Holographic interferometry of polarized and loaded metallic electrodes in aqueous solution Khaled Habib

Kuwait Institute for Scientific Research, Petroleum, Pet­rochemicals & Materials Division, Materials Applications Department, P.O. Box 24885, Safat 13109, Kuwait. Received 10 May 1989. 0003-6935/90/131867-02$02.00/0. © 1990 Optical Society of America.

A fundamental study of the effect of microscopic defor­mation on the corrosion behavior of a molybdenum elec­trode in 0.75-N KCl solution has been conducted.

It is well known that a metal electrode in aqueous solution acquires a potential as a result of chemical reactions near the metal-solution interface. When the electrode is elastically or plastically strained, a change of the interface chemistry leads to a change in the chemical reaction rate of the elec­trode. The change is almost always an increase of the anodic dissolution rate. Consequently, the idea that deformation accelerates the anodic dissolution rate of electrodes has gen­erated a strong interest among scientists and engineers to­ward understanding the basic mechanism behind this phe­nomenon. In fact the general interest of pursuing this phenomenon was mainly due to the relevance to stress corro­sion and corrosion fatigue of metals.1 As a result, numerous work has been carried out to investigate the effects of defor­mation on the anodic dissolution rate (corrosion rate) of metals2-6 and more specifically to understand the mecha­nism of stress corrosion and corrosion fatigue phenomena.7

By going through all the cited works in Ref. 7 on stress corrosion and corrosion fatigue, it would be obvious to any reader that all conducted studies concerning these two topics are qualitative in nature, and not one investigation thus far has attempted to quantitatively relate the anodic dissolution rate to the microscopic deformation of metal electrodes. It is worth noting that all the work was established based on the following:

(1) Monitoring the anodic current shift of a static or cyclic strained sample in an electrolyte under a control sample potential by using a potentiostat.

(2) Detecting any corrosion or deformation that may de­velop at the surface of the specimen by interrupting the stress corrosion or corrosion fatigue test at predetermined intervals of static or cyclic straining.

(3) Examining the surface of the specimen, during the interruption of the test, either in situ by an optical micro­scope or by a scanning electron microscope (SEM), where the sample is unloaded from the loading machine and removed from the electrolyte and cut to a suitable size to fit in the SEM. The SEM examination normally helps one to obtain high resolution images about the corrosion and microscopic deformation processes that take place at the metal surface.

One limitation of investigating the corrosion by micro­scopic techniques such as a SEM is the inability to observe the development of both processes continuously while the sample is statically or cyclically strained in aqueous solution. Hence, the above methods are only useful for a qualitative study. Therefore, a nondestructive method of observing the influence of microscopic deformation on the anodic behavior of electrodes was applied in this work to quantify this type of study.

Experimental procedures described elsewhere7-9 have been followed to relate the microscopic deformation to the corrosion of nickel electrode (wire) in a stress corrosion test. Nickel wire (99.85% Ni) of ~1-mm diameter was used. Prior to each stress corrosion test, the wire sample was first pol­ished, notched, and loaded up to a certain percentage of the yield strength of the material. Then the sample was im­mersed in the solution for 1 h during which time the open circuit potential of the sample reached a steady value. Fi­nally, the sample was polarized to an over potential with respect to the open circuit potential of the sample in the solution.

In the present investigation, the real time holographic interferometry was applied to nondestructively detect the microscopic deformation at a certain exposed surface area of the wire. In the meantime, the shift in the corrosion current of the polarized specimen was determined by a typical elec­trochemical instrument, i.e., a picoammeter.

Figure 1 shows a plot of the corrosion current density, Jcorr, vs deformation (displacement) of the nickel sample in a stress corrosion test in a constant load condition. During the test, the sample was loaded up to 93.7% of the yield strength of the materialand polarized at —542 mV with re­spect to the open circuit potential of the sample in 1-N H2SO4 solution. It is obvious from the plot that Jcorr in­creases nonlinearly as a function of deformation. Initially a deformation of 11.9 μm was developed after 1 h from the beginning of the test. Accordingly, Jcorr increased from 2468.15 μA/cm2 to 2547.8 μA/cm2. Subsequently, Jcorr again increased in responding to a deformation increase from

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Fig. 1. Plot of the corrosion current density vs deformation (dis­placement) of the Ni electrode in a stress corrosion test in a constant load condition. The number of each data point represents the time (hours) at which each point was recorded from the beginning of the

test.

11 to 18.8 μm. The final increase of Jcorr, despite the fact that both the deformation and the applied potential Eapp = -542 mV remain constant, is probably the result of another mechanism such as a diffusion process of ionic species from the electrode (source) to the solution (sink). In other words, the ionic diffusion process across the interface of the metal surface to the solution probably governs the flow of the electronic current through the metal.

In general, Fig. 1 exhibits an example of the mechano-chemical behavior of metallic electrodes in aqueous solutions obtained by implementing this novel technique. For more details on other applications of the new method, the reader is encouraged to refer to Refs. 10-12.

References 1. M. Fontana and N. Greene, Corrosion Engineering (McGraw-

Hill, New York, 1978), pp. 91-107. 2. T. P. Hoar, "The Anodic Behavior of Metals," in Modern As­

pects of Electrochemistry, J. Bockris, Ed. (Academic, New York, 1957), pp. 262-334.

3. A. Despic, R. Ralcheef, and J. Bockris, "Mechanism of the Acceleration of the Electric Dissolution of Metals During Yield­ing Under Stress," J. Chem. Phys. 49, 0926-938 (1968).

4. T.Hoar and J. West, "Mechanochemical Behavior," Nature London 181, 835-835 (1958).

5. T. Hoar and J. West, "Mechanochemical Anodic Dissolution," Proc. R. Soc. London Ser. A 286, 309-315 (1962).

6. D. Vanrooyan, "Stress Corrosion Cracking of Metals," in Pro­ceedings, First International Congress on Metal Corrosion (Butterworth, London, 1961), p. 309.

7. K. Habib, "Anodic Behavior of Metallic Electrodes During Cy­clic Deformation in Aqueous Solution by Holographic Interfero-metry," Ph.D. Dissertation, U. Iowa (1988), Ref. 8-30.

8. K. Habib, "Mechano-Electrometer I," U.S. Patent 7,200,242 (May 311988).

9. K. Habib, G. Carmichael, R. Lakes, and W. Stwalley, "Novel Technique for Measuring the Mechanochemical Behavior of Metallic Electrodes In Aqueous Solutions," in Proceedings, Frontier of Electron Microscopy for Material Science, Oak Brook, IL (1988).

10. K. Habib, "Novel Technique for Non-Distructive Evaluation of the Mechanochemical Behavior of Metallic Electrode In Aque­ous Solutions," in Proceedings, Twelfth World Conference on Nondestructive Testing of Materials, Amsterdam (1989), p. 508.

11. K. Habib, "The Initial Stage Behavior of Stress Corrosion Cracking of Ti-GAL-4V Wire In Aqueous Solution," in Proceed­ings, Seventh International Conference on Fracture, Houston (1989); Metall. Trans., to be published.

12. K. Habib, "Observation of Holographic Interference Fringes of a Polarized and Loaded Metallic Electrode In Aqueous Solution," in Fringe Analysis 89 (publisher, Loughborough, England, 1989), in press.

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