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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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Phosphating that won't remove galvanizing
Q. Hello,
I found your page very useful and interesting for my type of business. My question is: I have a powder TGIC coating installation, with a 3 step pretreatment (iron phosphating, washing and passivation). I coat pieces of galvanized iron (approx. 50 g/m2 or 7 microns of zinc layer), and my phosphating process is between 6 and 8 pH. I'm thinking that this pH is removing some of my zinc layer in the material, leaving it less protected against corrosion, but I can't lower the level of pH because then it won't remove the oil in the material. I Would like to know if there are some types of oil removers that I can use in my phosphating bath that do not remove the zinc layer?
Thank You,
- Santarem, Portugal
2003
A. Hi Pedro,
It is difficult to get a high-quality conversion coating on galvanized substrates using an iron phosphate. The fact that you have a three-stage washer makes the problem even tougher.
Are you running your phosphating bath at the correct pH? I believe that your pH is too high, but this is dependent on the chemistry of the bath. I suggest to keep your bath in the range specified by your supplier.
Reducing the pH should have little effect on cleaning ability because this type of phosphating product normally has detergents built into the product to do the cleaning and these will be effective at pH 4-6, which is the range where most iron phosphating baths are run.
I suggest to have your supplier provide you with phosphating options that have better detergency. You could also ask your supplier for a detergent additive that could be added to the bath on an as-needed basis.
- Naperville, Illinois
2003
A. Hi Pedro. Galvanized surfaces are prepared for powder coating all the time, but with a zinc phosphate process, not with iron phosphate. As George implies, you can't do that in 3 steps, you should have 5 or 7. Good luck.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
A. Any approved process that uses solvent will remove oil and grease without affecting zinc. Whether you can use it or not depends on many factors which cannot be considered due to your somewhat vague question.
Guillermo MarrufoMonterrey, NL, Mexico
2003
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