The authoritative public forum
  for Metal Finishing 1989-2024
  

  -----

Is it detrimental to pretreat alkaline zinc yellow with an iron phosphate?

2000

A customer of ours is getting spots were the chromate is coming off during a pre-paint cycle which includes:

1) A wash station with a mild spray cleaner. Temperature 152F. pH 10.6.
2) Spray rinse station.
3) A spray iron phosphate station. Temperature 132F pH 4.9.
4) Spray rinse station.
5) Sealant station pH 5.6. The reason you zinc plate and yellow chromate is for salt spray and good paint bonding. Is this pre-treatment necessary?

The claim is that shop dirts and soils that accumulate need to be cleaned off. Will this pretreatment affect the chromate and cause paint failure? I appreciate any help with this. Thank you.

Ken Hutchinson
Nico Products - Minneapolis, MN, USA

---

2000

Your cleaner is extremely hot, which makes it more aggressive. Your Phosphate pH is low, which makes it more aggressive. I won't say that the pretreatment is not necessary, but it may be substantially more aggressive than needed. A normal alkaline clean, Iron Phosphate, seal process will not ruin the zinc chromate coating, either something is wrong with the chromate coating, such as the cleaning step ahead of it, or your process is too aggressive. A good pretreatment supplier should be able to help you with no problem.

FYI, both the alkaline and phosphate stages are simply cleaning, you'll not get any phosphate over your chromated surface.

Jeff Watson
- Pearland, Texas

I'll agree with Jeff that chromated surfaces will not take a phosphate,

But I'm not at all surprised that there are spots where the chromate came off when sent through a 152 degree alkaline cleaning process.

Half measures like this do lead to problems. Leave the chromate on by not exposing the part to an alkaline cleaning solution, or make the solution strong enough that it get's all the chromate off.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2000