Two conversion coatings, two different purposes. Zinc phosphate carries soap in its porous structure and is the standard for cold forming; manganese phosphate works against sliding wear with a hard, dense layer. We compare crystal structure, coating weight and process differences.
Phosphating is not a single operation but a family of operations sharing the same chemical principle. When the metal cation in the bath changes, so do the composition, hardness, size and pore structure of the crystal growing on the surface. The two types most often met in industry are zinc phosphate and manganese phosphate. Both are conversion coatings, both grow out of the metal surface itself; but they are not alternatives to one another. They are designed for different jobs.
This article compares the two coatings at the level of chemistry and crystal structure, explains why zinc phosphate is the standard on wire and tube drawing and cold forging lines, when manganese phosphate really is the right choice, and the parameters to look at when deciding. If you want to refresh the basic mechanism of phosphating, what is phosphating is a useful starting point. A note for transparency: Kimfosan's phosphate coating range, KİMSOL, is zinc phosphate based; the manganese phosphate information below is impartial technical information given for comparison.
Both baths are acidic phosphoric acid solutions and both work through a local pH rise in the zone near the surface: metal dissolves, acid is consumed, dissolved phosphate salts exceed the precipitation limit and crystallise on the surface. What makes the difference is the cation in solution.
The essential distinction here is pore structure, and it determines the fate of the two coatings. The porous skeleton of zinc phosphate behaves like a store: it takes in a liquid or solid lubricant and gives it back to the contact surface under pressure. The denser, harder texture of manganese phosphate behaves less like a carrier and more like a wear surface; it holds oil but under hard contact it also works as a sacrificial layer itself.
Crystal size in both types depends on the quality of the activation stage. Weak activation gives a coarse, loose, dusting layer in either bath; we cover the subject in detail in the activation bath and crystal size.
In operations such as wire drawing, tube and profile drawing, cold forging and deep drawing, the contact pressure between die and metal is very high and the surface expands continuously throughout forming. The freshly exposed virgin metal surface has to be lubricated too. It is not possible for a liquid oil to hold film continuity under these conditions; the line is therefore built around drawing soap, a solid lubricant.
For the soap to do its job it needs a surface to key into. The porous crystalline skeleton of zinc phosphate does exactly that: the soap locks mechanically into the pores and, under the pressure inside the die, is fed from those voids to the contact zone. Zinc phosphate is also relatively soft and formable; as the surface elongates the layer cracks and redistributes rather than peeling off the metal. The coating itself also acts to some extent as a separator, reducing metal-to-metal contact and so the risk of cold welding (galling).
Manganese phosphate is not preferred for this work for two reasons: its pore structure is not as suited to carrying soap as zinc phosphate, and the layer is harder and more brittle; under heavy reduction it can crack and shed, building up in the die and the soap box. How the friction mechanism is set up in forming is explained step by step in friction in cold forming.
Kimfosan's zinc phosphate coating range, KİMSOL, is configured for this application class: 3–15 g/m² coating weight, 60–80 °C operating temperature, 5–15 minutes immersion, on carbon and low-alloy steels.
Manganese phosphate is used on machine parts that rub against one another. Typical application areas are engine and transmission components: gears, camshaft and tappet parts, piston rings, bearings, hydraulic components and certain fasteners. The aim is not forming but service life.
It works like this: when parts first start running together (the running-in period), the hard, dense manganese phosphate layer stops the surface asperities from scoring one another. As the layer wears it gently flattens the peaks on the opposing surface and lets the two parts bed in. At the same time the crystal structure holds oil; because the layer carries a thin oil reserve at the surface it reduces the risk of dry contact under boundary lubrication conditions. This is why manganese phosphate coated parts are generally put into service impregnated with oil.
A manganese phosphate layer is generally heavier and thicker than a zinc phosphate one, the bath runs at a higher temperature and the process time can be shorter depending on the application. The hardness of the layer is an advantage, but it is also what makes it unsuited to heavy plastic deformation. In other words, every point where manganese phosphate is strong is a disadvantage from the standpoint of wire drawing. This is not a difference in quality but a difference in design.
| Criterion | Zinc phosphate | Manganese phosphate |
|---|---|---|
| Crystal structure | Fine, needle-like/plate-shaped; distinctly porous skeleton | Blunt, densely packed; tight and hard texture |
| Layer appearance | Light grey, matt | Dark grey to black |
| Layer hardness | Relatively soft, formable; deforms with the surface | Harder and more brittle; not suited to heavy deformation |
| Typical coating weight | 3–15 g/m² (KİMSOL range) | Generally a heavier/thicker layer; set by the application |
| Typical application | Wire/tube/profile drawing, cold forging, deep drawing; pre-paint preparation | Rubbing machine parts: gears, cams, rings, bearings; running-in and wear resistance |
| Step after coating | Drawing soap (Kimkal) → drawing / cold forming | Oil impregnation → assembly and running-in |
| Lubricant compatibility | Carries solid soap in its pores; works as a soap–phosphate pair | Holds oil; acts as a reserve under boundary lubrication |
| Substrates | Carbon and low-alloy steel (KİMSOL); ferrous surfaces | Carbon and alloy steel, cast iron; ferrous surfaces |
| Process temperature | 60–80 °C (KİMSOL), immersion | Generally a higher temperature range, immersion |
The KİMSOL values in the table are the typical operating ranges of Kimfosan's zinc phosphate range. The manganese phosphate column summarises general industry practice; exact values vary with the product used and the service conditions of the part. The method for verifying coating weight is the same for both types; for the detail see coating weight measurement.
In surface preparation before painting, the aim is a thin, homogeneous, low-weight conversion layer for the paint to key into. Here the choice is generally iron phosphate (light duty, low cost) or zinc phosphate (more demanding corrosion conditions). Manganese phosphate was not designed for this work: because the layer is thick and hard it does not provide the thin, tight structure wanted under a paint film.
Kimfosan's focus is not preparation before corrosion paint but the carrier layer before cold forming. Although these two uses share the same chemistry, the target coating weight and crystal structure expectations differ; it is rarely possible to tune the same bath optimally for both.
| Application | Recommended coating | Why |
|---|---|---|
| Wire drawing (multi-pass, high reduction) | Zinc phosphate | The porous layer carries the soap; it deforms with the surface |
| Tube and profile drawing | Zinc phosphate | A continuous lubricating film is needed at die contact |
| Cold forging / deep drawing | Zinc phosphate | Reduces the risk of cold welding (galling) in heavy plastic deformation |
| Gears, cams, rings, bearings (rubbing parts) | Manganese phosphate | A hard, dense layer eases running-in, reduces wear and holds oil |
| Pre-paint surface preparation | Iron or zinc phosphate | A thin, homogeneous, low-weight layer improves paint adhesion |
| Short-term storage/transport protection | Zinc phosphate + suitable oil | The layer alone is not enough; it works together with a protective oil |
Choosing the phosphate type is not choosing a coating but choosing the step that comes after it. If soap is coming, zinc; if oil and sliding contact are coming, manganese.
In both cases the common factor determining layer quality is unchanged: the surface must genuinely be clean before coating. Neither zinc nor manganese phosphate will nucleate properly on an oily, scaled or rusty surface. The line sequence is always degreasing → rinsing/activation → phosphate coating.
If you would like to assess which phosphate configuration suits your substrate, reduction ratio and subsequent forming step, talk to our technical team. Producing phosphating chemicals and drawing soaps since 1982, Kimfosan manufactures under ISO 9001:2015, ISO 14001:2015 and ISO 45001:2018 certification at its 4,000 m² plant in Kartepe / Kocaeli; product technical data sheets are available through the document centre.
Both are conversion coatings, but their crystal structures and purposes differ. Zinc phosphate gives a fine, porous and relatively soft layer; because it carries drawing soap it is used in wire drawing and cold forming. Manganese phosphate gives a harder, denser, darker layer and is used, impregnated with oil, on rubbing machine parts for wear resistance and running-in.
Wire drawing needs a continuous lubricating film between die and metal, and that film is provided by solid drawing soap. The porous crystalline skeleton of zinc phosphate holds the soap mechanically and feeds it to the contact zone under pressure. The layer is also formable, so it does not peel as the surface elongates. Manganese phosphate has neither of these properties.
Generally on machine elements that rub against one another: gears, camshaft and tappet parts, piston rings, bearings and similar engine or transmission components. During running-in the layer helps the surfaces bed in, and by holding oil it reduces the risk of dry contact under boundary lubrication.
Iron phosphate or zinc phosphate is generally used before painting. The aim is a thin, homogeneous, low-weight layer for the paint to key into. Manganese phosphate was not designed for this work; its layer is thicker and harder than a pre-paint application needs.
Kimfosan's phosphate coating range KİMSOL is zinc phosphate based and configured for wire and tube drawing and cold forming applications. The manganese phosphate information in this article is impartial technical information given to help you select the right coating type. For a solution suited to your line, get in touch with our technical team.
Phosphate coating is not a single tank but a connected chain: degreasing, rinsing, activation, phosphating, soap and drawing. This guide summarises each stage end to end — what it does, its typical parameters and how it affects the next one.
Read →A zinc phosphate coating is a porous crystalline layer that grows on the steel surface through an acidic bath reaction. This guide explains how the layer forms, what coating weight actually means, and why the layer is indispensable in cold forming.
Read →In fastener production the surface expands several-fold; a liquid oil cannot carry that contact pressure. We explain how the porous phosphate layer and the soap film work together, and how the pair is selected by severity of deformation.
Read →