Rust Modifiers and Chromoly

[TC747paul]

Almost happy New Year to all! i am looking at the rust modifiers sold at the retail stores. Does anyone know if the rust modifiers (change rust to a black inert primer) damages chromoly strength or characteristics?

[Guest]

I used them on my pickup truck in the late 80's.

They were worthless. I would not use them on a truck again and definitely would not put them on an airplane.

I doubt the have any impact on alloy strength other than allowing it to continue to rust.

Suggest that you grit blast if possible then give a couple of coats of a good primer then one or two coats of a good enamel.

Dave R

[Scott]

What you're referring to is called pickling and is why chromoly steel and other steels are black when new. Various acids are used to produce the effect depending on the alloy.
The finish provides only temporary corrosion resistance.

Chromoly is susceptible to hydrogen embrittlement so unless you know for sure what's in the retail product you can't predict the consequences. The use of acids (which is basically what those retail produces are) for corrosion removal is risky, especially if it's not possible to prevent other components/materials from coming into contact with the chemical. Residue is also an issue so you need to be sure you can flush it all away after.

Bottom line is what Dave said. Mechanical removal (not sand, aluminium oxide works) followed by a good primer, and I would add a third step... once the primer has cured coat the area inside and out with LPS3 or ACF50 or similar.

[Robert Lees]

I bought a Winchester of 88% orthophosphoric acid when I renovated my first rusty car in the eighties. Mixed with water (which generated heat) it did a great job of turning surface rust in to an inert substance which could be painted.

I sold the car before the underlying rust reappeared; I had then assumed in my early engineering ignorance it would treat the whole of the rust, and no, it did not! I still have half of the contents of the Winchester left...awaiting its next opportunity.

So I am in agreement with the above posters....media blasting is the only answer (along with welding repairs to any weakened areas found).

Rob

[Robert Lees]

...and I would add a third step... once the primer has cured coat the area inside and out with LPS3 or ACF50 or similar.

If it's a fuselage frame, use Polyfibre "Tubeseal" to re-coat the inside (especially after any welding repairs) but NOT the outside if you are then fabric covering!

[Guest]

What you're referring to is called pickling and is why chromoly steel and other steels are black when new. Various acids are used to produce the effect depending on the alloy.
The finish provides only temporary corrosion resistance.

Chromoly is susceptible to hydrogen embrittlement so unless you know for sure what's in the retail product you can't predict the consequences. The use of acids (which is basically what those retail produces are) for corrosion removal is risky, especially if it's not possible to prevent other components/materials from coming into contact with the chemical. Residue is also an issue so you need to be sure you can flush it all away after.

Bottom line is what Dave said. Mechanical removal (not sand, aluminium oxide works) followed by a good primer, and I would add a third step... once the primer has cured coat the area inside and out with LPS3 or ACF50 or similar.

Well said, right on.

And sometimes when you do know we still can't predict. :-)

[TC747paul]

Thanks for the information everyone, I will find a company to remove the rust with aluminum oxide. Thanks again

[astjp2]

Not aluminum oxide, it will cause corrosion too, stick with silicon carbide, soda blasting, glass beads, or coal slag...

[Scott]

Was not aware of corrosion risks associated with aluminium oxide. Would certainly like to learn.
Aluminium oxide is specified in AC43.13-1b for both steel and aluminium which is one reason I use it (no need to change media for different parts).

I've never found soda to do an adequate job in corrosion removal but then I only have 110 Psi. Great for cleaning parts non-abrasively though.

Glass beads shatter and get everywhere, they also destroy the glass in the cabinet very quickly so you can't see! At least that's my experience. I also thought they had a higher tendency to embed in some materials but I could be wrong.