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Wire & Cable ASIA – September/October 2007

91

Wir & Cable ASIA – March/April

11

Less frequent changes of oil in the bolt heading

machines, as no soap is carried over to contaminate

the oil re-circulation system

To conclude, both wire manufacturers and wire users

benefit from the use of polymer coatings. This results in a

competitive advantage for both parties.

5 Electrolytic phosphating – a heavy

metal-free coating process

If a conversion coating (zinc phosphating) is required, then

the problems of disposal of the produced sludge and the

heavy metals that contaminate the effluent stream must be

addressed.

The solution to this problem is a calcium phosphate

coating applied electrolytically.

5.1 Electrolytic application of a calcium phosphate

coating

A calcium phosphate coating is also a conversion coating

but it differs fundamentally from the conventional zinc

phosphate coating used to date.

The coating solution is free from heavy metals such as zinc

or nickel and so, therefore, is the coating.

The coating can best be described with the formulation

CaHPO

4

. The principle of coating deposition on a

conductive surface is illustrated in

Figure 3

.

Since the part acts as a cathode, pickling attack on the

iron material does not occur.

Without this pickling attack no phosphating sludge will

form so this application process does not generate sludge.

Figure 4

shows phosphated and drawn cold heading wire.

It can be clearly seen that the calcium phosphate coats

are white prior to forming. After forming, the wire exhibits a

regular grey colour.

5.2 Benefits

Free from heavy metals so the effluent becomes easier

to manage

No sludge for disposal

Process temperature of approximately 25°C

Treatment time of 2–5 seconds allows for relatively

short plants

Required coating weight of 5–15g/m² can be adjusted

via the current density (A/dm

2

)

Higher drawing speeds for wire, and lower pressing and

ejection forces in cold heading and cold extrusion

No heavy metal-containing effluent in the de-

phosphating of fasteners prior to heat treatment

5.3 Outlook

A future treatment process for cold heading wire and solid

parts could combine the benefits of an electrolytic, heavy

metal-free phosphating with those of a polymer coating.

Figure 5

shows such a plant.

This will contribute to reducing the treatment times for

surface treatment from around one hour to less than

one minute.

Jude Burke

Chemetall UK – UK

Fax

: +44 1908 373939

Email

:

ukinfo@chemetall.com

Website

:

www.chemetall.co.uk

Consequently, the quality and productivity will appreciably

improve for both the surface treatment plant, and the bolt

manufacturing or cold extrusion plant.

This paper was presented at Istanbul Cable & Wire ’09 and

is reproduced here by kind permission of the IWMA.