Zinc phosphating bath

Hello!
I am a metal engineer. I study about converting coating(tri cationic phosphating bath).My coating bath ingredients is:

1.H3PO4:10(g/lit)
2.zinc oxide ⇦ on eBay or Amazon [affil link] :6(g/lit)
3.NICKEL NITRATE:3
4.MN(NO3)2:1
5.NaNO2(ACCELERATOR):1
ph=3 T=50-60(c) t=5-7(min)

My process stages consist of:oil cleaning-acid washing(sulfuric acid5%)-water rinse-phosphating bath-water rinse

The result coating is very smooth, dense and light gray color.but after two time immersion coating the bath is not work

good. My coating pieces are steel plates.(st37)
how can I improve the bath?
must I add other materials in working bath?
what are the best conditions of my bath?
what must I add to the bath for increase the salt spray time?
my bath sludge is very high. The way of coating is immersion. how can I solve this problem.

Hi Amir,

Your primary problem is that you appear to be using a "home-brewed" zinc phosphating bath. I suggest to contact a supplier of pretreatment chemicals and they can suggest a product that is properly formulated for your application.