normal wear and tear is pitting from cavitation, fatigue cracking, and abrasion from suspended solids in the water. steel elements are repaired by welding, usually with stainless steel rod. damage areas are cut or ground out, then welding back up to their original or an improved profile. old turbine runners may have a significant amount of stainless steel added this way by the end of their lifetime. elaborate welding procedures may be used to achieve the highest quality repairs.