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.
Degreasing is the cleaning stage that removes the film of oil, grease and dirt from a steel surface before the next operation. In wire, tube and profile drawing and cold forming lines, its real purpose is to prepare the surface for phosphating. Because the phosphate bath is not a cleaner: it does not clean a dirty surface, it simply tries to coat over it and records the contamination.
The surface of wire rod and semi-finished steel typically carries rolling oil, drawing oil left from the previous pass, protective oil, dust and oxide residue. Phosphating without removing that layer results in a patchy, voided coating. This is why degreasing is the first and decisive step of the phosphate coating process. This article covers how degreasing works, the difference between alkaline and neutral systems, typical parameters and how cleanliness is verified in the field.
Phosphating is a conversion coating: zinc phosphate crystals nucleate directly on the metal surface. If there is an oil film on the surface, the acidic phosphate solution cannot contact the metal and crystal nucleation never starts. The result is bright uncoated islands here and there, a loose and irregular layer, and consequently poorer soap pick-up in drawing.
The relationship can be summed up in one sentence: the quality of the phosphate layer is limited by the cleanliness of the surface it grows on. How phosphating works as a conversion coating and how the crystal grows is explained in detail in what is phosphating.
Degreasing chemicals remove surface oil by several mechanisms. Alkaline agents chemically break down saponifiable oils; surface-active agents (surfactants) split the oil into fine droplets, carry it away from the surface and stop it redepositing. Temperature speeds the process up by making the oil more fluid, and mechanical action (spray pressure, bath circulation, brushing) physically strips the dissolved soil from the surface.
In practice degreasing is done two main ways: immersion (the part is dipped in the bath and cleaned by circulation and time) and spray (pressurised solution strikes the surface, giving both chemical and mechanical action). Continuous wire and tube lines often use a combination of the two.
Degreasing systems divide roughly in two according to the pH of the solution:
Before wire and tube drawing, alkaline systems predominate, because rolling and drawing oil loads are generally high. The choice is made according to the type of contamination, the substrate and the downstream rinsing capacity.
| Parameter | Typical range | Notes |
|---|---|---|
| pH | 9–13 | By type of contamination and substrate |
| Temperature | 40–70 °C | Temperature speeds up oil dissolution |
| Concentration | 2–5% | Per manufacturer instructions and oil load |
| Time | 3–10 min | For immersion; shorter with spray |
| Method | Immersion / spray | Often both together on continuous lines |
The values in the table are typical ranges; final set points depend on the contamination load, line speed and the product used. A degreasing bath saturates with oil and dirt over time; when its cleaning power drops, a concentration top-up or a bath renewal is needed.
The rinse immediately after the degreasing stage is the most underrated step on the line. Alkaline film left on the surface, if not rinsed sufficiently, is carried into the following phosphate bath, neutralising it and upsetting the free acid balance. Most of the sudden drops in free acid and unexpected sludge increases seen in the field come not from the phosphate bath but from inadequate rinsing. We develop that link further in phosphate bath maintenance.
Whether degreasing has been adequate is hard to judge by eye; a thin oil film is invisible. There are a few checks used in practice on the shop floor:
A good phosphate layer starts with good degreasing. If the surface is not clean, no later stage can make up for it.
Degreasing is the first and decisive stage of the line, removing oil and dirt from the surface before phosphating. Because the phosphate bath is not a cleaner, the quality of the crystalline layer depends directly on the cleanliness of the surface. A degreasing stage running at the right pH, temperature and concentration, followed by adequate rinsing, is the precondition for a sound phosphate layer.
Kimfosan manufactures zinc phosphate coatings (KİMSOL) and drawing soaps (Kimkal), and provides field support so you can set up the whole line correctly, degreasing stage included. If you would like to review the degreasing, phosphating and drawing stages of your line together, talk to our technical team. In production since 1982, Kimfosan offers technical support tailored to process conditions from its plant in Kartepe / Kocaeli, Türkiye.
Degreasing is the cleaning stage that removes the film of oil, grease and dirt from a steel surface before the next operation. On wire and tube drawing lines its main purpose is to prepare the surface for phosphating, because the phosphate crystal can only nucleate and grow on clean metal.
The phosphate bath is not a cleaner; if there is an oil film on the surface the acidic solution cannot contact the metal and crystal nucleation never starts. The result is a patchy, voided and loose layer, and poorer soap pick-up in drawing. A clean surface is therefore the precondition for a good phosphate layer.
Alkaline degreasing uses high-pH solutions (typically 9–13) for strong, economical cleaning and is the most common choice for pre-drawing preparation where oil loads are heavy. Neutral or mildly alkaline systems work at lower pH and are used on sensitive surfaces or where alkaline residue causes problems, but their cleaning power is more limited.
The most practical method is the water-break test: on a clean surface water spreads as an unbroken, even film, while on an oily surface it splits into droplets and pulls back. Bright uncoated islands after phosphating also indicate inadequate degreasing. Bath concentration and rinse conductivity should be monitored regularly as well.
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 →Two numbers determine the coating weight and crystal structure of a zinc phosphate bath: total acid and free acid. What the point unit means, the titration steps, sludge and iron control, a daily maintenance list and a troubleshooting table.
Read →