There is no common industry standard that I can find for determining the depth of alpha-case on the surface of titanium alloy. If there is a universal standard, it is not being used. Boeing polishes the surface that is 90 degrees to the surface being measured, etches the polished surface with 10% nitric-hydrofluoric, and reads the depth at x200 to x500 magnifications. One of our casting suppliers adds ammonium bi-fluoride stain to show up oxygen after polishing; in a specific test case, got a reading that was about 0.002 in. (0.051 mm) more depth of alpha-case than we measured at Boeing from the same coupons in the one test we conducted jointly. So depth of alpha-case data is not directly comparable between test laboratories.

I have requested that our Material and Process Engineers take on the task of establishing an industry standard for measuring alpha-case thickness so the results will become more consistent between groups around the world.