Search for articles:
Fang Guan
, Xiaofan Zhai, Nan Wang
Corresponding authors:
Received: 2019-02-1
Revised: 2019-04-14
Accepted: 2019-05-21
Online: 2020-01-01
Copyright: 2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology
More
Abstract
Microbiologically influenced corrosion caused by sulfate-reducing bacteria (SRB) poses a serious threat to marine engineering facilities. This study focused on the interaction between the corrosion behavior of two aluminum alloys and SRB metabolic activity. SRB growth curve and sulfate variation with and with aluminum were performed to find the effect of two aluminum alloys on SRB metabolic activity. Corrosion of 5052 aluminum alloy and Al-Zn-In-Cd aluminum alloy with and without SRB were performed. The results showed that both the presence of 5052 and Al-Zn-In-Cd aluminum alloy promoted SRB metabolic activity, with the Al-Zn-In-Cd aluminum alloy having a smaller promotion effect compared with 5052 aluminum alloy. The electrochemical results suggested that the corrosion of the Al-Zn-In-Cd aluminum alloy was accelerated substantially by SRB. Moreover, SRB led to the transformation of Al-Zn-In-Cd aluminum alloy corrosion product from Al(OH)3 to Al2S3 and NaAlO2.
Keywords：
Corrosion caused by microorganisms is one of the main reasons for the failure of engineering materials immersing in seawater [1,2]. Sulfate-reducing bacteria (SRB), as one of the most corrosive bacterium, is wide spread in soil, pipelines and marine environments [3], and they are the most mentioned bacterial group involved in microbiologically induced corrosion [4].
SRB could induce corrosion in almost all metals, like iron [1], copper [5] and aluminums [6]. In aqueous anaerobic environment or environment with a low oxygen concentration, SRB use the organic matter in aqueous environment as a carbon source and reduce sulfate into sulfide. Corrosion can occur via chemical attack from hydrogen sulfide and extracellular polymeric substance secreted by SRB [7]. Hydrogenase-positive SRB are supposed to promoted corrosion by scavenging ‘cathodic hydrogen’ [8]. Hydrogen played a vital role as a cathodic hydrogen depolarizer in the cathodic reaction, and its removal constitutes a the rate-limiting step in cathodic reaction process [9]. Hydrogenase-positive SRB could accelerate metal corrosion by utilizing cathodic hydrogen [10,11]. However, recent studies have reported that SRB may act as biocathode that obtain electrons directly from metals by extracellular electron transfer (EET) [12], especially when there is lack of carbon sources [13], and the corrosion caused by EET was more severe than that caused by scavenging cathodic hydrogen [8] or metabolic corrosion [14]. Usually, addition of electron mediators [15,16] and carbon starvation [13,17] would also promote the EET of SRB and lead to pitting corrosion.
There is a close connection between metals corrosive behavior and SRB metabolic activities. Lots of studies [1,18] have been reported on charactering the corrosive behavior of metals in SRB medium, and of course the behavior varied with metals and environments. While few reports focused on the interaction between metals and bacteria. Javed et al. [19] found that high iron level in culture medium resulted in more attachment and exopolymer production of SRB. Lin [20] found that the presence of carbon steel promoted the growth of SRB and accelerated its metabolic activity. Our previous studies found that the SRB metabolic activity was promoted under the -0.85 VSCE polarization potential and thus resulted in the accelerated corrosion of steel [21]. While 5052 aluminum, which has the similar potential around -0.85 VSCE, has a much less promotion effect on SRB metabolic activity than -0.85 VSCE polarization or iron [22]. One possibility for explaining the phenomenon is that the passive film formed on 5052 aluminum surface was unsuitable for the EET of SRB from 5052 aluminum. So, is it possible that SRB metabolic activity got promoted if the passive film cannot form on aluminum surface?
Al-Zn-In-Cd aluminum alloy is a widely used in marine engineering facilities as sacrificial anode and no passive film is formed on its surface. In this study, Al-Zn-In-Cd and 5052 aluminum alloy were choose to study the interaction between the corrosion behavior of aluminum alloy and SRB metabolic activities. The electrochemical characteristics and corrosion morphologies of Al-Zn-In-Cd and 5052 aluminum alloy in SRB and sterile medium were analyzed in detail via electrochemical measurements and surface analysis. SRB metabolic activities were studied via the variation of sulfate concentration and growth curve.
The strain Desulfovibrio caledoniensis (D. caledoniensis) was separated from the rust layers of carbon steel. D. caledoniensis has been proven to be hydrogenase-positive in our previous work [23]. SRB was cultured in Postgate C (PGC) medium [24]. Before experiment, the PGC culture media was deoxygenated by purging with high-purity nitrogen for 30 min and autoclaved at 121 °C for sterilization. A 1% inoculation of 5-day-old bacteria was injected into the electrochemical cells.
The growth curves of SRB in different conditions were obtained using the optical density (OD) method using a spectrophotometer at 600 nm [25]. Concentration of SO42- was also analyzed to study SRB metabolic activity using ion chromatography system (883 Basic IC plus).
A three-electrode device was used for electrochemical measurements using the Gamry 1000 Electrochemical Analyzer (Warminster, PA, USA). 5052 aluminum alloy and Al-Zn-In-Cd aluminum alloy coupons measuring 10 mm × 10 mm × 10 mm were used as working electrodes, with the material chemical ingredient shown in Table 1, Table 2. A platinum plate and a saturated calomel electrode (SCE) served as the counter electrode and reference electrode, respectively.
Table 1 Chemical compositions of 5052 aluminum alloy (wt%).
| Si | Fe | Cu | Mn | Mg | Zn | Cr | Al |
|---|---|---|---|---|---|---|---|
| 0.25 | 0.4 | 0.1 | 0.1 | 0.4 | 0.1 | 0.15 | Balance |
Table 2 Chemical compositions of Al-Zn-In-Cd aluminum alloy (wt%).
| Zn | In | Cd | Si | Fe | Cu | Al |
|---|---|---|---|---|---|---|
| 2.5-4.5 | 0.018-0.05 | 0.005-0.02 | ﹤0.1 | ﹤0.15 | ﹤0.01 | Balance |
Electrochemical impedance spectroscopy (EIS) was conducted with frequencies ranging from 105 Hz to 10-2 Hz. The results were analyzed using ZSimp Win software. All the electrochemical tests carried out in this work were oxygen-free corrosion tests. After immersion for 15 days, the dynamic polarization was performed at a scan rate of 0.1 mV s-1. To avoid the delay in the process of potentiodynamic polarization test, the cathodic polarization was performed following the anodic polarization until the open circuit potential (Eocp) stabilized. All the experiments were performed in sterile or SRB-inoculated PGC medium.
Scanning electron microscopy (SEM) was used for morphologies observation of specimens before and after removing corrosion product, and X-ray photoelectron spectroscopy (XPS) (ESCALAB250 Surface Analysis System) for determining the chemical species on coupon surface. The procedure for SEM and XPS analysis were described previously [22].
The sulfate concentration in different conditions was measured to find the impact of 5052 aluminum and Al-Zn-In-Cd aluminum alloy on SRB metabolic activities, and the control system is that SRB cultured without any metal coupons. The results in Fig. 1 showed that the concentrations of sulfate in SRB culture medium with or without aluminum were almost unchanged during the first 3 days. Sulfate concentration dropped sharply on the 5th day, and then remained relatively stable from the 7th day. Compared with the control system, the sulfate concentration cultured with Al-Zn-In-Cd aluminum on the 5th day has the most rapid decline, followed by that cultured with 5052 aluminum.
Fig. 1. Sulfate variation in SRB medium over time (t) in different conditions.
The SRB growth curve (Fig. S1) also showed a rapid increase on the 5th day, However little difference was found between the SRB curves cultured with 5052 Al or Al-Zn-In-Cd aluminum alloy. To verify slight difference between the two systems, the most probable number (MPN) method [26] was used to get the exact number of SRB on the 5th day (Fig. S2), and the results showed that the concentrations of SRB were 1.1 × 106, 3.5 × 106, and 2.0 × 106 CFU/mL in culture medium without aluminum, with 5052 aluminum, and with Al-Zn-In-Cd aluminum, respectively. The pH (Fig. S3) in all the systems increased during the culturing time, with the rapidest increase observed on the 5th day. The change in pH variation was in accordance with SRB growth curve and the sulfite concentration variation, which indicated that the pH variation was caused by SRB metabolic activity.
Fig. 2 shows the evolution of Eocp for 5052 aluminum alloy and Al-Zn-In-Cd aluminum alloy in sterile and SRB medium for 15 days. The Eocp of 5052 aluminum alloy shifted positively in SRB medium, which can be attributed to the passive film formed on 5052 aluminum alloy [22], and this film protected metal from further oxidation. The decrease of the Eocp on the 15th day was probably caused by the metabolic activity of SRB and the detachment of the biofilm [27]. The acidic metabolites secreted by the SRB combined with Al, which might promote the process of Al3+ leaving the metal surface [28], leading to the negative shift of the measured Eocp.
Fig. 2. Eocp values obtained from 5052 aluminum alloy and Al-Zn-In-Cd aluminum alloy as a function of time in the culture media with and without SRB.
In the sterile medium, the Eocp of Al-Zn-In-Cd aluminum alloy shifted negatively by approximately 40 mV after being immersed for 3 days. Then the Eocp remained stable at approximately -1.0 V (vs. SCE). However, in SRB medium, a significant fluctuation in the Eocp of Al-Zn-In-Cd aluminum alloy was observed after two days, after which it remained relatively stable at -0.97 VSCE. These changes in Eocp may be related to the SRB metabolic activity [27]. The concentration of SRB was relative low over the first few days (Fig. S1), and therefore the electrochemical behavior of alloys was likely not affected by the metabolic activity of SRB. Thus, the Eocp of Al-Zn-In-Cd aluminum alloy in sterile and SRB inoculated PGC medium was similar. As the immersion time increased, the Eocp of Al-Zn-In-Cd aluminum alloy shifted positively and then remained stable.
Compared with that of 5052 aluminum alloy, the Eocp of Al-Zn-In-Cd was more negative during the immersion period. Previous study [29] have shown that only trace amounts of alloy element In were required to trigger attack, and lead to the pitting corrosion on aluminum surfaces. The more negative value of Eocp of Al-Zn-In-Cd than that of 5052 aluminum alloy may be related with the promoted dissolution of the aluminum alloy on Al-Zn-In-Cd aluminum alloy surface [30].
The electrochemical impedance spectroscopy (EIS) results are shown in Fig. 3, with experimental data (scatter dot) being consistent with the fitting line (straight line). The impedance diagram of 5052 aluminum alloy (Fig. 3(a)) showed a half-arc shape in which the impedance of the arc diameter increased from the 3rd until the 10th. The impedance represented by the capacitive arc gradually increased with immersion time. Subsequently, the Nyquist diameter decreased in the 15th day, which can be attributed to the partial deterioration of the oxidation film and the falling off of the biofilm [31]. However, the Nyquist diameters of 5052 aluminum alloy were much bigger than those of Al-Zn-In-Cd aluminum alloy during immersion, which indicated that the corrosion resistance of 5052 aluminum alloy is higher than that of Al-Zn-In-Cd aluminum alloy.
Fig. 3. Nyquist plots of the measured (symbols) and fitted data (lines) for 5052 aluminum alloy in SRB culture solution (a), and Al-Zn-In-Cd in sterile (b) and SRB media (c) for 15 days.
The Nyquist plots of Al-Zn-In-Cd aluminum alloy in sterile environment are shown in Fig. 3(b). The impedance diameter showed a capacitive semicircle in which the impedance loops increased over time, indicating that the corrosion rate decreased with time. The presence of an inductive reactance in the low-frequency zone on the 1 st day may be associated with the growth pits in the initial stages of the immersion [[32], [33], [34]].
In SRB medium, however, the Nyquist diameters for the Al-Zn-In-Cd aluminum alloy decreased over the first 10 days and then increased later. This process can be attributed to the SRB metabolic activity and the formation of corrosion product on metal surface. With the accumulation of corrosive metabolic products secreted by SRB and the subsequent reaction of Al-Zn-In-Cd aluminum alloy with those corrosive chemicals in aqueous environment, the corrosion resistance decreased with time over the first 10 days. However, in the end-stage of immersion, the deposition of corrosion product may act as a barrier for the direct corrosion of corrosive chemicals, thereby increasing the corrosion resistance [34,35]. Compared with those obtained in sterile medium, the lower diameter of the impendence loops of Al-Zn-In-Cd aluminum alloy in SRB medium were indicative of the accelerated corrosion.
Fig. 4 shows the Bode plots for both aluminum alloys. For 5052 aluminum alloy (as shown in Fig. 4(a)), the phase angle had one broad peak on the 1 st day, which can be considered to be joined by two time contants [22], and two greatly separate peaks on the other days. The high-frequency peak in the Bode plots was caused by the protective oxide film layer formed on sample surface [31], because a surface dielectric film normally has a faster and smaller time constant [36]. And the low-frequency peak can be attributed to the charge transfer process [37].
Fig. 4. Bold plots of the measured (symbols) and fitted data (lines) for 5052 aluminum alloy in SRB medium (a) and Al-Zn-In-Cd alloys specimens in sterile (b) and SRB (c) media.
The Bode plots of the Al-Zn-In-Cd aluminum alloy in sterile and SRB media are shown in Fig. 4(b) and (c), respectively. No significant change was observed in the characteristics of the Bode spectra of both samples, although the physical meanings may be different. The phase in the low-frequency region on the 1 st day (Fig. 4(b)) had a negative value, indicating the presence of an inductive reactance [32].
As shown in Fig. 5, the EIS plots of 5052 aluminum alloy and Al-Zn-In-Cd aluminum alloy in SRB medium were fitted with two time constants equivalent models with constant phase element Q instead of a capacitance, n is an empirical exponent that reflects the degree of heterogeneity on the sample surface, Rs is the solution resistance, Qf and Rf are the capacitance and resistance of the corrosion product, respectively, Qdl is the capacitance of the double layer, and Rct is the charge transfer resistance [38,39]. Two equivalent circuits (Fig. 6) were proposed for the EIS data of Al-Zn-In-Cd aluminum alloy in sterile medium, with model (a) used for the EIS plots of the 1 st day, and model (b) for the EIS plots of the other days. In Fig. 6, Rs, Qf, Rf, Qdl and Rct have been defined previously; Lpit is the inductive reactance of corrosion pits and Rpit is the resistance of pits [34].
Fig. 5. Equivalent circuits proposed for 5052 aluminum alloy and Al-Zn-In-Cd aluminum alloy in SRB culture media.
Fig. 6. Equivalent circuits proposed for Al-Zn-In-Cd aluminum alloy in sterile medium for the first day (a) and the other days (b).
The electrochemical kinetic parameters of 5052 aluminum alloy in SRB solution are shown in Table 3. The Rf increased with time, which may be related to the spatial distribution of biofilm combined with deposition of corrosion products. The attachment of SRB biofilm and the formation passive film on aluminum alloy surface may inhibit the direct corrosion attack from the corrosive chemical in SRB medium. The gradually increase of Rct over the first 13 days can be attributed to passive film farmed on 5052 aluminum surface. However, studies found that SRB could metabolic some corrosive anions, which played a major role in destroying the protection film on metal surface [40]. So, the decrease of Rct on the 15th day may be related to the SRB metabolic activity and the destruction of the passive film on 5052 aluminum alloy surface [41].
Table 3 Electrochemical parameters of the 5052 aluminum alloy sample in SRB medium.
| Time (day) | Rs (Ω cm2) | Qf (F cm-2) | nf | Rf (Ω cm2) | Qdl (F cm-2) | n | Rct (Ω cm2) |
|---|---|---|---|---|---|---|---|
| 1 | 6.39 | 9.94 × 10-6 | 0.92 | 2007 | 1.85 × 10-5 | 0.64 | 8.19 × 104 |
| 3 | 6.09 | 8.49 × 10-6 | 0.90 | 2095 | 2.33 × 10-5 | 0.64 | 2.00 × 105 |
| 5 | 6.20 | 7.89 × 10-6 | 0.89 | 3035 | 2.86 × 10-5 | 0.65 | 4.89 × 105 |
| 7 | 5.81 | 7.75 × 10-6 | 0.89 | 2988 | 3.07 × 10-5 | 0.65 | 6.47 × 105 |
| 9 | 6.44 | 7.90 × 10-6 | 0.87 | 3596 | 2.93 × 10-5 | 0.65 | 1.30 × 106 |
| 10 | 6.28 | 8.64 × 10-6 | 0.85 | 4824 | 2.79 × 10-5 | 0.67 | 1.30 × 106 |
| 13 | 6.94 | 9.23 × 10-6 | 0.84 | 4949 | 2.80 × 10-5 | 0.67 | 1.28 × 106 |
| 15 | 7.79 | 9.17 × 10-6 | 0.83 | 5145 | 2.83 × 10-5 | 0.68 | 4.40 × 105 |
Table 4 Electrochemical parameters of the Al-Zn-In-Cd aluminum alloy sample in sterile medium.
| Time (day) | Rs (Ω cm2) | Qf (F cm-2) | nf | Rf (Ω cm2) | Lpit (H cm2) | Rpit (Ω cm2) | Qdl (F cm-2) | n | Rct (Ω cm2) |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 3.93 | 8.03 × 10-6 | 0.90 | 300.0 | 9355 | 826.6 | 8.06 × 10-5 | 0.72 | 1503 |
| 3 | 6.221 | 1.18 × 10-5 | 0.89 | 537.4 | 8.77 × 10-5 | 0.62 | 2843 | ||
| 5 | 5.54 | 1.18 × 10-5 | 0.89 | 659.1 | 6.96 × 10-5 | 0.59 | 4487 | ||
| 7 | 5.46 | 1.15 × 10-5 | 0.90 | 604.3 | 6.04 × 10-5 | 0.55 | 5589 | ||
| 9 | 5.29 | 1.11 × 10-5 | 0.91 | 516.5 | 5.25 × 10-5 | 0.55 | 6029 | ||
| 10 | 5.32 | 1.24 × 10-5 | 0.90 | 862.3 | 5.14 × 10-5 | 0.56 | 6673 | ||
| 13 | 5.23 | 1.10 × 10-5 | 0.91 | 509.7 | 4.38 × 10-5 | 0.56 | 6938 | ||
| 15 | 5.43 | 1.18 × 10-5 | 0.90 | 783.3 | 4.51 × 10-5 | 0.55 | 8096 |
Table 5 Electrochemical parameters of the Al-Zn-In-Cd aluminum alloy sample in SRB medium.
| Time (day) | Rs (Ω cm2) | Qf (F cm-2) | nf | Rf (Ω cm2) | Qdl (F cm-2) | n | Rct (Ω cm2) |
|---|---|---|---|---|---|---|---|
| 1 | 3.484 | 7.25 × 10-6 | 0.9386 | 639.5 | 7.08 × 10-5 | 0.5968 | 4866 |
| 3 | 2.649 | 5.94 × 10-6 | 0.9269 | 342.8 | 4.37 × 10-5 | 0.6849 | 4455 |
| 5 | 2.415 | 4.83 × 10-6 | 0.9413 | 193.9 | 6.64 × 10-5 | 0.6370 | 4891 |
| 7 | 2.272 | 4.74 × 10-6 | 0.9386 | 184.6 | 1.07 × 10-4 | 0.5768 | 3273 |
| 9 | 2.185 | 4.42 × 10-6 | 0.9427 | 170.5 | 1.06 × 10-4 | 0.5712 | 2054 |
| 10 | 2.491 | 5.03 × 10-6 | 0.9292 | 193.0 | 1.14 × 10-4 | 0.5784 | 1897 |
| 13 | 2.603 | 5.91 × 10-6 | 0.9097 | 203.9 | 1.67 × 10-4 | 0.5270 | 2482 |
| 15 | 2.682 | 6.46 × 10-6 | 0.8991 | 230.1 | 1.81 × 10-4 | 0.5136 | 3894 |
The polarization curves for the 5052 and Al-Zn-In-Cd aluminum alloy in sterile and SRB inoculated medium are shown in Fig. 7. Tafel parameters of the 5052 and Al-Zn-In-Cd aluminum alloy exposed to media with and without SRB are shown in Table 6.
Fig. 7. Polarization curves with a scan rate of 0.1 mV s-1 of 5052 aluminum alloy and Al-Zn-In-Cd alloys exposed to media with and without SRB (i: current density).
Table 6 Tafel parameters of the 5052 and Al-Zn-In-Cd aluminum alloy specimens exposed to media with and without SRB (ba: Tafel slope of the anodic curve; bc: Tafel slope of the cathodic curve).
| Condition | 5052Al with SRB | Al-Zn-In-Cd alloys with SRB | Al-Zn-In-Cd alloys without SRB |
|---|---|---|---|
| Icorr (nA cm-2) | 63.1 | 9157 | 3952 |
| Ecorr(mV vs SCE) | -820.9 | -965.5 | -994.9 |
| ba (mV dec-1) | 191.5 | 50.24 | 49.1 |
| bc (mV dec-1) | 142.6 | 221.3 | 150.0 |
The cathodic polarization curves of Al-Zn-In-Cd and 5052 aluminum alloy were similar, indicating that the activation of passive film and the presence of SRB did not change the type of the cathodic reaction that occurred, but did accelerate the reaction rate. However, the anodic reaction of 5052 aluminum alloy was very different from that of Al-Zn-In-Cd aluminum alloy, which demonstrated that the mechanism behind the anodic dissolution process of 5052 aluminum alloy was different from that of Al-Zn-In-Cd aluminum alloy.
The Al-Zn-In-Cd aluminum alloy had a significantly higher corrosion current density, and more negative Eocp than that of 5052 aluminum alloy. The Al-Zn-In-Cd aluminum alloy in SRB medium had a higher cathodic current in the cathodic polarization curve than that in sterile medium, whereas no great difference was found in the anodic current curve. The higher corrosion current (icorr) of Al-Zn-In-Cd alloy in SRB-inoculated medium also indicated the positive role of SRB on corrosion of the alloy. These results are consistent with the impendence spectrum.
Fig. 8(a) shows the surface morphology of 5052 aluminum alloy in SRB medium, and amplified corrosion pits were shown in Fig. 8(b). Corrosion pits and SRB biofilms were clearly found on 5052 aluminum alloy surface and it was displayed that SRB cells gathered at the cracks and embedded within the corrosion products.
Fig. 8. SEM images of 5052 aluminum alloy in SRB medium (a) and Al-Zn-In-Cd alloys specimens (b) in sterile (c, d) and SRB (e, f) media.
The corrosion morphologies of Al-Zn-In-Cd aluminum alloy in sterile medium are shown in Fig. 8(c) and (d). The surface of the samples was relatively flat with some corrosion holes, while the amplified morphology image showed that long and deep corrosion cracks appeared on the surface of Al-Zn-In-Cd aluminum alloy. The protuberances on sample surface may be the matrix of the corrosion product, which was caused by the dissolution and precipitation of alloying elements on metal surface. Additionally, the corrosion products might have combined with the metal matrix to form galvanic corrosion pairs and acted as the cathode in the galvanic corrosion pairs [[48], [49], [50]], accelerating the corrosion of aluminum alloy.
Corrosion pits clearly appeared on Al-Zn-In-Cd aluminum alloy surface in SRB medium (Fig. 8(e) and (f)), indicating more severe corrosion than that observed in sterile medium and 5052 aluminum alloy in SRB medium. There were some flocs around SRB cells, which can be attributed to the extracellular polymer substances metabolized via SRB activity [5]. Localized pits on metal surface were found in the regions adjacent to SRB cells, indicating a close relationship between biofilm attachment, corrosion deposits and localized corrosion. The biofilm formed on metals prompted the crevice corrosion around the intermetallic inclusions of the aluminum alloy [51].
Fig. 9 shows the corrosion morphology of 5052 and Al-Zn-In-Cd aluminum alloy in sterile and SRB inoculated culture media after the removal of corrosion products. There were some scratches and corrosion holes on 5052 aluminum alloy surface in SRB medium (Fig. 9(a)). No scratches were observed on the surface of the Al-Zn-In-Cd aluminum alloy in the sterile environment, but a small area with non-uniform corrosion was found (Fig. 9(b)). However, in SRB medium (Fig. 9(c)), there were obvious corrosion holes on the surface of Al-Zn-In-Cd aluminum alloy with extensive corrosion areas and lots of pits. It was clear that Al-Zn-In-Cd aluminum alloy suffered the most serious corrosion among the samples tested.
Fig. 9. Surface morphologies of 5052 aluminum alloy in SRB medium (a) and Al-Zn-In-Cd alloys specimens in sterile (b) and SRB (c) media after removing corrosion products.
The wide XPS spectra of 5052 and Al-Zn-In-Cd aluminum alloy in sterile and SRB inoculated culture medium are shown in Fig. 10. The main elements on the surface of the samples immersed in sterile environment were Al, C, O, Na and Cl, indicating that aluminum oxide may be the main corrosion product in such environment. The existence of C and O elements in sterile medium may be caused by the adsorption of macromolecular organic in the culture solution. The relatively higher proportions of C, O, and N after SRB injection than that in sterile medium could be attributed to SRB biofilm formation on metal surface.
Fig. 10. Wide XPS spectra for surface of 5052 and Al-Zn-In-Cd aluminum alloys exposed to sterile and SRB media for 15 days.
Fig. 11 shows the high-resolution images of Al 2p spectra of 5052 and Al-Zn-In-Cd aluminum alloys in sterile and SRB inoculated culture medium. In the spectrum of 5052 alloy (Fig. 11(a)), the core-level Al 2p spectrum could be deconstructed into four peaks at binding energy (Eb) values of 75.4, 74.9, 74.4, and 73.6 eV, respectively caused by AlOOH, Al2O3, Al2S3, and Al(OH)3. In addition to AlOOH, Al2O3 and Al(OH)3, Al was detected on the surface of Al-Zn-In-Cd alloy sterile medium (Fig. 11(b)). On the other hand, in SRB medium (Fig. 11(c)), the peak caused by Al disappeared, but NaAlO2 (73.05 eV) and Al2S3 were detected, respectively. The high-resolution images of S 2p spectra of 5052 and Al-Zn-In-Cd aluminum alloys in SRB medium were shown in Fig. S4. Sulphides (160 and 162.5 eV), sulphates (169.5 eV) and Na2S (161.8 eV) were found on both alloys, the presence of organic compounds containing sulfur were also found on both alloys’ surface, which may be caused by the attachment of SRB biofilm.
Fig. 11. Al 2p spectra of 5052 aluminum alloy in SRB medium (a) and Al-Zn-In-Cd alloys specimens in sterile (b) and SRB (c) media for 15 days.
Table 7 shows that the Al was detected only on the surface of Al-Zn-In-Cd aluminum alloys in sterile environment. Moreover, Al2S3 was also only detected in SRB medium, for that the metabolic products of SRB were involved in the corrosion process of aluminum alloys, via the following reaction:
2Al3++3HS-+3OH-→Al2S3+3H2O (1)
Table 7 Results of XPS for different Al-positions on the 5052 Al alloy and Al-Zn-In-Cd alloys specimen surface in sterile and SRB medium (%).
| Energy (eV) | Element | 5052Al with SRB | Al-Zn-In-Cd alloys without SRB | Al-Zn-In-Cd alloys with SRB |
|---|---|---|---|---|
| 75.4 | AlOOH | 0.39 | 0.22 | 0.15 |
| 74.9 | Al2O3 | 0.15 | 0.31 | 0.05 |
| 74.4 | Al2S3 | 0.23 | 0.13 | |
| 73.6 | Al(OH)3 | 0.24 | 0.22 | 0.06 |
| 72 | Al | 0.13 | ||
| 76.0 | Al2O3 | 0.12 | 0.37 | |
| 73.05 | NaAlO2 | 0.24 |
Compared with 5052 aluminum alloy, which has good anti-corrosion attributed to the passive film formed on its surface, the Al-Zn-In-Cd aluminum alloy is quite active in seawater. When Al-Zn-In-Cd aluminum alloy is immersed in seawater, a very small anodic area was formed on its surface, and thus pits are initiated on the surfaces of Al-Zn-In-Cd aluminum alloy to accelerate the anodic reaction.
In the anodic process, with the continuous chemical reactions of metal with corrosive ions in aqueous environment [52,53], anodic ions entered solution as follows:
Al-3e-→Al3+ (2)
Zn-2e-→Zn2+ (3)
In-3e-→In3+ (4)
Cd-2e-→Cd2+ (5)
The cathodic reaction was the depolarization of hydrogen:
H++e-→H (6)
The generated [H] combined and formed a hydrogen film on the metal surface (Fig. 12). In sterile environments, the combination of [H] to generate H2 requires a high activation energy [9,54], and therefore [H] leaves the metal surface at a lower rate than that of the anodic dissolution process.
Fig. 12. Schematic of the corrosion mechanism for the Al-Zn-In-Cd aluminum alloy in sterile culture medium.
In SRB medium, electron can be transferred by SRB via EET, which typically involves $SO^{2-}_{4}$ as terminal electron acceptors. The schematic of the corrosion mechanism for the Al-Zn-In-Cd aluminum in SRB medium is shown in Fig. 13. As the acid dissociation constants pKa1 and pKa2 of H2S is 7.0 and 14.9, respectively [55]. So HS- is the most probable reacting sulfide species. And the cathodic reaction can be expressed as follows:
$SO_{4}^{2-}$ +8e-+9H+→HS-+4H2O (7)
Fig. 13. Schematic of the corrosion mechanism for the Al-Zn-In-Cd aluminum in SRB medium.
The [H] generated in the cathodic reaction could act as an electron transfer mediator in the EET of SRB, resulting in a large number of H leaving the metal surface, thus accelerating the cathode reaction and increasing the cathode current density (as shown in Fig. 7):
$SO_{4}^{2-}$+8[H]+H+→HS-+4H2O (8)
The cathodic reaction was accelerated by SRB, whereas the anodic reaction (the leaving of ions from metal surface) remained stable, which caused the dissipation of electrons on metal surface [24]. Thus, the corrosion potential (Ecorr) of Al-Zn-In-Cd aluminum alloy increased in SRB medium according to the mixed potential theory [56]. In the EET mechanism, the extracellular electrons were transported to the cytoplasm for the reduction reaction [2], which indicated that SRB cells acted as biocathode and electrons were obtained by SRB cells directly. This also led to the increase in Ecorr and accelerated corrosion rate of Al-Zn-In-Cd aluminum alloy in SRB medium. However, hydrogen depolarization theory or EET mechanism, to verify which theory played the major role in the corrosion behavior of Al-Zn-In-Cd aluminum alloy need further verification.
The metabolic activity of SRB changed the electrochemical properties of the interface between aluminum alloy and solution, affecting the anode and cathode reaction of the electrode and the transformation of corrosion product.
2Al3++3HS-+3OH-→Al2S3+3H2O (9)
(10)
The overall reaction then could be written as:
(11)
The interaction between 5052 aluminum corrosion mechanism and SRB metabolic activity has been discussed in our previous studies [22]. Compared with 5052 aluminum alloy, Al-Zn-In-Cd aluminum alloy has a smaller promotion effect on SRB metabolic activity. One possible reason may be related to the alloying elements in Al-Zn-In-Cd aluminum alloy, because studies have shown that the Zn ions can stimulate the antibacterial ability of materials [57,58]. However, the further studies need to be done to verity the possible mechanism.
Both the presence of 5052 and Al-Zn-In-Cd aluminum alloy promoted SRB metabolic activity, with the Al-Zn-In-Cd aluminum alloy having a smaller promotion affect compared with 5052 aluminum alloy. In SRB solution, a dense biofilm was formed on the surface of Al-Zn-In-Cd aluminum alloys, and the [H] generated in the cathodic depolarization process can be captured and utilized by hydrogenase-positive SRB, which promoted the cathode reaction and the positive shift of Eocp. SRB accelerated corrosion of aluminum alloy and led to the formation of sulfide and NaAlO2 compared to the sterile control.
This work was supported financially by the National Natural Science Foundation of China (No. 41806090), the Key Research and Development Plan of Shandong Province (No. 2018GHY115003), the National Natural Science Foundation of China (No. 41576080) and the China Postdoctoral Science Foundation (No. 2018M642707).
WeChat
/
| 〈 |
|
〉 |
