WARNING: This page is about a different car, the 2002 Ford Pickup, 2002 Ford F550 Super Duty, and 2002 Ford F450 Super Duty. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: View Of Sealing Element Worn/Hardened
Courtesy of FORD MOTOR CO.
The term describes hardening and chipping of the sealing ring.