EW January 2011

technical article

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 producing 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. 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 Chlorate/nitrate processes are

Comparison of the coating thickness of various lubricants 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

4 A new type of lubricant as an alternative to soaps and MoS 2

activator prior to the zinc phosphate is highly recommended. The activator provides sites for the nucleation of the zinc phosphate crystals, which pro- duces a refined, controlled, crystalline coating. This will lower the coefficient of friction and improve the adhesion of the coating to the metal surface. The fine crystal structure also increases the available surface area for the subsequent lubrication process.

The new lubricants are high performance polymer systems. Applied by immersion in an aqueous solution they produce thin organic coatings. results this lubricant is best used on top of a conversion coating. Application on bare material is possible but must be examined on a case-by-case basis and thus cannot be generalised. The chart above shows the coating thickness as compared to the conventional lubricants used so far. 4.1 Objectives of the new development The general target was to develop a coating that allows net shape forming without any negative impact on the tool life. This means that the forming accuracy should be appreciably better than that obtained when using soap, and should be at least equivalent to that obtained with MoS 2 . Moreover, the goal was to achieve a coating that is substantially cleaner and easier to remove. The lubricant concentrate should be classified in the lowest water hazard class and no additives were to be used that may present a potential corrosion risk. For maximum performance

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

Cross-section of a treatment plant

Wire and solid part

Cathode

Phosphating solution

Grid anodes

Feed calcium phosphate

Return flow

55

EuroWire – January 2011