Chemical Preparation of Surfaces

It is possible to clean and phosphate in a single operation. This process of acid phosphate cleaning combines the degreasing with a transformation of the metal surface by removal of oxide and the phosphating of the metal. The process can be used for metals such as steel, aluminium and zinc. This process produces an excellent surface for the adhesion of lacquers but it is not suitable for rubber-to-metal bonding. Tests have shown that the corrosion resistance of the components are not satisfactory. 

Excellent bonding can be achieved with zinc phosphate and mixed metal phosphates but the particle size and quantity of phosphate applied are very important. There are two main physical forms of phosphate that are used in commerce. One is an amorphous structure applied at a level typically below 4 g/m and the other an acicular (needle shaped) structure typically applied at a level of 15 g/m. The acicular form is used as an absorbent substrate for coatings and oils used to enhance the phosphate layer as protective coating. The acicular form is unsuitable for bonding as the crystal structure can fracture under the bonding agent primer. 

Phosphating should only be carried out on a flat surface, as a profiled surface, that has been grit blasted for example, may not be able to accommodate even packing of phosphate deposits and bare metal patches may be exposed [12]. 

The phosphating process nsnally involves cleaning of the metal, rinsing, surface activation, and phosphate conversion producing an amorphous layer before a final rinse and seal. This sequence is  

The surface activation step iii) is optional but is designed to produce a finer particle structure. The sealing process (passivation) in step vi) historically used chromate but environmental pressures have made this an optional step. Some alternative chemical seals must be proved to be suitable for bonding. Sealing is done to improve the water 

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