Zinc Phosphate Coating Prior to Wire Drawing
The primary benefit in phosphating of wires and sections for drawing and profiling lies in its superb adhesion and the survival of the coating even after several passes through a die. The crystalline nature of the phosphate coating offers an excellent substrate for the lubricant used, inhibiting film rupture of these.
Use of phosphated steel reduces wear of tools and dies and, in practice, significant increases in the working life of the diesis found. The phosphate film after drawing imparts a high surface finish to the wire at the same time affording a certain degree of corrosion protection during storage and transport.
In case of hot dip galvanized iron or steel wires which are then further drawn, phosphating has advantages to offer. With a heavy zinc coating of 80-150 g/m², there is a tendency for such wires to build up debris in the drawing die which results in increased wear rates of the die, a reduction in the quality of the drawn wire surface and the failure of brakes in the drawing operation. By phosphating in conjunction with galvanizing, all these problems are largely eliminated. The wire product has a somewhat darker appearance, but is ductile with a smooth, corrosion resistant surface.
In the production of thin, low carbon wires in a drawing machine, phosphating can increase the production rate while at the same time providing corrosion protection during storage and transport.
In drawing of phosphated steel wires, care must be taken to use correct phosphate coating thickness. If it is too great, enhanced friction will occur at the first drawing, resulting in bloom on the wire. On the other hand, though, the phosphate coating must be sufficiently which that its benefits persists even at the last of a number of drawings. As a general rule, for multiple drawing of high carbon wires, a residual phosphate film after the final drawing of 0.5 - 1.0 g/m² is desirable. Exceptions to this are those steel wires where a phosphate film is useful in some subsequent forming process. Thus wires for cold extrusion and cold forging require phosphate coatings of 5-15 g/m² as measured after the first calibrating pass.
Immediately before immersion phosphating of wire coils, an alkaline pre-rinse is often installed. This serves to neutralise residual acid traces on the incoming material. Such alkaline pre-rinses with added activating agents such as titanium salts have proved especially valuable in their promotion of very fine dense zinc phosphate coatings. Such finely crystalline coatings are less liable to lead to bloom on drawn wire than their coarser-crystalline forms.
Phosphating of wires is usually based on the immersion method in which coils of wire are lowered into individual baths. The phosphating tank is thus incorporated together with other post-treatment stages downstream of the pickling plant. A typical treatment sequence for wire bundles can be schematically shown as follows:
- Pickling, in several stages, with sulphuric acid or hydrochloric acid.
- Water rinse, for example with two dip tanks and a spray rinse.
- Water rinse.
- Post treatment.