Stainless steel is made by adding a lot of Chromium (at least 12%) and Molybdenum to steel alloy. The Chromium, when in contact with air forms a non reactive Chromium Oxide film. This film protects the surface iron from reacting with water and forming rust. If the scissor is scratched when wet, the Chromium cannot react with Oxygen in the air and form a new protective film, so stainless steel can rust when scratched underwater.
Many chemicals in the salon can also cause stainless steel to corrode. Some examples would be perm solution, hair color, and some sterilizers that do not contain rust inhibitors. Also high skin acid, (ex: women during their period) can cause discoloration on stainless steel tools. So don’t cut the tip off perm solution containers with your good scissors. Get a pair of cheap scissors in the discount store for that purpose. And keep your scissors clean.