Corrosion properties of nanocrystalline Co and Co-P alloys

In this thesis, the corrosion properties of electrodeposited nanocrystalline Co and Co- 1.1 and 2.1 wt% P alloys (7 to 20 nm grain size) were investigated in a wide range of solution pH by using polarization and electrochemical impedance spectroscopy techniques along with scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy analysis. In 0.1 M H₂SO₄ and 0.5 M Na₂SO₄ solutions (pH 1 and 3, rspectively), nanocrystalline Co showed higher anodic and cathodic reaction rates and reduced localized corrosion attack compared to coarse grained Co counterparts. An enhanced electrochemical reaction rate of nanocrystalline Co was correlated to a significant increase in grain boundaries and triple junctions. For pure Co, a catalytic mechanism has been suggested to rationalize a more favorable route for their anodic dissolution reaction. An addition of P leads to a significant increase of corrosion resistance of nanocrystalline Co-P due to the increased elemental P concentration On the corroded surface at E [sub oc] following an initial selective dissolution of Co. However, at higher anodic overpotential, the superior corrosion resistance of nanocrystalline Co-P did not last due to the formation of a porous and defective surface film. The excellent corrosion resistance of nanocrystalline Co-P deteriorated from heat treatment at 350 °C and 800 °C due to higher chemical heterogeneity. In a 3.56 % NaCl solution at pH 6.2, nanocrystalline Co-P showed a higher anodic dissolution rate than that of nanocrystalline Co due to a reduced adsorption area of chloride ions. However, the corrosion rate of nanocrystalline Co-P decreased in comparison to nanocrystalline Co due to a much smaller cathodic exchange current density for oxygen reduction on nanocrystalline Co-P. A physical model was elaborated to explain the different response of annealed nanocrystalline Co-P by considering the different adsorption properties of chloride ions. In a 0.1 M NaOH solution at pH 13, all tested Co and Co-P samples exhibited active-passive-transpassive behaviors with the onset of primary and secondary passivations. While anodic polarization curves for poly- and nanocrystalline Co had qualitatively very similar behavior, nanocrystalline Co-P showed higher passivation current densities due to their decreased passive film resistance and reduced film thickness. XPS analysis revealed that the passive film formed in the primary passivation ranges was consisted of mainly Co(OH)₂ with a thickness of >10 nm. The passive film formed during secondary passivation consisted of rather complex compounds such as CO₃O₄, CO₂O₃, Co(OH)₃ as well as Co(OH)₂ with a thickness of