Polarization curves of detached coatings were similar each other(Fig.9), even though corrosion potential values were more scattered with respect to the acid aqueous solution. Anodic passivity currents were in the range 1-4 uA/cm2, two order of magnitude higher than c.p. titanium.
Fig.9. Polarization curves: detached coatings and c.p. titanium(0.5MNaCl).
Polarization curves of coated samples are reported in Fig.10. Passive behaviour and passivity breakdown at potential values lower than in the case of substrate alone were observed. Interconnected porosities, typical of plasma-sprayed coatings, behaved as active sites for the onset of crevice phenomena on the substrate surface, leading to a lower breakdown potential.Breakdown of substrate passivity was confirmed by the hysteresis loops formed by the reverse scans of potentiodynamic curves and by the absence of pitting for the detached coatings. An increase of coating porosity was responsible for lower pitting potential(Fig.11): sample2N showed passivity breakdown at +80 mV, the more porous sample 9N showed an increase of anodic current at -30mV.
Fig.10. Polarization curves for coated samples:0.5 M NaCl aqueous solution.
Fig.11. Polarization curves of samples 2N (a) and 9N(b):0.5 M NaCl.
Potential values of uncoated steel substrate and detached coatings must be carefully interpreted to predict coated samples behaviour.A direct comparison lead to the wrong conclusion that coatings should exhibit anodic behaviour with respect to the substrate. This is misleading, since the presence of interconnected porosity and the resulting crevice corrosion induced on the substrate lead to acidification of pore solution, so that it was the substrate to exhibit an anodic behaviour.During the stabilization period prior to polarization curves, the corrosion potential of coated samples tended toward decreasing values.
Crevice corrosion was confirmed by metallographic investigations. As an example for the specimen 9N immersed for 170 h in the neutral solution, without any external polarization, large pits at the interface were evidenced(Fig.12).
Fig.12.Sample 9N cross-section after 170 h immersion in neutral solution.
As a general observation, reproducibility of the polarization curves in the neutral solution was poorer than in the acid solution.
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