Phosphating properties and types
Phosphating is a chemical conversion treatment that generates an adherent layer of insoluble phosphate crystals on metallic surfaces. The typical film thickness ranges from 1 µm (0.00004 in) to 30 µm (0.0012 in) depending on the type of phosphate applied. The reaction occurs by immersing the part in a dilute phosphoric acid solution containing iron, zinc or manganese cations; the acid attack on the base metal raises the interfacial pH and causes the precipitation of the corresponding phosphates.
The coating is mainly used to improve corrosion resistance, promote lubrication, facilitate adhesion of subsequent paints, or as a wear layer. It is one of the most widespread non-metallic coatings in the manufacturing industry.
Types of phosphating
Section titled “Types of phosphating”| Type | Main composition | Common application method | Dominant purpose |
|---|---|---|---|
| Iron phosphate | Amorphous or nanocrystalline iron phosphate (FePO₄) | Immersion or spray | Base for paint, light anti-corrosion protection |
| Zinc phosphate | Zinc phosphate Zn₃(PO₄)₂ | Immersion or spray | Corrosion resistance, lubricant reservoir, base for organic coatings |
| Manganese phosphate | Manganese phosphate Mn₃(PO₄)₂ | Immersion only | Wear resistance and lubricity in metal-metal contact |
Coating properties
Section titled “Coating properties”| Property | Iron phosphate | Zinc phosphate | Manganese phosphate |
|---|---|---|---|
| Typical thickness | 0,5 – 10 µm / 0.00002 – 0.0004 in | 5 – 25 µm / 0.0002 – 0.001 in | 5 – 30 µm / 0.0002 – 0.0012 in |
| Crystal structure | Amorphous or fine microcrystalline | Dense acicular crystals | Larger prismatic crystals |
| Porosity | High, good paint absorption | Medium‑low, controllable with activators | Medium, absorbs oils and waxes well |
| Corrosion resistance (unsealed) | Moderate (salt spray hours ASTM B117: 24–48 h) | Good (salt spray hours: 72–150 h) | Good (salt spray hours: 72–120 h) |
| Wear resistance | Low | Medium | High (ideal for bearings, gears) |
| Surface hardness | Low (~ 1 GPa / 145 kpsi) | Medium (~ 2 GPa / 290 kpsi) | High (~ 3 GPa / 435 kpsi) |
Process parameters
Section titled “Process parameters”Bath and operating conditions
Section titled “Bath and operating conditions”| Parameter | Iron phosphate | Zinc phosphate | Manganese phosphate |
|---|---|---|---|
| H₃PO₄ concentration | 2 – 10 % vol. | 5 – 15 % vol. | 5 – 15 % vol. |
| Bath temperature | 50 – 70 °C / 122 – 158 °F | 50 – 70 °C / 122 – 158 °F | 90 – 100 °C / 194 – 212 °F |
| Working pH | 4,0 – 5,5 | 2,0 – 3,5 | 2,0 – 3,0 |
| Immersion time | 2 – 10 min | 5 – 20 min | 15 – 30 min |
| Typical accelerator | Sodium nitrite (NaNO₂) or hydrogen peroxide | Sodium nitrite, chlorate or nitrate | Sodium nitrite or peroxide |
| Pre-activation | Optional (alkaline rinse) | Recommended (pre-dip with colloidal titanium) | Mandatory (titanium activator) |
| Post-treatment | Final acid wash + passivation with Cr(III) or polymer | Sealing with oil, wax or paint | Impregnation with lubricating oil or wax |
Applicable materials
Section titled “Applicable materials”| Base material | Iron phosphate | Zinc phosphate | Manganese phosphate |
|---|---|---|---|
| Carbon and low alloy steel | Excellent adhesion | Excellent adhesion | Excellent adhesion |
| Cast iron | Good adhesion | Good adhesion | Excellent adhesion and lubricity |
| Aluminum alloys | Poor (needs strong acid attack) | Acceptable with fluoride additives | Not recommended |
| Zinc and cadmium | Not applicable | Good adhesion | Not applicable |
| Stainless and high alloy steel | Generally not coatable (acid resistant) | Generally not coatable | Generally not coatable |
Advantages and limitations
Section titled “Advantages and limitations”| Aspect | Advantages | Limitations |
|---|---|---|
| Cost | Low operating and equipment cost process | Bath maintenance and contaminant control increase indirect costs |
| Corrosion resistance | Good economical protection in indoor environments; noticeably improves with subsequent sealing | Limited outdoor protection without additional organic coating; without sealing, resistance is inferior to other metallic coatings |
| Paint adhesion | Excellent mechanical anchoring for liquid and powder paints | If phosphate is too thick or porous, it can absorb paint unevenly |
| Lubricity | Manganese and zinc phosphate effectively retain lubricating oil in wear applications | Requires subsequent impregnation with oil or wax to be effective |
| Dimensional limitations | Barely modifies critical dimensions (minimum thickness) | Parts with complex geometries may suffer lack of coverage in deep cavities if not properly agitated |
| Environment | Modern processes use hexavalent chromium-free passivates (Cr³⁺) | Wastewater contains phosphates and heavy metals that require specific treatment |
Selection guide
Section titled “Selection guide”| Main requirement | Recommendation | Justification |
|---|---|---|
| Economical paint base on steel | Iron phosphate | Very low cost, thin thickness, good porosity that anchors paint |
| Maximum corrosion resistance with paint (automotive, appliances) | Zinc phosphate | Dense and resistant crystalline structure, formable for spray and immersion, excellent base for e-coat |
| Wear components: gears, cams, bearings | Manganese phosphate | Hard crystals that store oil, high resistance to galling and adhesive wear |
| Anti-corrosion protection for firearms and military equipment | Zinc or manganese phosphate (Parkerizing) | Durable and protective finish with oil retention; manganese gives higher surface hardness |
| Aluminum parts requiring phosphating | Zinc phosphate with fluoride | Fluoride ions activate the aluminum surface and allow formation of a zinc phosphate layer |
| Hardware items with dark decorative appearance | Manganese phosphate with oil | Provides a uniform matte black finish and some anti-corrosion protection |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the typical thickness of an industrial zinc phosphate coating?
Section titled “What is the typical thickness of an industrial zinc phosphate coating?”The thickness of a zinc phosphate layer ranges from 5 µm (0.0002 in) to 25 µm (0.001 in) depending on immersion time and bath formulation, with average thicknesses of 7–12 µm (0.0003–0.0005 in) achieved in spray processes for automotive applications.
What bath temperature is required for manganese phosphating?
Section titled “What bath temperature is required for manganese phosphating?”The manganese phosphate bath temperature is maintained between 90 °C and 100 °C (194 °F – 212 °F), close to boiling, which favors rapid crystallization and a controlled thickness layer.
How long does the iron phosphating immersion process take?
Section titled “How long does the iron phosphating immersion process take?”The standard immersion time for iron phosphating is 2 to 10 minutes, frequently 5 minutes, after which a coating of 1 to 5 µm (0.00004 – 0.0002 in) is obtained, suitable as a base for powder painting.
How many salt spray hours does an oil-sealed zinc phosphate coating withstand?
Section titled “How many salt spray hours does an oil-sealed zinc phosphate coating withstand?”A zinc phosphate coating sealed with anti-corrosion oil can exceed 96 hours of exposure in a salt spray chamber according to ASTM B117, while unsealed it barely reaches 48–72 hours.
At what maximum service temperature can a phosphate coating be used without degradation?
Section titled “At what maximum service temperature can a phosphate coating be used without degradation?”Phosphate coatings begin to dehydrate and lose protective effectiveness from 200 °C (392 °F), so they are not recommended for continuous applications above 150 °C (302 °F) without specific refractory sealing.
What is the approximate surface hardness of a manganese phosphate coating?
Section titled “What is the approximate surface hardness of a manganese phosphate coating?”The microhardness of a manganese phosphate coating is around 3 GPa (435 kpsi), sufficient to reduce the coefficient of friction and galling in gear steels if the layer is impregnated with lubricant.
References
Section titled “References”- efunda.com: https://www.efunda.com/processes/surface/conversion_coatings.cfm
- manufacturingguide.com: https://www.manufacturingguide.com/en/iron-phosphating