The phosphates used prior to painting are usually lightweight iron phosphates conforming to BS 3189:1973 Type 4 or DEF STAN 03-11/1 Class IV or zinc phosphates conforming to BS 3189:1973 Type 3 or DEF STAN 03-11/1 Class III. The heavier zinc and manganese phosphates can and are over painted but it would be more conventional to finish these with an oil type finish.
The choice of phosphate mainly depends on the conditions of service required by the end product. If the application is for internal use or low to moderate exposure requirements, iron phosphates would normally be used. If the application is for external use or a hostile environment, zinc phosphate is the preferred process.
LIGHTWEIGHT IRON PHOSPHATES
These processes can be applied to steel, zinc and aluminium in mixed metal production. Although no iron phosphate is produced on zinc or aluminium, the generic name is used to describe the process on all three metals.
The coatings produced from iron phosphate solution are amorphous in nature giving an iridescent appearance ranging from yellow through to blue. The conversion coating produced is very light, giving coating weights in the range of 0.2 to 0.8 gms per square metre, dependant on the type of process used.
The iron phosphate processes are very economical in use, easy to control, operate at low temperatures and do not present a major effluent problem.
These processes are predominantly applied to steel but will also treat steel, zinc and aluminium in mixed metal production.
The coatings produced on steel are crystalline, grey in colour and extremely adherent. The conversion coating produced is light to medium, giving coating weight usually in the range 1.5 to 6.0 gms per square metre.
Some zinc phosphate processes are susceptible to harsh cleaning methods which can be overcome by using a conditioning agent in the rinse prior to phosphating. An alternative would be to use a calcium modified zinc phosphate which produces fine uniform coatings even after harsh cleaning.
In comparison with lightweight iron phosphates, zinc phosphates are slightly more expensive to operate, requiring closer control, form more sludge as a by product and may give rise to effluent problems.
This process can be used in mixed metal production and treat steel, zinc and aluminium. The process produces lightweight coatings with very little sludge and can be used at low temperatures. The corrosion resistance is better than iron phosphate but not as good as zinc phosphate.
The choice of phosphate pretreatment is usually clearly defined by the end use of the product. Whether lightweight iron, zirconium or zinc phosphates are used, all are simple to operate, very economical in use and considerably upgrade the overall coating performance.
After phosphating the components are rinsed in two stages of water the first cold and overflowing to prevent excessive contamination, the second which may be hot containing a passivation treatment. The use of a final demineralised water rinse is advisable, particularly in hard water areas. If the work is to be electrophoretically painted a final demineralised rinse is essential.
For further technical information on our product range please call Brian on 01204 495818 or email us to see which process meets your individual requirements.
A typical example would be Phosguard LW 318 – High Performance Lightweight Iron Phosphate
PHOSGUARD LW 318 is a liquid, high performance, Lightweight Iron Phosphate conversion process designed to simultaneously degrease and phosphate in one operation. The process will treat iron, steel, zinc and aluminium in mixed metal production, however, when aluminium forms a significant part of the operation, other processes are preferred. PHOSGUARD LW 318 may also be used in the process stage of 5 and 6 stage plants.