphating or gray film phosphating.
The phosphating solution mainly consists of phosphoric acid, sodium fluoride, emulsifiers, and related additives. A gray phosphate coating forms on the workpiece surface and is primarily used as a base layer for subsequent finishing processes such as powder coating, spray painting, and electrophoretic coating.
The gray phosphate coating itself also provides a certain degree of corrosion resistance. Therefore, it may also be used independently as an anti-corrosion coating on component surfaces. Zinc phosphating is commonly applied to galvanized steel sheets, cold-rolled steel sheets, and aluminum sheets.
Manganese Phosphating
Manganese phosphating produces a black or dark gray surface finish and is therefore also known as black phosphating.
This process uses phosphating solutions containing manganese ions, which form a black phosphate coating on the workpiece surface. Among all phosphating treatments, manganese phosphating provides the best rust-prevention performance. It can serve as a long-term anti-corrosion treatment for mechanical components and is one of the most widely used protective phosphating processes.
Compared with other phosphating coatings, manganese phosphate coatings have a lower coefficient of friction. Therefore, black phosphating is especially suitable for components subjected to frequent friction or sliding contact, as it effectively reduces wear and friction. Typical applications include automotive components, fasteners, and various moving mechanical parts.
Phosphating Solution
Depending on the material type and phosphating purpose, different phosphating solution formulations may be selected. Common phosphating systems include zinc-based, zinc-manganese-based, and manganese-based formulations. According to operating temperature, they can be classified as room-temperature, low-temperature (40–50°C), medium-temperature (60–80°C), and high-temperature (85–100°C) phosphating solutions.
For mechanical parts and fasteners, phosphating is mainly intended to improve surface functionality. Therefore, medium-temperature or high-temperature zinc-manganese and manganese phosphating systems are commonly used. At present, most phosphating manufacturers use zinc-manganese phosphating solutions, which generally satisfy industrial requirements.
Table 1 Zinc Phosphating Formulations
| Component (g/L) | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| Manganese-iron phosphate salt (Parkerizing salt) [xFe(H₂PO₄)₂·yMn(H₂PO₄)₂] | — | 30–45 | 30–35 | 30–40 | — | 30–35 |
| Zinc dihydrogen phosphate [Zn(H₂PO₄)₂·2H₂O] | 30–40 | — | — | — | 30–40 | — |
| Zinc nitrate [Zn(NO₃)₂·6H₂O] | 80–100 | 100–130 | 80–100 | — | 55–65 | 55–65 |
| Manganese nitrate [50% Mn(NO₃)₂·6H₂O] | — | 20–30 | — | 15–25 | — | — |
| Nickel nitrate [Ni(NO₃)₂·6H₂O] | — | — | — | — | — | — |
| Free acidity FA (points) | 5.0–7.5 | 6–9 | 5–7 | 3.5–5.0 | 6–9 | 5–8 |
| Total acidity TA (points) | 60–80 | 85–100 | 50–70 | 35–50 | 40–58 | 40–60 |
| Temperature (°C) | 60–70 | 55–70 | 60–70 | 94–98 | 90–95 | 90–98 |
For nuclear-grade products, manganese phosphate treatment is specifically required. Recommended manganese phosphating formulations are shown below.
Table 2 Manganese Phosphating Formulations
| Component (g/L) | Formula 1 | Formula 2 |
|---|---|---|
| Manganese phosphate [Mn(H₂PO₄)₂] | 20–30 | — |
| Phosphoric acid (85% H₃PO₄) | 2–3 | — |
| Nitric acid (HNO₃) | 4–6 | — |
| Sodium fluoroborate (NaBF₄) | 1 | — |
| EDTA | 0.6 | — |
| Manganese-iron phosphate salt [xFe(H₂PO₄)₂·yMn(H₂PO₄)₂] | — | 30–40 |
| Manganese nitrate [50% Mn(NO₃)₂·6H₂O] | — | 15–25 |
| FA (points) | 3–5 | 3.5–5 |
| TA (points) | 35–45 | 35–50 |
| Temperature (°C) | 75–80 | 94–98 |
Major Components in Phosphating Solutions
(1) Zinc Dihydrogen Phosphate [Zn(H₂PO₄)₂·2H₂O]
Zinc dihydrogen phosphate is an excellent base salt for zinc-manganese phosphating systems. Its main advantage is rapid film formation, although the concentration must be properly controlled. Increasing its content also increases the acidity of the phosphating solution.
(2) Parkerizing Salt [xFe(H₂PO₄)·yMn(H₂PO₄)₂]
Also known as iron-manganese phosphate salt, Parkerizing salt promotes rapid phosphating, strong corrosion resistance, uniform crystal formation, and good coating appearance. However, prolonged use may cause solution aging, during which Mn²⁺ ions oxidize into Mn⁷⁺, reducing phosphating quality.
(3) Manganese Nitrate [Mn(NO₃)₂·6H₂O]
Manganese nitrate primarily supplies Mn²⁺ ions to the phosphating bath. It is often used together with Parkerizing salt to increase phosphating reaction speed, reduce treatment temperature, and improve coating stability.
(4) Nickel Nitrate [Ni(NO₃)₂·6H₂O]
Nickel promotes the formation of nickel phosphate crystal nuclei on steel surfaces, accelerating phosphate film formation. It also refines and homogenizes the coating structure, improving corrosion resistance and surface appearance.
(5) Zinc Nitrate [Zn(NO₃)₂·6H₂O]
Zinc nitrate mainly acts as a phosphating accelerator by supplying Zn²⁺ and NO₃⁻ ions. It increases phosphating speed, refines crystal structure, improves corrosion resistance, and stabilizes the phosphating bath. However, its concentration must be strictly controlled.
Phosphating Processes
The functional phosphating process for steel materials generally includes the following steps:
Chemical degreasing → Water rinsing → (Pickling) → Water rinsing → Surface conditioning → Phosphating → Water rinsing → (Passivation) → (Soaping) → Water rinsing → (Deionized water rinsing) → Drying → (Dehydrogenation treatment) → Oil sealing
(*Processes in parentheses are optional.)
1. Chemical Degreasing
Before phosphating, workpiece surfaces inevitably contain grease and oil contaminants. Oil contamination increases surface tension, inhibits phosphate film formation, reduces coating adhesion strength, and causes non-uniform phosphating. Therefore, surface oils must be removed before phosphating to ensure coating quality.
Degreasing methods are selected according to surface condition, cost, and wastewater treatment requirements. Organic solvents, acidic degreasers, and mildly alkaline degreasers may be used, with mildly alkaline degreasers being the most common.
2. Water Rinsing
This step removes residual contaminants remaining from the degreasing process and cleans the workpiece surface.
3. Pickling
Pickling is mainly used to remove oxide scale and rust from workpieces. For machined surfaces free of scale and rust, this step may be omitted. For high-quality applications, neutralization treatment may also be required after pickling.
4. Water Rinsing
This step removes residues remaining after pickling and cleans the workpiece surface.
5. Surface Conditioning
Surface conditioning refers to activating the workpiece surface in a solution containing conditioning agents before phosphating. This further cleans the surface, enhances surface activity, and forms numerous ultra-fine crystal nuclei, refining phosphate crystal formation and improving coating uniformity, density, and quality.
Depending on formulation, conditioning agents may be alkaline, acidic, or specially designed for manganese phosphate systems. Surface conditioning is mandatory for nuclear-grade products.
6. Phosphating
Phosphating is the most fundamental and critical process step and therefore requires the strictest quality control.
The phosphating solution composition must be strictly controlled within process requirements. After prolonged use, the bath composition may change due to aging and sludge formation. At this stage, chemical analysis and solution adjustment are required.
Changes in bath composition are also reflected in variations of total acidity (TA) and free acidity (FA). Therefore, monitoring TA and FA is an effective method for bath control.
Bath temperature is another key factor affecting phosphating quality. Temperature influences coating formation rate and film thickness.
- Excessively high temperatures accelerate reactions, coarsen crystal structure, increase porosity, and reduce corrosion resistance.
- Excessively low temperatures slow down coating formation and reduce coating thickness, making it difficult to meet process requirements.
7. Water Rinsing
This step removes residues from the phosphated surface. For high-quality phosphated parts, deionized water should be used to prevent mineral deposits from forming on the phosphate coating.
8. Passivation
Passivation dissolves protruding regions of the phosphate coating, making the coating morphology and performance more uniform. It also reduces coating porosity, thereby improving corrosion resistance and wear resistance.
Typical Passivation Formulations:
Formula 1
- Na₂CO₃: 4–6 g/L
- Temperature: 80–85°C
- Immersion time: 5–10 min
Formula 2:
- CrO₃: 1–3 g/L
- Temperature: 75–85°C
- Immersion time: 8–12 min
Formula 3:
- K₂Cr₂O₇: 50–80 g/L
- pH: 2–4
- Temperature: 60–80°C
- Immersion time: 8–12 min
9. Soaping Treatment
Soaping involves immersing the phosphated workpiece in a sodium stearate (soap) solution. A saponification reaction occurs between the phosphate coating and sodium stearate, forming a soap film that reduces porosity and improves corrosion resistance.
For general-purpose phosphated parts, soaping may replace passivation.
Common Soaping Formulations
Formula 1:
- Soap: 80–100 g/L
- Temperature: 40–50°C
- Immersion time: 10–20 min
Formula 2:
- Soap: 6–8 g/L
- Temperature: 80–90°C
- Immersion time: 3–5 min
After immersion, the surface should be wiped dry before use.
10. Drying
The purpose of drying is to remove moisture from the phosphated workpiece surface. Particular attention should be paid to holes and grooves where residual water may cause corrosion.
Drying methods may include natural drying or oven drying, depending on coating type and workpiece geometry.
11. Dehydrogenation Treatment
Hydrogen is generated during phosphating. Some hydrogen atoms inevitably remain trapped within the phosphate coating. These residual hydrogen atoms may reduce corrosion resistance and potentially cause hydrogen embrittlement.
Although the amount of residual hydrogen is relatively small, it can reduce material toughness, especially in high-strength materials.
Therefore, high-strength materials used in critical components should undergo dehydrogenation treatment after phosphating. For example, in nuclear power products, materials with tensile strength exceeding 1450 MPa should undergo heat treatment at 100°C ±5°C for more than 8 hours after phosphating.
12. Oil Sealing
Oil sealing involves immersing phosphated workpieces in oil, preferably hot oil at 80–100°C, for 3–4 minutes. This forms an oil film across the entire surface and allows oil to penetrate the pores of the phosphate coating, significantly improving corrosion resistance and long-term durability.
For nuclear power components, lubricating grease containing molybdenum disulfide (except for parts contacting primary circuit fluids) or graphite may also be applied to provide sealing protection.
Quality Control of Phosphating
Inspection of phosphate coating quality generally includes appearance, coating thickness, corrosion resistance, and wear resistance testing.
Appearance Inspection
Appearance inspection evaluates coating color, continuity, and integrity.
Under an illumination level of 500 lx (approximately equivalent to the illumination on a component surface located 30 cm from a 100 W bulb), visual inspection should confirm that the phosphate coating is uniform, continuous, and free from discontinuities or mottling.
Different materials and phosphating methods may produce different coating colors. If specific color requirements exist, the phosphating formulation should be properly selected beforehand.
Coating Thickness Inspection
For critical workpieces, phosphate coating thickness should be measured after treatment.
- Magnetic thickness gauges may be used for ferrous materials.
- Metallographic cross-sectional methods may also be used.
Because phosphate coatings are thin and brittle, precautions must be taken during sample preparation to prevent edge rounding or coating delamination.
Coating Weight Inspection
This test measures the coating mass per unit surface area (g/m²).
The calculation formula is: W = [P1 – P2)/S]*10
Where:
- ( W ) — coating mass per unit area, g/m²
- ( P1 ) — specimen mass after phosphating, mg
- ( P2 ) — specimen mass after coating removal, mg
- ( S ) — total specimen surface area, cm²
Coating thickness may also be approximately estimated from coating weight.
Table 3 Approximate Relationship Between Coating Thickness and Coating Weight
| Coating Thickness (μm) | 1–2 | 2–4 | 4–6 | 6 |
|---|---|---|---|---|
| Coating Weight (g/m²) | 1–2 | 2.2–4.4 | 6–9 | 12 |
Corrosion Resistance Testing
Corrosion resistance is mainly evaluated using corrosive media.
Sodium Chloride Corrosion Test
After storage in a drying chamber for 48 hours, the phosphated specimen is immersed in a 3% NaCl solution at 15–25°C for 1 hour. After rinsing and drying, no visible corrosion should appear.
Copper Sulfate Spot Test
According to GB/T 5936, copper sulfate solution is dropped onto the phosphate coating surface. No red copper deposition should appear within the specified time.
Typical reagent composition:
- CuSO₄: 5–8%
- Balance: water
Salt Spray Test
According to GB/T 10125, phosphated specimens are exposed to salt spray in a corrosion chamber. No corrosion should appear within the specified test duration.
Wear Resistance Testing
Wear resistance testing may be performed according to GB/T 5932 using either wet or dry friction methods.
The specific inspection methods and acceptance criteria should be selected according to the workpiece application requirements.
Process Planning for Phosphating
Since phosphating is typically the final manufacturing process, the following conditions should be satisfied before treatment:
- All manufacturing processes, including welding and heat treatment, must be completed.
- All required inspections and tests must be completed and meet technical specifications.
- The final machining process before phosphating must ensure that dimensions, tolerances, geometry, and surface roughness meet drawing requirements. For high-precision parts, allowance should be considered for coating thickness growth.
- Surface roughness before phosphating significantly affects coating quality.
Under identical phosphating conditions:
- Smoother surfaces are less susceptible to corrosion in the phosphating bath, resulting in slower coating formation, thinner coatings, lighter color, and denser structures.
- Rougher surfaces tend to form thicker and darker coatings, but coating uniformity decreases and the structure becomes more porous.
Therefore, appropriate surface roughness specifications should be carefully selected according to application requirements.
