Metal finishing Q&As since 1989

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IRON PHOSPHATE RUST PROTECTION

This question regards maximizing the time that between iron phosphate coating and painting before rust occurs. We are presently experiencing a delay between phosphating and painting, with the result that some of our parts are rusting before paint. Our pretreatment process consists of -

1. 5 stage immersion pretreatment (clean, tap water rinse, iron phosphate, tap water rinse, chromated seal,dry).
2. Iron phosphate solution is standard solution, conc. 3-5%, pH 3.5-4.5, temp. 120-130 °F, immersion time 5-10 min. Seal solution is 0.05 - 0.10%, pH 3.0 - 5.0, temp. 150-160 °F, immersion time 30-60 sec.

I am seeking suggestions regarding solution parameters, additives, alternate solutions, part storage practices, etc. that will delay the onset of rust before painting. Presently our parts start to rust one day after phosphating. We want to be able to go three days between phosphate and paint without rust formation. Thank you.

Thomas Kemp
Chemist - Erie, Pennsylvania, USA
2007

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Thomas,
It's usually recommended that you paint within 24 hrs of phosphating.
You only need about a minute and a half in the phosphate tank at about 2%, have you ever checked your coating weight?
The pH in the seal is too low, I'm surprised your not getting rust immediately after the seal.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina
2007