Maintaining your gear in good working condition is essential when canyoneering. If your gear fails, there is a high chance that you will die or get stranded.
The normal cause of wear for carabiners, belay devices, rappel devices and other steel or aluminum gear is from the rope running around or through it. The process is accelerated by dirty ropes, which have tiny particles of rock embeded in the fibers. In an abrasive canyon or cave, it's not unheard of to wear 50% of the way through an aluminum Figure 8.
There is a very prominent urban legend floating around that states that when you drop a steel carabiner or belay device, it will develop "micro-cracks" and must be thrown away. This is bologna. If you insist on getting rid of your gear because of microcracks, please send it to me for "recycling".
There are two normal causes of wear for ropes. The first is from rubbing on a sharp opject. This will cut through the sheath and expose the core aka "core shot". If this happens, the rope is not useable anymore and must be cut into smaller pieces. To prolong the life of your rope, use a rope protector whenever possible. The second cause of wear to ropes is environmental. Sunlight and extended moisture cause unseen damage to ropes. To prolong the life of your rope, always store it in a dry place out of direct sunlight.