The immobilization of cladding-derived stainless steel, zirconium, technetium-bearing residues, residual actinides, and noble-metal fission products takes place in stainless-steel–zirconium metallic waste forms through casting them into multiphase alloys. Durability assessment based on their nominal compositions alone is not possible since elements responsible for passivity, cathodic activity, and waste hosting segregate into separate phases upon solidification. This study aims to determine what surface-based parameters cause corrosion contrast of two 316L alloy-based reference alloys waste forms, RAW-2 and RAW-4, under acidic chloride polarization. Analysis of batched composition, surface phase fractions, phase-based chemistry, potentiodynamic results, and post-polarization surfaces of RAW-2, RAW-4, 316L, and palladium in air saturated 0.1 mmolal H$_2$SO$_4$ + 10 mmolal NaCl solution with pH = 4 is performed. In this analysis, there are three linked quantities that are considered: Fe-rich passivation reservoir provided by austenite and ferrite, ZrFe$_2$ burden represented by Cr-Mo deficient intermetallics, and noble-metal concentration that changes the mixed potential of alloy. Surface fraction of austenite and ferrite in RAW-2 is 33\% and 30\%, ZrFe$_2$-z, 20\%, ZrFe$_2$-u, 10\%, and ZrPd$_2$, 7\%. In the case of RAW-4, there is 50\% ferrite, 48\% ZrFe$_2$-z, and 2\% ZrFe$_2$-u. Surface value of passivity reservoir is 0.92 in the case of RAW-2 and 0.65 in the case of RAW-4, while ZrFe$_2$ burden is 0.16 and 0.26 accordingly. Calculated surface values correlate well with electrochemical results: RAW-2 alloy has corrosion potential $E_{\mathrm{corr}}=0.18$ V versus SCE and shows negligible anodic current in large potential range, while RAW-4 alloy has lower corrosion potential, $E_{\mathrm{corr}}=0.02$ V versus SCE, but fails near 0.42 V versus SCE due to ZrFe$_2$ intermetallic breakdown. Hence, durability is controlled by a balance between passive phases and vulnerable intermetallics on the surface, but not only by mixed-potential ennoblement effect. It is important that an environmentally resistant 316L-Zr metal waste form retains the Zr-rich waste host phases with minimal connectivity and neighboring Cr-Mo rich Fe phases capable of passivation in chloridic acidic environments.

metallic waste form,316L stainless steel,zirconium; ZrFe$_2$,Laves phase,passivity,chloride corrosion,molybdenum partitioning,mixed potential,nuclear waste immobilization