Oh my goodness. Ok so basically white scrolls are used as a failsafe for the slots on your equipment. When you use a white scroll with your normal scroll, if the scroll fails the item will not lose a slot. Obviously if the scroll works the item still loses a slot though. Another common misconception: White scrolls DO NOT protect your item from blowing up. If you use a 30% with a white scroll it can still blow up. This is why white scrolls are mainly used alongside 10% scrolls to create perfect equipment.
Say you use a white scroll with a 60% scroll on a clean weapon with 7 slots. If the 60% fails, white scroll saves your slot and weapon still have 7 slots. White scroll is used and disappears, right? If the 60% passes, does the white scroll still disappear?
Pro tip: If you get a WS, put it in your fricking storage immediately. You will otherwise use it by accident when you randomly scroll trash. It's inevitable, believe me.