Electrochemical Passivation of Ni-Fe-Cr-Mn-Co Multi-Principal Element Alloy: An Evaluation of the Fate of Alloying Elements During Aqueous Oxidation

The effects of alloying elements on protective oxide formation are well understood for conventional corrosion resistant alloys (CRAs) containing a limited number of major and minor alloying elements. However, little is understood regarding protective oxides formed upon exposure to aqueous solutions in the case of multiple principal element alloys (MPEAs), also known as, high entropy alloys (HEAs), and compositionally complex alloys (CCAs). These materials can contain more than 5 principal alloying elements. A limited number of MPEAs have been studied to determine the corrosion resistance and connect these properties to surface oxide passive films which regulate corrosion. Many of these studies focus on the overall corrosion behavior with few details on the MPEAs passive film which mediate oxidation and provide protection. Many merely report alloying elements found in the oxide and typically lack passivity analyses and/or oxide characterization using surface sensitive films and molecular scale interpretations. Moreover, there are few systematic studies that track the fate of each alloying element. Furthermore, incremental variations in passivating elements above and below threshold concentration for passivity without change in metallurgical phases in the underlying substrate is often not possible.
The research presented herein focused on the study of electrochemically formed passive films on Ni38Fe20CrxMn21-0.5xCo21-0.5x MPEAs (x = 22, 14, 10, 6 at. % - i.e., Crx-MPEAs) in acidified and alkaline chloride solutions. These alloys were designed as a part of a broader Energy Frontier Research Center from DOE using intuition, CALPHAD, and emerging data science methods. Binary Fe-Cr, Co-Cr, and Ni-Cr alloys were used to examine whether third element effects were activated in NiFeCrMnCo alloys. Various in-situ and ex- situ characterization techniques were utilized to investigate the electrochemical passivation behavior, dissolution behavior, and the chemical make-up of potentiostatically formed passive films on MPEA surfaces.
When polarized in acidic chloride electrolytes, Cr-MPEAs with Cr contents above the critical threshold concentration of typical binary Cr containing alloys exhibited passivity and formed a protective passive film enriched in Cr(III) species and trace amounts of Ni(II), Fe(II) (IV), Mn(II), and Co(II) cations. Ni enrichment was observed at the oxide/metal interface. Ni, Fe, and Co dissolved into solution during potentiostatic passivation resulting in Cr enrichment and not assisting in passivation. An in-depth analysis of Cr enrichment and depletion was conducted on passive films formed on both Cr-MPEAs and binary Cr alloys. Correlations between electrochemical passive properties, bulk Cr concentration, and Cr enrichment within the passive film were made. Notably, the Cr10-MPEA exhibited the highest Cr(III) enrichment and was found to exhibit depletion of Cr at the oxide/metal interface suggesting that enrichment is limited by diffusion of Cr through the altered zone. Trends in Cr enrichment with bulk Cr content observed in binary alloys was similar to those observed in MPEAs. Cr hydroxides were the dominate molecular species in all passive films but a small fraction of Cr spinels were detected in the Cr-MPEA passive films. Spinels were not detected in passive films formed on binary alloy surfaces.
The electrochemical passive behavior of the Cr-MPEAs in alkaline chloride solution was investigated using the same techniques discussed for acidic conditions. Passive films formed on the Cr-MPEA surfaces in alkaline conditions were all enriched with Ni and Cr and contained higher concentrations of Fe and Co cations compared to films formed in pH 4 electrolyte.
The influence of exposure time in an acidic Cl- environment on passive film formation on Cr22-and Cr14-MPEA surfaces was investigated at 100 s, 1 ks, 10 ks, and 86.4 ks and compared to Ni-24Cr binary alloy. All Cr-MPEA passive films were enriched in Cr(III) cations at all exposure times aided by chemical dissolution of Mn(II), Ni(II), Fe(II) in acidified chloride solution. As exposure time increased, the concentration of Fe(II) and Mn(II) cations increased within the Cr-MPEA passive films. The Cr22-MPEA passive film was enriched in Co(II) cations at early exposure times but became depleted at longer times of 10 ks and 86.4 ks. Passive films formed on Cr14-MPEA surface were enriched in Cr(III) and Co(II) cations at all exposure times within the bulk of the film. At longer exposure times Cr spinels began to form eventually becoming the dominant oxide species at exposure time of 86.4 ks. No spinels were detected in Ni-24Cr passive films at any exposure time. Passive films formed at exposure times of 10 ks and 86.4 ks exhibited better passivity than freshly grown passive films at similar film thickness suggesting annealing of point defects and resistance to morphological roughening.
This dissertation provides an improved testing protocol for understanding complex MPEAs as well as understanding on how alloying elements effect MPEAs passivity, what atomistic characteristics contribute to forming protective passive films, as well as, the attributes that provide better corrosion protection. The analysis presented provides a path forward for determining if a given third alloying element alters the enrichment of a strong passivator like Cr in a solvent such as Ni. Newly acquired information provides further insights into the fate of each principal alloying element during aqueous passivation and how each contribute to the passivation mechanism. Additionally, this improved understanding can help guide potential advancements in designing CRAs.