The influence of Nb addition on the passivity of CoCrNiNbx multi-principal element alloys

•CoCrNiNbx MPEAs are composed of FCC matrix and Nb-rich HCP dendritic phases.•Oxides on MPEAs consist of a Cr-enriched outer layer and an Nb-enriched inner layer.•CoCrNiNb0.2 achieves optimal corrosion resistance in 0.1 M H2SO4.•Nb0.2 stabilizes the passive film and reduces pitting sensitivity. The effect of Nb addition on the passivation behavior of CoCrNiNbx (x = 0 ∼ 0.5) multi-principal element alloy (MPEA) were investigated with surface characterization and electrochemical techniques. With the addition of Nb, the Nb-riched HCP dendrite phase gradually precipitates from the original FCC phase. The corrosion resistance of CoCrNiNbx MPEAs increases firstly and then starts to decrease, reaching an optimal value at x = 0.2. XPS depth profiles and ToF-SIMS characterization of the surface passive films demonstrate that the film structure is transformed from a single layer to the bilayer which consists of a Cr-enriched outer layer and an Nb-enriched inner layer. Analysis of the pitting and passivation behavior of the CoCrNiNbx MPEAs suggests that a moderate content of Nb thickens the passive film, promotes the production of Nb oxides and Cr2O3, and thus stabilizes the passive film and finally reduces pitting sensitivity.