Letter 4037

Galvanized steel-Iron phosphate pretreatment 

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I am interested in pretreating A-60 electrogalvanized steel in a five stage iron phosphate aqueous wash system. Parts will be painted with a polyester powder paint. Can I get 800 hours salt spray with this system. What chemistry should I use?

Jim Brock
Little Rock, Arkansas

Hi Jim,

Phosphating + powder coating on galvanized steel should be able to pass 800 hrs of the 0SS test if you get quality coatings. There are a number of commercial phosphating processes available in the market. Good luck!

Ling

L. Hao
Minden, NV

Hi, Jim

If electrogalvanized steel is treated with iron phosphatizer, the texture of treated surface will be zinc-phosphated-like what could withstand over 1000 salt fog testing hours.

Regards,

Jun

Jun Q. Zhang
Cincinnati, OH

Electrozinc is easily phosphated but amorphous coatings will not achieve the performance that you require. Paint delamination will occur in salt spray after 200+ hours from damaged areas. To achieve adequate performance in your 5 stage AP plant you will need either a chromate final rinse or one of the new technology silane-Zr rinses.

Roger Bridger
Surrey, UK CR9 4HT

ONE CAN ACHIEVE 800+ SALT SPRAY HOURS -- ASTM B-117 [link is to spec at TechStreet] WITH AN AMORPHOUS OXIDE COATING IN A 5 -STAGE IRON PHOSPHATE OPERATION, PROVIDED A QUALITY CHROME-FREE FINAL SEAL IS USED AND MAINTAINED. OUR CUSTOMERS ARE DOING IT.

RICH PODOLSKI
- LINCOLNSHIRE, IL

What you're after is possible. Like Rich said, you'll need a good final rinse, and a closely controlled process, but people are doing it several places now, consistently. Find a good pretreatment company and they should be able to tell you exactly how to get the job done.

Jeff Watson - Pearland, TX

I have found pretreatment of electrogalvanized sheet steel problematic. Where I used to work, we had dip tank system for precleaning/phosphatizing/sealing of parts to be powder-coated. The challenge was getting a soak cleaner that could do the job for all the parts we ran, without damaging the electrogalvanized coating. Most seemed to be too alkaline to allow sufficient degreasing, removal of grease-laden fingerprints etc. from the parts, without damaging the zinc coating. (Many of which were literally dripping with oil when we received them, I know...hard to believe, or the EG coating was contaminated with some sort of waxy substance (probably one of the newer environmentally friendly lubes for sheet metal break operations)...used to drive me crazy!) Any insight into this problem would be appreciated...After all, it would seem that the only reason to bother phosphatizing the part would be to ensure that the edges of the parts where they were cut in the breaking/forming operation were coated, would be the only reason to bother "treating" EG parts, were it not for the fact that EG coating seems to act like a magnet for any sort of oil or grease contamination. I would surmise that it may have helped to prevent outgassing under the powdercoat from the zinc coating?? Also, phosphate pretreatment of EG parts would tend to "repair" any damaged areas or scratches in the EG coating that occurred during the fabrication process. What do most operations use that regularly have to deal with EG parts...in the way of cleaning solutions that will remove the oils/grease/waxes from the EG coating, without damaging it??

Mike Solvie
Spokane, WA

800 hours is easily achieved when using the correct products. Fe phosphate may give these results but is dependent upon final rinse employed, Cr based may not be acceptable, try Zirconium or Silane based.

Alex Jennings
Hawarden, North Wales CH5 3PP

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