Abstract

The corrosion susceptibility of borided CoCrMo alloy exposed to a diffusion annealing process (DAP) and immersed during 7 days in a Hank's solution was evaluated, being compared with the results obtained in the borided CoCrMo alloy and the CoCrMo alloy. Firstly, the CoCrMo alloy was exposed to the powder-pack boriding process during 5 h at 1273 K, to obtain a biphasic layer (CoB-Co2B) with a total thickness around 33 μm. Then, the DAP was performed on the borided CoCrMo alloy during 2 h at 1273 K in an inert argon atmosphere to obtain a 29 μm of monophasic Co2B layer. The corrosion tests were performed by the potentiodynamic polarization technique to estimate the corrosion resistance (Rp) and the corrosion current density (jcorr) by means of the linear polarization and the Tafel extrapolation techniques, respectively. The results showed, in the 7th day of immersion in the Hank's solution, that the borided CoCrMo alloy exposed to the DAP exhibited a corrosion value ~22 times higher (~357 kΩ cm2) than that obtained in the borided CoCrMo alloy (~15 kΩ cm2), reaching a value close to the CoCrMo alloy (~385 kΩ cm2). The corrosion performance of the borided CoCrMo alloy exposed to the diffusion annealing process was attributed to i) the presence of the monophasic Co2B layer and high contents of Cr and Mo on the outer-zone of the borided surface, and ii) the depletion of the B2S3 and CrPO4 species at the surface of the borided CoCrMo alloy. Finally, the borided CoCrMo alloy exposed to the DAP exhibited a corrosion susceptibility around 7%, meanwhile the borided CoCrMo alloy obtained a value of 96%, approximately.

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