The job does not end where drawing ends: the soap and phosphate layer left on the surface determine the quality of the next operation. Weld porosity, loss of paint adhesion, uncoated areas in galvanising and residue in the heat treatment furnace all trace back to the same place.
When lubrication on a drawing line is set up correctly there is a layer deliberately left on the surface: phosphate crystals with a soap film settled into their pores. That layer does its job during drawing. The problem starts when the part goes on from drawing to welding, painting, galvanising or heat treatment, because most of those operations expect clean metal.
This article covers what kind of defect the residue creates in which operation, the removal methods, why soap and the phosphate layer call for different approaches, and how cleanliness is verified.
The surface after drawing is not made of a single material; there are several layers on top of one another:
This distinction has a practical consequence: “cleaning off the soap” and “taking the surface down to bare metal” are not the same job. The first is usually solved by alkaline washing; the second requires stripping the phosphate layer as well. How the layer bonds to the metal is explained in what is phosphating.
| Next operation | Soap residue | Phosphate layer | Typical defect |
|---|---|---|---|
| Welding (resistance / arc) | Must be removed | Generally must be removed | Porosity, spatter, irregular penetration |
| Hot dip galvanising | Must be removed | Must be removed | Uncoated areas, loss of adhesion |
| Paint / powder coating | Must be removed | Usually wanted | Blistering, peeling, cratering |
| Heat treatment (furnace) | Removal recommended | Case dependent | Burnt residue, surface staining, furnace fouling |
| Electroplating | Must be removed | Must be removed | Non-adherent plating, patchy appearance |
| The next drawing pass | May remain | Must remain | Unnecessary cleaning ruins the lubrication |
| Oiled stock / shipment | May remain | Must remain | Cleaning removes the protection |
The last two rows of the table matter: cleaning is not the right answer in every case. Cleaning the surface at an intermediate pass destroys the lubrication ground for the next draw; cleaning in stock removes the corrosion protection. We cover that second point in why phosphated wire rusts.
The most common way to remove soap residue is washing with a hot alkaline solution. The mechanism is the same as degreasing before phosphating: alkali and surfactants dissolve the film and mechanical action strips it from the surface. The parameters and verification methods in what is degreasing apply directly. Alkaline washing removes the soap but does not take off the phosphate layer.
If the phosphate layer must come off as well, an acid bath enters the picture. The layer dissolves and the surface goes down to bare metal. There are two risks here: the acid attacking the metal more than intended, and the unprotected surface rusting rapidly after stripping. The surface therefore has to move on to the next operation without waiting after acid stripping.
Brushing, shot blasting and mechanical peeling remove both soap and layer without chemistry. Producing no wastewater load is the advantage. Against that, they change surface roughness, carry dimensional and surface quality risks on thin sections, and require dust to be controlled.
Relying on the residue burning off in the heat treatment furnace is a common but risky approach. Soap does not evaporate completely; as a combustion product it can leave staining on the surface and deposits inside the furnace. On loads entering the furnace without controlled pre-cleaning, both part surface and furnace maintenance suffer.
| Method | Removes soap | Removes phosphate | Main caution |
|---|---|---|---|
| Alkaline washing | Yes | No | Rinsing and drying are essential |
| Acid stripping | Yes | Yes | Over-attack of the metal and rapid rusting afterwards |
| Mechanical (brush/blast) | Yes | Yes | Surface roughness and dimensional change |
| Ultrasonic washing | Yes | No | Effective on complex geometry, limited capacity |
| Thermal (burn-off in furnace) | Partly | No | Risk of staining and furnace fouling |
In practice most lines combine these methods: mechanical removal of free dust first, then alkaline washing, acid stripping if required, and rinsing at every stage. The sequence is built according to what the following process needs.
A thin soap film is invisible to the eye; “it looks clean” is not an acceptance criterion. There are a few checks that can be applied on the shop floor:
The real acceptance criterion for cleanliness is not the appearance of the surface but the result of the next process.
The most effective way to lower cleaning cost is to prevent more soap being carried than necessary. Overfilling the soap box, the wrong particle structure and choosing a product that leaves excessive residue all increase soap consumption and the cleaning load together. These two cost items are usually tracked separately but are fed from the same source. We cover the consumption side in reducing drawing soap consumption.
There is a balance in product selection too: a low-residue soap makes cleaning easier, but film strength must not be given away. The right balance is set according to the product's reduction and its post-drawing process — see drawing soap selection.
The surface after drawing carries a soap film and a phosphate layer; depending on the following process these are either a source of defects or useful. Soap is generally removed by alkaline washing, and if the phosphate layer must come off too, acid stripping or mechanical cleaning is required. The decision should be made not with the question “how clean must it be” but “what does the next process expect”; and verification is done not by the appearance of the surface but by the result of the following operation.
Kimfosan manufactures zinc phosphate coatings (KİMSOL) and drawing soaps (Kimkal). If you are seeing surface-related defects at the welding, painting or galvanising stage after drawing, talk to our technical team to review your line stages and product selection together. In production since 1982, Kimfosan offers technical support tailored to your process conditions from its plant in Kartepe / Kocaeli, Türkiye.
The most common method is washing with a hot alkaline solution; alkali and surfactants dissolve the soap film and mechanical action strips it from the surface. Removing free dust mechanically first, by brushing for example, reduces the washing load. Alkaline washing removes the soap but not the phosphate layer beneath it.
The phosphate layer is dissolved in an acid bath or removed mechanically by blasting or brushing. In acid stripping the metal must not be attacked more than intended, and the unprotected surface must go on to the next operation without waiting; a bare surface rusts quickly.
Yes. Organic residue left in the weld zone leads to gas evolution, causing porosity, spatter and irregular penetration. In both resistance and arc welding the weld zone must be free of soap residue, and in most cases the phosphate layer must be removed as well.
Usually no — the phosphate layer is a surface preparation that improves paint adhesion and is a wanted layer. What has to be removed is the soap film and loose residue on top of it. Paint applied to an oily or soapy surface shows blistering, cratering and peeling.
Visual inspection is not enough; a thin film is invisible. The water-break test is the most practical method: on a clean surface water spreads unbroken. Wash bath concentration and rinse conductivity should also be monitored, and the real verification made in the following process — weld porosity, paint adhesion tests or the count of uncoated areas in galvanising.
Not all the soap you buy reaches the wire surface; some is thrown out of the box, some is lost as dust, some cakes with moisture and is scrapped. We look at which variables move consumption in which direction, and why cutting back can raise total cost.
Read →Degreasing is the cleaning stage that removes rolling and drawing oil from the steel surface before phosphating. The phosphate bath does not clean a dirty surface; the crystal only grows on a clean one. We explain how degreasing works, the alkaline/neutral difference and how to verify cleanliness.
Read →Reactive soap reacts with the phosphate layer to form zinc soap; neutral soap leaves a physical film. Which suits which line is decided by drawing speed, reduction ratio, wire diameter and what the wire has to go through after drawing.
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