Srinivasan, S. (1986) Electrochemical aspects of metallic corrosion – characterization of pitting corrosion. PhD thesis, Madurai-Kamaraj University.

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Abstract

A satisfactory explanation of the problems of pitting corrosion has not been given till today, because of the fact that investigations have been carried out under different experimental conditions. The tendency of a metal to undergo pitting corrosion can be understood in terms of the composition of the metal or alloy and the environment or the composition of the solution to which it is exposed. The formation of passive film on the electrode plays an important role in understanding the nature and the magnitude of corrosion. The present study has been taken up with an idea of understanding the dependence of pitting corrosion on different parameters such as scan rate, speed of rotation of the electrode etc. For this purpose studies have been carried out using mild steel electrode in neutral solutions containing inhibitors such as sodium chromate, sodium Meta silicate and sodium benzoate in the presence of chloride. An elegant technique based on cyclic voltammetry has been employed to find the repassivation potential (Erp). In addition to this, impedance technique has been used in understanding the pitting corrosion. Cyclic voltammetric studies have led to following conclusions. i) The tolerance limit of chloride concentration is found to be greater under rotating condition than under stationary condition indicating the beneficial effect of increasing the mass transfer of inhibitors. ii) No systematic variation in Erp is observed with scan rate in all the systems under study. In general Erp shifts to more noble values as the rotation speed is increased at constant corrosion rates. In the present investigation, the effect of cations like Zn++, Mn++, Mg++ and Al +++ has been studies in the inhibiting system containing the border line concentration of chloride ion and 5 ppm of the respective salts. This study reveals the fact that once the pitting characteristics are revealed in the current potential curves, it is better not to state in terms of specific value of critical potential, since it depends on flow velocity, scan rate and the previous history. In the impedance measurements, Nyquist plots obtained for all the systems are not perfect semicircles, but are depressed and elongated. This is due to the absorption of the inhibitor and also due to the pitting condition of the metal electrodes. Bodes plot has clearly brought out the existence of a number of time constants or relaxation process. A comparison of the direct corrosion tests and impedance measurements clearly bring out the fact that the critical concentration of chloride could not be determined from impedance measurements, when the specimen is undergoing pitting corrosion. No significant change in the impedance plot is observed on adding cations to all the three inhibitor systems containing chloride ions. Impedance measurements in the potentiostatic mode shows that the film formed on metal consists of a barrier layer, immediately in contact with the surface and then a deposit layer. Inductive loop has been observed for benzoate-chloride system at low frequencies. The delayed action of silicate as an inhibitor is also brought out in the impedance measurements. Computer analysis of the impedance data is also made in this study.

Item Type:	Thesis (PhD)
Uncontrolled Keywords:	metallic corrosion; pitting corrosion
Subjects:	Corrosion Science and Engineering
Divisions:	UNSPECIFIED
Depositing User:	Dr. N Meyyappan
Date Deposited:	12 Jun 2012 05:57
Last Modified:	12 Jun 2012 05:57
URI:	http://cecri.csircentral.net/id/eprint/2800

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