Evaluation of electrochemical behavior of the passive film formed on Ni in 1N H 3 PO 4
solution
NesrineRamli1,, Farida Kellou-Kerkouche 1
1 Laboratoire d’Electrochimie-Corrosion, Métallurgie et Chimie Minérale,
Faculté de Chimie/ Université Des Sciences et de la Technologie Houari Boumediene (USTHB).
B.P 32, El Alia, Bab Ezzouar, Alger 16111, Algérie.
E-mail:nesrine-r@live.fr
Abstract- This paper focuses on electrochemical studies of nickel oxide, formed on metallic Ni electrode in 1N H3PO4 aqueous solution by means of anodic polarization at various temperatures. Open circuit potential (ocp), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and capacitance measurements are the principal techniques which are used. Mott–Schottky analysis shows that the passive film formed on nickel is p-type, corresponding to a preponderance of oxygen vacancies and nickel interstitials in the barrier layer.
The acceptor density and flat potential, of the semi- conductive passive layer growing on nickel surface in pho5sphoric acid solution, were determined.
Keywords: passive film, nickel oxide, electrochemical impedance spectroscopy, Mott–Schottky.
I. INTRODUCTION
Nickel and its alloys have been employed as structural materials in corrosive environments because of their excellent corrosion resistance. This latter property is mainly due to the protective passive film formed on the nickel surface 1, 2. On the other hand, nickel is widely used as an alloying element because it improves strength and toughness of some metals. Many studies have been conducted in acid solutions which are frequently used for pickling and removal of undesired scale and rust in several industrial processes [3–5].
In this context, the present work aims to study the electrochemical characteristics of the passive film formed by anodic polarization on Ni surface in 1N H3PO4 aqueous solution at various moderated
temperatures, by means of electrochemical techniques.
II. EXPERIMENTAL
The electrochemical measurements were performed in a glass cell using a conventional three- electrodes configuration: a saturated calomel electrode (SCE) as reference electrode, a platinum sheet as counter electrode (CE) and pure nickel with an active area of 0.28 cm2 as working electrode (WE). Prior to all measurements, it was abraded with a series of emery papers from 300 to 1200 grade. The specimen was washed thoroughly with distilled water and introduced quickly into the cell. The aggressive solution of 1N H3PO4 was prepared by dilution of analytical grade 85% H3PO4 with distilled water.
The electrochemical measurements were carried out using a PARSTAT 3000 and Voltalab PGP 201 (Radiometer Analytical) potentiostat / Galvanostat governed by VersaStudio and Voltamaster 4.0.program respectively. All electrochemical tests were repeated three times to ensure reproducibility of the measurements.
III. RESULTSANDDISCUSSION 3.1. Evolution of open circuit potential (ocp) or
free potential
Fig. 1 presents the open circuit potential or corrosion potential evolution versus immersion time in 1N H3PO4 solution at various temperatures: 20, 30, 40 and 50 °C.
0 500 1000 1500
-0.40 -0.35 -0.30 -0.25
20°C 30°C 40°C 50°C
E (mV/ECS)
Time (sec)
0 2 4 6 8 10
Fig. 1. Open-circuit potential versus immersion time for nickel oxide ,formed on metallic Ni electrode in 1N H3PO4.
It is obvious that whatever the temperature until 40°C, the potential shifted towards more positiive values, immediately after the metal immersion. This ennoblement can be attributed to the strengthening of the spontaneous oxide formed in air on the Ni surface and to its thickening resulting from interactions between the electrolyte and the metal surface [6]. During the first seconds of immersion, free potential of nickel at 20°C increases from -0.30 to -0.27 V/s and stabilizes at the latter value, this can be due to the presence of a passive film onto the metal surface. The sample operated with 30°C shows similar evolution, which suggests that oxide film continues to develop until it acquires a thickness that is stable in the electrolyte. This stability corresponds to the equilibrium of the partial reactions; the alloy oxidation which is under anodic control and the proton reduction which is related to the cathodic reaction [6]. The same phenomenon occurs at 40°C, however at 50°C a negative shift of the ocp is observed, this could be attributed to the dissolution of the nickel surface, indicating that the nickel oxide cannot develop at this temperature.
3.2. Potentiodynamic polarization
The effect of temperature on the potentiodynamic polarization curves in 1N phosphoric acid is highlighted in Figure 2.
-0.5 0.0 0.5 1.0 1.5 2.0
-10 -5
log i (A/cm
2 )
E(V/ECS)
20°C 30°C 40°C 50°C X Axis Title
Fig. 2. Potentiodynamic polarization of Ni electrode in 1N H3PO4 with potential scan rate 2 mV s-1 at (a) 20°C, (b) 30°C,(c)
40°C and (d) 50°C.
As it can be seen (fig.2) the temperature increase has not affected the shape of the current- potential curves of Ni in 1N H3PO4.
The cathodic polarization curves are similar; they are represented by well-defined parallel Tafel lines indicating that proton discharge reaction is governed by an activation process and that the temperature does not modify the reduction mechanism. All the anodic branches reveal a wide bearing current succeeding a brief activation of the metal and reflecting spontaneous passivation of nickel, which is probably due to the formation of a stable oxide film of NiO on the nickel surface. One can see that the anodic process is faster than that of the reduction.
Electrochemical parameters deduced from the Tafel technique are gathered in Table 1: values of the corrosion potential (Ecorr), the corrosion current density (icorr) and Tafel slopes (ba and bc) anodic and cathodic respectively, obtained from analysis of the potentiodynamic polarization curves in Tafel domain
TABLE 1: ELECTROCHEMICAL PARAMETERS OF NI ELECTRODE IN 1NH3PO4 WITH POTENTIAL SCAN RATE 2 MV S-1 AT (A) 20°C,(B)30°C,(C)40°C AND (D)50°C.
20°C 30°C 40°C 50°C Ecorr
(mV/ECS) -341.5 -358.8 -369.9 -381.7 Icorr
(µA/cm2) 0.12 0.27 0.58 1.05 ba
(mV/dec) 71.6 80.7 80.8 77.4 bc
(mV/dec) -111.2 120.3 -110.7 -122.4
It can be noticed that Ecor value decreases whereas the icorr increases with the temperature rise, indicating that nickel dissolution is accelerated and thus it becomes less resistant in these conditions.
3.3. Electrochemical impedance spectroscopy (EIS)
Fig 3 illustrates the temperature influence on the impedance spectra, for Ni /H3PO4 system, plotted in the complex plan of Nyquist plots. As it can be seen all the EIS plots display one capacitive arc. The presence of one semi-circle in the complex plan means that the metal dissolution is controlled by charge transfer process. Examining the EIS diagrams, it appears that the increase of temperature induced a decrease of semi-circle diameter indicating that the temperature rise has decreased the resistance of the oxide film formed on the electrode surface and thus accelerated nickel dissolution. This result is in good agreement with that obtained by Tafel method.
Fig. 3. Nyquist plots of the film formed on Ni in 1N H3PO4 at Efree.
The impedance spectra recorded at Ecorr (Fig.3) were analyzed by fitting to the electrical equivalent circuit model that is shown in the same figure. In this circuit, R1 is the solution resistance, R2 is the charge transfer resistance and CPE1 is a constant phase element which is used instead of capacitance when the metal surface presents roughness, inhomogeneity and porosity.
3.4. Mott–Schottky
Fig. 5 presents Mott–Schottky plots for the passive film on Nickel in 1N H3PO4 at 20°C and 50°C. The variation of 1/C2 versus applied potential shows a linear evolution in a wide potential range. This indicates the semiconductive properties of the oxide film in contact with electrolyte. The negative slope of the straight line relative to passivated Ni corresponds to p-type semiconducting characteristics [7].
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
0.2 0.4 0.6 0.8 1.0 1.2 1.4
Vfb Vfb
E (V/ECS)
1/C2 (F2 ) x10^10
20°C 50°C
1000 Hz
Fig. 5. Mott–Schottky plots for metallic nickel in Nickel Ni in 1 N H3PO4 solution at 20°C and 50°C.
The semi-conducting properties of NiO are elucidated from the capacitance (C) measurements.
The flat band potential (Efb) and the acceptor concentration (NA ) of the film are extracted from the Mott–Schottky relation (1):
𝐶12
=
2eε0εNA
{𝐸 − 𝐸
𝑓𝑏−
𝑘𝑇𝑒}
(1)Where C is the charge space capacity, ND the carrier density,e the electronic charge, ε0 the permittivity of vacuum, ε the dielectric constant of NiO (11.9), k the Boltzman constant and T the absolute temperature. The intercept of the plot with the potential axis and the slope give respectively a potential Efb of 1 V at 20°C and 1.34 V at 50°C, density NA of 1.08 1028 cm-3 at 20°C and 1.77 1028 cm-3 at 50°C. Obviously NA value increases with temperature which implies a higher current density in the passive domain. This result is in accordance with that of polarization curves.
3. CONCLUSION
The main electrochemical results obtained in this study are listed below:
1. The pure nickel present an anodic passive film in phosphoric acid at all temperature (20°C, 30°C, 40°C and 50°C).
2. The shape of the Nyquist plots and the cathodic polarization curves suggests that charge transfer controls the corrosion of Ni. It is remarkable that the increase of temperature in 1N H3PO4 solution does not change substantially the shape of the Log I-E plots and of EIS diagrams.
3. According to the Mot-Schotky technique, the film formed on the electrode surface (NiO) is a semi-conductor of p-type. The acceptor density increases with temperature, indicating that a passive film on nickel becomes more conducting.
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