Scanning Microscopy


The breakdown of passivity and localized corrosion of a Ni-20Cr-10Mo alloy was investigated. The methods employed were potentiodynamic polarization and SEM, and AES and EDX after potentiostatic polarization over a period of 20 hours in the passive and transpassive regions. The 1 μm finished as-cast specimens were polarized in aerated 0.1 M NaCl. The cyclic polarization curves revealed a critical pitting potential of 470 mv (SCE), while the protection potential was 300 mV (SCE). Using the potentiostatic polarization technique, nearly constant corrosion currents appeared, indicating that the whole surface was corroded uniformly. SEM pictures of samples, corroded at 650 mv, showed little pits under the oxide layer and a thinning down of the outer oxide layer. This lead to the opinion that the penetration as well as the adsorption mechanism determine the breakdown of passivity. EDX analysis and AES depth profiles showed an enrichment of Cr and Mo in the oxide. In contrast to oxidized samples, no second layer of Ni was found in the outer oxide region. In the transpassive region the relative amount of Cr and Mo in the oxide layer was higher than the one found in corresponding samples polarized in the passive region. The oxide thickness found was about 5 nm in the passive region (300 mV SCE) and about 250 nm in the transpassive region (650 mV SCE).

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