User blog comment:SilenceInTheLibrary/Annie's Future/@comment-4361160-20130826214228/@comment-179280-20130913013702

Nope. Reiner's armor is permanent. It's part of his skin. What we see him doing, though, is cracking and releasing parts of his armor to increase his speed. That's why everyone is so surprised in Chapter 44, because they didn't expect him to be so fast.