The role of microbes in the inhibition of the atmospheric corrosion of steel caused by air pollutants

2.1 Ambient air quality and pollution

The ambient air is, by nature, the source and store of life-supporting constituent gases for human respiration and existence as well as the life-support for the flora and fauna. The quality of the ambient air is of utmost importance as it affects the health and wellbeing of living things and their environment.

Due to human activities like industrialization (Adeniran et al. 2019; Fakinle et al. 2021; Odekanle et al. 2021), burning of fuels and vehicular activities (Fakinle et al. 2018), the ambient air quality can be compromised by the introduction of certain substances (which may become harmful when present beyond certain allowable quantities). These substances, over time, accumulate in the air and lead to the “contamination” of the ambient air. A number of substances are implicated in this “contamination” and in the consequent lowering of the air quality. These include gases like sulphur dioxide (SO₂), nitrogen monoxide (NO), nitrogen dioxide (NO₂), carbon monoxide (CO), carbon dioxide (CO₂) and particulate matter (PM). The particulate matter ranges in sizes from soot, to pollen, human air to fine beach sand. In more professionally accurate terminology, the “contamination” of the ambient air is known as air pollution, and the harmful “contaminants” introduced into the air by human activities are known as air pollutants.

2.2 Impact of air pollution

Air pollution is an issue of global concern which has undeniable impact on the atmosphere. Its impact transcends the local ambient air and often has far-reaching consequences on the tropospheric chemistry of the lower atmosphere, which has negative effects on the global climate.

From an engineering point of view, the impact of air pollution can be viewed in the light of the global professional practice of HSE (health, safety and environment). Air pollution affects the environment negatively, has harmful impact on human health and can lead to devastating outcome on the safety of structures. This tripodal impact is considered in this section.

2.3 Atmospheric corrosion of steel in polluted atmospheres

In recent years, several authors have studied the atmospheric corrosion of steel in different industrial atmospheric environments with a view to aid the understanding of the underlying corrosion mechanism(s) or to explore a protective method against corrosion for the materials or to propose a prediction model. Table 2 gives a summary of recent works which studied the effect of different polluted atmospheres on the corrosion of steel materials and the different purposes why the studies were conducted.

3.1 Cathodic inhibitors

The cathodic corrosion inhibitors stop the metal from reacting cathodically during the corrosion process. These inhibitors contain metal ions that, in the presence of alkalinity, can trigger a cathodic reaction, resulting in the formation of insoluble compounds that precipitate only on cathodic sites. They create a tight, adherent coating over the metal to prevent the diffusion of reducible species there. As a result, the surface impedance and diffusion restriction of the reducible species—specifically, oxygen diffusion and electron conductivity in these regions—increase. High cathodic inhibition is caused by these inhibitors (Dariva and Galio 2014).

3.2 Anodic inhibitors

Anodic inhibitors, also known as passivation inhibitors, work by blocking the anode reaction and promoting the natural passivation of metal surfaces. This is accomplished by creating a coating that is adsorbed on the metal. In general, when inhibitors and corrosion products interact, a cohesive and insoluble layer is produced on the metal surface (Dariva and Galio 2014).

3.3 Organic inhibitors

The adsorption on the surface to form a protective coating that pushes water away from the metal surface and shields it from deterioration is the basis for the mechanism of action of organic corrosion inhibitors. It is not just physical or solely chemical adsorption in this process. The distribution of charge in the molecule, the nature and surface charge, the chemical structure of organic inhibitors, and the type of hostile media (pH and/or electrode voltage) all have an impact on adsorption. The charged metal surface and charged inhibitor molecule interact electrostatically to cause physical adsorption. Chemical adsorption is related to the donor-acceptor interactions between unoccupied, low energy metal d-orbitals and free electron pairs (Brycki et al. 2018).

The adsorption of organic corrosion inhibitors on metal surfaces is a complex process that depends on several factors. Organic corrosion inhibitors with appropriate functional groups can form chemical bonds with metal surfaces and provide effective protection against corrosion. The concentration of the inhibitor and the pH of the solution are important parameters that influence the adsorption process. Some recent works (Chugh et al. 2021; Murmu et al. 2019; Saha et al. 2022) provide more insight on adsorption of organic corrosion inhibitors on metal surfaces.

As a side comment and for completeness’ sake, it is worthy of mention to note that a recent trend in the use of organic inhibitors is the burgeoning use of green inhibitors utilizing plant extracts for corrosion control (Oyewole et al. 2021, 2022, 2023). These environmentally-friendly extracts have been copiously deployed for the inhibition of metal corrosion in different acidic media (Elabbasy et al. 2023; Iorhuna and Ayuba 2023; Nesane et al. 2023; Othman et al. 2023). However, there is little work reported on their deployment for atmospheric corrosion.

Cathodic, anodic, and mixed corrosion inhibitors of inorganic or organic origin are now widely used (Pedeferri and Ormellese 2018). However, due to ineffectiveness and stringent environmental laws, the majority of chemical corrosion inhibition methods are losing appeal (Suma et al. 2019); this is why biological inhibition approaches are receiving more attention.

4.1 Microbes and atmospheric corrosion

An early evidence of the involvement of microbes in atmospheric corrosion and rather a common one is the influence of airborne microorganisms which settle on exposed metallic surfaces and thrive on the favourable conditions of humidity, and other environmental nutrients favourable for their growth which are found in the air like chlorides, phosphates and sulphates (Jeffrey and Melchers 2011). While this may be a common occurrence, there is little information on how the microbes cause atmospheric corrosion of metals. Only recently, in the work of Victoria et al. (2021), was a comprehensive review carried out on the mechanism of microbiologically-induced corrosion. This work however contained little to no information on microbiologically-induced atmospheric corrosion. However, since most of the microbes involved in MIC of metallic corrosion in other media are similar to those involved in MIC in the atmospheric environment or air medium, the former’s mechanisms can be taken as a basis or starting point for the study of the latter’s mechanisms. The study of the mechanisms of microbiologically-induced atmospheric corrosion in polluted air medium is a problem calling for further attention and the intervention of corrosion investigators and inquirers.

4.2 Mechanisms of microbial inhibition of atmospheric corrosion

The mechanism of microbiologically-influenced corrosion inhibition (MICI) is presumably significantly more intricate than conventional corrosion inhibition techniques, whether in industrial or natural settings. Numerous microbial impacts are present in this heterogeneous process, including oxygen consumption, competition for electron donors, blocking of corrosive species, and corrosion inhibitor secretion.

Zuo (2007) suggested categorizing MICI mechanisms into three groups: (i) the removal of corrosive agents, (ii) the inhibition of the adhesion or growth of dangerous microorganisms, and (iii) the production of protective layers by biofilms. Similar opinions and theories have been put forth by others, which claim that microorganisms play two roles in corrosion and can, in some circumstances, be used as a green strategy to prevent or mitigate corrosion.

A deeper understanding of the functions of microorganisms in corrosion processes has led to some interesting new breakthroughs and opportunities in the field of MICI in recent years. In order to facilitate the advancement of MICI and its practical industrial applications, Lou et al. (2021) offered an updated overview of the mechanisms that have been employed or may be effective for MICI technologies. The authors of the review divided MICI mechanisms into five kinds, as depicted in Figure 3. Of these five MICI mechanisms, mineralization has been found to be the major process of inhibitive action of Pseudomonas and is thus of interest to this work. However, other mechanisms have also been reported for some species of Pseudomonas.

Figure 3:

Classification of MICI mechanisms (Lou et al. 2021). Reprinted from Lou et al. (2021), with permission from Elsevier (copyright 2021).

Numerous mineral deposition processes involve microorganisms, and microbiologically induced mineralization is essential to chemical cycling in the environment. Numerous studies have demonstrated that various types of mineralized layers to differing degrees lower the danger of metal corrosion. A number of mineralized layers have been observed in the mechanisms of MICI which include phosphate mineralized layer, iron oxide mineralized layer and carbonate mineralized layer (Lou et al. 2021).

Phosphate mineralized layer formation is the key mechanism observed for MICI by Pseudomonas. A phosphate conversion coating is created through the chemical and electrochemical reaction known as phosphating. Under natural circumstances, some bacteria develop a layer of phosphate mineralization on material surfaces, showing a similar result to chemical phosphate treatment. Volkland et al. (2000) discovered that mild steel incubated in media containing various microorganisms (Rhodococcus sp. C125 or P. putida mt2) might develop a protective vivianite (Fe₃(PO₄)₂.8H₂O) surface coating.

The other four MICI mechanisms mentioned by Lou et al. (2021) include: microbial respiration to consume corrosive substances, formation of an extracellular polymeric substance (EPS) protective layer, competitive microbial corrosion inhibition, and the secretion of corrosion inhibitors by bacteria. Besides the formation of microbiologically-induced mineralized layer, MICI can also occur as a result of these other four mechanisms.

In some cases, corrosion-inhibiting microbes operate through competition with other corrosion-causing microbes. For example, in the oil and gas industry, nitrate injection into oil fields creates a competition which helps to combat sulphate-reducing bacteria. This is because nitrate is an important source of nitrogen for microorganisms, as such; the introduction of nitrates into oil fields stimulates the growth of nitrate-reducing bacteria which compete with sulphate-reducing bacteria for the available nitrate. Hence, this makes the former a very potent inhibitor of the corrosion that would have been caused by sulphate-reducing bacteria.

Sometimes, when microbes inhibit corrosion, they secrete substances which act as inhibitors. For instance, some bacteria secrete biosurfactants which have been found to have better biodegradability, lower toxicity, and higher environmental compatibility than conventional chemical surfactants. Zin et al. (2018) studied the inhibitive effect of rhamnolipid biosurfactant secreted by Pseudomonas sp. PS-17 on the corrosion of Al–Cu–Mg aluminium alloy under artificial acid rain conditions. The authors found the biosurfactant to be effective in inhibiting the aluminium alloy corrosion under the synthetic acid rain conditions, with the inhibition efficiency increasing with biosurfactant concentration.

Another way microbes inhibit corrosion is through the use of dissolved oxygen for their respiration. Dissolved oxygen acts as a depolarizing agent and promotes corrosion in aqueous environments. Microorganisms that are aerobic or facultatively anaerobic can get energy through aerobic respiration. A low-oxygen or oxygen-free region is created when oxygen is consumed close to a metal surface, which prevents the cathode reaction and prevents corrosion. In Jayaraman et al. (1997c), the researchers’ findings showed that Pseudomonas fragi protection reduced steel corrosion losses by 10–50 % compared to those in the sterile media. The biofilm of the live bacteria provided a low-oxygen barrier that prevents the metal substrate from corroding or oxidizing.

Perhaps, the earliest known mechanism of MICI is the formation of protective biofilms containing extracellular polymeric substances (EPS). The cultivation of non-corrosive microbes to create a natural EPS protective layer on the material surface to prevent corrosion was the major focus of early work on MICI.

4.3 MICI by Pseudomonas bacteria species

Table 3 summarises previous works which had utilized Pseudomonas to inhibit corrosion of metals in different media ranging from aqueous, acidic, sea-water and seawater-mimicking media. However, there are limited (if any) literature evidence for the use of the microbe to inhibit corrosion in gaseous corrosive media, especially under polluted atmospheric conditions.

4.4 MICI by P. putida

P. putida is a Gram-negative bacterium that has a wide range of uses and applications as shown in Figure 4 (Volke et al. 2020). Besides its significance as biofilm-forming bacteria, it has been explored for industrial uses which include bioremediation, cell factory and biopolymer accumulation, among others.

Figure 4:

The significant scientific and industrial uses of P. putida (Volke et al. 2020). Reprinted from Volke et al. (2020), with permission from Elsevier (copyright 2020).

As a bioremediation agent, P. putida can be used to degrade hydrocarbons found in oil spills and thus has a major application in the oil and gas industry.

P. putida has also found application as a cell factory. Engineered microbes with biosynthetic pathways that are streamlined to manufacture desired compounds from renewable carbon sources are known as microbial cell factories (Cho et al. 2022). P. putida has demonstrated to be a superior bacterial host for the production of medicines, bulk chemicals, and polymers (Weimer et al. 2020).

In this work, P. putida would be investigated as a MICI agent owing to its promising performance in inhibiitng corrosion in recent works (Suma et al. 2019) by forming a protective biofilm on the surface of metals. This bacterium produces an extracellular polymeric substance (EPS) that can form a protective film on the surface of metals, preventing them from coming into contact with corrosive agents. The EPS produced by P. putida contains proteins, polysaccharides, and lipids, which can interact with metal surfaces and inhibit corrosion.

Several studies have investigated the potential of P. putida as a corrosion inhibitor, including the use of P. putida biofilms and EPS as protective coatings on metal surfaces. These studies have demonstrated the effectiveness of P. putida in inhibiting corrosion in various environments, including marine environments, oil and gas pipelines, and cooling water systems (Li et al. 2023).

The earliest work found in utilizing P. putida to inhibit corrosion in metals was that by Jayaraman et al. (1997b). Later in 2000, Volkland et al. incubated mild steel coupons with a medium containing the microbe and found out that the corrosion rate was reduced significantly. Recently, Suma et al. (2019) passivated mild steel with the biofilm of the microbe and observed that the corrosion rate of mild steel was decreased by 28-fold in comparison with the control.

4.5 The mechanism of MICI exhibited by P. putida

P. putida has been shown to inhibit MIC through several mechanisms. One of the most important mechanisms is the formation of a protective biofilm on the metallic surface. The biofilm acts as a physical barrier between the metal and the corrosive environment, thereby reducing the rate of corrosion.

P. putida also produces extracellular polymeric substances (EPS), which are polymers that form a matrix around the bacterial cells. The EPS can bind to the metallic surface and form a protective layer that prevents corrosion. In addition, the EPS can adsorb corrosive species, such as hydrogen sulfide and iron ions, reducing their availability for corrosion reactions.

Another mechanism by which the microbe inhibits corrosion is by consuming corrosive species. For example, it can oxidize iron ions, producing iron oxide, which is a less reactive species than iron ions. Similarly, it can oxidize hydrogen sulfide, producing sulfate, which is less corrosive than hydrogen sulfide. It can also induce the precipitation of metal sulfides, which are less corrosive than metallic surfaces. For example, P. putida can induce the precipitation of iron sulfide on steel surfaces, which protects the steel from corrosion.

Finally, P. putida can produce molecules that inhibit the growth of other microorganisms that are involved in corrosion. For example, it can produce antibiotics that kill or inhibit the growth of sulfate-reducing bacteria, which are known to cause MIC.

4.6 Application of P. putida for corrosion inhibition in outdoor structures

Corrosion-inhibiting microorganisms, like P. putida, are a promising approach to mitigating the effects of atmospheric corrosion on outdoor metallic structures. These microorganisms can produce a biofilm on the surface of the metal, which acts as a protective layer against corrosion-causing agents in the environment.

To apply corrosion-inhibiting microorganisms to outdoor metallic structures, a few steps can be taken. First, the metal surface must be cleaned thoroughly to remove any dirt, rust, or other contaminants that could interfere with the adhesion of the microorganisms. Then, a solution containing the microorganisms can be applied to the surface of the metal. This solution can be sprayed, brushed, or otherwise distributed across the surface.

Once the microorganisms are applied, they will begin to grow and produce a biofilm on the surface of the metal. The growth and activity of the microorganisms can be monitored over time to ensure that they are effectively inhibiting corrosion. It may also be necessary to periodically reapply the microorganisms to maintain their effectiveness.

Overall, the use of corrosion-inhibiting microorganisms shows great potential for protecting outdoor metallic structures from the effects of atmospheric corrosion. However, further research is needed to determine the optimal conditions for the application and maintenance of these microorganisms.

4.7 Climate effects on microbial inhibition of atmospheric corrosion

Atmospheric corrosion is a major challenge faced by industries that rely on metallic materials. Microbial activity has been found to be a contributing factor to atmospheric corrosion, as it can create corrosive environments by producing acidic metabolites. However, the relationship between microbial activity and corrosion is complex, and it is influenced by a variety of factors, including climate.

Climate can affect the rate and extent of microbial inhibition of atmospheric corrosion. For example, temperature and humidity can affect the growth and activity of microorganisms, with some microbial species thriving in warm and humid environments, while others preferring cooler and drier conditions. Additionally, rainfall can wash away microbial colonies from metallic surfaces, reducing their corrosive impact.

Understanding the impact of climate on microbial inhibition of atmospheric corrosion is crucial for developing effective corrosion mitigation strategies. By considering the environmental factors that influence microbial activity, it is possible to design materials and coatings that are resistant to microbial-induced corrosion. Therefore, research into this area is important for developing sustainable and long-lasting solutions to the problem of atmospheric corrosion.

During a rain event, the corrosion-inhibiting microbes on metallic structures will continue to function as they do under normal conditions. These microbes consume oxygen and create a protective layer on the surface of the metal, preventing the formation of rust and corrosion. During an extended dry period, the corrosion-inhibiting microbes may become less effective due to a lack of moisture. Without water, the microbes may become dormant and not consume enough oxygen to maintain their protective function. Additionally, the absence of rain may cause dust and debris to accumulate on the surface of the metal, which can create a barrier and prevent the microbes from accessing the metal.

To prevent these issues, it is important to ensure that the microbial treatment is properly maintained and that moisture is available to the microbes, even during dry periods. This may involve periodic watering of the metal surface or the use of a supplemental moisture source. Additionally, regular cleaning and maintenance of the surface can help to prevent the accumulation of debris and ensure that the microbes can access the metal.

6.1 Future outlook

The use of corrosion-inhibiting microbes like P. putida is still in the early stages of development. However, with advances in biotechnology, there is a growing interest in using these microbes for industrial applications. The potential benefits of using P. putida include reduced maintenance costs, improved equipment lifespan, and increased operational efficiency. As research in this field progresses, we may see an increasing adoption of these microbes in various industries.

6.2 Recommendations

Here are a few recommendations on the use of P. putida for industrial applications.

1. Conduct thorough research: Before using P. putida or any other microbial-based corrosion inhibitor, it is essential to conduct thorough research to understand its effectiveness, limitations, and potential risks.

2. Consider the application: Different industries may have different requirements for corrosion inhibition, and it is essential to consider the specific application when selecting a microbial-based inhibitor. For instance, the microbial-based inhibitor may work well in some applications but may not be effective in others.

3. Monitor performance: It is crucial to regularly monitor the performance of the microbial-based inhibitor to ensure that it is effective in preventing corrosion. This can involve testing the equipment or infrastructure for corrosion, as well as monitoring the microbial population.

4. Follow best practices: When using P. putida or any other microbial-based inhibitor, it is important to follow best practices for handling, storage, and application. This can help to ensure that the inhibitor is effective and safe to use.