To What End? A Review of Recent Trends in Microbiologically Influenced Corrosion Research 1990 to 2023 Open Access

Microbiologically influenced corrosion (MIC) is typically defined as the modification of the rate of degradation of materials due to the presence and/or activity of microorganisms.^1,2  While in many cases an emphasis has been placed on microorganisms causing increases in the rates of corrosion, in some cases their presence can also lead to corrosion inhibition. A wide range of microorganisms, including bacteria, archaea, algae, and fungi, have been shown to affect corrosion rates,^3,4  with major associated financial and human/environmental safety problems observed across a wide range of industries and environments.

Some of the earliest reports on MIC can be traced back to the late 1800s and early 1900s. The discovery of the sulfate-reducing bacterium Spirillum desulfuricans (now known as Desulfovibrio desulfuricans) as reported by Beijerinck in 1894⁵  is often referred to as seminal in the origins of the field of MIC. While the first direct report of a study of bacteria influencing corrosion is usually linked to Gaines in 1910,⁶  there are even earlier reports^7-8  indicating that others were also working on this topic around this time. Another key early paper on MIC was that by Von Wolzogen Kuhr and van der Vlugt in 1934, which introduced the theory of cathodic depolarization, an early attempt to explain rapid corrosion associated with MIC. In the early to mid-1900s important relevant research was undertaken focusing on the physiology of microorganisms associated with MIC (Postgate, Starkey, Butlin, etc.). Work on MIC continued over the next decades, largely in the laboratory, with researchers such as Booth, King, Miller, Ashton, and Williams producing reports with a general focus on the study of pure strains of microorganisms (e.g., sulfate reducers). In the 1970s and 80s this work was continued by Little, Videla, Beech, Mansfield, Flemming (to name but a few), and many others, and there was a growing interest and finally recognition by industry that microorganisms can influence and cause serious material degradation problems.

One of the topics discussed in this paper is the philosophy behind publishing scientific research. While many researchers believe that the fundamental concepts of what is generally known as “science” and the “scientific method” are relatively straightforward, in reality, the history, development, and definitions of “science” and associated research are complex and evolving topics.^9-10  An example of a somewhat recent definition of science is “the pursuit and application of knowledge and understanding of the natural and social world following a systematic methodology based on evidence”.¹¹  But what does this mean in reality, and how is it translated in application? The interpretation can be different in different laboratories and influenced by social, cultural, and economic factors. One associated concept directly relevant to the current paper is the communication of scientific research. This process has, and continues to evolve, with research articles in scientific journals today having significant prominence and associated prestige. This is driven in recent times, partly, some might say largely, by the use of scientific publications in a range of modern quantitative performance indicators (e.g., h-index) in determining grants, promotions, and university rankings.^12-13  Hence, there is significant pressure on researchers nowadays to increase the number of scientific articles they produce, which can come at the expense of quality. This pressure is an issue that is common to researchers in most countries across the globe, especially those at academic institutions.

The potential impact and quality of a scientific article is often associated with the journal in which it is published, which in turn is ranked by a variety of measures, such as how many times articles in that journal are cited. The aforementioned performance indicators often take into account the ranking of the journal in which a researcher has published, and hence many scientists/engineers are incentivized to try and publish in higher-ranked journals. But what gets published in these journals? This is largely determined by a reviewing process, where theoretically independent experts from the field help to evaluate submissions to determine suitability. The term “theoretically” is used here deliberately, as in reality, independence is a relative term, and all reviews are inherently subject to varying levels of personal bias.¹⁴  This, for example, can include disagreements on the scientific methods that should be used in a study or the relative “novelty” of the work performed. This is all happening against the backdrop of a massive expansion in the number of scientific articles being published, which is resulting in greater demands on reviewers, and reviewers having less time to allocate to the reviewing task.¹⁵  Linked to the aforementioned discussion are increasing concerns of a “replication crisis” in science, potentially driven by the current focus on “novel” research.^16-17 

The current work is an analysis of journal articles published on the topic of MIC and a comparison of content over time. There have been a number of previous general reviews^1,18-19  and bibliometric studies^20-22  written on MIC, and while they can provide important qualitative and subjective information, they tend to lack specific information on the details and trends of experimental methods used. This paper takes a slightly different approach from the somewhat standard bibliometric studies and reports an examination of the content of scientific articles on the topic of MIC reported in five highly ranked journals over a recent approximately 3-y period (January 2021 to October 2023) and compares with similar data obtained for journals for both the 1990 to 1999 and 2000 to 2010 decades to show trends over time. Information on authorship, the specific aspects of MIC studied, test arrangements, and analysis methods was all examined.

The key aims of this paper are to investigate:

• changes in publishing volumes and the countries that authors publishing on MIC are affiliated,

• the topics of MIC research currently being studied/reported and how do they differ from historical research, and

• have there been any changes in experimental practices in MIC research over time?

Using the information obtained, the results will be discussed, and hopefully spark some debate, in relation to current publishing practices and the question of “to what end”—what is the point of the MIC research in the context of scientific publishing?

To obtain information on the number of scientific articles published on microbiologically influenced corrosion, a Scopus search was performed using any of the terms “microbially influenced corrosion,” “microbiologically influenced corrosion,” “biocorrosion,” “microbial corrosion,” “microbially induced corrosion” in the article abstract. A discussion about some of the limitations of these types of searching methods is provided in the Appendix. Yearly data was available for up to, and including, the year 2022. For the sake of comparison, a similar search was performed using the general term “corrosion”. There is a considerable difference in the absolute numbers of articles published on MIC compared to corrosion in general, hence, to help visualize the results, the absolute numbers of articles published vs. year (since 1990) were normalized by dividing by a quasiaveraged number of articles published in 1990. There is a reasonable fluctuation in the year-by-year number of articles published. To account for this, the normalizing value was determined using an exponential fit to the raw data and then calculating the value corresponding to the 1990 period.

Five scientific journals—Corrosion Science, npj Materials Degradation, Bioelectrochemistry, International Biodeterioration and Biodegradation, and Corrosion, were chosen to analyze the detailed content of MIC-related research articles. These journals were chosen on the basis that they have historically and/or recently published articles on MIC and are relatively highly ranked in terms of both impact factor and the subject categories in which they were grouped (see Table 1). The search function on the individual website of each journal was then used to find and download all articles that for the journal that contained the terms “microbiologically influenced corrosion” or “microbial corrosion” or “microbially induced corrosion”. Each of these articles was then manually scanned to find the information of interest. Details of the journals and number of articles studied for each of the periods of interest were provided in Table 2. A total of 229 articles were examined, with at least 60 articles in each of the periods of interest.

Figure 1 shows that the trends of the numbers of MIC and general corrosion scientific articles being published since 1990 are well characterized by exponential increases. Similar growth behavior of scientific publication numbers has been observed in previous research and found to follow an exponential trend.^23-24  Using the exponential fits to the data to smooth out yearly fluctuations shows that the number of publications on MIC has increased by a factor of twelve compared to an increase of a factor of seven for corrosion in general over the 1990 to 2022 period. In absolute numbers, over the 5 y from 1990 to 1994 approximately 24 journal articles on MIC were published per year, whereas over the five years from 2018 to 2022 the number is approximately 200 per year. It is worth noting that these numbers only relate to journal articles and do not include conference papers and book chapters, which contribute significantly to the literature output on MIC.

The results of the analysis of the number of authors listed on MIC journal articles over time are shown in Figure 2. This shows that the average and maximum number of authors per journal article more than doubled during the recent 3 y period compared to the 1990 to 1999 period.

There have also been some major changes over time in relation to which countries the authors of articles published on MIC in the target journals are affiliated (see Table 3). In the 1990s authors from institutions based in Europe and North America dominated the work reported, combined making up 82% of the papers published. There has been a massive shift, however, in recent times, with about 83% of the articles published during the recent analysis period (January 2021 to October 2023) having at least one author based at a Chinese institution. Approximately 64% of these recent articles were published with all authors linked to a Chinese institution. While North American-authored papers have maintained a reasonably consistent percentage of the articles, the numbers from European, South American, and Indian-based authors have dropped considerably.

The rise in research publications on MIC by authors from Chinese institutions follows several broader research and development trends as well as changes in economic status and research and development (R&D) spending.^25-27  For some context, in 2020, China made up about 18% of the world’s population, compared to the 4.2% contribution by the USA. During the period covered by the current review (1990 to 2023), the percentage of global GDP shared by China has increased from 1.75% to 16.9%. China has also reportedly increased the amount it spends on R&D by approximately 14.1%, between 2000 and 2020, compared to an increase of about 1.9% and 1.6% in the USA and Germany, respectively, over the same period. In 2020, the amount spent on R&D by China was approximately USD$563 billion, which is rapidly approaching the amount spent in the USA, which was USD$672 billion. Australia, on the other hand, makes up only 0.33% of the world’s population (2020), 1.7% of the world’s GDP (2020), but still managed to contribute about 8% of the publications on MIC in the journals studied in the latest period.

The trend of increasing numbers of research publications by Chinese authors is not just limited to MIC but is being seen across a range of areas of science, engineering, and medical research. According to the Nature Index, which tracks research contributions published in a selection of nearly 100 high-quality natural science journals, in 2015, authors from China made up about 7.5% of the articles compared to approximately 21% by authors from the USA. In 2022, however, this had changed with authors from China now making up about 19% of the articles compared to about 17.5% by authors from the USA.²⁸ 

Each of the articles studied was allocated a primary topic that described the key focus of the work presented (see Figure 3). It is not too surprising that research on fundamental mechanisms of MIC was highly ranked for each of the periods studied, as this type of research often falls within the highly “novel” scope/requirement that many highly ranked journals tend to require for articles. Along the same lines, articles reporting on field studies are often seen as being less “novel” by such journals, and this might explain the drop-off of papers being published in these specific journals. The number of reviews related to MIC published in these journals has also dropped off, possibly for similar reasons. In addition to work that has been allocated as “fundamental mechanisms,” the recent trend has been to publish work related to test media effects, environmental conditions, and new/novel materials.

To provide some more information about what areas of research are being published, the authors investigated the recent group of publications (January 2021 to October 2023) to determine which papers specifically focused on two topics that they speculated were “hot” topics, those being electron transfer mechanisms and carbon starvation.

In the context of MIC, the term electron transfer mechanism relates to a range of processes²⁹  which, unfortunately, have had a variety of different names assigned, creating some confusion. An early example of this research is the work on the relatively rapid corrosion observed with metallic iron acting as an electron donor for tests performed with the Desulfopila corrodens IS4 strain.³⁰  Analysis indicated that up to 19% of recent MIC articles in the journals studied were on this topic. Work is being reported on a range of aspects of electron transfer mechanisms, including various combinations of mediators, materials, and microorganisms. Given that there are a large number of combinations of mediators, materials, and microorganisms that can be studied, some caution possibly needs to be applied by researchers and reviewers to think about which combinations are appropriate/best to study. It should also be noted that while the topic of microbiological electron transfer has been around for quite some time,³¹  there is ongoing discussion and some controversy about the precise mechanisms involved and what role this plays in MIC observed in the field, which involves mixed-species microbial communities.³² 

Work on the effects of nutrient sources/test media on the metabolism of MIC-relevant microorganisms and also on the associated corrosion has been an active area of MIC research over many years.^33-36  Recently, the authors of the current paper noticed a general increase in related papers discussing carbon starvation effects on MIC. This often involves modifying the levels of citrate, lactate, and yeast extract in the test solutions. A general Scopus search using (“microbially influenced corrosion”, “microbiologically influenced corrosion”, “biocorrosion”, “microbial corrosion”, or “microbially induced corrosion”) and (“carbon starvation” or “carbon source starvation”) indicated that there were 104 articles in 2023 on this topic. The relative percentage of articles focusing on this topic for journals studied in the current work in the recent analysis period was about 5%.

As shown in Figure 4 sulfate-reducing prokaryotes (SRP) have consistently been one of the main types of microorganisms that have been studied and reported in MIC articles in the journals of interest over the different time ranges investigated in this work. The specificity of the identification of the sulfate-reducing prokaryotes used in tests has ranged from unspecified sulfate reducing bacteria isolated from the field through to specific culture collection strains, the latter becoming more common over time. The predominant genera of SRP studied have been Desulfovibrio desulfuricans and Desulfovibrio vulgaris, which have been reported at a similar rate. Pseudomonas sp. are the second-highest genera of microorganisms studied.

Another interesting trend shown in Figure 4 is that reports of MIC studies using a consortium of microorganisms have been dropping over time in the target journals. One possible explanation for this is that the fundamental studies often reported in these journals tend to use a single species of microorganisms to study specific phenomena. Traditionally, the vast majority of MIC research in the journals studied was performed using various species from the bacteria group, in many cases identified simply by a physiological trait (e.g., sulfate reduction). In recent times, identification of the microorganisms used in MIC studies is to the genus or species level, likely due to improved isolation techniques and increased access to relatively low-cost identification methods (i.e., DNA sequencing). Over time, there has been an increase in the number of articles that report MIC experiments using nontraditional/novel species of microorganisms. This includes an increase in MIC studies that use microorganisms other than bacteria, such as archaea, including methanogens. Along similar lines to a previous discussion, there are a huge variety of microorganisms that could be studied, but what is relevant, rather than simply being studied as they might meet the publishing threshold for novelty, needs to be questioned.

A summary of the types of material studied in MIC articles, in the target journals over time, is shown in Figure 5. The most common materials used were variations of carbon/mild steel, which were the focus of just over 50% of the articles in the most current period studied. Stainless steels were the next most common material investigated, followed by copper/copper alloys. A reasonable trend in recent times has been the study of modifications of alloy combinations, such as the addition of copper to carbon or stainless steels,³⁷  and articles on novel metals, such as high entropy alloys.³⁸  A key aim of these “new” materials is to find alloys that retain the key properties required for a target application (e.g., mechanical strength) while being less susceptible to MIC than the traditional alloys used for the application.

The duration of MIC tests reported in the target journals for different periods is summarized in Figure 6. For the two older periods (1990 to 1999 and 2000 to 2009) there was a relatively even spread of test durations, including shorter (less than 14 d), medium duration (15 d to 30 d), and longer test durations (>30 d). For the recent analysis period, the vast majority of papers (∼70%) were for tests that were 14 d or less in duration, and less than 10% involved test durations of more than 30 d. While some of the recent data can be attributed to initial (short-term) attachment studies, this makes up only a relatively small percentage of the tests performed/reported. This change in test duration appears to be one of the key shifts in recent MIC studies.

MIC involves the interplay of microorganisms, materials (e.g., metals), and media/environment. Hence, it is often recommended that the analysis and identification of MIC in both laboratory and field tests incorporate multiple lines of evidence (i.e., analysis methods) from these three different disciplines.^39,40  Figure 7 provides an overview of the types of analysis methods that have been used in MIC articles in the journals studied over time. There appear to be two key takeaways from the data obtained. First, the use of nearly all of the individual analysis techniques examined has grown over time. Second, the average number of analysis methods in an individual paper has risen from 3.3 (1990 to 1999), to 3.8 (2000 to 2009), and then to 7.4 in the most recent period studied. This means that authors are incorporating a lot more analysis techniques in combination now, and reporting these, when compared to the past. Some of the most common test methods used nowadays include scanning electron microscopy (SEM), pit depth analysis, electrochemical impedance spectroscopy (EIS), and potentiodynamic scans. Only about a third of papers provide information on planktonic or sessile microbial numbers, and there were no cases found of authors performing detailed tests to check for any microbiological contamination taking place during testing.

As discussed in the Authorship Information section there has been a massive increase in the number of papers published on MIC in recent times. This appears to be partly driven by increased output from researchers at Chinese institutions, which is likely related to additional spending on research and development in general in China. The overall increase in volume of publications, often exponential, is something that is happening in many areas of science and engineering, and not just MIC.^13,15,41  This is likely driven by pressures on researchers across the globe to meet target metrics, imposed by various institutions, governments, and funding bodies. The number of articles being produced and the underlying volume of associated effort raises a number of potential challenges and opportunities. One potential outcome is that, given the increased efforts, we might be able to solve some of the key challenges associated with understanding MIC (e.g., mechanisms involved) and be able to mitigate MIC faster (e.g., identifying when and where it might occur, developing prevention methods). But does this require some sort of coordinated effort, or is the current ad-hoc and somewhat competitive nature of current research going to provide the same outcome?

A significant challenge brought about by the increased number of MIC articles being produced is the ability of researchers to keep up with all of the MIC research that is reported. Gone are the days when an individual researcher can read in detail a reasonable proportion of the articles being published. This makes it increasingly difficult to keep up with the latest research and to be able to “see the forest for the trees”. This also causes problems with the reviewing process, with increasing reports of journal editors having difficulty in recruiting enough qualified reviewers to review all of the submissions. At the same time, researchers have limited time to perform reviews, so any increase in the number of papers they accept to review is likely to result in less time and detail spent on each article. The problems with current models and the future of scientific publishing are very hot topics of discussion^42-45  and it will be extremely interesting to see what, if any, changes come about to address the issues. The development of artificial intelligence tools to assist with these difficulties may provide a means of dealing with these challenges and again, it will be interesting to see how this progresses. Another proposed solution is the concept of open-ended publications, where a research study on a topic can have additional studies added on, rather than multiple individual papers being published on essentially the same topic.⁴¹ 

One of the key changes in MIC research identified in the current study was the recent reduction in the duration of tests that were reported in the target journals (down to 14 d or less for ∼70% of papers in the most recent period analyzed). A fundamental question related to this point is given that the microbiological/physiological, chemical and corrosion processes are typically time-dependent, is there a way of providing guidance on the recommended duration of MIC tests? How long, for example, does it take for a MIC-related biofilm to reach some sort of steady state? A blanket recommendation is highly unlikely as the exact test duration will no doubt depend on the specific nature of the research problem being tackled and the test configuration. The questions remain, however, about how well short tests match longer-term MIC in the natural environment, and can the relatively small pit depths reported in many short-term MIC experiments be extrapolated to what happens longer-term in the real world. This discussion point also touches on some fundamental questions about testing arrangements and how laboratory studies, often performed in a batch process, can simulate the real-world environment with the associated changes in nutrient supply. Why has there been a general trend of a reduction in test durations? An author of the current paper also wonders what a previous reviewer who commented that “42 days is a pretty short time to evaluate microbiological corrosion” would think about the current trend of short-term testing.

As noted in the Analysis Methods Used section there has been a substantial increase over time in the number of analysis methods used in individual research studies, more than doubling from the 1990 to 1999 period to the 2020 to 2023 period. In some ways, this is understandable as researchers have increased access to tools that can provide crucial insights into microbiological, metallurgical, and chemical processes taking place in their experiments. Including multiple lines of evidence in MIC research is increasingly acknowledged as best practice; however, journal articles often have limits of length and the number of graphs and tables that can be included. If more techniques are included in a paper, does this mean that there is enough space for an appropriate level of analysis of each of the methods performed? Or are the results being provided simply because the authors are under the impression that the article will not be accepted without specific test methods included, hence potentially creating formulaic research? While in some ways it is understandable that authors might want to hedge their bets to increase publication likelihood, it can be at the expense of papers becoming generic and containing unnecessary information.

An associated question relating to analysis methods used in MIC studies is whether all analysis techniques are created equal or strictly necessary for all MIC studies. For example, one of the authors of the current paper recalls past reviewers who separately stated that “MIC is defined as an electrochemical process, electrochemical data must be provided, which is still the most efficient and correct way to monitor MIC”, and “According to me no corrosion study is complete without the detailed analysis of the Tafel plots”. There is no doubt that electrochemical methods can provide important information about fundamental processes involved in MIC; however, there are limitations associated with electrochemical methods^1,46-48  that can be overlooked, and while useful information can be obtained, it does not mean that they are necessary by default. The limited analysis provided in some papers on the electrochemical tests used in MIC studies would suggest that electrochemical testing is sometimes included only on the basis that it is expected. A similar discussion could be held in relation to the “best” (if this can be defined) microbiological counting methods to use, which can be another bone of contention for some reviewers.

Like all good scientific articles, it is worth discussing the limitations of the current study. One limitation is that it only analyzed a subset of the journals that publish articles on MIC. Nowadays, there is an increasing number of journals that publish on MIC-relevant work. The specific journals analyzed in the current work tend to publish research that is deemed to be “novel” and more fundamental, laboratory-based in nature, which may bias some of the results obtained. However, we would argue that this grouping of journals represents a substantial portion of cutting-edge research on MIC and depicts the broader efforts underway in this field. The specific search terms used to find papers in the journals analyzed and to investigate the trends in volume of MIC papers being published (Trends in Volume of Microbiologically Influenced Corrosion Papers Being Published section) can also affect the outcome. More detail on this aspect is provided in the Appendix. This work also relied on manual extraction of key information from individual papers, which was time-intensive and, in some cases, required a certain level of personal interpretation. While there is potential for certain papers being missed from the analysis, the potential for manual errors, and potential debate about interpretation here and there, the authors are confident that the data obtained represent the key trends of MIC research. The rapid advances being made in artificial intelligence mean that it is likely that in the near future similar types of analysis will be able to be performed using such tools, however, it was decided that for the current research, the “old-school” manual version analysis was a reasonable approach.

The final discussion point relates to the broader question of “to what end” in relation to performing and reporting research on MIC. As discussed, the rapidly increasing number of journal articles on MIC is linked to numerous pressures on researchers to publish. But how can, or should, the field guide researchers to maximize the likelihood that work performed helps to advance the field rather than just being another metric box ticked? Previous reviews of MIC have provided some excellent discussions on some of the key problems and knowledge gaps existing in the field.^1,18-21,49  In relation to the work undertaken in the current paper, we have raised a range of additional questions, for example, in relation to the point that there are endless combinations of microorganisms, materials, and media conditions that could be studied in relation to MIC. But does this mean that every single one is a worthwhile topic for investigation? Or would it be helpful if there were some general guidelines or basis as to which types of combinations might be useful? And does a new combination of microorganism, material, and media in itself reach the threshold for the “novelty” criterion often thrown around for high-impact journals as a criterion to reject/accept papers? Other proposed solutions include the move to publishing open-ended papers⁴¹  and the recommendation that we should simply publish fewer and more higher-quality papers.⁵⁰  These are questions for the broad community of MIC researchers. But how do they get answered, and how might the community come together to develop some sort of coordinated effort to solve major MIC challenges? There are examples of international networks (e.g., the EuroMIC network, www.euro-mic.org) being developed to try and foster such communication and collaboration, and while they are having an impact, they are not currently truly global in representation. Another possible useful contribution would be the development of a set of priority questions to be posed to the MIC community, similar to the recent efforts published on biofilms.⁵¹ 

A detailed review of journal articles was performed to find quantitative information on trends in MIC research reported since 1990. The key takeaways from this study were:

• Like many other research fields, there has been an exponential increase in articles on MIC since 1990. This raises several challenges and opportunities for those working in this field, as well as industries that suffer from associated problems.

• Chinese-affiliated authors are currently leading the way in the number of papers published on MIC (83% in the most recent period analyzed), which is potentially linked to the significantly increased spending on R&D by China since 1990.

• Researchers have increased the number (>7 on average in a recent period studied) of experimental techniques used in combination in individual MIC studies.

• The duration of MIC experiments has reduced over time, with approximately 70% of papers having tests being performed for 14 d or less for in the most recent period analyzed.

• The broad community of MIC researchers is recommended to come together to help tackle the challenge of overpublishing and look at ways it can provide recommendations for coordinated efforts to solve major MIC challenges.

This work was inspired by conversations with numerous colleagues working on MIC, who all noted significant changes in the style and content of research articles on MIC over time, increasingly so in recent years. The authors would also like to acknowledge the COST (European Cooperation in Science and Technology) Action European MIC Network—New paths for science, sustainability and standards (Euro-MIC) [CA20130] for supporting numerous discussions that helped to shape the development of the paper.

As discussed in the main paper, the choice of specific search terms used can affect the outcome of a research database search. To show some examples of this, searches of the Scopus database were performed with a variety of search terms, and the number of identified papers was recorded (see Table A1). The results highlight how the various terms used to describe the microbiological degradation of materials can create some confusion. They also show the huge and increasing number of papers that are being published that refer to this topic. Searches of abstract text only were also performed, as sometimes the general topic of MIC might be discussed in a paper without it being the actual focus of the paper.

Figure A1 shows a cumulative graph of the durations of MIC tests for articles in the recent (January 2021 to October 2023) analysis period, with results for tests up to 40 d in duration being shown to highlight the large number of tests being performed with relatively short durations. The data shows that approximately 70% of papers published have test durations of less than 14 d.