Author: Skip Schaefer
In my 49 years of heating aqueous based chemical solutions, heating phosphates is the toughest without a doubt. And we're not talking about historical Parkerizing a gun. Most quality metal finishers dip the metal into a hot bath, rather than spray the phosphate coating.
Phosphate conversion coatings are used on steel parts for corrosion resistance, lubricity, or as a foundation for future coatings. If not done properly the base layer will not chemically bond with the protective coating. Phosphoric acid and phosphate salts are applied and chemically reacts with the surface of the part being coated to form a porous layer. Besides steel, phosphate coatings are usually used on cadmium, silver, tin, aluminium, and zinc. The phoshate coating main types are: zinc, iron, and manganese.
Problems usually arise during the various metal finishing steps:
The temperature of the phosphate solution, insufficient accelerator, poor pre-cleaning, and insufficient processing time in the hot phosphate bath is most likely to effect phosphate crystal growth. Be sure to measure the coating weight, see MIL-DTL-16232.
If you find rust spots, usually red or dark brown it's typically cleaning issues. See if your pickling acid is effective. I suggest to improve the concentration, temperature or add agitation. And of course, rinse with lots of deionized clean water.
Some specs dont allow acid pickling. In this case use a alkaline derusting agent or abrasive blasting.
There are several ways to heat phosphates; I’m not going to list them in an order as to preference because each has its own pro and con.
All the immersion heater systems have a liquid level safety to shut the heaters off in the event of evaporation, tank leakage, or chemical hose failure. There are no right answers, only intelligent choices.