Potential Dependent Mn Oxidation and Its Role in Passivation of Ni₃₈Fe₂₀Cr₂₂Mn₁₀Co₁₀Multi-Principal Element Alloy Using Multi-Element Resolved Atomic Emission Spectroelectrochemistry

Abstract

The identity of passivating oxides on multi-principal element alloys is of great interest as their optimization offers the potential for exceptional corrosion resistance in aqueous solutions over a broad range of potential and pH. This study focuses on a non-equiatomic Ni38Fe20Cr22Mn10Co10 solid solution alloy and tracks the fate of each alloying element during linear sweep voltammetry, low and intermediate potential holds in the passive potential domain as well as during open circuit relaxation after anodic polarization in slightly acidified Cl− solution. Ni dissolves at all potentials investigated in this work, Fe and Co are incorporated into oxides or hydroxides in low concentrations whilst Cr and Mn are enriched at passive potentials. At low passivating potentials, Mn(II) dissolves and is incorporated in minor amounts in oxides containing large concentrations of Cr(III). Considerable enrichment in Mn(II)-species occurs relative to Cr(III) in the oxide at 0.1 V vs SCE. Electrochemical impedance spectroscopy suggests the presence of layered oxides with marginal passivation at high Mn(II) levels. The formation of these oxides depends on a combination of thermodynamic and kinetic factors as well as the sequence of passivation.

Recent insights in corrosion science from atomic spectroelectrochemistry

“…Noticeable contribution of Mn to the oxide layer was confirmed using XPS analysis in both the studies mentioned above. [82,83] Nevertheless, incongruent dissolution and selective enrichment of Cr were observed in both the cases mentioned above, similar to Cr-containing conventional alloys. Although the passivity of these alloys was attributed primarily to Cr, thermodynamic predictions suggested the formation of complex spinel oxides.…”

“…Based on the dissolution profiles and XPS results, the relatively poor corrosion performance was attributed to higher Mn content in the MPEA. To shed more light on the fate of Mn, Han et al [83] studied the dissolution behavior of Ni 38 Fe 20 Cr 22 Mn 10 Co 10 in 0.1 M NaCl (pH ∼ 4). They reported that both Cr and Mn enrichment occurred during active to passive transition, reflecting that Mn contributes to the surface film.…”

“…Most alloys are found to dissolve incongruently initially during anodic polarization in the passive region, whereby selective enrichment of key passive elements was reported such as Cr in Fe-Cr, Ni-Cr, and MPEAs. [31,32,40,83,90] ASEC studies have unveiled the role of minor alloying elements on the passivity of Cr-containing alloys exhibiting Cr 2 O 3rich passive films. For example, Mo and Ni were found to dissolve selectively during anodic polarization of Ni-Cr-Mo alloy [40,107] and SS316L [19] in different electrolytes, until "critical" thickness of Cr 2 O 3 rich "n-type" passive film was reached in both cases.…”

Elementally Resolved Dissolution Kinetics of a Ni-Fe-Cr-Mn-Co Multi-Principal Element Alloy in Sulfuric Acid Using AESEC-EIS

“…34 In the passive film, Mn may be also present given that the dissolution behavior reached a quasi-steady state for t > 5000 s. However, such a film would not be stable as the Mn dissolution rate was higher than other alloying elements and did not show a decaying profile with time, similar to the previously observed case for the same MPEA at pH = 4.0 in 0.1 M NaCl. 34,35 It should also be noted that no potential dependent oscillation was observed for all elemental dissolution rates during AESEC-EIS at 0.60 V vs SCE, different from that at E oc shown in Fig. 5.…”

“…[27][28][29][30] Atomic emission spectroelectrochemistry (AESEC) has demonstrated to be a powerful technique to track the fate of each alloying elements for MPEAs during electrochemical measurements. 21,[31][32][33][34][35] The fate of each element, whether in metal, oxide or solution during spontaneous dissolution, activation and passivation, was ascertained by the AESEC technique. AESEC enables the determination of atoms which are oxidized but not dissolve in solution, i.e., may have reached the surface as an oxidized insoluble species, via AESEC mass-charge balance.…”