Corrosion behaviour of CoCrMo alloys in 2 wt% sulphuric acid solution

The corrosion behaviours of γ- and É-phase CoCrMo alloys with fcc and hcp crystal structures, respectively, were investigated in 2 wt% H ₂SO₄ solution via polarization testing. The polarization curves of the two alloys overlapped each other from the passive to the transpassive regions, showing typical transpassive dissolution characteristics that led to passivity breakdowns for both alloys. However, the microstructural evolutions of the sample surfaces in the transpassive region were remarkably different, with dissimilar corrosion-susceptible regions detected for the two alloys. X-ray photoelectron spectroscopy analysis indicated that Cr contributed significantly to the formation of the corrosion product films, resulting in trilayer structures on both the fcc and hcp sample surfaces. Scanning electron microscopy and 3D laser scanning microscopy observations suggested that the existence of precipitated phases was the main reason for the differences in the corrosion features between the hcp and fcc samples. The second phases rich in Cr and/or Mo in hcp Co-Cr-Mo alloys result in a heterogeneous microstructure distribution of alloy matrix, which can act as preferential initiation sites for the breakdown of passive film during the transpassive region. Inductively coupled plasma-optical emission spectrometry results indicated that the ratios of the dissolved elements for both samples roughly approximated the composition of the initial alloys. However, for the hcp sample, the amounts of dissolved Cr and Mo were somewhat lower than those in the initial alloy composition due to the selective dissolution of alloy matrix with either low Cr or low Mo content near either the Cr-riched σ phase or the Mo-riched phase. Cr is a key factor in determining the passive current of the alloy; whereas, in the transpassive region, Mo exerts its role by inhibiting the local dissolution of metals via attachment to the outermost surface of the ]chromium oxide film. The overlapped anodic current densities during transpassive region in fcc and hcp samples are mainly ascribed to the oxygen evolution, which dominantly contributed to the high anodic currents in both samples.