WCA March 2011

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.

Comparison of the coating thickness of various lubricants

Lubricant layer g/m 2

Soap

MoS

Polymer

2

Figure 2 ❍ ❍ : Comparison of the coating thickness of various lubricants

Cross-section of a treatment plant

Wire and solid part

Cathode

Phosphating solution

They are: Nitrite/nitrate • Chlorate/nitrate • Nitrate/air (Fe side process) •

Grid anodes

Feed calcium phosphate

Return flow

Nitrite/nitrate is the most widely used outside Central Europe. The system operates at high temperatures pro-

Figure 3 ❍ ❍ : Cross-section of a treatment plant

The sludge produced is hard and dense, and settles readily at the bottom of the tank. These processes are nickel and nitrite free.

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.

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.

89

Wire & Cable ASIA – September/October 2007 Wir & Cable ASIA – March/April 11