Influence of Substrate Dislocation on Passivation of Pure Iron in pH 8.4 Borate Buffer Solution

Abstract

The influence of substrate dislocations on the passivation behavior of pure iron in a pH 8.4 borate buffer solution was investigated. Cold-rolling was employed to induce dislocation in the annealed substrate. The dislocation density on the cold-rolled substrate depended on the reduction ratio ͑Red͒ and was maximum at Red = 60%. The electric charge consumed in the passivation and the passivity-maintaining current increased with an increase in the dislocation density. Electrochemical impedance spectroscopy revealed that the dislocation exposed to the iron surface increased the donor density in the passive film and reduced the chargetransfer resistance for the film aging. It was also demonstrated by radio frequency-glow discharge optical emission spectroscopy that a thick passive film formed on the cold-rolled iron compared with that on annealed iron. These results suggest that substrate dislocations enhance the reactions at the substrate/film interface, resulting in the formation of a highly defective passive film.Industrially important metallic materials such as steels and aluminum alloys usually undergo a modification of the metallographic texture by the working process of cold-rolling. Although many metallographical studies have revealed the mechanical characteristics of materials deformed by cold-rolling, it has not been clarified that the corrosion resistance of the material's surface is degraded by rolling. The degradation of corrosion resistance seems to be mainly caused by phase transformation and alloy segregation, 1-7 though the dislocation and/or the strain of the material's substrate are also detrimental to the corrosion resistance. 8,9 However, there are no consistent changes in pitting and corrosion potentials 4,6,10,11 and in the dissolution rate for the worked steels. 3,12-14 These inconsistent results seem to be due to the differences in the working degree and the materials' compositions and phases. It is very difficult to investigate only the effect of individual metallographic textures induced by working on the corrosion behavior of alloys because alloys easily cause phase transformation and/or alloy segregation at grain boundaries. Thus, it is important to investigate each metallographic parameter with pure metals, causing no phase transformation and no precipitation.The corrosion resistance of metals strongly depends on the passive film formed on the surface. The characteristics of a passive film formed on pure iron in a pH 8.4 borate buffer solution have been investigated in detail using various techniques such as coulometry, 15,16 electrochemical impedance spectroscopy ͑EIS͒, 17-19 ellipsometry, 16,20 surface-enhanced Raman spectroscopy, 21,22 extended X-ray absorption fine structure, 23,24 X-ray diffraction, 25 and X-ray photoelectron spectroscopy. 26 Although details are still not clear completely, the iron passive film is composed of a spinel structure such as ␥-Fe 2 O 3 and/or Fe 3 O 4 , having the property of an n-type semiconductor with a donor density on the order of 10 26 m −3 17-19 a...

The effect of pre-deformation on corrosion resistance of the passive film formed on 2205 duplex stainless steel

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Effect of Tensile Stress on the Passivity Breakdown and Repassivation of AISI 304 Stainless Steel: A Scanning Kelvin Probe and Scanning Electrochemical Microscopy Study

“…Concerning the oxide film, the deformation increases the doped charge density (concentration of donors and acceptors) in the passive films of stainless steels [18][19][20]41]. For pure iron, dislocations exposed to the iron surface increase the number of donors, resulting in a highly defective passive film [42]. In this particular study, after tensile straining and repassivation, SKP showed that the surface of the AISI 304 steel did not return to the initial level of passivity and that the potential was 30 mV lower than that of the unstrained reference area (Figure 14…”

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