Yu
Wang
a,
Hongfeng
Liao
b,
Li
Gan
b,
Zhengxiu
Liu
c,
Ziqiang
Tang
c,
Xiaoran
Zhao
c,
Yubin
Zeng
*a and
Chunsong
Ye
*a
aDepartment of Energy Chemistry Engineering, School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China. E-mail: zengyubin@whu.edu.cn; csye@whu.edu.cn
bJingneng Shiyan Thermal Power Co., Ltd., Shiyan 442000, China
cBeijing Jingneng Energy Technology Research Co., Ltd., Beijing 100022, China
First published on 16th November 2023
Corrosion control is vital for the safe operation of a circulating cooling water system. The biological method is a novel treatment method performed by adding a microbial agent to achieve corrosion control. In this study, microbial agents, including Bacillus cereus, Pseudomonas, Bacillus subtilis, and Thiobacillus denitrificans, were selected to investigate the anti-corrosion effect and mechanism of the biological method. The results indicated that the anti-corrosion efficiency of the microbial agent on Q235B carbon steel ranged from 8.06–9.02%, while that for 316L stainless steel was 62.80%. The anti-corrosion mechanisms of functional bacteria were also revealed, which included (a) the formation of a biofilm to isolate oxygen and corrosive substances, (b) the inhibition of a cathodic reduction reaction and reduction of corrosion current, and (c) the inhibition of sulfide corrosion.
Water impactCorrosion control is vital for the safe operation of a circulating cooling water system. Biological treatment is a novel, efficient, and environmentally friendly method used for corrosion control achieved through adding a microbial agent. Our study provides a deeper understanding of the roles of functional bacteria, allowing for the advanced development and application of the microbial agent used for water treatment. |
Adding a chemical corrosion inhibitor is a common method for circulating cooling water treatment.3 Organic phosphonates, chromate, and molybdate are common corrosion inhibitors, which are usually used in conjunction with scale inhibitors. Chemical methods have good anti-corrosion and anti-scaling effects.4,5 However, the extensive use of chemical agents may cause problems, such as deterioration of water quality. Related studies have shown that the application of corrosion and scale inhibitors led to the total phosphorus content of circulating cooling water exceeding 3 mg L−1.6 This could increase the risk of the water quality exceeding effluent standards. Therefore, the development of an environmentally friendly method for corrosion control has become a research priority in circulating cooling water treatment.
The biological method is used for circulating cooling water treatment by means of adding a specific microbial agent and utilizing the synergistic effect of functional bacteria metabolism.7 Chen et al.8 used compound microbial preparation (CMP) with nitrobacteria, Bacillus subtilis (B. subtilis), photosynthetic bacteria, and Thiobacillus denitrificans (T. denitrificans, TDN), to treat circulating cooling water. The results showed that CMP had good corrosion and scale inhibition effect, and the corrosion inhibition rate on Q235 carbon steel reached 99.69%. Hu et al.9,10 evaluated the corrosion inhibition behavior of Bacillus cereus (B. cereus) on Q235 carbon steel in simulated cooling water, whose anti-corrosion efficiency was 73.88%. The results showed that the addition of B. cereus increased the charge transfer resistance of the metal and reduced the corrosion current density, allowing it to attach to the surface of Q235 carbon steel to form a tight biofilm.
Some research studied the anti-corrosion effect of microbial secretions. Li et al.11 studied soluble extracellular polymeric substances (s-EPS) secreted by B. cereus as a new biological agent with anti-corrosion and anti-scaling effects. The results showed that the corrosion rate of s-EPS (40 mg L−1) on 316L stainless steel in an artificial seawater medium reached 91.16% in 30 d. Jayaraman et al.12 studied the corrosion inhibition of aluminum 2024 using the formation of biofilms by B. subtilis WB600 and Bacillus licheniformis, respectively. The results indicated that the polyaspartate and γ-polyglutamate, which originated from biofilms, slowed down the pitting corrosion. The carboxylic acid group could be capable of chelating or coupling aluminum ions or aluminum oxides that were present at the metal–solution interface via hydrogen bonds, dipole–dipole, and coulombic interactions. Gunasekaran et al.13 found that the corrosion rate of carbon steel significantly decreased due to the formation of a surface biofilm in a phosphate-buffered basal salt solution containing Pseudomonas cichorii. The FTIR results proved that the biofilm protects the metal due to the ferrous ions and EPS that formed the Fe-EPS organo-metal complex layer. However, differences exist in the anti-corrosion effect of biofilm formed by microbial functional bacteria on different metals. The process of biofilm formation by functional bacteria and the interaction between biofilm and metal have not yet been clearly recognized. Therefore, it is important to clarify the anti-corrosion mechanism of functional bacteria for engineering the application of microbial agents.
In this paper, Q235B carbon steel and 316L stainless steel were selected to study the anti-corrosion effect of microbial agents and reveal the mechanism of functional bacteria. The corrosion rates of metal coupons with and without the microbial agent were compared by a rotary coupon test. A corrosion electrochemistry test monitored the attachment process of microorganisms onto the metal surface and calculated the anti-corrosion effect of the microbial agent. A biofilm observation test observed the formation process of the biofilm. Our study provides a deeper understanding of the anti-corrosion mechanism of functional bacteria, allowing for the advanced development and application of the microbial agent used for corrosion control in circulating cooling water.
(1) |
(2) |
3650 is the conversion factor; m0 and m1 are the weights of the coupon before and after corrosion (g); A is the surface area of the coupon (cm2); t is the corrosion time (d); ρ is the density of the material coupon (g cm−3); va1 and va0 are the average corrosion rate with and without the microbial agent (mm a−1), respectively.
The coupon with attachment after corrosion was dried under natural conditions. The surface attachment on the coupon was detected by energy dispersive spectroscopy (Aztec Energy EDS, Oxford Instruments) for the elemental component and analyzed by XPert Pro X-ray diffraction (XRD, Malvern Panalytical) and K-Alpha X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific) for the material component. The coupon was further pickled according to the Chinese Standard GB/T 18175-2014.15 The pickling of the coupon was conducted to remove the corrosion product and surface attachment. The coupons after corrosion and after pickling were observed by MIRA 3 field emission scanning electron microscopy (SEM, Tescan) for microscopic morphology.
The working electrode was installed in the beaker of the rotary hanging-piece corrosion device, and the working surface was immersed in solution. The circulating cooling water was used as a test solution. In one group, a liquid microbial agent was added with a concentration of 50 mL L−1 and carbohydrate daily with a concentration of 75 mg L−1. The other group did not add an agent. The solution was kept at a constant temperature of 36 °C for 30 d, and the rotation speed was set to 100 r min−1. The working electrode was removed at regular intervals for the electrochemistry test. The test solution was taken out from the beaker as a test electrolyte. All electrochemical tests were performed at the electrochemical workstation.
The OCP was the electrode potential measured when the metal was not carrying current. The EIS test was performed at a stable OCP and a sinusoidal signal of 10 mV was applied to the electrode. The frequency range was 0.1–105 Hz. The OCP and EIS tests were non-destructive, which allowed for the continuous monitoring of the influence of microorganisms on the electrode and the analysis of the corrosion behavior of the electrode. The potentiodynamic polarization method was scanned at the potential range of −0.5 V to +0.5 V (vs. OCP) with a scan rate of 1 mV s−1. Potentiodynamic polarization was a destructive test and only measured at the end of the test. The anti-corrosion efficiency (ηc) could be calculated according to corrosion current density (icorr), and the equation was as follows:
(3) |
The coupon-attached biofilm was removed from the beaker every 5 d, and soaked in a phosphate buffer solution containing 3% glutaraldehyde for 4 h to immobilize the biofilm. Then the coupon was dehydrated successively with an ethanol gradient: 25%, 50%, 75%, 90%, and 100% for 10 min.15,16 The coupon-attached biofilm was observed by SEM and optical surface profilometer (OSP). The organic component of the biofilm was analyzed by Fourier transform infrared spectroscopy (FTIR), where a diamond crystal was used as the reflection accessory.
Fig. 1 shows the appearance image and SEM image of Q235B and 316L coupons after corrosion and pickling. There was no significant difference between the surface of the Q235B coupon with and without microbial agent after corrosion. The Q235B coupon generated a corrosion product layer that covered the whole surface of the coupon. The scattered attachment appeared on the 316L coupon surface without the microbial agent after corrosion. This could be a particle impurity originating from the circulating cooling water, which remained on the metal surface. A layer of film appeared on the 316L coupon surface with microbial agent after corrosion because the functional bacteria could attach to the metal surface and form a biofilm layer. The surface of the Q235B coupon after pickling with and without microbial agent was uneven and had no metallic luster, but the surface had no obvious pit. The corrosion process of Q235B carbon steel with and without microbial agent in circulating cooling water was uniform corrosion, rather than pitting corrosion. The surface of the 316L coupon after pickling with and without microbial agent was flat and had metallic luster. The 316L stainless steel with and without microbial agent in circulating cooling water had no obvious corrosion behavior.
The XRD images of surface attachment on the Q235B and 316L coupons are shown in Fig. 2. The diffraction peaks of Fe3O4 magnetite crystal (PDF Card No. 99-0073) appeared in attachment of Q235B coupon with and without microbial agent, corresponding to 30.43°(220), 35.43°(311), 43.05°(400), 56.94°(511), and 62.52°(440), respectively. The results indicate that Q235B carbon steel underwent oxygen consumption corrosion and generated the corrosion products of Fe3O4. The diffraction peaks of Cr0.19Fe0.7Ni0.11 austenite crystal (PDF Card No. 33-0397) appeared in the attachment of 316L coupon with and without microbial agent, corresponding to 43.58°(111), 50.79°(200), and 74.68°(220), respectively. In fact, a Cr0.19Fe0.7Ni0.11 austenite crystal was the base of 316L stainless steel rather than an attachment.
The elemental component for the surface attachment of Q235B and 316L coupons by EDS analysis is shown in Table 1. Four elements of C, O, S, and Fe were present in the attachment of the Q235B coupon after corrosion. The C, O, and Fe were derived from the Fe3O4 corrosion product, which was confirmed by the XRD results. The element, S, was detected because Q235B carbon steel may have generated a small amount of FeS corrosion product. Sulfate-reducing bacteria (SRB) is a common corrosive microorganism in circulating cooling water. It could consume cathodic hydrogen through an enzyme called hydrogenase to generate H2S. The accumulation of corrosive H2S on metal leads to metal corrosion, which generates the FeS corrosion product.18,19 The growth environment of TDN of functional bacteria is similar to SRB. TDN could convert the corrosive H2S into non-corrosive SO42−, thereby reducing the corrosion of sulfide to metals.20 EDS results show that the content of S for Q235B attachment decreased after adding the microbial agent. This proves that the proportion of FeS in the corrosion product decreased and functional bacteria had the effect of inhibiting sulfide corrosion.
Element | Without microbial agent | With microbial agent | ||
---|---|---|---|---|
Quality percentage/% | Atomic percentage/% | Quality percentage/% | Atomic percentage/% | |
(a) Q235B coupon | ||||
C | 3.65 | 9.83 | 3.90 | 10.10 |
O | 23.65 | 47.87 | 26.12 | 50.83 |
S | 0.62 | 0.50 | 0.18 | 0.14 |
Fe | 72.08 | 41.80 | 69.81 | 38.93 |
(b) 316L coupon | ||||
C | 4.21 | 16.39 | 33.54 | 61.57 |
N | — | — | 0.57 | 0.84 |
O | 0.94 | 2.74 | 11.02 | 15.16 |
P | — | — | 1.22 | 0.84 |
Si | 0.43 | 0.72 | 0.22 | 0.16 |
S | 0.58 | 0.85 | 0.97 | 0.66 |
Cr | 17.05 | 15.34 | 9.63 | 4.08 |
Mn | 1.04 | 0.88 | 0.33 | 0.12 |
Fe | 66.2 | 55.46 | 37.16 | 14.61 |
Ni | 9.55 | 7.61 | 5.33 | 1.97 |
Eight elements, C, O, Si, S, Cr, Mn, Fe, and Ni, were present in the attachment of 316L coupon without microbial agent after corrosion, which was mainly derived from the 316L stainless steel base. After adding the microbial agent, the extra elements, N and P, were detected in the attachment. Furthermore, the content of C and O significantly increased. This may be due to the formation of biofilm on the surface of 316L stainless steel by functional bacteria. The main components are microbial flora and an extracellular polymeric substance (EPS) secreted by functional bacteria (B. cereus, Pseudomonas, and B. subtilis). An EPS is mainly composed of polysaccharides, proteins, and small amounts of phospholipids, humic acids, and nucleic acids.21 Therefore, the content proportion of C, N, O, and P is higher in the attachment of 316L coupon with microbial agent.
The material proportion of Fe for the surface attachment of Q235B coupon by XPS analysis is shown in Fig. 3. The characteristic peaks of Fe3O4 and FeS appeared at 710.8 eV and 713.6 eV, respectively.22–24 Therefore, the main components of the corrosion product for Q235B carbon steel are Fe3O4 and FeS. In addition, the proportion of FeS in the corrosion product with the microbial agent was 7.27%, which was lower than that without the microbial agent (16.64%). The functional bacteria could inhibit the production of FeS, which has an anti-corrosion effect on Q235B carbon steel. This is consistent with the conclusion of the EDS analysis.
Fig. 3 High resolution Fe 2p3/2 spectra for the attachment of Q235B coupon under different conditions. (a) Without microbial agent; (b) with microbial agent. |
Furthermore, the proportion of Fe3O4 in corrosion products was much higher than that of FeS. Therefore, oxygen consumption corrosion is the main type of corrosion for Q235B carbon steel in circulating cooling water, rather than sulfide corrosion. Although the functional bacteria have the capacity to inhibit the corrosion of sulfide, the anti-corrosion efficiency on Q235B carbon steel is low. The functional bacteria could not inhibit the occurrence of oxygen consumption corrosion.
Fig. 4 Change of OCP for different electrodes. (a) Q235B without microbial agent; (b) Q235B with microbial agent; (c) 316L without microbial agent; (d) 316L with microbial agent. |
There was a significant difference between the change of OCP for the 316L electrode with and without microbial agent. The OCP of the 316L electrode without microbial agent remained unchanged, while that with microbial agent was positively shifted. This positive shift of OCP indicates that the microorganisms have attached to the metal surface, and the shift value is positively related to the number of microorganisms attached to the electrode surface.25 The change of OCP for the 316L electrode is mainly caused by the attachment process of functional bacteria rather than the corrosion process. The OCP remained unchanged for 0–10 d, and the functional bacteria slowly attached to the electrode. After 10 d, the positive shift in OCP was obvious as the amount of functional bacteria increased and biofilm gradually formed on the electrode surface. For the 316L stainless steel electrode, the functional bacteria could attach to the metal surface and form biofilm.
Fig. 5 shows the Nyquist diagram of the Q235B electrode and the 316L electrode with and without microbial agent at different times. There was no significant difference between the Nyquist diagram of the Q235B electrode with and without microbial agent. The extra semicircle occurred in the high-frequency region of the 316L electrode after adding a microbial agent, which was caused by functional bacteria forming a biofilm on the electrode surface. The corresponding EIS data were fitted using the equivalent circuit, as shown in Fig. 5. The one-time constant was used to represent the process of oxide film formation on the electrode of Q235B without microbial agent, Q235B with microbial agent, and 316L without microbial agent. The two-time constant was used to represent the process of double film on the electrode of 316L with microbial agent. The outer layer was biofilm and the inner layer was oxide film. CPE was usually used instead of the capacitor element when the data deviated from ideal capacitance behavior due to the non-uniformity of the scale layer. In the equivalent circuit, Rs represents the solution resistance, Rct represents the charge transfer resistance, CPEdl represents the constant phase element of the metal/medium double layer, Rf represents the biofilm resistance, and CPEf represents the constant phase element of the biofilm layer.
According to the equivalent circuit, the impedance data were fitted in Zview software and the relevant parameters are shown in Table 2. The Rs value of the solution changed slightly and was in the range of 32.79–79.16 Ω cm2. The Rct value of the Q235B electrode with and without microbial agent shows a trend of increasing first before slightly decreasing because the oxide film of the corrosion product gradually forms on the Q235B electrode during the initial 0–15 d. As a consequence, the corrosion is inhibited. However, this oxide film is unstable, and its protective effect of metal gradually weakens with time.
Time/(d) | R s/(Ω cm2) | CPEdl/(F cm−2) | n dl | R ct/(Ω cm2) |
---|---|---|---|---|
(a) Q235B without microbial agent | ||||
5 | 32.79 | 4.22 × 10−6 | 0.99 | 70.95 |
10 | 34.50 | 1.67 × 10−6 | 0.99 | 161.98 |
15 | 39.02 | 1.58 × 10−6 | 0.99 | 195.98 |
20 | 38.26 | 1.72 × 10−6 | 0.98 | 194.55 |
25 | 38.68 | 1.56 × 10−6 | 0.99 | 175.46 |
30 | 35.52 | 1.62 × 10−6 | 0.99 | 164.09 |
(b) Q235B with microbial agent | ||||
5 | 60.87 | 1.37 × 10−6 | 0.99 | 207.93 |
10 | 76.83 | 8.85 × 10−7 | 0.97 | 379.28 |
15 | 69.42 | 9.67 × 10−7 | 0.97 | 357.58 |
20 | 67.29 | 1.01 × 10−6 | 0.97 | 343.03 |
25 | 64.53 | 1.02 × 10−6 | 0.97 | 333.40 |
30 | 66.01 | 9.62 × 10−7 | 0.98 | 331.15 |
(c) 316L without microbial agent | ||||
5 | 52.23 | 2.57 × 10−5 | 0.88 | 9.62 × 105 |
10 | 54.55 | 2.25 × 10−5 | 0.86 | 4.50 × 106 |
15 | 58.22 | 2.49 × 10−5 | 0.85 | 3.53 × 106 |
20 | 55.18 | 2.43 × 10−5 | 0.85 | 2.27 × 106 |
25 | 79.16 | 2.43 × 10−5 | 0.85 | 2.10 × 106 |
30 | 73.43 | 2.31 × 10−5 | 0.86 | 2.89 × 106 |
Time/(d) | R s/(Ω cm2) | CPEf/(F cm−2) | n f | R f/(Ω cm2) | CPEdl/(F cm−2) | n dl | R ct/(Ω cm2) |
---|---|---|---|---|---|---|---|
(d) 316L with microbial agent | |||||||
5 | 71.60 | 8.92 × 10−6 | 0.93 | 194.14 | 2.60 × 10−5 | 0.90 | 1.63 × 106 |
10 | 74.02 | 1.51 × 10−6 | 0.92 | 468.40 | 2.45 × 10−5 | 0.86 | 7.51 × 106 |
15 | 69.84 | 1.52 × 10−6 | 0.92 | 464.74 | 2.44 × 10−5 | 0.88 | 7.76 × 106 |
20 | 74.44 | 1.51 × 10−6 | 0.92 | 470.47 | 2.52 × 10−5 | 0.88 | 7.18 × 106 |
25 | 72.12 | 1.55 × 10−6 | 0.92 | 455.63 | 2.46 × 10−5 | 0.85 | 7.24 × 106 |
30 | 76.57 | 1.41 × 10−6 | 0.93 | 526.48 | 2.46 × 10−5 | 0.85 | 7.44 × 106 |
The Rct value of the 316L electrode without microbial agent increased before slightly decreasing, which was consistent with the changing trend of the Q235B electrode. For 316L, the electrode with microbial agent, the CPEf value decreased from 8.92 × 10−6 to 1.41 × 10−6 F cm−2, and the Rf value increased from 194.14 to 526.48 Ω cm2 after 30 d. The increase in the Rf value indicates that the integrity of the biofilm gradually increased. Furthermore, some studies have shown that the CPEf value is inversely proportional to the film thickness.15,18,26 Therefore, the functional bacteria could attach to the 316L electrode surface and form the biofilm, which is consistent with the result of the OCP change. The Rct value of the 316L electrode with microbial agent reached a maximum of 7.76 × 106 Ω cm2 at 15 d and thereafter remained unchanged. Therefore, the biofilm can maintain a long and stable protective effect on 316L stainless steel. In addition, the Rct value of the 316L electrode with microbial agent is higher than that without microbial agent, indicating that the formation of biofilm inhibits the charge transfer and diffusion process of oxygen molecules. The functional bacteria have an anti-corrosion effect on 316L stainless steel.
The biofilm formation of functional bacteria on the metal surface is one of the important factors for anti-corrosion. The biofilm could isolate the metal from oxygen molecules and inhibit the cathodic reduction reaction; thereby, the biofilm acts as a physical barrier layer that prevents the corrosive substances in the circulating cooling water from coming in contact with the metal and inhibits the metal from being damaged by corrosive substances.27–29 Moreover, the type of metals and microorganisms influence the biofilm formation process. The results show that biofilm could only be formed on 316L stainless steel but experienced difficulty forming on Q235B carbon steel. When Q235B carbon steel comes in contact with the circulating cooling water, oxygen consumption corrosion will rapidly occur and generate the corrosion products of Fe3O4. The process of biofilm formation is relatively slow. The surface of Q235B carbon steel is covered by corrosion products before the attachment of functional bacteria. Therefore, the functional bacteria could not inhibit oxygen consumption corrosion by biofilm formation, and the anti-corrosion efficiency on Q235B carbon steel is low.
The new Q235B and 316L electrode and the electrode after 30 d of test were used for potentiodynamic polarization. Fig. S1† shows the potentiodynamic polarization curve of different electrodes. The corrosion current density (icorr), corrosion potential (Ecorr), anode Tafel slope (βa), and cathode Tafel slope (βc) could be calculated by fitting the Tafel lines. The fitting parameters are shown in Table 3. The icorr value of Q235B electrode with and without microbial agent after 30 d were 5.70 × 10−5 and 6.20 × 10−5 A cm−2, respectively. The anti-corrosion efficiency of the microbial agent on Q235B carbon steel was 8.06%, close to the rotary coupon test. The microbial agent had an anti-corrosion effect on Q235B carbon steel. However, the icorr value of the electrode after 30 d was higher than that of a new electrode. Q235B carbon steel was corroded in the circulating cooling water with and without microbial agent.
Type | i corr/(A cm−2) | E corr/(V vs. SCE) | β a/(mV dec−1) | −βc/(mV dec−1) | |
---|---|---|---|---|---|
Q235B electrode | New electrode at 0 d | 7.21 × 10−6 | −0.63 | 182.31 | 578.95 |
Without microbial agent after 30 d | 6.20 × 10−5 | −0.50 | 376.29 | 329.18 | |
With microbial agent after 30 d | 5.70 × 10−5 | −0.42 | 405.98 | 318.29 | |
316L electrode | New electrode at 0 d | 1.70 × 10−7 | −0.09 | 848.04 | 134.81 |
Without microbial agent after 30 d | 1.89 × 10−7 | −0.08 | 1115.20 | 135.66 | |
With microbial agent after 30 d | 7.03 × 10−8 | 0.19 | 509.62 | 87.48 |
The icorr values of the 316L electrode with and without microbial agent after 30 d were 7.03 × 10−8 and 1.89 × 10−7 A cm−2, respectively. In addition, the icorr value of the 316L electrode after adding a microbial agent decreased compared to that with the new electrode. The microbial agent had an anti-corrosion effect on 316L stainless steel, and the anti-corrosion efficiency was 62.80%. This is because the formation of biofilm inhibits the diffusion process of oxygen molecules, resulting in a decrease in the limiting diffusion current density (iL) of oxygen. Since the corrosion is controlled by the diffusion process of oxygen, the icorr is equal to iL. The results show that the icorr value of the electrode with microbial agent is lower, which proves the above conclusion.
Fig. 6 SEM images of the coupon surface at different times. (a) 0 d; (b) 5 d; (c) 10 d; (d) 15 d; (e) 20 d; (f) 25 d; (g) 30 d; (h) 30 d-high magnification. |
The 3D topography images were obtained by using OSP to observe the biofilm formed on the coupon surface at different times, as shown in Fig. 7. The coupon surface at 20 d was smooth and had no obvious fluctuation. Some convex regions appeared on the coupon surface with time, which reflected the continuous attachment of functional bacteria. The convex regions gradually increased, and the surface height of the coupon gradually increased. The attached functional bacteria could form a biofilm layer and gradually cover the coupon surface. To quantitatively describe the thickness of biofilm, the height distribution in the observation region was statistically analyzed. Table S3† shows the surface roughness parameters of the coupon at different times, including the surface arithmetical average height (Sa) and the surface root-mean-square height (Sq). The results show that the Sa and Sq values increased in 0–20 d, the surface roughness of the coupon increased, and the thickness of the biofilm increased. The values of Sa and Sq remained unchanged after 20 d. At 20 d, the functional bacteria formed a dense and complete biofilm layer structure on the surface of the 316L coupon, which is consistent with the SEM results. It could be inferred from the roughness parameters that the thickness of the biofilm layer is about 1 μm. The dense biofilm layer formed by functional bacteria could effectively prevent the corrosive substances in the circulating cooling water from making contact with the metal, thus achieving an anti-corrosion effect.
Fig. 7 3D topography images of the coupon surface at different times. (a) 0 d; (b) 5 d; (c) 10 d; (d) 15 d; (e) 20 d; (f) 25 d; (g) 30 d. |
Fig. S2† shows the FTIR spectrum of biofilm on the coupon surface after 30 d. The results show that the characteristic absorption peaks of the N–H amino group and O–H hydroxyl group appeared at 3286 cm−1, CO bond appeared at 1644 cm−1, NH2 bond appeared at 1539 cm−1, the amine appeared at 1026 cm−1, and carboxylic acid group appeared at 1411 cm−1. These were typical characteristic peaks of a protein secondary structure.32–34 The vibration band of C–O appeared at 1261 and 1082 cm−1, which was typical of the characteristic peak of a polysaccharide.35 In addition, the characteristic absorption peaks caused by the C–H bond stretching vibration appeared at 2958 and 798 cm−1. The results indicate that the components of biofilm were protein and polysaccharide, which was EPS secreted by functional bacteria.
Fig. 8 Schematic diagram of the anti-corrosion mechanism for functional bacteria. (a) Biofilm formation; (b) corrosion current reduction; (c) sulfide corrosion inhibition. |
The positive shift in the OCP value and the increase of the Rf value prove that biofilm formed on the 316L electrode after adding a microbial agent. The Rct value of the 316L electrode with the microbial agent is higher than that without microbial agent. This indicates that the formation of biofilm isolates the oxygen molecules and inhibits the consumption corrosion of metal. The SEM images and 3D topography images illustrate that the functional bacteria could attach to the 316L coupon surface and secrete EPS to form biofilm. A dense and complete biofilm layer covers the 316L coupon surface after 20 d. The thickness of biofilm is about 1 μm and the main components of the biofilm are protein and polysaccharide.
Moreover, related studies have shown that not all EPS produced by microorganisms results in a protective effect on metal. Stadler et al.36 found that EPS produced by Desulfovibrio alaskensis had a corrosion inhibition effect on pure iron and carbon steel under anaerobic conditions; however, EPS produced by Desulfovibrio vulgaris and Desulfovibrio indonesiensis did not protect metals. Therefore, it is of great significance to select functional bacteria that could produce EPS that do protect metal. In our study, the B. cereus, Pseudomonas, and B. subtilis were used as functional bacteria, proving that they could form biofilm and produce an anti-corrosion effect on 316L stainless steel.
Suma et al.37 reported that the protective action of Pseudomonas putida RSS on a mild steel surface was enhanced by biofilm formation. The results indicate that no trace of corrosion was present even after 12 months of immersion, with a negligible corrosion rate of 3.01 × 10−2 mmpy. However, this property of biofilm producing/biopassivating did not appear in our study. The Q235B carbon steel is corroded and covered by corrosion products before the attachment of functional bacteria. The biofilm secreted by functional bacteria is difficult to form on Q235B carbon steel.
The results show that functional bacteria could form biofilm on the metal surface, thereby inhibiting corrosion. However, biofilm could only be formed on 316L stainless steel, and it had difficulty forming on Q235B carbon steel. The positive shift of the OCP value and the increase in the Rf value proved the formation of biofilm on the 316L electrode after adding a microbial agent. The SEM images and 3D topography images show that a dense and complete biofilm layer covered the 316L coupon surface after 20 d. The thickness of biofilm was about 1 μm and the main components of biofilm were protein and polysaccharide. The change in the Rct value of the 316L electrode after adding a microbial agent indicated that the formation of biofilm isolated the oxygen molecule and corrosive substances, which inhibited the metal corrosion.
The attached functional bacteria and biofilm on metal surfaces could consume oxygen molecules through respiration and inhibit the diffusion process of oxygen. The icorr value of the 316L electrode with microbial agent was lower, which proved that functional bacteria could inhibit the cathodic reduction reaction and reduce the corrosion current.
The TDN of functional bacteria could convert the corrosive sulfide produced by SRB into a non-corrosive sulfate, thereby inhibiting the corrosion of sulfide to metal. The analyses of EDS and XPS showed that the proportion of FeS corrosion products produced by Q235B carbon steel decreased after adding a microbial agent. Although the functional bacteria had the effect of sulfide corrosion inhibition, the anti-corrosion efficiency on Q235B carbon steel was low. The functional bacteria could not inhibit oxygen consumption corrosion because the biofilm experienced difficulty forming on Q235B carbon steel.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ew00629h |
This journal is © The Royal Society of Chemistry 2024 |