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 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 buil 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,
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 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 sulfuric acid or hydrochloric acid.
Water rinse, for example with
two dip tanks and a spray rinse.