For about 50 years prestressing steels are used in Switzerland. They are in tendons, in fitted anchors and for some years also in stay cables. The level of internal prestressing steels is important for the structural safety, serviceability and durability of the relevant building and should be detectable and assessed. The options to non-destructively determine the corrosion state of clamping and anchoring systems are still relatively limited. Corrosion damage occurred in prestressing steels can gain valuable experience through the accurate evaluation of causation.