Microbiologically influenced corrosion in uncoated and coated Mild Steel

The corrosion resistance of coatings.

Immersion cycle test in E -Coli medium

Corrosion medium containing E. Coli bacteria was used to immerse Zn-Ni-Cu and Zn-Ni-Cu-TiB2 coated samples as well as uncoated ASTM A-36 Steel (control sample) in order to evaluate the efficacy of corrosion-resistant coatings. Six sample specimens in all were utilized for the test; the second specimen of each sample was used to gain a deeper understanding of the data and lower the likelihood of error. 101.6 mm × 63.5 mm × 2 mm was the specimen’s size as shown in Fig. 1(a) and Fig. 2(a), 2(c), 2(d), 2(e), 2(f). The average value of the outcome is found in the final readings of every sample. Eight.6 g/l MgCl₂:2 H₂O, 1.2 g/l CaCl₂ :2 H₂O, 0.01 g/l H3BO₃, 0.01 SrCl₂, and 0.5 G/L Na₂ SO₄ were added to 250 ML beakers containing 3.5% NaCl to inoculate the standard-incubated E Coli Bacteria. Following this, around Each 250 ml beaker was filled with 70 ml of this bacterial solution, the samples were added at the same time, and the entire setup was maintained under the same circumstances for 28 days (672 h). Every beaker containing specimens had the same amount of bacterial solution. The samples were taken out and cleaned with distilled water every seven days (168 h), dried in an oven, weighed, and then returned to the solution. A measuring balance (LIBROR –AEG − 120) was used to determine each sample’s weight with an accuracy of 0.000001 g. The samples were washed with distilled water, cooked in an oven for 28 days (672 h), then promptly weighed once again using the identical weighing scale.

Shows the mass loss of uncoated/coated samples in the E Coli medium for 28 days (672 h).

When compared to the uncoated ASTM A-36 Steel sample, it was discovered that both coated samples (Zn-Ni-Cu and Zn-Ni-Cu-TiB2) had minimal mass loss, suggesting that coatings had effectively stopped microbial corrosion attack. On the surface, the ASTM A-36 Steel without coating had a significant level of bacterial adhesion. On uncoated ASTM A-36 Steel samples, bacterial colonies had flourished, most likely due to Fe on the surface. Following the immersion cycle test, Fig. 2(b) displays the bacterial adhesion with uncoated ASTM A-36 Steel. Compared to uncoated samples, coated samples exhibited resistance against microbiological deterioration. Table 2 presents the data collected during the immersion cycle test.

The Zn-Ni-Cu and Zn-Ni-Cu-TiB2 coated specimens exhibited minimal surface bacterial adhesion. Cu, a component of coatings that has been shown to kill bacteria when it comes into touch with its surface, was present in the coated samples¹⁸. According to reports, Cu2O/CuO generated during immersion prevents bacteria from adhering to the substrate surface, lowering their density and, consequently, the rate of corrosion¹⁹. According to reports, bacteria are killed by ZnO, a byproduct of zinc corrosion action, which adheres to the surface and releases reactive nascent oxygen²⁰. Using a pH meter (P-100 Cole Parmer), the pH of the E Coli solution was measured at the beginning and end of the experiment. and it was found that the pH was 7.2 at the beginning of the immersion test and decreased to 5.8 at the end of the experiment. The pH dropped as a result of the release of H + ions into the mixture. Tejero et al. (2019) have observed comparable findings regarding pH lowering.

This illustrates the corrosion mechanism on an uncoated mild surface in an E. Coli culture containing 3.5% NaCl.

Here Fe²⁺ reacts with OH⁻ as

The presence of O₂ from atmospheric air help Fe(OH)₂ oxidizes to Fe(OH)₃ by the following reaction

This is insoluble and reacts with O₂ causing the further reaction

The reduction in pH of the solution is because of the following reaction

Here (α-FeOOH) is a rust form known as goethite, and Fe₂O₃ is maghemite, Lepidocrocite (γ-Fe³⁺O(OH), the chemical formula for both goethite and lepidocrocite is the same but different structure. These forms of rust are stable and produced when ASTM A-36 Steel is immersed in a 3.5% NaCl- E coli medium. The presence of goethite, lepidocrocite, and maghemite is ascertained in XRD analysis for both coated and uncoated ASTM A-36 Steel in Sect. 3.1.2. Increased H⁺ ion concentration in solution leading to decrease in pH as noted by pH meter is because of Eq. 6 taking place in solution medium. Fe²⁺ transformation into Fe³⁺ results in the formation of the soluble layer which is easily removed. Fe²⁺ transformation into Fe³⁺is facilitated by microbes²⁰. Bacterial attachment with ASTM A-36 Steel has been reported before as well²¹.

Whereas the following reactions take place in coated samples

This ZnOH₂ formed has been reported to kill bacterial cells[Zare et al., 2018] as depicted in Fig. 2(d, f).

This prevents the removal of Fe²⁺ ions to escape from the surface of steel hence reducing the rate of corrosion (cathodic protection). Some other reactions reduce oxygen areas

Given that the HVOF thermal spray technique contains certain pores. Certain pores serve as anodic sites, whereas sites next to them serve as cathodes and are the sites where a redox reaction occurs. These pores allow Cl- and OH-ions to pass through the coating into the substrate and initiate pitting corrosion. The efficacy of HVOF coatings is somewhat decreased by the presence of pores.

The extracellular electron must enter the cell from the outside because the oxidation of the electron donor occurs outside the cell while the reduction of the electron acceptor occurs inside. Extracellular electron transfer (EET) is the term for this electron movement across the cell wall, and the general process underlying the related metal corrosion. TIB₂ And Cu prevent electron transfer thereby reducing corrosion.

By stopping soluble corrosion products from permeating into the solution’s bulk, the copper layer aids in the formation of a protective layer on the surface. By stopping soluble corrosion products from permeating into the solution’s bulk, the copper layer aids in the formation of a protective layer on the surface.

Zinc’s Antimicrobial Properties: The element zinc itself possesses fungicidal and bactericidal properties. Because zinc ions interfere with cellular functions like protein synthesis and produce toxic microenvironments around the microbial cells, they can interfere with the growth and activity of some microbes. Because of this antimicrobial action, bacteria are less able to colonize the surface, which limits the creation of biofilms and the corrosive consequences that go along with them.

The TiB2 phase’s smoothness and hardness make it harder for microbes to stick to the surface, which further inhibits the formation of biofilms. Limiting the growth of biofilms, which are the main source of MIC, can significantly lower corrosion rates. Additionally, TiB₂ possesses characteristics that may impair the production of biofilms by interfering with microbial processes.

The TiB₂ phase may produce microenvironments that are less favorable for the growth of microorganisms. For example, TiB2’s hardness and inertness can inhibit the growth of localized corrosion cells, lowering the risk of pitting or crevice corrosion—both of which are frequently made worse by microbial activity.

Similarly, from the above data corrosion rate of every specimen was calculated using the Eq. 

Where W = mass loss of specimen in mg.

A = Area of the specimen (Exposed area To Corrosion Solution) in cm².(64 cm²).

T = Time of Exposure in Hrs. (672)

ρ = Density of material in gm/cm³ (7.85)

Using Eqs. 1 and 2 we calculated the corrosion rate and coating of each specimen, which are given in Table 2.

It is important to note that the mass loss in uncoated ASTM A-36 steel is greater each week compared to the combined mass loss in Zn-Ni-Cu on ASTM A-36 steel and Zn-Ni-Cu-TiB2 on ASTM A-36 steel. Figure 1 displays mass loss and Fig. 2 shows optical micrographs before and after testing.

Shows the micrographs of specimens before and after the immersion cycle test. (a)uncoated ASTM A-36 Steel before the test, (b) uncoated ASTM A-36 Steel after the test, (c) Zn-Ni-Cu on ASTM A-36 Steel before the test, (d) Zn-Ni-Cu on ASTM A-36 Steel after the test, (e) Zn-Ni-Cu-TiB₂ before the test, (f) Zn-Ni-Cu-TiB₂ after the test.

Characterization of uncoated/coated samples post immersion testin E coli bacterial medium

To undertake a more detailed surface analysis of coating corrosion mechanisms. FESEM (Gemini SEM 500) was used to assess samples both before and after the immersion cycle test. The surface morphology of the uncoated ASTM A-36 Steel sample altered considerably, indicating that the corrosion resistance was inadequate. In contrast, corrosion resistance to Cl- ions in NaCl and E Coli solution was discovered in Zn-Ni-Cu on ASTM A-36 Steel and Zn-Ni-Cu-TiB2 on ASTM A-36 Steel. Providing a masking effect on ASTM A-36 steel surfaces, which considerably slows corrosion propagation. Coated surfaces in particular exhibit some localized corrosion.

This is what accounts for the HVOF coating’s porous nature. Figure 3 depicts the FESEM micrographs obtained before and after the immersion cycle test. Figure 4 (a), (b) depicts the spread of isolated bacterial colonies on an uncoated mild surface following an immersion cycle test. Rust precipitate found on the surface of uncoated ASTM A-36 Steel contains Fe3 + ions that react spontaneously with the metal substrate, oxidizing it. Section 3.1 details the chemical process in question. Bacterial colonies ranging in size from 2.4 mm have been found on the surface of uncoated ASTM A-36 Steel, indicating the severity of the corrosion attack.

Depicts SEM micrographs of specimens before and after an immersion cycle test. (a) uncoated ASTM A-36 steel before the test; (b) uncoated ASTM A-36 steel after the test; (c) Zn-Ni-Cu on ASTM A-36 steel before the test; (d) Zn-Ni-Cu on ASTM A-36 steel after the test; (e) Zn-Ni-Cu-TiB2 before the test; (f) Zn-Ni-Cu-TiB2 after the test.

(a, b) Shows a higher magnification of uncoated ASTM A-36 Steel specimen following the immersion cycle test.

Figure 4(b) shows the bacterial penetration and colonies formed on uncoated ASTM A-36 Steel. Similar findings have been reported by²². E Colibacterial attachment with rough surfaces has been reported to cause severe corrosion²³. Cu acting as virucidal and killing bacteria cells is a well-known phenomenon now²⁴. The FESEM micrographs manifest the changes E Coli bacteria have made into the interface of both uncoated and coated ASTM A-36 steel.FESEM images of both coatings show the mechanism of corrosion prevention is by barrier effect. wherein the passage of Cl⁻¹ ions across the interface is prevented by constituents of both coatings.

XRD analysis

For a better understanding of the intensity of corrosion on the surface of uncoated/coated ASTM A-36 Steel after the immersion cycle test (672 h,28 days). XRD analysis was undertaken using (Rigaku, a smart lab) having a beta filter at a scanning rate of 2.000 deg/min, and a scanning angle of 20⁰ to 90⁰. Maximum diffracted peaks were observed in the uncoated mild specimen. Where lepidocrocite, magnetite, and goethite (all forms of rust) were observed in high intensity. Whereas in Zn-Ni-Cu and Zn-Ni-Cu-TiB₂ coated ASTM A-36 Steel little forms of rust in lower intensity were observed. Figure 5 (a, b,c) shows XRD analysis before and after each sample. Here L = Lepidrocite, G = Geothite, M = Maghemite.

Shows the (a) uncoated ASTM A-36 Steel before and after the test.(b) Zn-Ni-Cu coated ASTM A-36 Steel before and after the test.(c) Zn-Ni-Cu-TiB₂ coated ASTM A-36 Steel before and after the test.

Bacterial attachment

After 24 h of immersion in medium, E Coli adhesion to the substrate was tested, and it was discovered that bacterial proliferation and growth in uncoated ASTM A-36 Steel was significantly higher than in Zn-Ni-Cu and Zn-Ni-Cu-TiB2 coated ASTM A-36 Steel. After 24 h, the samples were withdrawn from beakers, cleaned with distilled water, and studied using an optical microscope Leica (MODEL DM 6000 M paired with Image J software for fluorescence of bacterial cells).

Figure 6 displays the optical micrographs. Bacterial cells have successfully attached themselves to the uncoated mild surface, and their development is broadly diffused across the entire surface, resulting in the formation of bacterial colonies, which then create polymers required for gene replication and sustenance. However, no such bacterial colonies can be seen in any coated specimen. Small individual cells can be observed clinging to the surface and linked.

Shows the optical micrograph of uncoated/coated specimens after 24 h in an E Coli medium.