Phosphating at ambient temperature and at high temperature both have weaknesses. Room temperature phosphating produces large amounts of sediment and slime, shows poor wear and corrosion resistance, and can produce fungus. Phosphating at high temperatures (145-160 degrees F) will result in high energy consumption, as well as greater chemical consumption. Phosphating processes at lower temperatures (110-120 degrees F) has shown the strengths in producing a fast coating forming rate, low energy consumption, manageable sludge beds, and good corrosion resistance.
Iron phosphate conversion coatings are achieved by making contact with a ferrous metal by an acidic phosphate solution that will react with the metal surface. The iron in the metal substrate reacts chemically with the solution and redeposits on the surface to form a low weight coating that adheres closely as an amorphous iron phosphate coating which serves as an excellent base for painting.
This process is typically used as a spray application requiring at least 20 psi. Chemical concentration in the bath is typically 2-3% with a ph of 3.0-4.5. Some low temperature phosphate processes require the part to be precleaned in a heated alkaline cleaneing stage, but not always necessary. Some phosphating processes have as many as 5 stages and some as few as 2. A typical 5 stage process has alkaline cleaner in stage 1, rinse in stage 2, phosphate in stage 3, rinse in stage 4, and RP/seal in stage 5.
Processes can be 2,3,4,5 stages depending on process needs. Producing phosphate conversion coatings at low temperatures has proven to be practical and economically desirable.