Characterization of Oxide Films on Stainless Steel Surfaces of Boiling Water Reactor Primary Cooling Systems

Double oxide layers were observed on the stainless steel coupons. The outer layer consisted of well-developed crystals (diameter: 0.5 to 1.0 üm, probable major oxide form: NiO Fe₂O₃), while the inner layer consisted of smaller particles [diameter: 0.1 to 0.3 μm, probable major oxide form: NiO-(Cr,Fe)_2O3]. The depletion of chromium and nickel and the accumulation of impurities in the water, such as ⁶⁰Co, zinc, and copper, were observed in the outer layer. For the carbon steel the chromium-rich layer existed at the outer/inner interface. It was considered that the outer and inner layers were a corundum-type oxide (Cr,Fe)₂O₃. The outer layer seemed to be formed by a wet corrosion mechanism (dissolution-precipitation of metal), while the inner layer seemed to be formed by a dry corrosion one (reaction of metal with oxygen diffused through the oxide layers from the water). More than 80% of the ⁶⁰Co in a coupon was included in the outer layer, and it was supposed that this was deposited mainly during wet corrosion.