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Removal Of Titanium/Phosphoric Acid Reaction Products

 

During a routine cleaning of interior bays and cavities on a military aircraft, untrained personnel used a brightener (phosphoric acid, 1 - 5%) instead of the standard degreaser. Rinsing did not occur for nearly 6 hours. Internal titanium bulkheads and panels were contaminated. In addition to grain boundary intrusion and discoloration, a white powdery substance remains on the surface that is totally insoluble in water. When the powder is mechanically removed and hydrolized, the resultant pH is in the 3 to 4 range. We are searching for a silver bullet that can neutralize the surface and dissolve the precipitant. If not, much of the contaminated areas must be dismantled and cleaned by hand using a mild alkaline soap.

Jim Cobb
aeronautics - Palmdale, California, USA

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I did a quick search on google and found a PhD thesis at www.unige.ch/gap-b/diplo/kerem/pads.html quantifying the degree to which titanium oxide surfaces absorb phosphate. I didn't take the time to study it or track down any references, but inferred that the adsorbed phosphate is not going to simply wash off.

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

Have you tried steam? The phosphoric acid cleaners are typically used for de-scaling metals and ceramic surfaces & contain surfactants, buffers & phosphoric acid. This is only conjecture due to my lack of knowledge of the particulars, but since the cleaners are formulated with soluble components--even if these dry out they should be soluble in water; maybe warming the water might loosen the powder. Titanium is very resistant to mineral acids (with the notable exceptions of concentrated sulfuric and hydrochloric acids) and I doubt plain 1% phosphoric acid in cold water will attack pure titanium, but alloys including aluminum may not fare as well. Ferrotitanium alloys which are widely used in aircraft may react with phosphoric acid; if the powder you scrape off is soluble in acids and if the bulkheads are iron alloys, you might want to look into the possibility of some ferric phosphate being formed somehow. Ferric phosphate is usually derived from a reaction of ferric chloride ⇦this on eBay or Amazon [affil links] with sodium phosphate, but since we are talking about an expensive piece of machinery here, it bears investigation. If this is indeed the case, a warm, mild acid solution (again, not nitric or sulfuric) might remove the powder, but you will probably be etching the surface further & I would be concerned with structurally damaging the substrate, in this case a structual component of an aircraft. Whatever you do, you should probably bring in your metallurgy department on this one.

Dale Woika
- Bellefonte, Pennsylvania, USA

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You say you hydrolized the powder, which is completely insoluble in water. How did you do this? Have you tried some of the hair of the dog? (Tried the same brightener again to see if you can remove the powder, then rinse before it dries again).

  
Tom Pullizzi
Falls Township, Pennsylvania
 

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Dear Jim, decoating of titanium layers works in neutralized or slightly alkaline (therefore destabilized) hydrogen peroxide (~10-20%) yielding water soluble Peroxotitan-cation of orange yellow colour. This unstable TiO2++ complex cation decomposes later to TiOxOH precipitate. Do not fear if it gets warm and foams. Do not inhale the fog!

Good luck and best regards,

Dr. Andreas Szabo
- Ludwigsburg, Germany

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Jim:

If the Doctor's recipe is not feasible, try with diluted ammonia ⇦this on eBay or Amazon [affil links] in water. It is a very potent solvent mixture for many metallic salts (including sulphates and chlorides) although I don't know exactly how it will work with phosphates. Again, personnel have to use special respiratory protection.

Guillermo Marrufo
Monterrey, NL, Mexico