Electrochemical Noise during Pitting Corrosion of Aluminum in Chloride Environments

Abstract

Aluminum shows vigorous fluctuations in current and potential in chloride containing media close to its pitting potential. To determine the source of this noise, we used aqueous solutions, gels with a pH indicator, and currents between interconnected electrodes, to locate pits and study currents generated at freely corroding potentials. We demonstrated that there is a transition in the characteristics of the noise. Transitions occur when pits continue to remain active, and also when more than one active pit is present because of the electrochemical communication that takes place between them. The major source of current and potential transients with aluminum is due to the growth process in active pits, rather than metastable pitting at the passive surface.Aluminum and its alloys are susceptible to pitting corrosion in chloride environments. Fluctuations in potential and/or current are observed that are associated with the pitting processes. 1-6 These fluctuations generally are termed electrochemical noise, and have been predominantly interpreted in terms of metastable pitting events. The present study concludes that care must be taken in analyzing these events in this manner, because, in general, most of the noise is a result of the growth process of active pits.Fluctuations or electrochemical noise have been related to a wide range of corrosion phenomena. [2][3][4][5][6][7][8][9][10][11] The objective of many studies has been to derive the mechanisms involved in the breakdown of passivity and the initiation of pitting. The processes taking place during the pitting of stainless steel 7-11 are better understood than the pitting of aluminum. It is generally accepted for stainless steels that electrochemical transients are due to an initiation process, a short period during which the pit grows, and repassivation of the metastable pit. [7][8][9][10][11] The open-circuit potential ͑OCP͒ during metastable pitting of stainless steel in a NaCl solution shows a rapid drop, followed by a significantly slower rise to the original value. The current for the early stages of pit growth is derived from the interfacial capacitance of the entire passive surface. 8 The slow recovery in potential after repassivation of a metastable pit is due to recharging of the capacitance by the cathodic oxygen reduction reaction. Thus, the electrochemical transients in stainless steel can be modeled as the consequence of superposition of these metastable pit events. 2,8 However, the open-circuit electrochemical noise of aluminum often shows features of a buffered potential, 12,13 indicating that the mechanisms during the pitting processes are different from those of stainless steels. As described below, we demonstrate that the electrochemical transients during the pitting of aluminum initially originate from processes similar to those on stainless steel, but later are dominated by the electrochemistry within the pit and the interactions between pits once a number of pitting sites remain active. ExperimentalTest electrodes were fabricated from Ͼ...

Effect of sulphate and molybdate ions on pitting corrosion of aluminium by using electrochemical noise analysis

“…More recently, Pyun et al [8][9][10] investigated the pitting corrosion in terms of the statistical variation in the pitting potential and pit embryo density, on the basis of a stochastic theory. Since the fluctuations in potential or current associated with the pitting process can be easily observed in the experimental data, electrochemical noise measurement has been widely used for monitoring, as well as studying corrosion processes [11][12][13][14][15][16]. However, little attention has been paid on the analysis of electrochemical noise based upon a stochastic theory [17][18][19][20].…”

In Situ Corrosion Studies:Tracing Interconnecting Threads

“…Corrosion products were still present on the old pit on Electrode 3 which had slowly lost its anodic component during the period of 150 to 350s as seen from the difference in current flow for Electrodes 1 and 3 Figure 11A. Electrode 1 showed no indications of pitting which was also deduced by the close correlation of its cathodic current with the variations of the open circuit potential for the three coupled electrodes (37,38).…”

“…Scans of Al surface did not however show any correlation between the appearance of pits and the noise as was seen with Fe (34) and stainless steel (24). Only recently was it realized that virtually all the noise with Al (and probably its alloys) originated from pit growth and very little from passive film breakdown and repair events (37,38).…”

High Strength Al-Alloys: Microstructure, Corrosion and Principles of Protection

“…An assumption behind the electrochemical work is that stable pits are on the extreme end (in an extreme value statistics sense) of the metastable pitting events as can be seen in Table 8 for total charge passed, pit stability product and the pit size. There have been other studies that suggest that the current transients are fundamentally different involving activationrepassivation events within active pits (Sasaki, Levy et al 2002). Some studies have also tried to connect phenomenological measurements to the current transients such as through measuring the volume of the trench around an IM particle to the total charge passed.…”