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Original article
12 (
1
); 134-141
doi:
10.1016/j.arabjc.2018.01.019

Effect of immersion time on electrochemical and morphology of new Fe-Co metal-metal glassy alloys in acid rain

,
 Chemistry Department, College of Science, Taibah University, Al-Madinah Al-Monawarah, Saudi Arabia

⁎Corresponding author. kabdalsamad@taibahu.edu.sa (Khadijah M. Emran)

Received: , Accepted: ,
© The Author(s) 2018
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Electrochemical behavior of two Metal-Metal glassy alloys Fe78Co9Cr10Mo2Al1 (VX9) and Fe49Co49V2 (VX50) (at.%) were investigated at different immersion times in artificial acid rain at 20 °C using electrochemical techniques as electrochemical impedance spectroscopy (EIS) and cyclic polarization (CP). The morphology and composition of alloy surface were investigated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) measurements and atomic force microscopy (AFM). VX9 alloy has high corrosion resistance more than VX50 due to good protective elements as Cr 10% that create chromium oxide. Immersion measurements during (0.5–12 h) reveal the critical time for VX9 alloy is about 3 h to begin corrosion resistance while the VX50 alloy take a longer time to form protective film. The susceptibility of studied alloys for localized corrosion was observed by cyclic-polarization data due to passive film of two layers (inner compact and outer prose). The VX50 alloy has low value of charge transfer resistance due to exposure to more pitting attack. The AFM and SEM images found that average surface roughness (Ra) of VX9 alloy is lower than VX50 alloy.

Keywords

Acid rain
Fe-Co bulk Metallic glasses
Immersion time
EIS
EFM
SEM
1

1 Introduction

Bulk metallic glasses (BMG) are known to exhibit the unique mechanical, physical properties and high corrosion resistance (Inoue, 2001). Unlike crystalline metals, the metallic glasses are featured with amorphous structural, which that do not possess long-range structural order (Suryanarayana and Inoue, 2011). Amorphization of an alloy from the liquid state essentially involves the kinetic suppression of nucleation and growth from an under cooled melt. A glass is a solid which does not crystallize upon cooling fast by solidification enough from the melt and can be formed at cooling rates of 106 K/s, that attributed to these glassy alloys do not contain the crystalline properties such as grain boundaries, dislocations, and stacking faults (Raj, 2005; Miller and Liaw, 2007).

BMGs in metal–metal systems were first prepared in the early 1990s by the stabilization of supercooled liquid (Inoue et al., 1988, 1989, 1990). The BMGs are very important in the industry because it use in many applications to there properties as magnetic (Inoue et al., 2003), engineering, physical and chemical properties (Suryanarayana and Inoue, 2011; Wang et al., 2004), also in biomedical applications (Li and Zheng, 2016). The BMGs had been studied in many media such as a neutral media in absence and presence of chloride ions (Emran and AL-Refai, 2017a), in acid (Emran and AL-Refai, 2017b) and base (Emran and Al-Refai, 2018) solutions. Fe-based BMGs show the most attractive because of their commercial importance (Pang et al., 2002) and unique combination of excellent soft magnetic properties (Guo et al., 2008), ultrahigh strength (Guo et al., 2010), relatively low preparation costs (Guo et al., 2009) and good corrosion resistance (Gostin et al., 2010). In the Fe-based BMGs with high content of the metalloid elements usually brings in side-effects on the mechanical properties and it was found these alloys exhibit high plastic strain and fracture strength (Gu et al., 2008). Fe, Co, Ni-based ferromagnetic amorphous alloys always attracts a great deal of attention due to not only their excellent promising magnetic and mechanical applications, but also their significantly low cost (Anantharaman and Suryanarayana, 1987).

Guo et al. (2013) have been investigated the structure, thermal stabilities and corrosion resistance of Fe43Co4.5Cr16.5Mo16.5C14B4Y1.5 (at.%) bulk metallic glass in the acid rain solution. The high corrosion resistance in simulated acid rain was attributed to its unique chemical compositions and structure. Another novel Fe-based (Fe41Co7Cr15Mo14C15B6Y2) BMG alloy exhibit spontaneous passivation with very low passivation current density in simulated acid rain, that due to the passive elements such as Cr element (Wang and Chao, 2012). The corrosion behavior of Zr-based bulk metallic glasses in different artificial body fluids was investigated and found that BMGs exhibited excellent corrosion resistance in various artificial body fluids and as some additives in alloying elements lead to enhance the resistance (Liu et al., 2006). The corrosion resistance of Fe71−xCrxMo3.5Ni5P10C4B4Si2.5 (x = 0, 4, 8, 10, 12, 14 at.%) amorphous alloys was investigated in aerobic 3.5 wt% NaCl solution at ambient temperature. The study reveals that minor alloying of Cr by 4 at.% can significantly increase the corrosion resistance in this solution, where the thermal stability of that alloy increases slightly with appropriate Cr content. Moreover, the corrosion resistance increased with increases ratio of Cr element that attributed to the formation of thick oxide film especially when x = 12 the alloy exhibits the best corrosion resistance (Li et al., 2016).

The object of this work is to study effect the different immersion times in artificial acid rain at 20 °C during (0.5–12 h) for Fe78Co9Cr10Mo2Al1 and Fe49Co49V2 glassy alloys and follow the electrochemical behavior and morphology changes of these alloys for ruling on suitable climate for use in industrial applications.

2

2 Experimental

The electrochemical experiments were carried out by Gamry Instruments, USA (potentiostat/galvanostat-Interface 1000™). The electrochemical measurements were measured at least three times to confirm from the experimental data. All the potentials are referred to the saturated Ag/AgCl electrode. The EIS measurements in all experiments were extended from 650 kHz to 0.1 Hz, with 10 mV peak to peak sine wave being the excitation signal. Cyclic polarization curves were recorded at potential sweep rate of 1 mV for VX9 and VX50 alloys with potential forward sweep was from cathodic to anodic direction (−600 mV to 1200 mV and −900 mV to 1200 mV, respectively) and the potential reverse sweep from anodic to cathodic direction (1200 mV to −600 mV and 1200 mV to −900 mV, respectively) after impedance run at different immersion times (0.5–12 h) in artificial acid rain of pH 3.5. The surface film formed on studied alloys (VX9 and VX50) were examined after electrochemical measurements by XPS analysis with AlKα at 150 W of X-ray power (Kratos Axis Ultra DLD). The alloys surfaces have been investigated by using scanning electron microscope SEM, X-ray spectroscopy EDX microanalysis hardware with 10.23 keV (Superscan SS-550, Shimadzu company, Japan made) and AFM (digital instrument CP-II, Veeco company, USA made) techniques in acid rain.

Working electrodes (surface area 1 cm2) were cut from sheets (0.35 mm thick) of Fe78Co9Cr10Mo2Al1 and Fe49Co49V2 metal-metal glassy supplied by Vacuumschmelze by rapid solidification technology to cast thin metallic ribbons process step directly from the molten metals. The corrosive solution is an artificial acid rain. Its composition and characteristics are given in Table 1. All chemicals were analytical grade obtained from Aldrich chemical co.

Table 1 Characteristics of the artificial Acid rain.
Artificial Acid rain
Conductivity (20 °C) 41.8 μS
pH 3.5
Sulfuric acid 31.85 mg/L
Ammonium sulfate 46.20 mg/L
Sodium sulfate 31.95 mg/L
Nitric acid 15.75 mg/L
Sodium nitrate 21.25 mg/L
Sodium chloride 84.85 mg/L

3

3 Results and discussion

3.1

3.1 Electrochemical impedance spectroscopy measurements (EIS)

The Nyquist and Bode plots of the VX9 and VX50 alloys were measured at different immersion times (0.5, 1, 3, 6 and 12 h) in artificial acid rain of pH 3.5 at 20 °C as shown in Fig. 1(a and b). The Nyquist plots in Fig. 1(a and b) reveal that testing VX9 and VX50 alloys have negative impact in the beginning. The arc of the capacitive loops decrease in size as well as the corrosion resistance reach to minimum value after 1 h for VX9 alloy and after 3 h for VX50 alloy. After that the diameter of capacitive loops increased with increase the exposure time in acid rain.

Nyquist plots for VX9 and VX50 alloys at different immersion times in artificial acid rain, with the appropriate equivalent circuit.
Fig. 1
Nyquist plots for VX9 and VX50 alloys at different immersion times in artificial acid rain, with the appropriate equivalent circuit.

The data obtained by analysis the impedance spectra using equivalent circuits shown in Fig. 1 and summarized in Table 2 (χ2 about 10−3–10−4). At high frequency, in EIS plots, it can observe the intercept with the real axis corresponding to the solution resistance with average value about 70.25 Ω cm2 for VX9 alloy and 73.78 Ω cm2 for VX50 alloy resulting of ohmic or solution resistance. That maybe caused by a combination of low electrolyte concentration or low temperature, where the ohmic resistance know as the sum of the solution resistance and the resistance of the current collector, the active mass and the transition (Van der Vliet et al., 2010; Aswathy et al., 2015).

Table 2 The results from impedance and cyclic polarization measurements for the both alloys at different immersion times in artificial acid rain.
Alloy Impedance Cyclic polarization
Immersion time (h) Rs (Ω cm2) Q f/s × 10−6 (Ω−1 Sn cm−2) Rf/s (Ω cm2) n W × 10−6 (Ω−1 S0.5 cm−2) Qm/f × 10−6 (Ω−1 Sn cm−2) Rm/f (Ω cm2) n Rct (Ω cm2) Icorr (µA/cm2) Ipass (mA/cm2) Epass (mV) Epit (mV) Epp (mV) −bc ×10−3 (V/decade) CR (mmpy)
VX9 0.5 55.87 482.3 502.4 0.59 542.5 179.5 33.39 × 103 0.98 33.89 × 103 0.631 0.004 −13 404 65 56 0.007
1 78.05 623.3 356.2 0.64 648.7 244.2 29.78 × 103 0.88 30.14 × 103 0.695 0.003 23 416 47 56 0.009
3 77.83 310.5 536.1 0.41 653.2 241.2 48.68 × 103 0.96 49.22 × 103 0.546 0.005 −60 410 101 58 0.006
6 64.44 188.7 539.3 0.61 670.1 141.1 50.80 × 103 0.99 51.34 × 103 0.464 0.004 −67 428 80 52 0.005
12 75.04 119.5 554.4 0.71 661.2 96.27 76.60 × 103 0.99 77.15 × 103 0.320 0.005 −30 719 156 56 0.003
VX50 0.5 69.56 116.9 422.8 0.78 – 269.2 2.67 × 103 0.68 3.09 × 103 3.87 2.04 292 835 −442 60 0.089
1 78.94 136.6 411.6 0.72 – 299.2 2.30 × 103 0.71 2.71 × 103 5.25 2.16 351 830 −426 643 0.121
3 81.07 192.9 330.7 0.75 – 349.8 2.07 × 103 0.72 2.40 × 103 5.86 2.76 399 816 −458 69 0.136
6 63.90 147.6 403.4 0.77 – 319.8 2.18 × 103 0.73 2.58 × 103 5.60 2.09 381 816 −439 66 0.129
12 69.45 114.1 425.1 0.65 – 204.9 2.76 × 103 0.81 3.19 × 103 1.95 1.94 256 803 −468 68 0.045

The small capacitive loop in Nyquist plots of VX9 alloy, Fig. 1(a), indicate the rapid dissolution process for alloys due to O2 reduction, followed by along diffusion tail at low frequency which attributed to the porous film and a diffusion process. The arc of the capacitive loops decrease in size as well as the corrosion resistance reach to minimum value at 1 h. The porosity of protective layer decrease or heal with increasing the immersion times above 1 h and the diffusion tail shift to the left direction that reflects the increasing corrosion resistance at 12 h. The critical time also observed in case of VX50 alloy after 3 h immersion. After that the diameter of capacitive loops and relaxation time increased with increase the exposure time in artificial acid rain as illustrated in Fig. 1(b). This resistance caused by the corrosion products, which provided protective layer to protect the alloys surface against the corrosion.

The impedance plots were explained by using two time constants models, the first found at high frequency and related to the outer porous layer resulting from FeO and Fe3O4 for VX9 alloy (where the diffusion occur) and the inner layer (barrier layer) resulting mainly from Cr2O3 and Cr(OH)3 and minority from MoO2 and Al2O3. FeOOH, Fe2O3 and Fe3O4 exist in outer layer of VX50 alloy and inner layer build up of Co2O3 and CoO for VX50 alloy. These results are agree with the XPS analysis as it will be discuss in XPS analysis.

According to composition of inner layer, the VX9 alloy has high corrosion resistance than VX50, Table 2, due to include good protective elements as Cr that create a good protective layer from chromium oxide (or chromium oxyhydroxide) on alloy surface. The average n values (0.59 for VX9 and 0.73 for VX50) extent of exposure to acid rain indicate a semi homogeneous layer on VX50 alloy and a less homogeneous layer on VX9 alloy (outer layer).

The chromium oxide (Cr2O3, inner layer) is responsible of the high resistivity of the VX9 alloy in artificial acid rain. It has been calculate thickness of oxide films (µm) through Eq. (1).

(1)
C dl = ε ε ° A d where d is the thickness of the protective layer, ε is the dielectric constant of the oxide film (ε of Cr2O3 = 30), ε° is the vacuum permittivity (8.85 × 10−12 Fm−1) and A is the surface area of the working electrode (1 cm2). Therefore, the oxide film thickness formed on the VX9 alloy surface increases with increased immersion times from 1.09 to 2.76 µm (1–12 h) but the oxide film formed on the surface of the VX50 alloy is very thin.

3.2

3.2 Cyclic polarization measurements (CP)

Cyclic polarization curves in Fig. 2(a and b) illustrate the ability of VX9 and VX50 alloys to form protective oxide layer in artificial acid rain. The VX50 alloy in Fig. 2(b) shows delay in the establishment of pseudo passive layer compared to VX9 alloy, Fig. 2(a). The corrosion and passivation parameters are summarized in Table 2. The maximum values of the corrosion rates for VX9 alloy are about 0.009 mmpy at 1 h for VX9 alloy and 0.136 mmpy for VX50 alloy at 3 h which is about 3 and 3.02 time lower the corrosion rate at 12 h, respectively. The corrosion current density (Icorr) reduced by increasing the immersion time after 1 h for VX9 alloy and 3 h for VX50 alloy as in impedance measurements, where the Icorr extracted by extrapolating the cathodic Tafel line to corrosion potential (Ecorr) (Amin, Khaled, and Fadl-Allah, 2010). According to standard expressions for corrosion rate, the value of VX9 alloy classified as outstanding state as result to get on value of corrosion rate <0.02 mmpy and the VX50 alloy classified as good state as result to get on value of corrosion rate in range (0.1–0.5 mmpy) (Fontana, 1987). The outstanding corrosion resistance of VX9 alloy is attributed to the spontaneous formation of a chromium oxyhydroxide (chromium oxide) mixed with iron oxides thin passive layer on the surface, where the Ipass is independent of the applied potential. Generally, the passive film formed on two alloys can be described by a bi-layer model, inner layer, which gave a high corrosion resistance and the defective outer layer. These results are confirmed with EIS measurements.

The cyclic polarization curves for VX9 and VX50 alloys at different immersion times in artificial acid rain.
Fig. 2
The cyclic polarization curves for VX9 and VX50 alloys at different immersion times in artificial acid rain.

It is known through previous works that the bi-layer passive films formed by selective dissolution for those ions migrating under the control of an electric field, where the Fe element migrate faster through the film and preferentially dissolved at the outer film while Cr remains enriched in inner passive film (Kim et al., 2011). The chromium oxide passive film act as protective and compact layer that caused by the amounts of Cr in inner corrosion products dissolved and then deposited in the pits (Kim et al., 2011). The decreasing of corrosion resistance for VX9 alloy in artificial acid rain at 1 h can be attributed to decrease in Cr2O3 and increase in Cr(OH)3 on the alloy surface (Jakupi et al., 2011). The less resistance of the passive film formed on VX50 alloy surface attributed to Co and Fe oxides. The Co and Fe dissolved and diffused quickly during the corrosion process, so the amount of Fe and Co in the inner and outer passive film are relatively different. The result is a small amount of Fe and Co in outer corrosion products while a large amount in inner corrosion products explains the resistance in the inner passive layer this result is agree with EIS measurements.

It can be seen from curves of the VX9 and VX50 alloys they exhibits similar polarization behavior as evidenced by obtained Tafel parameter (−bc) which are close to each other. From the experimental data of cyclic-polarization measurements refers that both two alloys probably undergo localize corrosion. Decrease the corrosion rate for studied alloys due to accumulation of the corrosion products on the pits of alloys i.e. healing the pits. The passivation area up to minimum value at a critical immersion time for VX9 and VX50 alloys (383 and 417 mV), respectively. The VX9 alloy getting less difference value between the Epass and Ecorr (178 mV) after exposure for longer time in artificial acid rain (12 h) compare to VX50 alloy with low tendency for transform to passivation state at all exposure times.

3.3

3.3 X-ray photoelectron spectroscopy (XPS) measurements

The resistivity behavior under acid rain of pH 3.5 at 20 °C conditions was further examined by XPS analysis of the surface film of studied alloys (VX9 and VX50) after impedance and polarization measurements.

The XPS spectra of two alloys after exposure to artificial acid rain solution show significant peaks of iron, chromium and oxygen for VX9 alloy and iron, cobalt and oxygen for VX50.

The components of samples surface are detected through described the bi-layer. The highest binding energy peak of the Feox2p3/2 for VX9 alloy at 709 and 710 eV can be attributed to the presence of iron oxides on the surface as Fe3O4 and FeO in outer porous layer (Moulder et al., 1995; Biesinger et al., 2011). The peak corresponding to the Crox2p3/2 spectrum, Fig. 3(a), reveal to Cr2O3 at 576 eV and Cr(OH)3 at 577 eV (Moulder et al., 1995) or hydrated chromium oxyhydroxide film [CrOx(OH)3–2x·nH2O] which formed in range (576.3–579.7 eV) (Arab et al., 2014) in inner layer. Low intensity was found for peaks of Mo and Al, where the Moox 3d5/2 and Alox 2p spectra, found the highest binding energies peaks at 231 and 74 eV, respectively, that refers to presence of MoO2 and Al2O3 on the surface of the VX9 alloy (Moulder et al., 1995; Schroeder et al., 2004).

(a and b) The X-ray photo-electron spectra, the SEM micrograph at critical immersion time (c) at 1 h, (d) at 3 h, the EDX spectra (e and f) at 12 h and the AFM images at critical immersion time (g) at 1 h, (h) at 3 h for VX9 and VX50 alloys, respectively, in artificial acid rain.
Fig. 3
(a and b) The X-ray photo-electron spectra, the SEM micrograph at critical immersion time (c) at 1 h, (d) at 3 h, the EDX spectra (e and f) at 12 h and the AFM images at critical immersion time (g) at 1 h, (h) at 3 h for VX9 and VX50 alloys, respectively, in artificial acid rain.

The components of film surface of the VX50 alloy as outer porous layer resulting from FeOOH, Fe3O4 and Fe2O3 of the Feox2p3/2, where located at the highest binding energy peak (711 eV) (Moulder et al., 1995; Biesinger et al., 2011). The highest binding energy peak for VX50 alloy of Coox2p3/2 was in 779.9, 780 and 781 eV attributed to CoO, Co3O4 and Co(OH)2 on the alloy surface as inner layer as shown in Fig. 3(b). The Co-containing compounds are considered to be related with a decrease of corrosion rate due to a decrease in the anodic reaction (Moulder et al., 1995; Biesinger et al., 2011).

For Oox 1s spectrum, the highest peak located at 530 eV for two alloys can be attributed to formed of oxide, and at this binding energy the O2– could be related to the bond with dissolved metal.

3.4

3.4 Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX) measurements

SEM technique was used to observe the surface morphology of VX9 and VX50 alloys before and after exposure in artificial acid rain at different immersion times.

It has been observed the surfaces of VX9 and VX50 alloys before exposure to artificial acid rain are uniform and homogeneous. The surface appearance in Fig. 3(c) at critical immersion time, shows a homogeneous with isolated pits on the surface as well as in other immersion times due to the chromium oxide healing the pits. The reason for the appearance a homogeneous surface for VX9 alloy is healing the pits as result to presence the chromium oxide on its surface. This result confirms with results obtained from AC and DC measurements, which have proven the high resistance and lower corrosion rate for VX9 alloy.

The localized attack shows on of VX50 alloy surface after exposure in artificial acid rain, which produced small pits on alloy surface. The pitting attack becomes severe at the critical immersion time (3 h), Fig. 3(d), that inferred from the small black spots on VX50 alloy surface denoting pitting corrosion on the outer passive layer. Then this phenomenon become less pronounced at longer immersion times, where it is highly declines the pits on surface that reflect increases the corrosion resistance of this alloy. This lead to difficult arrive the solution to alloy surface.

From the analysis of the EDX spectra of two alloys (VX9 and VX50), found the values of mass fractions of chemical elements on the two alloys surface (wt%). It can be concluded that increase immersion period (12 h), Fig. 3(e), decreased iron element (75.28%) on VX9 alloy. The mass fractions for chrome increased (11.72%) and the oxygen achieved the highest value (3.22%). This shows the film layer formed on the VX9 alloy surface has a highest resistance as result to increase chromium oxyhydroxide (chromium oxide) content in this film. This due to the fact that the presence of the Cr 10% leads to increase resistance of VX9 alloy in artificial acid rain. The values of mass fractions for iron and cobalt are close to each other in VX50 alloys for all immersion periods in artificial acid rain. This indicates the two compounds are formed the film layer on VX 50 alloy. The mass fractions of oxygen achieved the highest value (6.69%) after 12 h Fig. 3(f). That lead to form a lot of oxides in film layer and give the alloy surface more resistance in artificial acid rain.

3.5

3.5 Atomic force microscopy (AFM) measurements

The AFM technique was used to study morphology and the surface change of VX9 and VX50 alloy resulting due to corrosion processes in artificial acid rain at different immersion times (0.5, 1, 3, 6 and 12 h) of pH 3.5 at 20 °C. The parameters extracted from the AFM technique are summarized in Table 3. The values of average roughness Ra for VX9 alloy increased to highest value from 59.59 to 91.90 nm as exposure time increased from 0.5 h to 1 h as illustrated in Fig. 3(g). For VX50 alloy, Fig. 3(h), this parameter increased to highest value from 51.89 nm to 166.0 nm as exposure time increased from 0.5 h to 3 h. The lowest value of Ra was obtained at 12 h for two alloys (38.45 and 46.22 nm, respectively).

Table 3 The AFM parameters of VX9 and VX50 alloys surfaces at different immersion times in artificial acid rain.
Alloy VX9 VX50
Immersion time (h) Ra (nm) Rp (nm) Rt (nm) Rv (nm) Ra (nm) Rp (nm) Rt (nm) Rv (nm)
As-received 33.02 73.91 123.3 49.09 43.58 82.08 139.3 57.22
0.5 59.59 285.2 383.9 98.7 51.89 113.2 276.0 162.8
1 91.90 157.8 353.7 195.9 106.3 283.1 458.8 175.7
3 45.48 106.5 197.9 91.4 166.0 454.0 748.0 294.0
6 42.12 131.0 220.0 89 106.0 308.0 511.0 203
12 38.45 85.47 172.7 87.23 46.22 181.9 277.7 95.8

A comparison between two alloys found that the alloy VX50 have the highest Ra value, indicates greater corrosion rate are than the VX9 alloy as observed in Fig. 3(g and h). This is correspond with the electrochemical measurements.

A significant difference between the maximum and the minimum values of Rt (total roughness) and Rv (the deepest valley) for two alloys, the value of Rt = 353.7 nm at 1 h and 172.7 nm at 12 h for VX9 alloy and for VX50 alloy the value of Rt = 748.0 nm at 3 h and 277.7 nm at 12 h. This indicates that the surface of the two alloys at a critical time allow to reach the highest roughness, while with the length of exposure in the artificial acid rain this surfaces become less roughness. As for the Rv gave values of VX9 alloy are 195.9 nm at 1 h and 87.23 nm at 12 h and for VX50 alloy are equal to 294.0 nm at 3 h and 95.8 nm at 12 h. Also this reflects that the surface of the two alloys at a critical time become more deeper. The surface roughness decreases significantly with increasing exposure in artificial acid rain. That proves the two alloys are good corrosion resistance in this solution.

4

4 Conclusions

Fe-Co-based bulk metallic glasses of Fe78Co9Cr10Mo2Al1 and Fe49Co49V2 in simulated acid rain different immersion times have been studied in simulated acid rain and investigated the passivation behavior for both alloys at different immersion times. Immersion measurements times (0.5–12 h) in artificial acid rain at 20 °C for VX9 and VX50 alloys reveal a negative impact in the beginning. It has been found that the critical time for VX9 alloy is about 3 h to begin corrosion resistance while the VX50 alloy take a longer time to form protective film. This resistance caused by the corrosion products, which provided protective layer to protect the surface of the alloys against the corrosion. The pseudo passive film formed on two alloys can be described by a bilayer model, inner layer resulting mainly from Cr2O3 and Cr(OH)3 and minority from MoO2 and Al2O3 for VX9 alloy and Co2O3 and CoO for VX50 alloy, which gave a high corrosion resistance and the defective outer porous layer resulting from FeO and Fe3O4 for VX9 alloy and FeOOH, Fe2O3 and Fe3O4 for VX50 alloy.

Acknowledgments

The authors would like to thank Dr. Hartmann Thomas from Vacuumschmelze company for providing the specimens. Also, the authors would like to thank the Kink Abdulaziz City of Science and Technology (KACST) for financially supporting part of this research under Contract (GSP-37-21).

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