Electrochemical corrosion study of La(Fe_11,6-xSix_1,4Mn_x)H_1,5 in diverse chemical environments

Highlights

• linear sweep voltammetry of La-Fe-Si alloys, focusing on La(FeMnSi)₁₃H_1,5

• electrolytes: citrate, carbonate and TRIS buffers, NaCl and NaOH solutions

• impact of commercial corrosion inhibitors and defined protective compounds

• assessment of protection strategies for operation in magnetocaloric cooling

Abstract

La-Fe-Si-based alloys represent a promising material class for magnetocaloric cooling at ambient temperatures, but contain highly oxophilic elements and are chemically sensitive, which impairs their continued operation in aqueous heat exchange media. The development of protection strategies ensuring long-term stability necessitates a comprehensive understanding of the material's corrosion characteristics. The present work focuses on a such an assessment for hydrogenated La(FeMnSi)₁₃ containing α-Fe to enhance it's mechanical properties. Linear sweep voltammetry served as the main analytical tool and was performed in preferably buffered electrolytes with pH values reaching from moderately acidic to strongly alkaline, in the presence and absence of corrosion-enhancing species (chloride, chelators). α-Fe, La₁Fe₁Si₁, and La(FeSi)₁₃ were employed as reference materials to clarify the passivation pattern of the material in carbonate buffer. Specific chemical compounds with clear mechanistic impact (phosphate acting as precipitate former, hydrazine as oxygen scavenger) were tested alongside commercial corrosion inhibitors to investigate their effects. Our objective is to provide a systematic evaluation of the corrosion properties of the alloy system, building on previous investigations and taking into account its general materials chemistry.

Introduction

Satisfying the gloabally quickly rising need for cooling in an environmentally benign fashion is challenging, given its considerable energy demand, the negative impact of coolants, and the maturity of the existing solutions, in particular vapor compression refrigeration [1,2,3]. The latter technique utilizes the most prevalent thermodynamic cooling cycle and looks back at over a century of development and widespread commercialized application, but is approaching its efficiency limits. Due to the high level of existing optimization, further efficiency gains are restricted in their extent and / or involved in their execution (e.g., expanders coupled with strongly pressurized transcritical CO₂ [1]), hampering the aspired technological transformation. This issue can be resolved by switching to alternate cooling mechanisms. which are not bound by the intrinsic thermodynamic limitations of the vapor compression cycle, such as the magnetocaloric effect [4,5].

Recent develpopments in tailored magnetocaloric materials have fuelled the prospects of room temperature magnetic refrigeration, which simultaneously promises an increased energy efficiency and the replacement of disadvantageous gaseous coolants with aqueous heat exchange fluids [4,6,7]. Still, the commercial implementation is hamstrung by the complexity of the technique (e.g., concerning the system design, fluid and heat transfer management, magnet and material optimization), as well as by the chemical and mechanical sensitivity of important magnetocaloric materials [4,8,9].

Due to their low raw material cost, elemental criticality and toxicity [3], large isothermal entropy change and tunable Curie temperature (Fig. 1), lanthanum-iron-silicon-based alloys represent a promising material class for magnetocaloric cooling [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. One main obstacle preventing implementation of La-Fe-Si alloys so far has been their poor mechanical stability [18], [19], [20], although recent work has shown that this issue can be mitigated [17]. To ensure continued and reliable operation, magnetocaloric materials must be stable in aqueous heat transfer media, which aside their environmental benefits exhibit attractive physicochemical properties (e.g., excellent thermal capacity and good thermal conductivity), thus promoting performance [8]. Alternatives such as silicone oil are much less aggressive, but trade corrosion stability for lower specific heat or increased viscosity [21].

Unfortunately, in aqueous environments, La-Fe-Si alloys are highly prone to corrosion: The low nobility of the constituting elements, particularly of the rare earth component, create a strong driving force towards oxidation (standard electrode potentials: E^θ(Fe/Fe(II) = -0,447 V, E^θ(La/La(III) = -2,379 V [22]). This issue is exacerbated by the inefficient passivation of the dominating component iron, which in typical alloys constitutes 75-82 at% of the material [21,[23], [24], [25], [26], [27], [28], [29], [30]]. While the corrosion behavior of La-Fe-Si alloys has been subject of several electrochemical investigations [21,[23], [24], [25], [26], [27], [28], [29], [30]], the existing efforts are somewhat fragmented, focusing on singular electrolytes and few measurements, using distilled water as a comparably gentle but low-conductivity medium, or resorting to commercial corrosion inhibitor mixtures, which offer application-relevant insights, but do not allow for a mechanistic analysis due to their unknown composition and concentration.

This study aims at providing a more systematic assessment of the corrosion properties of La-Fe-Si alloys in diverse chemical environments, including different pH values, the presence of coordinating and corrosion-promoting ions, and commercial as well as single-component corrosion inhibitors, using linear sweep voltammetry as the main analytical tool. Based on the experimental results, recommendations for general protection strategies are devised to limit the corrosive attack on La-Fe-Si alloys, which can be used to develop and optimize aqueous heat exchange media for application in magnetic refrigeration.