Phosphating of metals: Knowledge for purchasers & engineers
Phosphating is one of the most widely used chemical surface treatment processes in the metal sector - and also one of the most underestimated. If you know the differences between zinc, manganese and iron phosphating, you can make better procurement decisions and avoid expensive reworking.
What is phosphating?
During phosphating, the metal surface reacts with phosphate in an acidic bath.
It reacts chemically with the surface of the workpiece.
A crystalline or amorphous conversion layer is formed. This layer consists of metal phosphates. Firmly bonded to the base material, not just applied.
The phosphate layer itself is practically non-conductive. However, its microporous structure allows current to flow to the substrate. This makes it ideal as a pre-treatment for cathodic dip coating and powder coating. It is a hindrance to resistance welding - joining takes place before phosphating.
The process is a conversion coating. Unlike galvanic processes, no foreign material is applied. Instead, the base material is converted into a protective compound. Suitable materials are primarily steel and cast iron, as well as zinc and aluminum to a limited extent, depending on the process variant.
At a glance: What is phosphating used for?
- Corrosion protection (alone or as an undercoat)
- Bonding base before painting, powder coating or cathodic dip coating
- Run-in aid and friction reduction for moving parts
- Preparation for cold forming (drawing, cold extrusion)
Phosphating: Process explained
The quality of the phosphate coating depends largely on the pre-treatment. Oils, scale, rust or anti-corrosion grease on the workpiece surface prevent the formation of a uniform coating. Typical pre-treatment steps are
Depending on the application, phosphating is followed by
- Passivation (chromium(VI)-free according to RoHS/REACH)
- Oiling or waxing (especially with manganese phosphating)
- Painting, KTL, powder coating or wet paint
- Drying oven for paint systems
Phosphating: Advantages and disadvantages
| Advantages | Disadvantages |
| Cost-effective compared to galvanic processes | No independent long-term corrosion protection without post-treatment |
| Excellent paint adhesion - better than bare steel | Phosphate sludge as hazardous waste - plan for disposal costs |
| Dimensionally stable: no relevant dimensional change to the component | Not suitable for all materials (stainless steel, copper, non-ferrous metals) |
| Easily scalable - drum and rack products possible | Process-sensitive: pre-treatment must be precisely adhered to |
| Suitable for many geometries due to immersion application - consultation with the coater recommended for cavities | Short time window between phosphating and painting |
| Multifunctional: corrosion protection, primer and tribology in one | Limited coating hardness - no substitute for hard chrome plating or nitriding |
⚠️ Important note: Hydrogen embrittlement in high-strength steels
Acid pickling can cause hydrogen embrittlement in high-strength steels (Rm > 1000 MPa) - such as springs, class 10.9/12.9 screws or bearing steels. VDA 235-104, DIN EN ISO 9588 and ISO 4042 therefore prescribe tempering at 190-230 °C.
The duration is 2-24 hours. Tempering must be carried out within 4 hours of treatment. As an alternative, more and more Zinc flake systems (Geomet, Delta-Protekt) are used, which do not require acidic staining.
Phosphating: Process comparison
Zinc phosphating
Zinc phosphating is the most commonly used phosphating process.
It takes place at 30 to 60 °C. This produces a gray, crystalline zinc phosphate layer on the steel surface. According to DIN EN ISO 9717, the layer is specified as a mass per unit area in g/m²:
- 1.5-4.5 g/m² (approx. 1-4 µm) as KTL pre-treatment in the automotive industry
- 5-10 g/m² (approx. 3-8 µm) for corrosion protection with oil or wax post-treatment
- up to 20 g/m² for cold forming
Classic trication zinc phosphating was the industry standard for decades. It is still widely used in existing plants. In new systems today, zirconium and silane-based thin-film processes (e.g. Bonderite M-NT, Oxsilan, TecTalis) often replace the classic Zinc phosphate pre-treatment.
The crystal size of the zinc phosphate layer can be specifically controlled. Fine-grained layers are created by accelerators, nickel or manganese additives - they are ideal as a paint primer. Coarser crystals are better suited for forming operations or as a lubricant carrier.
Manganese phosphating
Manganese phosphating takes place at 88 to 98 °C. This is significantly hotter than zinc or iron phosphating. It forms a manganese phosphate layer in dark gray to black.
The layer has a clearly porous structure. The layer is specified as a mass per unit area:
- 7-30 g/m² (approx. 5-20 µm) in industrial standard
- up to approx. 25 µm in special cases
Higher layer thicknesses are not advisable. The Manganese phosphate layer becomes brittle from approx. 25 µm and loses its adhesive strength.
The pores of the manganese phosphate layer absorb oil very well. This makes Manganese phosphating the first choice for highly stressed components. Transmission parts, crankshafts, camshafts, piston rings and firearm parts benefit from the combination of running-in aid and permanent oil binding.
Without re-oiling, corrosion protection is limited. For this reason, manganese-phosphated components are almost always treated with an anti-corrosion oil.
Iron phosphating
Iron phosphating runs at lower temperatures (40-70 °C). It is much easier to handle than the zinc or manganese processes. The resulting layer is very thin and non-crystalline (amorphous). The layer thickness is specified as a mass per unit area:
- 0.2-1.0 g/m² (approx. 0.2-1 µm)
This thin layer alone offers only moderate protection against corrosion. Its main area of application is as a favorable preparation for subsequent painting. It is used when very high demands are not placed on corrosion protection.
Iron phosphating is often used in mass production lines (e.g. for household appliances or furniture frames). One advantage is that the operating costs are low. The process water can be treated more easily than with zinc or manganese baths.
Phosphating of metals: All materials
Not every metal is equally suitable for phosphating. Suitability depends on whether the material can form a stable conversion layer with the phosphating solution. The decisive factor here is the iron content in the base material.
1. phosphating of steel - ✅ Very suitable
Steel and structural steel are the classic applications for phosphating - all three processes work reliably and deliver reproducible coating qualities.
Zinc phosphating is used as a primer before cathodic dip painting, powder coating or wet paint. Manganese phosphating is suitable for highly stressed components such as shafts, gear wheels and springs. Iron phosphating is the most economical option for mass-produced parts with subsequent painting.
For purchasers, steel from phosphating is the most straightforward case - no process change, no special bath chemistry, full process control.
2. phosphating of cast iron - ✅ Very suitable
Cast iron behaves chemically similar to steel and can be easily phosphated. Manganese phosphating is particularly suitable for Engine blocks, cylinder liners, crankcases, timing chain components and pump parts.
The dark, oil-absorbing layer protects against inlet wear and corrosion. The somewhat rougher cast structure requires careful degreasing and pre-treatment. This is because mold release agent residues from the casting process can impair the coating quality.
3. phosphating of steel - ✅ Well suited
Galvanized steel - hot-dip galvanized or electrogalvanized - can be phosphated, but requires an adapted bath chemistry. The acid concentration must be reduced so as not to over-pickle the zinc surface. The resulting phosphate layer serves as an excellent primer before painting and significantly improves the adhesive strength compared to bare galvanized parts. In the automotive industry, phosphated galvanized steel is a proven combination for the highest corrosion protection requirements.
4. phosphating of aluminum - ⚠️ Conditionally suitable
Aluminum can be used in modern Multi-metal zinc phosphating baths (trication Zn-Ni-Mn) with added fluoride can also be processed. This is the current state of the art in automotive body production. Without fluoride, aluminium dissolves in the acidic bath and destabilizes the bath chemistry.
Pure iron phosphating as an inexpensive adhesion promoter before powder coating or wet paint is possible, but increasingly untypical. More frequently used today are Zirconium/silane-based conversion coatings, chromium(III) passivation or anodizing - especially for higher corrosion protection requirements.
Buyers should inform the job coater in advance if aluminum and steel are to be mixed in one batch.
5. phosphating of zinc die casting - ⚠️ Conditionally suitable
Zinc die casting behaves in the same way as aluminum: only iron phosphating is suitable for practical use, and only as a paint primer. The surface of die-cast parts is often uneven. This is due to blowholes, parting lines and residues of mold release agents. Thorough pre-treatment is therefore particularly important.
Where high corrosion protection requirements apply, zinc die casting is generally not an ideal material for phosphating. Here, galvanic processes or painting without a phosphating step are recommended.
6. phosphating of magnesium - ❌ No longer common
Magnesium is used in industrial practice Hardly ever classically phosphated. Classic methods such as the DOW-7 method (MIL-M-3171) are largely history today.
Plasma electrolytic oxidation (PEO/MAO) is state of the art. It is offered, for example, in the form of magoxide coatings, keronites or tagnites. Chromium(III) conversion coatings are also included.
Examples include Magpass-Coat and SurTec 650 for magnesium. Zirconium-based conversion processes are also used as paint primers.
Anyone who needs to have magnesium components coated should look specifically for specialists in PEO/MAO or Cr(III) passivation. Traditional phosphating companies are not the right contact here.
7. phosphating of stainless steel - ❌ Not suitable
Stainless steel cannot be phosphatized effectively. The natural chromium oxide layer (passive layer), which gives stainless steel its corrosion protection, prevents the chemical reaction with the phosphating solution.
Breaking up this layer through aggressive pre-treatment is technically possible, but not economically viable. And destroys the corrosion properties of the material.
Chemical passivation is the standard surface finishing process for stainless steel. Today, citric acid in accordance with ASTM A967 is preferred.
8. phosphating of copper, brass & bronze - ❌ Not suitable
Copper materials react too strongly and uncontrollably in the phosphating bath. The resulting copper phosphates destabilize the bath and lead to unusable coatings.
For copper, brass and bronze, galvanic processes such as tin plating, nickel plating and gold plating are used instead. Direct lacquering without a phosphating step is also used.
Buyers should pay particular attention to mixed assemblies made of steel and copper materials: Copper parts must not enter the phosphating bath.
Phosphating vs. other corrosion protection methods
When is phosphating the right choice? And when is it better to use galvanizing, passivation or organic coatings?
Phosphating is optimal if the coating is to be used as a System solution with lacquer or oil is intended. And not as corrosion protection in its own right.
The coating alone cannot withstand prolonged exposure to moisture. Protection is created in combination with a top coat or a high-quality corrosion protection oil.
This is often superior to electroplated zinc coatings in terms of adhesive strength and tribological properties.
Compared to passivation, phosphating offers thicker layers and better mechanical properties. Compared to electroplating, there is no need to use heavy and precious metals, which reduces costs and environmental requirements.
Relevant standards and specifications
The following standards are of central importance for procurers and designers when phosphating services are put out to tender or drawings are specified:
- DIN EN ISO 9717:2018-03 - Phosphate conversion coatings on metals (main standard; has replaced the older DIN 50942)
- AMS 2480 - Zinc phosphating as a paint primer (aerospace)
- AMS 2481 - Heavy phosphating (manganese/zinc) for wear and run-in protection (aerospace, defense technology)
- MIL-DTL-16232G - US military specification for phosphate conversion coatings (Type M = manganese, Type Z = zinc, Class 1-4); active and valid
- DIN EN ISO 9227:2017-07 - Salt spray test (NSS, AASS, CASS) to evaluate corrosion protection
- OEM-specific factory standards (e.g. Volkswagen TL standards, BMW GS standards) - binding for automotive suppliers; the OEM specifies the exact reference standard in the supplier portal.
The tender should always state the required coating mass (g/m²). Also state the process (zinc, manganese or iron).
Also specify the post-treatment. Also specify the corrosion protection requirements. For example, a salt spray test in accordance with DIN EN ISO 9227.
Checklist for purchasers: Requirements for the job coater
- Certification according to DIN EN ISO 9001 or industry-specific (IATF 16949 for automotive)
- Proven bath control and documented process parameters
- In-house testing equipment for coating thickness (XRF or cross-section) and corrosion resistance
- Experience with the required process variant (zinc / manganese / iron)
- Environmental certification and proven REACH-compliant bath chemistry
- References from our own industry or for comparable part geometries
- Clear information on delivery times and minimum batch sizes
What does phosphating cost? Influencing factors for buyers
Phosphating is one of the more cost-effective surface treatments. The unit costs are influenced by several factors:
Part geometry and wall thickness: Complex cavities require a higher process effort for complete wetting.
Lot size: Phosphating is particularly suitable for large series and mass-produced parts - rack or barrel products reduce unit costs considerably.
Choice of procedure: Manganese phosphating is more expensive than iron phosphating due to higher temperatures and bath maintenance.
Pre-treatment effort: Heavily contaminated or rusted parts increase the costs for cleaning and pickling processes.
Follow-up treatment: Additional oiling, passivation or direct painting have a significant impact on the overall costs.
Environmental and disposal costs: Phosphate baths produce sludge (phosphate sludge) that must be disposed of as hazardous waste - costs that job coaters factor in
Phosphating: Industries & Applications
Phosphating is one of the few surface processes that is firmly established in series production across all industries. The combination of low costs, high process speed and multifunctionality makes it the first choice.
This applies wherever steel has to be coated in large quantities.
| Industry | Typical components | Procedure | Primary goal |
| Automotive | Bodywork, springs, transmission parts | Zinc, manganese | KTL primer, run-in |
| Mechanical engineering | Shafts, housings, hydraulics | Zinc, manganese | Corrosion protection, tribology |
| Defense technology | Weapon parts, locks | Manganese | Wear protection, optics |
| Aerospace | Structural components, fasteners | Zinc, manganese | Corrosion protection according to AMS |
| Household appliances | Sheet metal parts, housing | Iron | Paint primer (powder) |
| Furniture / Construction | Steel furniture, profiles, frames | Iron | Paint primer, throughput |
| Standard parts / screws | Screws, springs, stamped parts | Manganese, zinc | Drum goods, bulk price |
Phosphating at FACTUREE
As a digital procurement platform for drawing parts, FACTUREE offers access to a qualified network of specialized contract coaters. These companies take on the phosphating of individual components through to series production.
Are you looking for a contract coater for phosphating in your area? For regional inquiries, we can find the right company directly and easily. Be it phosphating in Berlin, Munich, Hamburg or other locations. Alternatively, we can draw on our nationwide and international network if capacity, specialization or certification have priority.
In addition to steel and cast iron, we also cover specialized applications. Phosphating of galvanized steel in accordance with current industry standards (e.g. VW TL 183, BMW GS 90010, IATF 16949) and aerospace standards (AMS 2480/2481). Contact us - we will clarify feasibility and availability quickly and directly.
Your advantages with procurement via FACTUREE: Qualified network of certified electroplating companies (DIN EN ISO 9001, NADCAP, etc.) Quick quotations for standard and special coatings | Transparent specific requirements directly in the inquiry Support with the selection of standards (DIN EN ISO 4042, DIN 50961 etc.) Flexibility with quantities: Prototype to large series
