Sludgeless Phosphating

? Please give us some more detail on what you know about it, Shankar. New developments are usually proprietary and trade secret, so I rather doubt that anyone will tell you how to do this in a public forum.

A. Dear Shankar

Check temp. in your bath, toner value and you will definitely get sludge at the bottom as Phosphating is a conversion coating.

Regards

Anurag

A. Sankar,

Phosphating is a conversion coating process and metal dissolution is the basis of coating formation in all conversion coating processes. Zinc phosphating baths are formulated in such a way that it contains zinc phosphate and some amount of free phosphoric acid. When steel is immersed in the phosphating bath, the free phosphoric acid attacks the steel and initiates the conversion coating process. The consumption of free phosphoric acid due to the this reaction leads to a local raise in the pH at the steel-phosphating solution interface which enables conversion of soluble primary phosphate into insoluble tertiary phosphate, which in turn deposits on the steel surface. The ferrous ions generated in the phosphating bath following dissolution of steel get oxidized to ferric ions by the accelerators. The ferric ions combine with phosphate ions and get precipitated as ferric phosphate (sludge) at the bottom of the tank.

The ferric phosphate sludge will not form in the absence of the accelerator in the phosphating bath. But, the phosphate coating weight will be very low and it is not practical to use such baths in industrial operations.

Sludge free phosphating processes are reported in the published literature and patents. One such process is called cathodic phosphating, in which the job is made as the cathode and a suitable potential is applied using a D.C. source. Since this process does not involve metal dissolution it will not generate any sludge. It is an eco-friendly process and it has many advantages. You can find more details about this process in the following literature reports.

"Formation of zinc-zinc phosphate composite coatings by cathodic electrochemical treatment"
Surface and Coatings Technology, Volume 200, Issues 12-13, 31 March 2006, Pages 4117-4126 doi:10.1016/j.surfcoat.2005.04.022

"Corrosion resistance of phosphate coatings obtained by cathodic electrochemical treatment: Role of anode- graphite versus steel", Progress in Organic Coatings, Volume 55, Issue 4, 1 April 2006, Pages 355-362 doi:10.1016/j.porgcoat.2006.01.009.