M. Janik-Czachor

The analysis of electrochemical methods for the determination of characteristic potentials of pitting corrosion

Abstract Electrochemical methods for the determination of characteristic potentials of pitting are summarized and results of measurements of these potentials are discussed. Two characteristic potentials have to be distinguished: the potential of pit nucleation Enp, and the critical potential of pitting, Ecp. The physical meaning of these potentials is elucidated. Whereas both Enp and Ecp values are important from the theoretical standpoint, Ecp is more useful for practical purposes, since it indicates the range of potentials more negative then Ecp within which the pitting is surely excluded.

Electron microprobe study of the corrosion pits nucleation on Fe-16cr single crystals

Abstract Pit nucleation on Fe-16Cr single crystals was studied under controlled electrochemical conditions. As shown by microscopic and electron microprobe investigations, the pits are formed predominantly at inclusions of chromium oxide. The effect of oxide inclusions on the pit formation is discussed in the light of the results obtained in this study.

Pitting corrosion of single crystals of the Fe-16 Cr alloy in solutions containing Cl− ions

Abstract Pitting corrosion of a Fe-16Cr alloy in solutions containing Cl − ions was studied on {100}, {110} and {111} faces of single crystals. Near the breakdown potential there occurred anisotropic dissolution of the alloy and the pits took the form of regular polyhedrons. The lowest dissolution rate was observed with the {110} face (the most closely packed crystal plane) and with the less closely packed {100} plane. These planes formed the bounding faces of regular pits. The nucleation of pits occurred at subboundaries, at spots where bubbles of O began to grow, and at the three-phase boundary between metal-solution-gas.

Corrosion behaviour of low-manganese stainless steels

Abstract Electrochemical measurements have shown that low-manganese (∼0.2% Mn) stainless steels with low (⩽1.2% Mo) or no molybdenum exhibit a higher pitting resistance in neutral sodium chloride (0.005–2 M) than the analogous commercial steels containing 1.4 ∼ 2% Mn and ∼2.8% Mo, or the special laboratory heats of similar composition, but, with a high S content, up to ∼0.2%. Microscopic examinations, X-ray microprobe analysis and SAM investigations have revealed a correlation between pitting resistance of the steels, the composition of the non-metallic inclusions, and the composition of the passive film formed at the inclusions and at the adjacent matrix. Electrochemical measurements in acidic and neutral sulphates have shown that the low-manganese steels generally exhibit higher ability to passivate than the commercial ones. In contrast to the behavior in neutral solutions, also in those containing chlorides, the passivity of a modified steel containing neither Mo nor Ti is less stable in acidic media than that of the other novel steels.

The stability of the passive state on AlMo sputter-deposited glassy metals

Abstract The anodic behavior of sputter-deposited Al-Mo glassy alloys containing 15–45 at% of Mo were investigated in 0.05–1 M NaCl by using electrochemical, microscopic and surface analytical methods. The alloys exhibited a stable passivity within a large potential region. AES revealed the presence of oxidized Al, Mo and O within the passivating film. The alloys underwent pitting at and above an anodic potential, which was about 1200 mV higher than the corresponding pitting potential for Al metal. The pitting potential for Al-Mo glasses increased by ∼200 mV with a 10 at% increase in Mo content and decreased by ∼50 mV with a ten-fold increase in chloride concentration in the electrolyte. A tentative mechanism of pit nucleation in Al-Mo glasses is discussed.

Inter-relationships between alloy composition, passive film composition and pitting behaviour of Al-Mo and Al-Mo-Si metastable alloys

Abstract Potentiodynamic polarization has been employed to determine the pitting potentials of Al-Mo and Al-Mo-Si metastable alloys. A significant increase in the pitting potential of aluminium is found through addition of molybdenum, to 30 at.%, which is further enhanced, by ~ 600 mV, with addition of 6 at.%Si. Auger electron spectroscopy indicates formation of relatively pure alumina passive films, containing negligible amounts of molybdenum and silicon species, under the selected conditions of polarization. The absence of molybdenum and silicon species in the anodic film is attributed to the enrichment of the alloying elements in the alloy, which is a feature of anodic oxidation of many aluminium alloys. The presence of enriched alloy layers, formed by prior oxidation of aluminium, is revealed by transmission electron microscopy of galvanostatically formed passive films. The increased pitting potentials are suggested to be associated with the enrichments of molybdenum, or molybdenum and silicon, in the alloy, although the precise mechanism is not disclosed.

Passivity of metal-metalloid glasses

Abstract Recent progress in fundamental (and applied) research in the area of passivity of amorphous metals is summarized. Electrochemical results characterizing ability to passivate and stability of the passive state of glasses with and without Cr are compared. Effect of structure and composition, particularly the effect of metalloid components, on passivity is considered. Surface analytical results aimed at characterization of the chemical state of passive film constituents are discussed.

On the composition of passive films on homogeneous alloys

Abstract Implications of the Kirchheim model describing passive films formed on homogeneous alloys are discussed. A comparison of the theoretical predictions with the results for NiZr glassy alloys are given.

On phenomenological description of the anodic behavior of homogeneous alloys

Abstract Recent progress in understanding of the anodic behavior of homogeneous alloys is briefly summarized. A comparison between the theoretically predicted by Kirchheim and experimentally measured,by different authors, Cr-contcentration in passive films on Fe-Cr alloys is given.