The effect of Mn addition on corrosion behaviour of equiatomic CoCrFeNi high entropy alloys in NaCl solution

Abstract

The corrosion behaviour of equiatomic CoCrFeNi and CoCrFeNiMn high entropy alloys manufactured by the vacuum arc melting were studied in 3.5 wt-% NaCl solution. And the reasons for the differences in corrosion resistance were investigated in terms of microstructure, electrochemical behaviour and oxide film. The results show that the two alloys exhibit different corrosion characteristics, with CoCrFeNi alloy displaying typical passivation characteristics and CoCrFeNiMn alloy exhibiting activation behaviour. The CoCrFeNi HEA with compact and protective passive film has lower corrosion current density, larger film resistance and polarisation resistance, meaning a lower corrosion rate and superior corrosion resistance compared with those of CoCrFeNiMn alloy. Moreover, CoCrFeNiMn alloy forms a discontinuous and loose corrosion product film with higher Mn content and lower contents of Cr, Fe and Co compared to the matrix, which decreases its protection performance. The addition of Mn changes the electrochemical behaviour, and higher Mn oxides/hydroxides within oxide film are unfavourable to forming a stable passive film.

Influence of the Mn Content on the Corrosion Behaviour of HEA CoCrFeN-iMnX (X=5, 20, 35 at.%) Prepared via MA+SPS

“…3). This behaviour is similar to the observations of Zhang et al [9], confirming the negative effect of the Mn on the corrosion resistance of the CoCrFeNi-based alloy. On the other hand, both the Mn5 and Mn20 alloys were showing the formation of a barrier layer that was stable up to E b of -105.0 and -191.3 mV/ACLE, respectively.…”

“…There are only a few scientific reports describing such features, even for the well-known CoCrFeNiMn alloy. Considering the aqueous corrosion, the situation is even more disturbing since these reports are focused on materials/layers prepared by significantly different methods, such as laser cladding [8], and arc melting [9], while only one publication [10] is devoted to the CoCrFeNiMn alloy deposed as a layer via mechanical alloying. Considering the ability of HEAs to fine-tune the properties just by changing the content of particular elements, they might become highly universal materials, that can be either used as materials for implants or as structural materials for extreme conditions.…”