The pre-coating applied before drawing sets the ground the soap keys into. Borax and lime are carrier coatings that merely sit on the surface, while phosphate is a conversion coating chemically bonded to the metal. That difference decides how far up the reduction scale you can go.
In wire, tube or profile drawing, the surface entering the die is not bare metal: it carries a pre-coating for the soap to key into. The three options most often met on the shop floor are borax coating, lime coating and phosphating. All three are described as “preparing the surface for drawing”, but their working principles differ fundamentally, and that difference determines how far up the reduction scale your line can run without trouble.
This article covers the distinction between a carrier coating and a conversion coating, where borax and lime are sufficient, at what point phosphate becomes mandatory, and what to watch for when moving between the two systems.
During drawing, as the wire passes through a die opening smaller than its own diameter, it sees both considerable pressure and elongation across the surface. If die and metal make direct contact, friction, heating and adhesion begin; the die wears and the surface scores. The function of the pre-coating is not to prevent that contact on its own but to create a porous, retentive ground on which the dry drawing soap can key in. We explain that mechanism in detail in friction in cold forming.
In other words a pre-coating is not a lubricant in itself. Its job is to prepare the surface so the soap can form a film. If the pre-coating is weak the soap cannot key in; if the soap cannot key in, metal-to-die contact begins. This is the first link of the chain.
Borax and lime are carrier coatings applied to the surface as a solution and then dried. The wire is passed through a hot solution, the water evaporates and a thin salt film is left on the surface. That film does not bond chemically to the metal; it sits on it. Its adhesion is physical, so its resistance to mechanical loading is limited.
Phosphating is a conversion coating: the acidic zinc phosphate solution reacts with the iron at the surface, part of the metal dissolves and zinc phosphate crystals nucleate and grow in its place. The layer is not placed on the metal, it derives from it. It therefore behaves as though locked into the surface, and as the wire elongates and thins in the die the layer moves with it. How the crystal grows is explained step by step in what is phosphating.
Borax coating is cheap, the line is short and chemical control is simple. With tank temperature and concentration held, it gives an acceptable carrier film. It also dissolves easily: removing it from the surface in a following process (before galvanising or heat treatment, say) is far easier than with phosphate. Because it contains no zinc, it creates no zinc load on the wastewater side.
Its limits come from the same place. Because the film is not bonded to the surface, it breaks up inside the die at high reduction and in multi-pass drawing; protection falls away as the passes go on. Its durability drops at high line speed and high die temperature. Borax film also attracts moisture: in a coil stored in a damp environment, surface condition and drawing performance deteriorate together.
Lime coating (dipping in milk of lime) is simpler and cheaper still; its main function is to neutralise acid residue and leave a thin carrier. It is still used on low-reduction, coarse wire work, but its retention and surface quality are the lowest of the three options.
| Criterion | Lime | Borax | Zinc phosphate |
|---|---|---|---|
| Coating type | Carrier (physical) | Carrier (physical) | Conversion (chemical bond) |
| Bond to the metal | None | None | Yes — crystal grows out of the surface |
| Soap retention | Low | Medium | High |
| Suitable reduction | Low | Low–medium | Medium–very high |
| Multi-pass drawing | Not suitable | Limited | Suitable |
| Cold forging / upsetting | Not suitable | Not suitable | The standard choice |
| Surface quality | Low | Medium | High and repeatable |
| Ease of stripping | Very easy | Easy | Requires an extra step |
| Line investment | Lowest | Low | High (multi-stage line) |
| Waste load | Lime sludge | Salt load | Zinc + phosphate + sludge |
The table is not a ranking but a matching tool. Rather than saying “phosphate is always better”, the sounder question is: what is the total reduction on this product, how many passes are there, what is the line speed, and what operation does the part go to after drawing?
On the phosphate side there is a further choice of layer type: we compare zinc against manganese phosphate in manganese or zinc phosphate.
The chemical cost of a borax line is low; looked at from there, phosphate appears expensive. But total cost on a drawing line is not just the price of chemistry. Die change frequency, break count, line downtime, scrap rate, surface-related returns and soap consumption all enter the same calculation. Because soap keys in better on a phosphated surface, soap consumption per tonne generally falls; we cover that link in reducing drawing soap consumption.
The pre-coating decision is not a comparison of chemical prices but a comparison of die life and scrap rate.
The transition does not end with filling a tank with phosphate. Phosphate coating requires a multi-stage line: degreasing, rinsing, pickling, rinsing, activation, phosphating, rinsing and neutralising. If one of the stages is missing, the layer will not form at the required quality. You will find the sequence and the function of each stage in the phosphate coating process step by step.
Borax and lime are carrier coatings that sit on top of the surface; cheap, simple and easy to strip, but unable to survive as deformation increases. Zinc phosphate is a conversion coating derived from the metal; its investment and waste management are heavier, but it is the only practical option that gives repeatable results at high reduction, in multi-pass drawing and in cold forging. The right decision is made by reading the total reduction of the product and the post-drawing process together.
Kimfosan manufactures zinc phosphate coatings (KİMSOL) and drawing soaps (Kimkal). To assess whether your existing borax or lime line is adequate for your product, or to plan a move to phosphate, talk to our technical team. In production since 1982, Kimfosan offers technical support tailored to your process conditions from its plant in Kartepe / Kocaeli, Türkiye.
Borax is a carrier coating that dries onto the surface; it forms no chemical bond with the metal. Phosphate is a conversion coating formed by the reaction of an acidic solution with the iron at the surface, and the layer derives from the metal. For that reason phosphate stays on the surface at high reduction and in multi-pass drawing, while a borax film breaks up and sheds.
Borax gives an acceptable carrier film on low and medium reduction drawing with few passes and line speeds that are not very high. It is also advantageous in applications where the coating has to be stripped easily after drawing. It falls short at high reduction, in multi-pass drawing and in cold forging.
Yes, particularly for neutralising acid residue and for low-reduction coarse wire work. However, its soap retention and surface quality are the lowest of the three options; as surface quality and die life targets rise it gives way to borax or phosphate.
In terms of chemistry and line investment, yes, it is higher. But making the comparison on chemical price alone is misleading. Because soap keys in better on a phosphated surface, die life lengthens, breaks and scrap fall and soap consumption per tonne generally drops. Total cost should be compared with these items calculated together.
Phosphating requires a multi-stage line: degreasing, rinsing, pickling, rinsing, activation, phosphating, rinsing and neutralising. Alongside that, a bath analysis routine (total/free acid, accelerator), coating weight control and sludge/wastewater management must be planned. If one of the stages is missing, the layer will not form at the required quality.
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 →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.
Read →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 →