What about coatings? Several coating materials purport to protect parts from formation of alpha-case during thermal processing of titanium alloys. Based on several research papers and on my experiences, I note the following:

• None of the coatings allowed by the specification are effective barriers against diffusion of oxygen (alpha-case) or hydrogen during thermal treatment of titanium alloy in the 1000 F 600 F (538 C 871 C) ranges.

• The studies concluded that use of coatings did not reduce the depth of alpha-case for a given heat-treat furnace.

• Most of the suppliers doing stress relief on parts after welding or forming have chosen to omit coatings.

• Although the coatings tested did not reduce surface and sub-surface alpha-case, they did make the surface scale easier to remove (in non-vacuum furnace applications). Easier cleanup after thermal processing may be a sound reason to use coatings in non-vacuum furnace processes.