During the operation of water-moderated reactors having zirconium-alloy clad fuel elements, exposure to high-temperature water or steam may cause corrosion of the cladding. Corrosion deposits which form on heat-transfer surfaces are likely to result in “hot spots,” and if the corrosion is severe, structural damage may occur. The best quality alloy, such as is used in the fabrication of the fuel elements, is initially corrosion-resistant and may be tested by visual inspection. However, the fuel elements are subsequently fabricated into a subassembly by welding. Structural changes resulting from the welding operations, improper rinsing after pickling, or surface contamination due to incomplete removal of iron used in rolling operations may result in loss of corrosion resistance. It is necessary, therefore, to test each complete subassembly in high-temperature water or steam before it is used in the reactor to be sure that no areas exist where corrosion is rapid.