plating, anodizing, & finishing Q&As since 1989
Phosphoric acid pickle
A customer of ours is having problems with steel springs that were powdercoated at our facility. We have been doing the same process on this and other similar parts for years.
We prep the parts by going through an alkaline cleaner, rinse, 15 - 20 % phosphoric acid, rinse, iron phosphate, rinse, and chromic acid seal before powdercoat. The only difference is we prepped the parts in the phosphoric acid for up to 12 hours. this was needed to remove the scale from the parts. The material that the parts were made of was ASTM A229 [affil link] oil tempered spring steel (600 degree F stress relieved 1/2-1 hour) Could the phosphoric acid cause a weakness in the spring steel when left for this extended amount of time.
- Spokane, Washington, USA
I've been working with Phosphoric acid since the '70s and have never heard of it weakening spring steel. The 12 hour phos acid soak sounds very long. If the scale is that difficult to remove, you might want to try another chemical such as inhibited hydrochloric acid (muriatic acid)for a shorter time.
I would expect you to see evidence of chemical corrosion on the springs if the chemical had caused the problem.
Could there have been a temperature excursion somewhere in your processing?
- Vista, California, USA
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