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Wire & Cable ASIA – September/October 2007

89

Wir & Cable ASIA – March/April

11

1 State of the art

The diagram below illustrates what

is currently described as the state of

the art process sequence.

1.1 Explanations regarding the

state of the art

Initially the parts are pickled, rinsed

and activated. An alternative to

pickling is mechanical descaling.

This is followed by a zinc phosphate

conversion coating, which is obtained

by means of a chemical reaction.

For many manufacturing processes

a conversion coating is essential to

achieve high quality, cost effective

large-scale production.

The parts being processed are again

rinsed, neutralised and finally a

lubricant is applied.

The lubricants are water-soluble

soaps that react with the conversion

coat or, alternatively molybdenum

disulphide (MoS

2

) dispersions. The

complete process sequence takes

between 60 and 90 minutes.

It is worth noting that the quality of

the phosphate, and the subsequent

performance results, are a reflection

on how much care has been taken in

the early process stages.

2 Zinc phosphates

Currently there are three types of

zinc phosphate commonly in use.

These are defined by their means of

acceleration.

They are:

Nitrite/nitrate

Chlorate/nitrate

Nitrate/air (Fe side process)

Nitrite/nitrate is the most widely used

outside Central Europe. The system

operates at high temperatures pro-

ducing sludge that settles at the bottom of the process

tank. The most common form of accelerator is sodium

nitrite. These processes commonly contain nickel, which

acts as a refining agent.

Chlorate/nitrate processes are used for specific situations

where either the operation is more suited to this method

of acceleration, such as intermittent working, or the

phosphate produced meets a particular demand.

Sludge is also produced in the process tank by this system.

Nitrate/air (Fe side processes) operate at lower temperatures

and produce their sludge in an external tank into which a

controlled supply of air is introduced.

The sludge produced is hard and dense, and settles readily

at the bottom of the tank. These processes are nickel and

nitrite free.

3 Activation

To obtain the maximum benefits from a zinc phosphate

coating the use of an activator prior to the zinc phosphate

is highly recommended.

The activator provides sites for the nucleation of the zinc

phosphate crystals, which produces a refined, controlled,

crystalline coating.

Comparison of the coating thickness of various lubricants

Lubricant layer g/m

2

Polymer

MoS

2

Soap

Figure 2

: Comparison of the coating thickness of various lubricants

Cross-section of a treatment plant

Return flow

Feed calcium

phosphate

Grid anodes

Phosphating solution

Cathode

Wire and solid part

Figure 3

: Cross-section of a treatment plant