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Problem aluminum phosphating mild steel

Q. Hello there,

First post here. I am a PhD student from Germany.
I want to develop aluminium overlay coatings to protect steel parts in turbines. The phosphate coatings contain 25% phosphates, 45% aluminium powder particles, 30% water and are applied by spraying and drying.
Before, the solutions contained Cr(VI) but we need to go chrome(VI)-free using phosphates with cations like Al, Zn, Mg, CrIII.
My question is: after spraying the slurries on mild steel the acid reacts resulting in the formation of bubbles, blisters and pinholes in the film. When I increase the pH value by adding more salts it gets better but the films get more brittle with bad adhesion and cracking.
What can I do to suppress the etch reaction and or the formation of bubbles?

Thank you very much and greetings from Bavaria,

Alex

Ferdl Meyer
- Munich, Germany
October 9, 2024

A. Hi Ferdl,

Chrome is very valuable towards deterring corrosion of aluminum, so CrIII within the phosphating formulation or as a final passivation is important, but I can't actually offer a formulation for you.

The steel must react with the phosphating solution -- that's how phosphate conversion coatings work: they consume some of the steel in creating a "conversion coating" not just a topping on the steel. You are beyond my education level and pay grade, but perhaps a better approach is some kind of actual phosphate conversion coating (iron or zinc) on the steel, followed by the slurry with aluminum particles rather than combining the appetizer with the main course this way?

Luck & Regards,

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

thanks for publishing my post and for answering.

1. The idea with a primer layer is good. But to provide electrical contact to the basematerial it must contain aluminium particles also. What type of Fe or Zn-phospahte would you recommend that shows minor hydrogen evolution. Can you recommend a product (Hemkel) or an approximate formulation with parameters (pH, FA, TA,...)?

2. I was thinking about using additives to minimize the formation of hydrogen bubbles. I tested many substances with no success. Can you recommend any additives for that? Or can you recommend some literature that can aid?

Thanks for helping!

Kind regards, Ferdl

Ferdl Meyer [returning]
student - Munich, Germany
October 10, 2024

A. Hi, Ferdl
My guess is you mean development of a paint/coating with aluminum powder and inorganic binder based on phosphoric acid. There are few such products on the market, mostly specified for aerospace and industrial turbines. This site abstains from publishing of companies names, so I suggest you look for paint/coating manufacturers for aerospace market and try to cooperate with them.
Good luck!

Alex Sirota
Process Engineer - Lod, Israel
October 10, 2024

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