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Electrochemical Response of Nickel in Artificial Seawater
E. Garcia Ochoa; J. Genesca Llongueras*; J.Uruchurtu Chavarin
Instituto de Investigaciones Electricas
*Facultad de Quimica, UNAM.
email: juch@iie.org.mx
Electrochemical Response of Nickel in Artificial Seawater
E. Garcia Ochoa; J. Genesca Llongueras*; J.Uruchurtu Chavarin
Instituto de Investigaciones Electricas
*Facultad de Quimica, UNAM.
email: juch@iie.org.mx
This work presents electrochemical results of nickel immersed in artificial seawater under laboratory conditions. Reverse sweep polarization, electrochemical impedance spectroscopy and electrochemical potential and current noise were performed. The electrochemical behaviour shows a passive region, a critical pitting potential at 200 mVsce and a repassivation potential at 70 mVsce. The Nyquist diagram presents a capacitive semicircle and an inductive loop associated to the migration and adsorption of chloride ions forming a salt layer under which a localized attack in the form of pitting is developed. No rupture-repassivation transients were observed suggesting that the localized pitting attack mechanism is controlled by the local oxide film dissolution and salt film formation. Spectral analysis of current noise signal shows a characteristic frequency at 280 mHz probably associated to salt film formation promoting pitting corrosion. The electrochemical measurements can help to elucidate a possible corrosion mechanism.
Different pitting corrosion mechanisms have been proposed including the salt film formation (1-3). According to the authors, pitting initiation is related to the formation of a salt film between the base metal and the electrolyte solution at localized sites over the surface.
The stable coexistence of an active state within a pit and a passive condition surrounding it at a same potential, suggests the existence of a high ohmic resistence. This barrier consists either of corrosion product deposits in the pit mouth, or the presence of a salt film over the pit internal surface (1-9).
This work presents electrochemical results of nickel immersed in artificial seawater obtained under laboratory conditions.
A Shlumberger 1286 electrochemical interphase and a 1253 gain analyser coupled to a PC computer by means of an RS 232 was used for all electrochemical measurements.. A saturated calomel reference electrode (sce), a graphite auxiliary electrode and a working electrode (metal samples) were used in a three electrode set-up.
The samples were fabricated from a 99.94% nickel sample. The 1 cm3 were mounted in a polyester resin, leaving an area of 1 cm2 exposed on only one face. Electrical connection was achieved by means of a brass rod screwed to the mount; the rod was isolated from the electrolyte by a glass tube previously set in the polyester mount. The area exposed was ground with silicon carbide paper up to 1200 grit, then washed with distilled water and degreased with ethanol and dried under an air stream.
Reverse polarization curves were performed at a sweep rate of 20 mV every 6 seconds. Electrochemical noise measurements were obtained every 0.7 seconds, and to obtain an "ensemble" of 2048 data points an averaging was performed over 5 time series. Impedance measurements were performed with a 20 mV (rms) amplitude sinusoidal waveform in the frequency range 10 kHz to 10 mHz.and the results are presented as Nyquist plots. All measurements were performed after immersion and at different polarizing conditions.
Artificial seawater was prepared under British Standard (BS2900) as the working electrolyte. The detailed experimental procedure was presented elsewhere ( 10,11).
The main cause of localized corrosion is the failure of the passivating layer to restore after disruption. This work aimed at elucidating the factors affecting the localized attack of nickel in artificial seawater by means of electrochemical techniques.
Figure 1 presents a reverse sweep anodic polarization curve of nickel immersed in artificial seawater. Starting at the corrosion potential (-220 mV), the active region goes up to 40 mV where the current density decreases. According to MacDugall (12) this region corresponds to the nickel dissolutiion rection as follows:
which is followed by the hydration and hydrolisis processes for nickel ions (7). At 40 mV a passivation region starts up to 200 mV coresponding to the formation of Ni oxide up to 200 mV where the current density increases drastically indicating a possible pitting process (12). At this point the pitting potential is established and afterwards the polarization sweep is reversed appearing an hysteresis loop presenting higher current densities until the repassivation potential is reached at about 70 mV. The results obtained established the pitting parameters and were verified under the EDAX and scanning electron microscope (13).
Several researchers suggest that the main role of chloride ions on the nickel pitting corrosion process is to deposit and strongly adsorb themselves in localized sites over the metal surface in order to inhibit the Ni oxide formation and hence repassivation (14-18). To corroborate this, impedance measurements were performed at the passivation potential (40 mV), in the passive region (120 mV), at the pitting potential (200 mV) and beyond (240 mV).
Figure 2 presents the Nyquist plot obtained at 40 mV showing a high frequency a resistive capacitive semicircle and a distorted resistive inductive loop was formed at low frequencies. The semicircle corresponds to the nickel dissolution and subsequent nickel oxide formation.while the inductive loop is associated to an adsorption process (19). Migration most likely under the electric field present, precipitation and adsorption of chloride ions at the more active local sites occur over the oxide metal surface (7).
At 120 mV (figure 3) an increased semicircle at high frequencies was formed, while at lower frequencies a second small resistive pseudo capacitive semicircle appeared. The result suggests a passive condition associated to the formation of nickel oxides (high frequency semicircle), while the low frequency semicircle to local charge transfer processes forming the salt film at local sites identified as NiCl2.6H2O which is more stable than NiCl2 (7,21).
At the pitting potential (200mV) a similar Nyquist plot (not presented) was obtained but with a smaller and less defined second semicircle compared with the previous condition. A higher rate of nickel dissolution and salt film formation can be ascribed to the second semicircle once a critical pitting depth has been reached and a stable salt film reached a teady state (22).
Finally above the pitting potential (240 mV) the Nyquist plot (figure 4) presents a semicircle increasing its diameter and an adsorption inductive loop at lower frequencies, pehaps associated to the nickel dissolution and chloride adsorption to maimtain the steady state of the porous salt film within the bottom of the growing pits (7) .
The AC impedance plots obtained suggest a reaction controlled most likely by migration and adsorption of chloride ions at local sites forming a salt layer. The presence of at least one adsorbed species is indicated by an inductive loop and a pseudo capacitance by a second low frequency semicircle related to the local dissolution of nickel in the presence of a porous salt film(7,19,20).
To establish the pitting initiation conditions for the nickel oxide present, a series of electrochemical potential/current noise measurements were performed below (180 mV), at the pitting potential (200 mV) and above (240 mV) and their corresponding current densities. These are illustrated in figures 5 and 6.
Figure 5 presents (a) the potential noise and (b) the current noise time ensemble and their corresponding spectral density analysis below the pitting potential. Both ensembles present basically low frequency oscillations. Regarding the electrochemical potential noise measurements, the high frequency components are almost non-existent and the spectra present a smooth slope. The electrochemical potential noise measurements remained the same regardless the polarizing potential conditions considered, therefore only this result is presented.
More changes were observed in the current noise measurements. Some high frequency components are present in the current noise and reflected in the spectrum where a characteristic frequency at 280 mHz can be observed probably associated to the nickel oxide dissoltion and salt film formation. A similar observation was reported elsewhere(23,24).
(b)
Figures 6 correspond to the current noise measurements obtained at the pitting potential. Similar behaviour was registered above the pitting potential. The behaviour observed is similar to the one presented before, although the current noise shows an increase in the frequency bandwidth considered. This can be associated to a general increase in the nickel oxide dissolution under the precipitated chlorides, as the potential is raised towards the pitting potential.The noise measurements obtained present a low frequency behaviour associated to difussion and species adsorption under the presence of an oxide film or corrosion products over the surface, and the presence of a salt film formed within pits(10,11).
The results presented previously suggest that the pitting mechanism of nickel is controlled by adsorption of chloride ions and the development of a salt film since no rupture-repassivation transients were observed during noise measurements. Therefore it can be concluded that pitting corrosion is not controlled by the film breakdown-repassivation of the oxide; but instead by the nickel dissolution and hydrated nickel chloride formation at local sites which inhibits repassivation. Once the pitting conditions are firmly established the salt film formed is stabilized within the pits (13).These results supports the model recently proposed(7).
From the electrochemical results obtained it can be concluded that pitting of nickel in artificial seawater proceeds by migration, precipitation and adsorption of chloride ions at local sites over the metal oxide surface promoting localized nickel dissolution and salt film formation. A characteristic frequency (280 mHz) appears to be related to the salt film formation. Once the pitting process is established adsorption of chloride ions controls the local dissolution kinetics.
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