The effects of surface roughness, chloride, and molybdate on the corrosion behavior of iron in bicarbonate/carbonate solutions

• Ahmed S. Alshamsi, Afra G. AlBlooshi, Almaha S. Alshamsi, Asma Y. Alkaabi, Yasmeen S. Elnasiri and Mouza M. Aldhaheri

Department of Chemistry, College of Science, United Arab Emirates University (UAEU), Al Ain, UAE

Abstract: The effects of surface roughness, chloride ions, and molybdate ions on the corrosion behavior of pure iron was thoroughly investigated in bicarbonate/carbonate solutions at 22°C. Open circuit potential versus time, polarization resistance versus time, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) were used for this investigation. The results demonstrated the presence of up to 2% molybdate ions does not enhance the corrosion resistance of pure iron in chloride-free bicarbonate/carbonate solutions. Moreover, molybdate ions and surface roughness have little effect on the passivity of iron in chloride-free bicarbonate/carbonate solutions. The effect of molybdate and surface roughness, however, changed in the presence of chloride. Iron with relatively smooth surface finish showed better corrosion resistance than iron with relatively rough surface finish. Moreover, the presence of molybdate improved the corrosion resistance of iron when chloride was present. The passive current densities decreased while the pitting potential increased in the presence of molybdate for both smooth and rough surface finish. SEM images taken after two days of immersion (at the open circuit potential) in chloride-containing solutions did not show any signs of pitting corrosion. SEM images taken after the polarization tests showed pitting corrosion in chloride-containing solutions in the absence of molybdate. Interestingly, SEM images taken after polarization did not show signs of pitting in chloride-containing solutions when 2% molybdate was present. The presence of molybdate enhanced the iron resistance to localized corrosion. Finally, the constant phase element can be treated as a non-ideal capacitor in chloride-free solutions for both the relatively smooth and relatively rough surface finish. In chloride-containing solutions, the constant phase element can be treated as a non-ideal capacitor for the relatively smooth surface finish, but not the relatively rough surface finish.

Keywords: iron, corrosion, pitting, bicarbonate, carbonate, chloride, molybdate, electrochemical impedance spectroscopy, surface roughness, SEM

Int. J. Corros. Scale Inhib., 2019, 8, no. 4, 835-849 PDF (4 601 K)
doi: 10.17675/2305-6894-2019-8-4-4

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