The effect of high-temperature water chemistry and dissolved zinc on the cobalt incorporation on type 316 stainless steel oxide

Abstract

Oxidation tests on Type 316 stainless steel were performed under hydrogen water chemistry and normal water chemistry for 500 hours with continuous injection of a 59 Co solution with and without 5 ppb of Zn injection. The present paper identifies the resultant oxides, analysed using analytical electron microscopy and complementary surface microstructural characterisation techniques. Zn injection has been shown to reduce Co incorporation in the inner oxide layer under both water chemistry conditions. The secondary effects of Zn injection on the oxide film growth have also been investigated and discussed.

Effects of Zinc Addition on the Corrosion Behavior of Pre-Filmed Alloy 690 in Borated and Lithiated Water at 330 °C

“…The reason for the inconsistency is not clear at this time, but the conflicting results may be due, in part, to differences in thickness measurement methods, test materials, and conditions. However, it is clear that the oxide thicknesses on fresh stainless steels and nickel-based alloys exposed to zinc conditions were significantly thinner than those of samples that were exposed to zinc-free conditions [ 7 , 11 , 14 , 18 , 21 , 53 ]. In this work, subsequent exposure of the pre-filmed Alloy 690 to zinc water resulted in a significant decrease of the corrosion and corrosion release rates, as shown in Figure 10 .…”

“…First, zinc-incorporated compounds can be formed directly from solutions containing metal cations such as zinc, iron, chromium, and cobalt. It should be noted that the free energy of formation of zinc-incorporated spinels is lower than that of zinc-free spinels [ 17 , 53 ]. In other words, formation of zinc chromite (ZnCr 2 O 4 ) is thermodynamically preferred to that of non-zinc chromites (i.e., NiCr 2 O 4 , FeCr 2 O 4 , CoCr 2 O 4 ).…”

Behavior of Zinc Incorporation into Fuel Deposits in Borated and Lithiated Water with Dissolved Zinc

“…Therefore, it is believed that the zinc-incorporation process includes the selective dissolution of metal cations from the lattice sites and solid-state diffusion through the vacancies. It was also reported that the free energy of the formation of zinc-containing spinel oxides on stainless steel was larger than the energies for the zinc substitution of divalent cations such as nickel, iron, and cobalt [ 11 ]. That is, the formation of zinc-incorporated spinel is thermodynamically favored.…”

“…Zn 2+ ions have been reported to replace Ni, Fe, Co, or radioactive Co ions in spinel-type oxides formed on nickel-based alloys and stainless steels under RCS conditions [ 7 , 8 , 9 , 10 ]. Because the zinc-incorporated oxides are thermodynamically more stable and protective [ 5 , 11 , 12 ], the corrosion and corrosion release rates of the RCS materials are significantly mitigated, resulting in a reduction of the radiation source term.…”

“…Accordingly, as of 2014, there were 85 PWRs worldwide, and most BWRs have implemented the zinc injection technology into the RCS [ 17 ]. As mentioned above, a lot of research on zinc addition has been performed from the viewpoint of the corrosion rate [ 5 , 6 , 18 , 19 ], corrosion release rate [ 4 , 5 , 6 , 10 , 20 ], oxidation kinetics [ 7 , 12 , 21 , 22 , 23 ], dose rate [ 20 , 24 , 25 , 26 ], and zinc incorporation [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 20 ] in the oxide films of Ni-based alloys and stainless steels in laboratories and operating PWRs. However, there is relatively little open literature on the effects of zinc injection on fuel deposits, possibly because of experimental difficulty in laboratories and extremely high-level radiation from burned fuel assemblies.…”