I have been using WD40 on various metal surfaces for over 30 years
(including every hinge and drawer runner in my home) and have never
ONCE had an adverse reaction. And that AP900 shaft was treated
shortly after I received the mount in 2007; no deterioration at all.
Not trying to be argumentative, only recounting MY experience.
PS: My neighbor is a remodeling contractor. He routinely uses WD40
on a major investment; his tools.