A Brief Introduction to Microbial Corrosion in the Oil Industry

Corrosion is a thermodynamic process. This simply means corrosion is a natural process that can only be controlled and never be stopped completely.

Metals are to be extracted from their ores because metals in their ores have the lowest level of energy (lowest level of “restlessness”). After being submitted to mining engineering processes, the metal with a higher level of purity goes through extractive metallurgy processes. It is during these processes whene extra electrons are added into the metal.

This is the nature of all reducing processes. The added electrons seek ways to break loose and leave the metal. These electrons need to do so to lower the energy of the metal that has already increased by the addition of these extra electrons. The process by which these electrons free themselves from metals is called corrosion. Many factors such as, but not limited to, applied stresses or action of some bacteria can help accelerate or facilitate corrosion.

The main character of electrochemical corrosion is that, under industrial conditions, three elements are needed for it to happen:

1. An anode, from where the electron is to be emitted

2. The cathode, where the emitted electrons are absorbed

3. An electrolyte, which is a medium through which ions can move. Electrolyte is a water or water-bearing medium (for instance, the water cut in oil or gas fluid moving in a pipeline).

Microbiologically influenced corrosion (MIC), alternatively addressed as microbial corrosion and abbreviated as MIC, is electrochemical corrosion in which the impact of certain bacteria can contribute to enhancing corrosion or under many circumstances, even decelerate it.

MIC has been reported in oil and gas treating facilities such as refineries and gas fractionating plants, pipeline systems, and exporting terminals. MIC is one of the common failure modes for pipeline corrosion, and around 70% of pipeline internal leaks are reportedly due to localized corrosion mainly and involving MIC.

MIC From Another Viewpoint

Involvement of micro-/macro-organisms is the main feature of MIC processes. These organisms are divided as below:

1) Bacteria

2) Archaea

3) Fungi

4) Algae

These organisms can contribute to corrosion through various electrochemical corrosion mechanisms.

Archaea normally can grow and show activity at higher temperatures (above 600°C); bacteria such as sulfate-reducing bacteria (SRB) or iron-reducing bacteria or possibly Clostridia can contribute to corrosion via electrical MIC (EMIC).

EMIC, in simple terms, means that the bacteria releases a part of its body as a nanowire where they can take electrons from the metal directly. This makes the bacteria become the cathode and the metal the anode and in the case of SRB, corrosion rates around 36 mpy can be expected (as per standard NACE RP0775-2005 corrosion rates above 5 mpy are considered severe corrosion conditions).

Algae can be a potential problem on nonmetallic substrates such as concrete or composite.

Bacteria can survive extreme conditions, high temperatures, and pressures up to 1000 bars. They can be active in a pH range of 5 to 9.5. Studies on the soil samples taken from around the Chernobyl power plant has shown that the soil bacteria can demonstrate unusual biofilm formation behaviors that can have significant impacts on the role they play on corrosion. This must be a point of vital importance to nuclear power plants.

A lot of work is currently being carried out in the field of MIC. By considering the number of papers published on MIC each year, one can say that almost every 10 days, two papers with a research topic related to MIC is augmented to the MIC literature.

Methods To Control MIC

MIC is electrochemical corrosion in nature. Therefore, in almost all aspects of the methods by which electrochemical non-MIC corrosion is cured, MIC is also treated. To explain it in more detail, let us first list the ways by which electrochemical corrosion and not particularly MIC is treated.

Modifications with regards to MIC to each of these methods are mentioned in parentheses:

• Physical methods such as coating (coating with impregnated biocide, superhydrophobic coating and coatings with various electrical charges)
• Chemical methods such as use of corrosion inhibitors (biocides)
• Electrical methods such as cathodic protection (use of a 100 mV overpotential vs. Cu/CuSO₄ reference electrode)
• Mechanical methods such as use of pigs to destroy scales and sediments
• Material upgrade and design modification (removing the points in an asset where slowing down of the fluid may happen)
• Software measures (such as modeling of MIC processes)

In practice and based on the working conditions, a combination of the above can be used. It is possible to use a suitable coating to manage external corrosion in the form of MIC and at the same time use of appropriate combination of biocides (e.g., dual regime) to treat internal corrosion efficiently. The external corrosion can also be managed by combination of coating and application of cathodic protection where, for a pipeline, the normal applied -850 mV (vs. copper/copper sulphate reference electrode) will need to be more negative by a 100 mV to make it -950 mV. Even better, it may also be possible, again in a pipeline, to remove excess number of horizontal branches (to remove points where decelerating of the fluid may happen) and upgrade the material of construction from the susceptible carbon steel to more resistant classes of steels. In parallel to this, the operator can also use a corrosion prediction software to assess the present state of the asset and predict its probable behavior in the future.

While the above paragraph may be used to draw an ideal picture of MIC corrosion management, in practice not all of those factors are observed (particularly due to the imposed costs).

Where To Expect MIC More?

The rule of thumb to find out where to expect MIC in a given plant: Anywhere water exists as liquid, with suitable level of nutrient (particularly high total dissolved solids [TDS]), pH, and temperature to provide growth and activity of bacterial species. This water must also possess the required hydrodynamics (very low linear fluid flow) and particularly in contact with rough surfaces.

In a refinery, for instance, MIC risk can be expected with a high likelihood in:

• BOL of pipelines (irrespective of the material of construction)
• Cooling systems
• Underground fire water rings and systems
• Post-hydrotested pipelines (with electric resistant welding), particularly if the hydrotest medium is seawater
• Reverse osmosis systems, particularly on membranes

What makes all these seemingly different assets and equipment similar to each other in terms of MIC risk is the existence of MIC-facilitating factors. In a fire water line, stagnant water of chemically poor-treated nature can be leading into MIC. The conditions, despite vast differences that may exist, may be categorized as the same. For instance, MIC risk in a hydrotested pipeline that is subjected to wet lay-up can be likened to the situations that exist in a fire water ring: in both assets relative anaerobic conditions may exist so do growth conditions for bacterial species.

In fact, if we can identify the similarity between MIC growth and activity patterns despite equipment and assets being very different from each other, this can be leading into creating much more efficient methods to contain MIC in terms of the issues with which it is in challenge.

Problems Related to MIC

There are four domains in which MIC still needs to prove its answers toward the ever-increasing challenges:

• Definition and standards
• Identification
• Treatment
• Monitoring

1. Definition and Standards. With regards to definitions, let us consider the term biofilm. Biofilm is the heart of MIC literature, and it refers to a state where freely swimming bacteria (planktonic bacteria) enter into an irreversible phase to find food (nutrient) not in the bulk water phase but on the surfaces. One of the reasons is that when the fluid becomes very slow in its linear flow velocity (less than 1.5 m/s or 5 ft/s), the nutrient materials sink under the effect of their weight on surfaces. Therefore, the bacteria which used to find these nutrient in the bulk water before and cannot find them anymore but on the surfaces, change into a state where they land on these surfaces and are named as “motionless bacteria” or “sessile bacteria” to distinguish them from their previous planktonic state.

In addition to their state, sessile bacteria also differ from planktonic bacteria genetically; this means that the sessile bacteria will have features such as being able to feed on very little food and showing higher metabolic and growth when necessary.

These bacterial establishments on the surfaces by time form a net or trap to collect nonbiological objects. This establishment by time becomes thicker and after a while (hours to days) can form a diffusion barrier against the entrance of food and gases from the bulk. Therefore, overtime, the layer thus-formed will act as a loosely existing coating that based on several factors such as the hydrodynamics of the fluid may be damaged locally. Such physically damaged spots will create electrochemical cells such as differential aeration cells, resulting in localized corrosion and pitting.

The term biofilm, which was coined in late 70s, implies that such establishments are mainly made up of biological material and that they act like film. Neither of these features are correct: most of the fabric of biofilms are made from non-biological material and instead of having a film-like, sound nature, biofilms have fluffy natures that is far from being a layer.

That is why I suggested the alternative term “Temenos” in 2020, which is a Greek term meaning “cut off” to put emphasis on the fact that when this establishment is formed, conditions inside and outside of it could be vastly different from each other. Thus if the bulk water has a neutral pH or oxygenated, there is no need for the same conditions to exist under the “biofilm.” I believe temenos is a much better replacement for the term biofilm.

There are standards related to MIC similar to standards for other corrosion processes such as atmospheric corrosion or corrosion under insulation. The table below shows some of these standards:

Some examples of MIC standards

These examples are all very useful to a large extent but also suffer from some serious drawbacks. For instance, most of them are too wordy and inclined more toward microbiology than engineering. There are many other defects this author has expressed. Much more can be said about definitions used in MIC literature as well as the related standards and sometimes misleading impacts that they can have on users, but we will stop here.

2. Identification. It is very important to recognize and identify MIC because that can have significant impact of both the costs involved and the severity of consequences to be expected. Below are some of the issues related to identification of MIC:

1. Equating pit morphology with mechanism—it is still believed that observing certain shapes of pits can be the cardinal evidence to convince us that the involved mechanism is MIC. Nowadays, it is believed that relying on pit morphology alone as a guide to estimate the corrosion mechanism being MIC could be highly misleading. It is necessary to collect more evidence and pit morphology can be secondary evidence that in combination with other evidence need to be used to make sure if the corrosion mechanism is MIC-related.
2. Safe number of bacteria—it is still the holy grail of many operators to find out if there is a safe number of bacteria that can be taken as the basis for which no particular treatment or intervention will be necessary. There is an extra issue here and it is that in many routine tests for MIC risk assessment it is the number of planktonic bacteria (expressed as cells/ml) and not the sessile bacteria (expressed as cells/cm²) that is taken as the main measure for evaluating of MIC severity. The safe number delusion states that there is a safe threshold for both planktonic and sessile bacteria numbers. This number does not exist despite that some operators may have certain guides for their activities in this regard. But even those guides must be met with care, and they cannot be regarded as universal measures that can be used in every refinery or in every industry.
3. In no standards of MIC, significance of the morphology of the bacteria involved is taken into consideration. Certain morphologies such as being a bacillus will render the bacteria more resistant to chemical treatment (application of biocide).

There are many more issues related to identification of MIC that in addition to intrinsic shortcomings of available methods (culture-dependent methods such as MPN) or culture-independent methods (such as q PCR or DGGE) increase the margin of error.

3. Treatment. This can be categorized in four groups: Application of coating types, administration of biocide types and the regime of application, cathodic protection as well as other measures. All these methods have their pros and cons, but the point is that to get the best results, it is often needed that these measures are used in parallel. Due to factors such as cost, normally not all the measures are applied and even if they applied, most of the time the application may not be carried out in the way it is meant to. For instance, some biocides must not be used with some corrosion inhibitors (such as use of chlorine as a biocide with phosphate as a corrosion inhibitor: the latter feeds bacteria and the former is to kill the bacteria). Use of tools such as intelligent pigs, if not applied sensibly and with required precaution, may not only aggravate the existing MIC problem but also facilitate other corrosion processes such as stress corrosion cracking.

4. Monitoring. This is perhaps the most important measure to contain MIC. There is no clear guidelines to allow efficient monitoring. Perhaps the best way to monitor MIC is to arrange a monitoring schedule based on a suitable set of parameters and corrosion management model(s). Both planktonic and sessile bacteria species must be assessed within certain time intervals. Before that, it is necessary to determine what bacteria (and based on the working temperature ranges, archaea) groups must be taken as the guide. Fluctuations in the numbers of planktonic and sessile bacteria coupled with TDS values can give a better idea about the likelihood of MIC. Many other factors as such must be taken into consideration. In this respect, having an MIC model that has the capability of being quantified can greatly help. The most important factor here is to find out when Temenos formation is to take a serious shape so that it will interact with the normal process for which the asset has been designed (for example, drop of thermal efficiency in a heat exchanger).

Summary

MIC is electrochemical corrosion in which micro-organisms have contributions. Such contributions could be in the form of accelerating corrosion or even slowing it down. Issues with MIC starts from its very central concepts (such as the wrong term of biofilm) to existing shortcomings in the related standards and issues related to identification, treatment, and monitoring. In this short review, in addition to emphasizing the significance of MIC for oil industry, topics such as where to expect MIC and problems related to identification, treatment, and monitoring of MIC were briefly addressed.