Summary for cold phosphating Discussion
Cold phosphating is discussed here as a way to form phosphate coatings at lower-than-usual temperatures. Ramchander asked what the solution contains and how low-temperature phosphating is possible. Dan Brewer recalled, somewhat tentatively, that a cool phosphate might use a methylene chloride carrier with a suitable accelerator and phosphate, but noted it was not widely used because of the solvent. Roger Bridger gave the clearest practical answer: zinc phosphate can be done at room temperature with a standard nitrite-accelerated, nitrate-rich solution, about 1 g/l sodium nitrite, and free acid reduced to 5% of the total acid/free acid ratio. He said the process works because of the high oxidizing content and lower acidity, but sludging increases and bath loading must be kept low; typical times are about 15 minutes at 20–25 °C. For cleaner pretreatments, he suggested a titanium-containing refining predip. In a later electro-phosphating question, Shankar Omprakash Khandelwal offered only a general hint to try oxidizing agents.
Can anybody tell me what is cold phosphating? and what does the cold phosphating solution contain. How is it possible to get the phosphating at low temperatures.
Ramchander
– Hyderabad, Ap, India
2002
First of two simultaneous responses —
The details are pretty hazy here, but I seem to remember that a cool phosphate could be formed using a methylene chloride carrier and a suitable accelerator along with the requisite phosphate. Maybe someone else can shed some more light on my memory darkness and expand on this. This isn’t in wide use because of the solvent.
Dan Brewer
chemical process supplier – Gurnee, Illinois
2002
Second of two simultaneous responses — 2002
Hi,
It is possible to zinc phosphate at room temperature using
“standard” nitrite accelerated solution, high in nitrate, 1 g/l sodium nitrite and the free acid reduced to 5% (Total acid: free acid ratio). The process works because of the high oxidising content and the reduced acidity. Sludging will be higher and bath loading must be kept low to avoid overloading the process. Process times will be typically 15 minutes at 20-25 °C. If using an alkaline or acid cleaner, a titanium containing refining predip may be required to ensure complete, fine grain coatings.
Roger Bridger
– Croydon, UK
Q. Dear All ,
We are trying to develop electro phosphating with on line application (not batch phosphating). We are able to achieve coating of 4 to 5 gm/sq. mtr but at higher temperatures of 65 to 70 °C. We would like to achieve similar results at room temperatures (say 30 to 35 °C ).
We are using zinc phosphate self-accelerated solution with TA 150 and FA 15. Operating at lower temperatures leads to blackening of bath (self-acceleration stops).
Can we add some chemicals or additives to achieve the required coatings at room temp?
Pease suggest. Thanks.
Murali R
Development, Projects – Chennai, Tamil Nadu, India
October 23, 2012
A. Wow, Murali sir,
You are trying to be a revolutionary, all the best. But I am sorry, I have no solution to your query at present.
But I’ll hint: keep trying some oxidizing agents.
Shankar Omprakash Khandelwal [deceased]
– Coimbatore, TAMILNADU, INDIA
It is our sad duty to pass along the information that the knowledgeable Mr.Shankar was lost to Covid/Corona.
He was awarded with honorable medal by then Indian president Ms. Pratibha Patil. R.I.P.