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Phosphate coating weights vs. sludge quantity

We are a steel processing plant using phosphate immersion tanks as our carrier coating. Our chemical supplier has added an additional chemical into the bulk phosphate which is intended to increase coating weights.

Will increased coating weights have an effect on the amount of sludge that develops?

Joe Pizzo
Steel Processing - Romulus, Michigan
2004

My guess is that they increased the level of the promoter or accelerator. Only the manufacturer can answer the question since they are the only ones that not the chemistry. A wild guess would be yes, IF you let the bath set idle for periods at temperature. If it is perfectly operated, I will guess that it is NO. Do you let the tank set with nothing in it at lunch and during breaks and shift change? If so, you are costing your company problems sources in bath stability and quality of the product.

James Watts
- Navarre, Florida
2004

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Being a conversion coating, metal dissolution is the basis for phosphate coating formation. The presence of accelerators such as nitrate, nitrite, etc. prevent the excessive build up of ferrous ions in the bath and oxidize the ferrous ions to ferric ions, which in turn gets precipitated as ferric phosphate (sludge). Hence increased amount of metal dissolution will certainly enhance the amount of sludge formation. Any chemical that are deliberately added to the phosphating bath in order to increase the extent of metal dissolution might help in enhancing the phosphate coating weight, however, this results in increase of sludge as well.

T.S.N. Sankara Narayanan
- Chennai, Tamilnadu, India
(ed.note Nov. 2017: The good doctor has a fascinating blog at https://advancementinscience.wordpress.com)
2004