What is phosphate coating on steel and what is it used for?

How does a phosphate coating form on steel?

A phosphate coating forms on steel when a dilute phosphoric acid bath dissolves a little iron, the acid near the surface is used up, and the local rise in pH makes insoluble metal phosphates crystallize on the part. In the bath, free phosphoric acid attacks iron at microanodes on the steel while hydrogen evolves at microcathodes (Narayanan, 2005). The resulting local depletion of free acid raises the pH at the metal and solution interface, which shifts the bath equilibrium toward insoluble tertiary phosphates that deposit rapidly (Narayanan, 2005).

Because part of the base metal is converted into the film, the coating is an integral part of the surface rather than a layer laid on top (Narayanan, 2005). The reaction involves a minor etching of the basis metal (Rudy, 2022). Without help the reaction is slow, because hydrogen polarizes the cathodic sites, so practical baths use accelerators such as nitrite, nitrate or chlorate (Narayanan, 2005).

Surface preparation decides much of the result. Zinc phosphate solutions have no soil-cleaning capacity, so oil and grease must be removed beforehand (Rudy, 2022). For heavy coatings to MIL-DTL-16232H, phosphating comes after all machining, forming, welding and stress-relieving heat treatment, but before hydrogen embrittlement relief, unless the contract says otherwise (US DoD, 2020).

What are zinc, iron and manganese phosphate coatings?

Zinc, iron and manganese phosphate coatings are the three main types of phosphate coating, and each forms a different deposit that suits a different job (Rudy, 2022). Iron phosphate is the lightest and is used under paint; zinc phosphate ranges from light paint bases to heavy oil-holding coatings; manganese phosphate is the heavy coating for sliding parts. Iron phosphate is also the simplest bath to control, by titration and pH measurement, while zinc and manganese baths need total acid, free acid and iron titrations (Rudy, 2022).

The table below shows which phosphate type does which job on steel and which specification names it.

The key split is light versus heavy: MIL-DTL-16232H states that light phosphate coatings for a paint base are covered by other specifications such as TT-C-490 (US DoD, 2020). TT-C-490 is at revision J (6 September 2024) as of 15 October 2026 (DLA, 2026); the TT-C-490 types and values in this post are from revision G (2019), so confirm them against revision J. ISO 9717:2024 is the international specification for phosphate conversion coatings on ferrous materials, aluminium, zinc and their alloys (ISO, 2024). How to choose between them is covered in iron, zinc or manganese phosphate: which to use.

Does a phosphate coating protect steel from rust on its own?

A phosphate coating protects steel from rust on its own only for a short time, because the crystal layer is porous; the oil, preservative or paint on top provides most of the protection. MIL-DTL-16232H states that heavy phosphate coatings are intended as a base for supplemental coatings which provide the major portion of the corrosion resistance (US DoD, 2020).

The salt spray minimums in MIL-DTL-16232H show the gap. Without supplementary treatment, Type M and Type Z coatings in classes 1 to 3 must show no corrosion after 1.5 and 2 hours of salt spray respectively (US DoD, 2020). With oil or preservative, Type M Class 2 must last at least 48 hours and Type Z Class 2 at least 72 hours (US DoD, 2020). Chemically converted Class 4 coatings must last 24 hours bare, and 72 hours only if a supplementary treatment is applied (US DoD, 2020).

The pores explain both sides. Fissures and channels between crystals reach the base metal, with porosity of the order of 0.5% to 1.5% of the surface (Narayanan, 2005), yet the same pores let the coating absorb and hold lubricant (Narayanan, 2005). Narayanan's 2005 review describes a hot (70 to 80 °C) dilute chromic acid rinse as the usual seal for the pores (Narayanan, 2005), but MIL-DTL-16232H states that materials containing hexavalent chromium should not be permitted on coated end items unless drawings or contract say otherwise (US DoD, 2020).

What does a phosphate coated part look like?

A phosphate coated steel part looks gray to black for zinc and manganese phosphate, and blue to iridescent for iron phosphate (US DoD, 2020; Rudy, 2022). The crystals are fine: MIL-DTL-16232H requires a uniform crystalline texture with a pattern not readily visible to the unaided eye, gray to black and not mottled (US DoD, 2020).

The same specification lists what a good coating must not show: white stains from dried phosphating solution, rust and fingerprints (US DoD, 2020). It also lists what is not a defect: brown or orange stains from a chromic acid rinse, and color variation caused by heat treatment, cold work or base metal composition (US DoD, 2020). TT-C-490G sets a similar rule for zinc phosphate paint bases, which must be gray to black and free of smut, powder, corrosion products and white residues (GSA, 2019).

On an oiled part, the oil is part of the protection. MIL-DTL-16232H notes that supplemental oil may be removed from phosphate coated items by contact with absorbing material (US DoD, 2020), so packaging and handling matter. The coating also adds thickness: coated items must meet drawing dimensions with allowance for the phosphate coating buildup (US DoD, 2020).

Phosphate coating vs other conversion coatings

Phosphate coating is one of three chemical conversion coating families in Narayanan's review, alongside chromating and oxalating, named after their phosphate, chromate and oxalate constituents (Narayanan, 2005). All conversion coatings form by a chemical reaction that turns part of the base metal into the film, without an external current (Narayanan, 2005). What sets phosphating apart is a reasonably hard, electrically non-conducting phosphate layer that is considerably more absorptive than the metal (Narayanan, 2005).

The pretreatment specification TT-C-490G shows where phosphate sits among the alternatives under paint. It lists zinc phosphate and aqueous iron phosphate next to organic pretreatments, inorganic pretreatments with more than 50% inorganic compounds in the dried film, and conversion coatings on aluminum validated under MIL-DTL-5541 (GSA, 2019). For bare steel parts that need a dark finish and oil instead of paint, the usual comparison is black oxide, covered in phosphate coating vs black oxide.

The practical choice follows the job: paint adhesion points to iron or light zinc phosphate, oil-held rust protection to heavy zinc phosphate, and break-in of sliding parts to manganese phosphate. The specified coating mass for each use is set out in phosphate coating weight by type. When a phosphated part rusts or a paint film lifts, Lubinpla can work through your process records, data sheets and test results and return one cited document for your review.

Related terms

• Conversion coating: a film formed by chemical reaction with the base metal, not deposited on top of it.

• Phosphating, phosphatizing or Parkerizing: other names for applying a phosphate coating to steel.

• Supplementary treatment: the oil, preservative, dye or paint applied over a phosphate coating.

• Accelerator: an oxidizing additive, such as nitrite or chlorate, that speeds coating formation.

• Black oxide: a dark conversion coating for steel that is also used with oil.

• Phosphor coating: a light-emitting layer on lamps and LEDs, unrelated to phosphate coating on steel.