Iron, zinc or manganese phosphate: which coating should you use?

This guide covers phosphate conversion coatings on carbon and low-alloy steel, under paint, powder or oil. It does not cover phosphating aluminum or zinc-coated steel, phosphate-free pretreatments such as zirconium, or parts whose drawing already names a specification, type and class, where the drawing governs.

Which conditions decide the phosphate coating type for steel?

Five conditions decide the phosphate coating type for steel: the final finish (paint or oil), the service corrosivity category, the salt spray hours on the drawing, sliding or forming contact, and the stages your pretreatment line has. The finish comes first. Iron phosphate is amorphous and does not absorb oils, lubricants or waxes, so it serves only as a corrosion underlayer and bonding base for organic coatings, while zinc and manganese phosphates are crystalline and hold oil (Rudy, 2022).

Exposure comes next. ISO 12944-2:2017 defines the atmospheric corrosivity categories (ISO, 2017), and a paint maker's guide to ISO 12944 describes C3 as urban and industrial atmospheres with moderate sulfur dioxide and coastal areas with low salinity, C4 as industrial and coastal areas with moderate salinity, and C5 as aggressive industrial atmospheres and coastal areas with high salinity (Cloverdale Paint, accessed 2026). Contact matters for moving parts: manganese phosphate is used on camshafts, gears and piston rings to prevent galling during break-in (Rudy, 2022). The line decides what you can run, because zinc phosphate needs a conditioning rinse and more stages than iron phosphate (Products Finishing, 2008).

The table below compares the three phosphate coatings on the properties that decide fit.

Which phosphate coating fits which steel part?

The phosphate coating that fits a steel part follows from its finish, its exposure and its contact: iron phosphate for indoor painted parts, light zinc phosphate for painted parts outdoors, heavy zinc phosphate for oiled parts in storage or forming, and manganese phosphate for sliding parts. The decision table below sets out which phosphate coating to specify for each set of conditions.

The row most often misread is the C4 to C5 row: the 240 to 1,440 hours cover C4 Low to C5 High durability, C5 Very High uses the cyclic ageing test, and the hours apply to the whole paint system in an ISO 12944-6:2018 test, not to the phosphate alone, so a pass belongs to the pretreatment, primer and topcoat together (ISO 12944-6:2018, via Cloverdale Paint, accessed 2026). ISO 12944-6:2018 assesses paint systems for carbon steel structures on uncoated, hot-dip galvanized or thermal-sprayed steel (ISO, 2018), so for phosphated parts treat the hours as a target agreed with the buyer. For a line of three or five stages, iron phosphate is the practical choice, because a zinc phosphate line needs a conditioning rinse and a minimum of seven stages (Products Finishing, 2008).

Which phosphate coating gives the best corrosion protection under paint?

Light zinc phosphate gives the best corrosion protection under paint of the three phosphate coatings; heavy zinc and manganese phosphates are mainly oil-holding bases, though MIL-DTL-16232H allows heavy zinc phosphate where both paint and supplemental oil are required (MIL-DTL-16232H, 2020). A 2022 overview states that light to moderate zinc phosphates provide the best corrosion protection versus the other phosphates, which is why they are ideal as a pretreatment before paints and powder coats (Rudy, 2022). TT-C-490G gives zinc phosphate Type I a coating weight of 150 to 500 mg/ft² (1.6 to 5.4 g/m²) for use under organic coatings (TT-C-490G, 2019). TT-C-490 is at revision J (6 September 2024) as of 15 October 2026 (DLA, 2026); the TT-C-490 values in this guide are from revision G (2019), so confirm them against revision J before writing a drawing note.

More coating weight does not mean more protection under paint. MIL-DTL-16232H states that light phosphate coatings used for a paint base are covered by other specifications such as TT-C-490, and that its heavy coatings are intended as a base for holding supplemental coatings (MIL-DTL-16232H, 2020). Heavy zinc coatings build crystal upon crystal, which makes them more susceptible to physical failure (Rudy, 2022). A 2017 answer in Products Finishing adds that the zinc phosphate TT-C-490 type requires grain refinement, and that smaller, denser crystals give a more adherent and corrosion-resistant base for paint (Products Finishing, 2017).

The phosphate is only one layer. ISO 12944-6:2018 test results are intended as an aid in selecting paint systems, not as exact information for determining durability (ISO, 2018), so compare phosphate types on panels painted with your own primer and topcoat.

When is manganese phosphate the right choice for steel parts?

Manganese phosphate is the right choice for steel parts that slide or mesh under load and need a break-in coating that holds oil, such as gears, camshafts, piston rings and bearing surfaces. Manganese phosphate is applied to steel for wear resistance and to prevent galling and control wear during break-in (Rudy, 2022). Its coarse, black crystalline structure improves the retention of rust-preventive oils and lubricants (Rudy, 2022).

MIL-DTL-16232H sets the choices by class. Type M Class 2 is impregnated with lubricating oil or rust preventive and may be specified to prevent wear and assist the break-in of bearing surfaces; with oil it must pass at least 48 hours of salt spray (MIL-DTL-16232H, 2020). Class 4 is chemically converted and is recommended for reducing sliding friction in preference to Class 2, and it reduces the variation in torque versus tension usually found with threaded fasteners (MIL-DTL-16232H, 2020).

Manganese phosphate is not a corrosion coating on its own. Without supplementary treatment, Type M Class 3 must pass only 1.5 hours of salt spray (MIL-DTL-16232H, 2020), and the specification notes that the supplementary coating provides most of the corrosion protection. Type M is more resistant to alkaline environments than zinc phosphate but should not be exposed above 121 °C (250 °F) (MIL-DTL-16232H, 2020).

Is iron phosphate enough for outdoor painted steel?

Iron phosphate is enough for outdoor painted steel only when the complete paint system has been tested and approved for the corrosivity category; without that evidence, zinc phosphate is the safer base outdoors. Iron phosphate coatings are the lightest phosphate coatings and provide a corrosion protection underlayer and a bonding base for organic coatings (Rudy, 2022), but a 2008 Products Finishing answer states that pretreatments alone are generally not considered an adequate coating to survive even a mildly corrosive environment, and that long-term, high-corrosion-resistance performance will usually require zinc phosphate with a primer and topcoat (Products Finishing, 2008).

Approved exceptions exist. A pretreatment maker reported in 2025 that its iron phosphate followed by a heavy-metal-free passivation achieved QUALISTEELCOAT approval for corrosivity classes C4 and C5 at one paint shop (Kluthe, 2025). That approval covers that process combination at that shop, so it does not transfer to another line or paint.

Coverage also limits any phosphate. In a 2017 case, castings with zinc phosphate and epoxy powder rusted on internal surfaces; the expert's answer was that meeting a 500-hour ASTM B117 salt spray requirement there would need something beyond zinc phosphate alone, such as a dip paint (Products Finishing, 2017). For outdoor parts with hollow sections, ask how the paint reaches every surface before choosing the phosphate.

What should you ask a supplier before changing phosphate type?

Ask a phosphate supplier six questions before changing phosphate type, and record the answers in the process sheet.

1. Which specification, type and class will the coating meet? MIL-DTL-16232H asks the buyer to state the type, class and supplementary treatment (MIL-DTL-16232H, 2020), and the contract should name the TT-C-490 revision it invokes.

2. How many stages does the process need? A zinc phosphate line needs a minimum of seven stages, including a conditioning rinse, and some lines stretch to ten (Products Finishing, 2008).

3. What coating weight will you control, and how often? TT-C-490G requires coating weight to be controlled and tested every four hours of line operation (TT-C-490G, 2019).

4. Which salt spray results exist with my primer, topcoat or oil, and on which test?

5. How is the bath controlled? Iron phosphate needs titration and pH, while zinc and manganese need total acid, free acid and iron titrations (Rudy, 2022).

6. What do the safety data sheet (SDS) and the local wastewater permit require? The zinc phosphate stage generates considerable sludge that must be removed and disposed of (Products Finishing, 2008); handle bath additions and sludge as the SDS and your permit direct.

When should you call a specialist about a phosphate coating change?

Call a coatings or corrosion specialist about a phosphate coating change when the part is hardened, close-fitting or under a contract specification, because each case adds rules beyond the phosphate type. For steels of 39 HRC or harder, MIL-DTL-16232H requires an embrittlement relief heat treatment at 98 to 107 °C for at least eight hours, or a rest of 120 hours at room temperature before use, unless the order states relief is not required (MIL-DTL-16232H, 2020). Parts of 39 HRC or harder that were machined, ground, cold formed or cold straightened after heat treatment must also be stress relieved at 177 to 204 °C for at least four hours before coating, unless otherwise specified (MIL-DTL-16232H, 2020). TT-C-490G states that ferrous materials of HRC 39 or greater shall not be cleaned or derusted with phosphoric acid compounds unless the materials and process have been pre-approved (TT-C-490G, 2019).

Close fits need a check too. MIL-DTL-16232H warns that coating buildup may cause apparent interference on assembly where close fits are specified (MIL-DTL-16232H, 2020), so a change from iron to heavy zinc or manganese phosphate can affect threads and bores. Chromium also matters: for all types and classes, unless contraindicated on drawings or contract, MIL-DTL-16232H states that materials containing hexavalent chromium should not be permitted on coated end items (MIL-DTL-16232H, 2020). A specialist can set the coating weight, the test plan and the drawing note together. Working such a change from your own TDS, SDS and line history is the kind of case Lubinpla structures into one cited document for your review.