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Yellow rust formation before zinc phosphating of tubes

Q. Dear All,

We are seeing yellow rust formation after we do the HCl Pickling & rinse 1 & 2. we have cascading facility for the rinse to maintain the pH of the bath, but still the tubes are turning yellow before the activation.

And also we are observing non-uniform coating of Phosphate in the Tubes.

I request your opinion on the above.

thanks for your valuable time

Kaarthic Natarajan
Product Designer - China
August 18, 2008

A. Sir,
If you use HCl you will get this problem of yellowing, use Phosphoric acid based derusting along with inhibitors and surfactants.
The rough coating of phosphating is due to 2 reasons.
1) you are not using a surface conditioner, use it.
2) The formulation problem of the mother liquor by the supplier, Check if your Total acid and Free acid are in correct ratio.

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.