Resistance of Titanium Alloys to Hydraulic Fluids at Elevated Temperatures

The phosphate ester hydraulic fluids (Skydrol and Hyjet) used on civil aircraft can undergo a hydrolytic decomposition at temperatures above about 300F (150C) in the presence of moisture, producing phosphoric acid that can cause corrosion and hydrogen absorption in titanium alloys. Alpha-beta alloys, including Ti64, are particularly susceptible to attack, which continues to limit their usage in elevated temperature structures that may be subject to exposure to hydraulic fluid leakage. Testing of alloys for resistance to decomposed hydraulic fluid is problematic due to the formation of tar-like residues that can either impede or accelerate the corrosion based on random differences in residue morphology. This presentation will discuss the results of testing various titanium alloys against Skydrol and also for testing directly in the active decomposition product, phosphoric acid. It was found that TIMETAL 21S (Ti-15Mo-2.7Nb-3Al-0.25Si) offers the best overall resistance for both test methods.