Comparison of internal stresses and air oxidation induced by quenching of partially Cr-coated and uncoated optimized ZIRLO™ part II: Stress assessment

The internal stress states of Optimized ZIRLO (OPZ) samples are evaluated post-quenching from increasing temperatures to determine the role of chromium coatings, oxidation, and microstructural reorienting on internal stress state. Before substantial oxide development (under 700 °C), uncoated tubular samples freely expand and do not accumulate significant global internal stress. As a stable, passivizing oxide develops (700–900 °C), a complex stress state develops with both the interior and exterior oxides in hoop compression. The internal tensile stress increases until the sample fully oxides, and biaxial stress is released through tortoiseshell-like cracking mechanisms. When a chromium coating is introduced, the initial stress states at low temperatures are higher than the uncoated samples due to Cr-coating application techniques and material differences between Cr and OPZ. At higher temperatures (700–1100 °C), the Cr-coated samples undergo a much simpler stress state than the uncoated samples due to the tensile hoop stress state of the Cr-coating caused by differing thermal expansion and the compressive state of the internal oxide caused by a Pilling-Bedworth ratio > 1. The subsequent film cracking mechanisms also differ from the uncoated samples. The smoother stress state experienced by the Cr-coated vs uncoated OPZ samples allows the Cr-coated samples to maintain the circular geometry that is vital to their protective application in nuclear reactors.