The structure of alloying materials is often found in the form of amorphous, crystalline or metastable. Amorphous alloys are metals and metal alloys with no long range atomic order; also known as glassy alloys, metallic glasses, or non-crystalline alloys.
Formation of alloys have long been used to form new materials with specific properties depending on the alloying elements, such as how nickel can be used for corrosion resistance, cobalt for magnetic memories, and chromium for decorative purposes. The structure of these alloys can be tailored by controlling the process parameters, such as current density, pH, temperature, and bath composition. It was found that the alloy composition induces the structural and corrosion behavior of amorphous alloys.1-10
Due to the lack of long range order and their degree of atomic disorder, amorphous alloys exhibit unique physical, chemical, and mechanical properties, all of which make them superior to their crystalline counterparts. The non-metal atoms, such as B, Si, C, and P are often used as amorphizing agents11,12 and are usually employed to reduce metals, such as Fe, Co, Ni, Cu, Pb, and Re in the aqueous solution to form composite coatings.
The amorphizing agents usually have dimension less than the metals they bond with.1,2,4,7
Several studies reveled the corrosion resistance of Ni− metalloid amorphous alloys and the passivation ability of them in different media.1,4,5,6,9,10-22
The present work aims to investigate the beneficial effect of Cr presence on the electrochemical and passivation properties of bulk Ni-base glassy alloy in H2SO4 solutions.
Specimens and Test Solutions
Ingot Ni92.3Si4.5B32, Ni82,3Cr7Fe3Si4.5B3.2 and Ni75.5Cr13Fe4.2Si4.5B2.8 (at %) alloys were supplied by Vacuumschmelze with physical properties summarized in Table 1. Rounded specimens were cut from a foil (2.5-7.5 mm width and 20-50 μm thickness) with a working area of (2.4 mm2).
Each experiment was carried out with a new strip. The electrodes were decreased with alcohol and rinsed several times with double distilled water and finally cleaned in an ultrasonic bath (Model LF2003, 50160Hz, Dal Trozzo, Italy). The electrodes were connected to copper specimen holder and immersed in the test solution with out drying.
Table 1.Some physical properties of studied alloys
As corrosion medium, aerated solutions of H2SO4 (Panreac) were used. Appropriate concentrations of the acid were prepared by dilution using be-distilled water.
Electrochemical measurements have been achieved by connecting the electrochemical cell to ACM Gill AC instrument and to a Samsung computer (Bridge DVD ASUS 8X max). In the conventional cell three − electrode assembly, containing naturally aerated H2SO4 solutions, the working electrode was nickel-base glassy alloy (2.4 mm2), Table 1, with platinum wire and saturated calomel electrodes were used for counter and reference electrodes, respectively. Experiments were carried out in stagnant solutions at 30 ℃ ±0.1 temperature.
After immersion of the specimen, prior to the impedance measurement, a stabilization period of 15 min was found, which proved sufficient for Ess (the steady-state potential). The AC frequency range extended from 30 kHz to 0.1 Hz, a 10 mV peak to peak sine wave being the excitation signal. The impedance spectra were fitted using a non-linear least − squares fitting procedure developed by Boukamp.23
Potentiodynamic polarization curves were obtained with a potential sweep rate of 1 mV/s. The potential was sweep from cathodic to anodic directions after impedance run. Corrosion current densities (Icorr) were calculated by extrapolating the Tafel lines to corrosion potential (Ecorr). Passivation parameters: critical passivation current density (Icc), critical passivation potential (Ecc), passivation potential (Epass), passivation current density (Ipass) were determined from the potentiodynamic polarization curves.
After the electrochemical test in 3.0 M H2SO4, chemical compositions of the alloy surface were measured by XPS using a multi-technique surface analysis system (MAX200, Leybold) with MgKα and AlKα at 100W of x-ray power. The pressure in the analysis chamber during sample analysis was less than 10-8 m bar.
The nickel base specimens coated by a film of 50 nm of gold by sputtering, have been investigated by using a XL20−PHILIPS SEM. The data obtained from these experimental methods were compared.
RESULTS AND DISCUSSION
Electrochemical Impedance Spectroscopy (EIS)
The electrochemical impedance spectroscopy (EIS) helps to isolate the individual components describing the particular processes and properties of phases and interfaces, electrolyte resistance, charge transfer resistance (the fast process), double layer capacitance, adsorption and diffusion (the lowest process).24
The Nyquist diagrams were characterized by depressed capacitive loops with the theoretical centre located below the real axis. So, the measured capacitive response is not generally ideal (i.e., a pure capacitor). A constant phase element (CPE) is then introduced for the spectra fitting, instead of an ideal capacitance element. The impedance expression of CPE is given by:
where Q and n are frequency - independent fit parameters, j = (-1)½ the imaginary number and ω = 2nf the angular frequency in radius. The factor n, the CPE exponent, is an adjustable parameters.
For n = l the CPE describes an ideal capacitor with Q (mF/cm2) equal to the capacitance C and the phase angle θ equal to 90o and for n = 0 the CPE is an ideal resistor and the phase angle equal to 0o. When n = 0.5 the CPE represents a Warburg impedance with diffusion character and for 0.5 < n < 1 the CPE describes a frequency dispersion of time constant due to local inhomogeneties in the dielectric material. Inductance characteristics presents when n = -1. In general, it is believed that the CPE is related to some type of heterogeneity of the electrode surface as well as to the fractal nature (roughness or porosity) of the surface.25,26 Analysis of the impedance spectra was done by fitting the experimental data to equivalent circuit. The quality of fitting to the equivalent circuit (EC) was judged firstly by the Chi- square (χ2) values and secondly by the error distribution versus frequency, comparing experimental with simulated data.
Fig. 1.Impedance plots of Ni-base glasses alloys in different concentrations of H2SO4.
The impedance spectra recorded for alloy I in 1.0, 3.0, 6.0, 9.0 and 12.0 M H2SO4 solutions are shown in Fig. 1(a) where one capacitive loop is observed irrespective of the sulphat ion concentration with an open end at low frequency at 1.0, 3.0 and 6.0 M H2SO4. The corresponding Bode - phase plots are shown in Fig. 1(a') where one time constant is obtained, this reveals that there is only one reaction controlling the corrosion process. This reaction is the homogeneous corrosion of the surface in contact with the solution.27 Only when [H2SO4] equals to 12.0 M the impedance plot is a semicircle fallowed by an inductive loop.
The spectra were fitted using (EC) shown in Fig. 2(a) (one time - constant circuit) with circuit description code (CDC) R(RctQ) in the range (1.0-9.0) M and using (EC) shown in Fig. 2(b) at 12.0 M with the (CDC) R(Q1 [Rct1(Q2Rct2)]). Excellent agreement between the fitted and experimental data was obtained (χ2 = 10-3-10-4).
Table 2 shows the fit parameters for alloy I in different concentrations of H2SO4. It is clear from the data that the alloy resistance (Rct values) decreases indicating more dissolution. A sudden decrease occurred at 9.0 M of acid, and the corrosion rate in mm/y goes to opposite direction. The extent of dissolution becomes greater (about 75.7%) as the acid concentration increases to 12.0 M. The size of the semicircle is reduced with an indication of localized attack. The n values listed in Table 2 are close to 1 (~ 0.86) at (1.0 to 9.0) M of SO42- ion, and it may considered to behave as a capacitor. Values of Q are regularly increase with [H2SO4] increase related to more porosity of the protected layer allowing Ni+2 to diffuse through it. The Q values were found to be lower than that in HCl solutions.28
Fig. 2.Electrical equivalent circuit for the interpretation of experimental impedance diagrams of passivated alloy III at 3.0, 6.0 and 9.0 M in H2SO4. Rpass: electrolytic resistance through the passive film; Cf: film capacity; Rct: charge transfer resistance; CPE: double layer capacity: (a) Equivalent circuit model for the metal suffering general corrosion. Rs: solution resistance; Rct1: charge transfer resistance; Q: double layer capacitance; (b) Equivalent circuit model for the metal suffering localized attack. Rs: solution resistance; Rct1: charge transfer resistance; Q1: double layer capacitance. Rct2: electrolytic resistance through the pit; Q2: the pit capacity.
Table 2.Electrochemical kinetic parameters and corrosion rate obtained by EIS technique for Ni-base metallic glasses alloys in naturally aerated H2SO4 solutions at 30 ℃
Nyquist plots and Bode - plots of alloy II in [H2SO4] solutions are presented in Figs. 1(b,b') respectively. EIS spectra obtained consist of capacitive arc with pseudo inductive loop regardless of the concentration of SO42- ions. The phase angle θmax is around 90o suggesting that the electrochemical process occurring at high frequency favors the formation of the passive film.29 A combination of a capacitive and a pseudo - inductive loop is notecited at the low frequency. Such a combination can be explained to be due to a competitive behavior between acceleration and deceleration (inhibition) process on the alloy surface by the intermediate species.30
The results in Table 2 indicated that the alloy resistance decreases as the [H2SO4] increases up to 6.0 M and then its values increase resulting high values of Rct and low values of corrosion rate (mm/y), but they are still lower in compared with its value in HCl solutions.28 The increase in the capacity (Q) of the protective layer at 6.0 M HCl indicated the increase in the porosity this layer. The acid diffuses through the layer and reached the alloy at alloy/film interface. Where the low value of Q at high acid concentration reflexes high protective thick film layer of protective film formed on the alloy.
The obtained values of n (~0.86) indicates that the CPE element is associated with low film capacitance and slight low homogeneous of the surface may due to the computation between both the healing and pitting on the oxide surface.
For impedance spectra of the alloy III in different [H2SO4] solution, the semicircle representing the protection film merges with the charge transfer loop. Hence, the EIS spectra for the corrosion test shown in Figs. 1(c,c') are described by a simple equivalent circuit as shown in Fig. 2.
The lowest frequency capacitive loop seems to change to pseudo inductive loop at 12.0 M H2SO4. A pseudo inductive time constant is normally observed for a passive electrode when an increase of the surface fraction causes an increase in the local current at the film / solution interface.
Fig. 3.Potentiodynamic polarization curves of Ni-base metallic glasses alloys different concentrations of H2SO4.
As a whole, the tendency to resistance (Rct values) decrease as [H2SO4] increases. Corrosion resistance at 12.0M H2SO4 is lower by about 69.76% (3.3 times) than at 1.0 M H2SO4. On the other hand, addition of SO42- in range (1.0 to 12.0)M results in a regular increase in CPE values indicated the accelerating effect of this ion. These results indicated that the solution reaches the metallic surface by penetrating through the pores or because of the film thickness dissolution causing the decrease of the total system impedance. This hypothesis can be confirmed by time decrease of the capacitive arcs in the Nyquist diagrams and increase the corrosion rate mm/y. The n values listed in Table 2 were close to 1(~0.92) and the wider relaxation time constants with maximum phase angle of 85o indicates the capacitor behavior of the alloy III surface which is attributed to a dissolution controlled process.
Potentiodynamic Polarization Study
The potentiodynamic polarization curves of alloy I in H2SO4 solutions in the concentration range (1.0-12.0) M H2SO4 are shown in Fig. 4(a). The cathadic polarization curves are shifted towards increase in hydrogen evolution reaction with increasing acidity of the acid. Also Icorr values goes in same direction as shown in Table 2.
From curves in Fig. 4(a) it can be observed that the alloy exhibit active - passive behavior at 1.0, 3.0 and 6.0 M H2SO4 solutions at potentials (Ecp = 193, 386 and 442 mV (SCE)) respectively. Values of both the passivation tendency Icc, and degree of passivation IPass, of the alloy increase appreciably with the increase of acid concentration as shown in Table 3. This may indicate that the increase of [H2SO4] will accelerate the Ni dissolution in both the active and passive region. Rising of Icc values with the increase of [SO42-] indicated that the rate of anodic dissolution on in the active region increases with increasing acidity of the acid, while the increase of Ipass values within the passive potential range as the potential shifted in the positive direction can be attributed to the dissolution action of the acid on the passivation film layer and / or the increase in the porosity of protective layer with the increase in the acid concentration.
Pseudo passivation is exhibited by the alloy at 1.0, 3.0 and 6.0 M of H2SO4 acid attributed to the stimulation action of low concentration of SO42- which adsorbed at the uncovered small anodic area, increasing the cathode / anode area ratio, resulting in a strong shift of potential towards positive Ep values and a concentrate attack on the un passivated regions to create pits and pores resulting in porous film (Scheme 1) formation on the alloy surface.
Fig. 4.The photoelectron spectra obtained for the metallic glassy alloys before and after the experiment in 3.0 M H2SO4 solution.
Table 3.Electrochemical parameters of polarization for metallic glassy alloys in naturally aerated H2SO4 solutions at 30 ℃
Le Canut et al.29 pointed out that with high [SO42-], or in presence of sulphide S2- ions on the layers lead to thinner and porous layer. In our case the pores in protective layer possibly due to the presence of nickel sulphides at the interface, which would tend to destabilize the passive layer. These S2- would be the result of sulphate reduction in reducing conditions. The metal surface turned grayish black and the lest solution become green after the polarization experiments.
The current versus potential curves for alloy II (which contains Cr) in H2SO4 solutions in the potential range from hydrogen evolution to transpassive dissolution are shown in Fig. 3(b). The cathodic polarization curves are shifted towards higher current densities as H2SO4 concentration increases. In the anodic part of the potentiodynamic Tafel region followed by a wide constant and low current plateau, which corresponds to a passive region, meaning that the alloy is spontaneously passivated. An active - passive behavior at potential values < -80 mV (SCE) and transpassive phenomena at potential values < 800 mV (SCE) was observed.
The current - potential curves of many metal - electrolyte systems consist of a peak followed by a plateau.27,31 There is a good agreement that in the rising portion of the curve the metal is dissolved and becomes progressively covered by a blocking species (generally an oxide); current attains a maximum and then drops to a much lower values, once a complete monolayer of the blocking species is formed. Further potential increase, beyond the peak, induces thickening of the oxide film and decrease Icorr and Ipass values. Corrosion rate on the electrode depends on the diffusion rate and competitive between formation and destroy of protective layer on the electrode.
The kinetic parameters calculated from the polarization curve are summaries in Table 3. It is clear from this Table that the corrosion rate (Icorr values) increase as [H2SO4] increase up to 6.0 M, and then the resistance occur.
The onset potential of transpassivity for alloy II in the studied conditions defined as the applied potential slightly increased, the current density started to increase rapidly, this has been found to lie between 800-1000 mV (SCE). The transpassive region is usually associated with the evolution of a gas, particularly oxygen, or metal dissolution and/or breakdown of the passive film leading to localized corrosion. The type of reaction actually operative in a particular case is determined by the nature of metal / electrolyte interface and the potential at which the process ensues.32 In this study in H2SO4 solutions in the transpassive region evolution of gas on the surface was observed on the outside of the formed film which suggested to be oxygen according to the following reaction:33
On the other hand, according to Pourbaix diagram for Cr at potential higher than 1300 mV (SCE), the Cr3+ can be oxidized to Cr6+ (H2CrO4, CrO42-, Cr6+- 4H2O) to form soluble compounds.34 The parallel polarization curves at the different acid concentrations in transpassive region indicated that the dissolution posses occur with the same mechanism.
Although there was a systematic increase in Ecp values and decrease in the passivation tendency (pronounced increase in Icc values) and potential needed to reach the passive region for alloy II is directly proportional to the acid concentration, values of degree of passive state (Ipass values) in the presence of SO42- ions indicated deterioration effect of these ions on the passive state at lower concentrations. Adsorption and subsequent in corporation of SO42- ions within the passivation oxide layer may lead to the formation of a highly porous protected film allowing high rate of anodic dissolution in the passive region. Diggle et al.35 and Abdel- Aal and Osman.36 reported that SO42- ions and some other anions allow some anodic dissolution, resulting in the formation of porous layer of significant thickness. This passive layer strongly inhibit the anodic dissolution of the metal on both active and passive region at high concentrations of SO42- and makes it’s passivation more feasible. At the highest examined concentrations of SO42- ion (9.0 and 12.0) M the passivation degree has remarkable increase (decrease in Ipass values) suggesting homogeneous adhered non porous protective passive layer on the alloy in spite of low tendency of passivity. This phenomenon assumed that the alloy surface in these solutions covered of both accelerator (S-2) and decelerator (Cr3+) species. The accelerator species decrease the tendency of passivation and decelerator species may increase the passivation degree at high potentials. This behavior shows good agreement between impedance and polarization measurements.
In SO42- solutions the test solutions turned colorless to light green yellowish after the polarization experiments, and the alloy surface become gray. The second passive region observed after transpassive zone at 9.0 and 12.0 M H2SO4 can be attributed to the formation of a new oxide phase. Chaudhry and Sing37 pointed out that the second passive region of Ni is due to either different type of adsorption processes on the electrode surface or oxide film transformation. Such mechanism of transformation is consistent with the assumption that Ni goes in to the solution as Ni2+ although the passive film in the transpassive region is considered to be higher valent oxides associated with oxygen evolution.
The cathodic polarization curves of alloy III, Fig. 4(c), are shifted regular towards high current densities as pH of acid solution decrease. In the anodic region, SO4 2- ion doesn’t preventing passive layer formation. The alloy spontaneously passivated after a potential larger than 25 < Epass < 290mV (SCE). Passivation current density around 0.0075 < Ipass < 0.0099 mAcm-2 as shown in Table 3. The active - passive behavior exhibited by the alloy may indicate the role played by competitive adsorption of inhibiting SO42- and the aggressive S2- ions. The fixed values of Ipass in a wide passive region, about 739 mV(SCE) reflex stability of the passive oxide film.
As shown in Table 3, the passivation tendency (Icc) and the degree of passivation (IP) of alloy III in 2SO4 solutions increase with the increase of acid concentration. This may indicated that the increase of acid concentration results in an acceleration of Ni dissolution in both the active and passive regions. Decreasing in the potential needed to reach the passivity region and increasing of Ipass value within the passive potential range as the potential shifted in the positive direction can be attributed to the dissolution action of the acid on the passivation oxide layer and / or the increase in the porosity of the oxide layer with the increase in the acid concentration.
The current starts to increase due to transpassive oxidation at 877 < Eb < 1080 mV(SCE) associated with the evolution of oxygen. No change in the slope of the valtammogram can be seen, this may be indication for the same mechanism of the passive film dissolution. Bojinov et al.38,39 reported that alloying of Ni with Cr can lead to an increase susceptibility to general corrosion in highly oxidation environments via dissolution of Cr in transpassive region. They also confirm that the transpassive dissolution is a complicated process including electron and ion transport through a surface film. Multi steps electrochemical reactions at the film / solution interface and transport of reaction products in the bulk solution. In addition, they found that the tendency for a secondary passivation in the transpassive region decreases with increasing chromium content in Ni-Cr alloys. This may be connected with the formation of a Cr oxide − based film, on which does not occur on the secondary passivation at high potentials in the transpassive region. Bojinov et al.39 pointed out that susceptibility of alloy Ni80Cr20 for secondary passivation was lower than that for alloy Ni90Cr10 in H3PO4 solution. This result explains the continuo increasing the dissolution process of alloy III by increase the acid concentration and disappear of secondary passivation comparing to alloy II in the same conditions. As it was explained, before the oxidation of Cr3+ to soluble ion (Cr3+ - 4H2O) at more positive potential reveled the transspassive region at the end of anodic scan. Because of the dissolution of both of Ni and Cr the test solution turned colorless to yellowish at the end of polarization experiment and the alloy surface was smooth but not shiny.
X-ray Photoelectron Spectroscopy (XPS)
XPS analysis was conducted for the specimens before and after impedance and polarizations measurements in 3.0 M HCl open to air experiment at 30C to clarify the difference in surface composition between alloy I (Cr − free), alloy II (7% Cr) and alloy III (13% Cr). The peak binding energy of Ni 2p3/2 and S 2p3/2 were approximately 852.8 eV and 162.2 eV, respectively, after the treatment (after the experiment) for alloy I as shown in Fig. 4. No change exhibited for Si 2p3/2 and B 2p3/2 before treatment (asreceived) and after treatment.
These data confirmed that pseudo passivation was exhibited by the alloy I at 1.0, 3.0 and 6.0 M of H2SO4 may attributed to porous nickel sulphide (NiS).
For alloys II and III the peak binding energy of the Ni 2p3/2 were about 852.9 and 853 eV respectively, before and after the treatment. The Cr 2p spectrum of these two alloys give binding energy of the Crm 2p3/2 and Crox 2p3/2 electrons at 574.2, 575.3 and 576.8, 577 eV respectively which is in agreement with values obtained by many authors1,2,40-44 Therefore the Crox in the surface film will be in the trivalent state. On the other hand, the peak binding energy of S2p3/2 electron was observed at 162.9 and 162.5 eV respectively for the alloys II and III indicated the presence of NiS on the alloys surfaces.
According to analysis, the corrosion resistance and the limiting current show in anodic polarization curves of nickel base glassy alloys II and III, is attributable mainly to the formation of hydrated chromium oxyhydroxide film [CrOx(OH)3-2x nH2O] which formed in range (576.3−579.7 eV).1,2,9,12,40-45 This oxide layer is associate with little amount of NiS.
Oxide Film Thickness
The average thickness of hydrated chromium oxyhydroxide film on alloy II and III was calculated using equation (2).
where dox is the passive layer thickness, Cox is the capacitance of oxide layer, A is the sample surface area (~2.4 cm2), ε is the dielectric constant of the oxide (assumed to be ε = 3045-47), εo is the permittivity of free space (εo = 8.854 × 10-12 Fm-1) and Cox is the passive film capacitance is known (in this study n values are nearly to unit so we can assumed that CPE (Q) is equal to C).
The film capacitance values in Table 2 the average oxide thickness formed on alloy II and III calculated at different concentration of H2SO4 and listed in Table 4.
Table 4.Calculation the oxide thickness on alloy II and III from impedance data
It is clearer from Table 4, that the thickness of the oxide film is several nanometers (nm) and grows linearly with decrease the acids concentration. Also the table explains the protective nature of Cr2O3 which [CrOx(OH)3-2xnH2O]. The oxide film grows on alloy III is more thicker than that film formed the surface in alloy II. Therefore, the overall decrease in the corrosion resistance and the increase in the capacitance for alloy II were both correlated with the decrease in the effective thickness of the oxide layer.
Scanning Electron Microscope (SEM)
After polarization measurement, the surface of the amorphous alloy I in 3.0 M H2SO4 in Fig. 5(a) shown a mud-cracked and prose structure of the oxide film . This explained the high passive current density in passive region. The higher magnification is shown the pits and cracks in the oxide layers, Fig. 5(b,c), interpreting the presence of pseudo − passivation behavior.
Fig. 5.SEM morphology of for Ni-base metallic glasses alloys at different magnifications in 3.0 M H2SO4.
The morphology of Alloy II is observed in Fig. 5(d) show the damaged oxide layer after the excrement. The less homogeneous oxide film in Figs. 5(e,f) reflect the lower values of n in impedance measurements which confirm the competitive between the accelerator and inhibition process.
Figs. 5(g,h) show the Alloy III surface with nearly compact passive layer covering most of it. The higher magnification, Fig. 5(i), is shown some damage in oxide layer. These porous may the center where the O2 evolutes in transpassive region.
The electrochemical behavior of amorphous Ni(Cr)FeSiB alloys in H2SO4 solutions at five concentrations from 1.0 to 12.0 M at 30 ℃ were investigated. The following conclusions can be withdrawn:
1. Increase of H2SO4 concentration increases the corrosion rate of alloy I and III. Critical concentration behavior appeared in alloy II.
2. Pseudo - passive film formed on the alloy I (Cr free alloy) in low concentrations of H2SO4 solutions may attributed to grayish black color porous nickel sulphide (NiS).
3. Chromium-bearing alloys show active - passive region and spontaneously passivated with low passive current densities at all studied acid solutions, and the passive region become wider and more stable in low concentrations.
4. The formation of a hydrated chromium oxyhydroxide passive film on the amorphous alloys II and III surface is responsible for their high corrosion resistance.
5. In spite of beneficial effect of Cr for corrosion resistance, it can lead to increase susceptibility to general corrosion in highly oxidizing environment. In contract, the tendency of secondary passivation at high potentials in the transpassive region decreases with increasing Cr content due to the dissolution of chromium compounds at potentials higher than 1300 mV(SCE).
6. The low CPE (Q) values in the case of alloy III reflex the high capacitive behavior of the alloy surface and thicker oxide layer on this alloy compared with alloy II.