Corrosion of CrN-Coated Steels for Nuclear Reactors in Liquid Pb-Bi

Corrosion tests of CrN-coated steels for nuclear reactors were conducted in liquid lead-bismuth (Pb-Bi) at 450°C and 550°C for 3000 h to investigate the applicability of CrN coating to a liquid Pb-Bi environment. CrN coatings on F82H (Fe-8Cr-2W-0.3V-0.04Ta-0.1C) and 316SS exhibited good compatibility in liquid Pb-Bi during corrosion test at 450°C. The CrN coating layer suffered heavy damage such as cracking and spalling, and showed no effectiveness as a protective layer in corrosion test at 550°C. Nickel and chromium in 316SS dissolved into Pb-Bi through the damaged coating layer at 550°C. The cracking and spalling were not found after heating CrN-coated steels in Ar gas at 550°C. It is considered that stresses caused by the difference of thermal expansion coefficients between CrN and steels led to cracking and spalling of the CrN coating through corrosion attack by liquid Pb-Bi at 550°C.