Microbial corrosion does not refer to a single type of corrosion but instead describes a process that encourages and accelerates other forms of corrosion. 

The factors involved in the propagation and proliferation of microbial corrosion are common across a range of industries in freshwater, saltwater, and other environments that might encourage microbial growth.

In this guide, we’ll dive into how microbial corrosion works, outline common microbes and bacteria and their impact on corrosion, and highlight how microbial corrosion impacts various industries.

What is Microbial Corrosion?

Microbial corrosion is the general process in which the presence of biological organisms or microorganisms brings about corrosion. 

You might also find microbial corrosion referred to as:

  • Bacterial corrosion
  • Biocorrosion
  • Microbiologically influenced corrosion (MIC)
  • Microbially induced corrosion (MIC)

While not a single form of corrosion itself, microbial corrosion can accelerate a number of other corrosion processes, including:

Microbial corrosion can occur in various environments, including seawater, freshwater, soils, foodstuffs, demineralized water, sewage, aircraft petrol, human plasma, and process chemicals. 

This means it’s a potential concern for a vast range of industries.

What Causes Microbially Induced Corrosion?

Microbially induced corrosion requires two elements to be present:

  • Microbes which can initiate the attack
  • An environment conducive to the growth and spread of said microbes.

Exact microbes — and the resulting damage they cause — vary. However, bacteria are often a common source of microbial corrosion.

Most bacteria — both aerobic and anaerobic — fit into one of four categories:

  • Sulphate-reducing bacteria: Includes Desulfovibrio and Desulfotomaculum
  • Slime-forming bacteria: Includes Pseudomonadaceae, Enterobacteriaceae, Micrococcaceae, and Bacillaceae
  • Iron-oxidizing bacteria: Includes Siderocapsa, Gallionella, Sphaerotilus, sheathed iron-oxidizing bacteria, and Hyphomicrobium
  • Sulphur-oxidizing bacteria: Includes Thiobacillus, Beggiatoa, Chlorobium, and Chromatium

Each group of bacteria will exhibit different responses and encourage or accelerate different forms of corrosion. 

The table below outlines some of the common bacteria types and the actions through which they initiate corrosion.

BacteriaActionExhibited Responses
GallionellaConverts soluble ferrous ions to insoluble ferric ionsIncreased corrosion due to the production of iron oxide deposits
Crenothrix
Spaerotilus
Desulfovibrio
Clostridium
Thiobacillus
Hydrogen sulphide producersCorrodes metals, reduces chromates, destroys chlorine, and precipitates zinc
Thiobacillus
Nitrobacter
Sulphuric acid producers or nitric acid producersCorrodes metal

Regardless of the bacteria, environment, or materials involved, microbial corrosion progresses across three phases:

  • Microbe attachment
  • Growth of initial pit and tubercle
  • Maturation of tubercle and pit

In most cases, this growth and progression will appear in localized patches as microbes tend to settle and colonize in distinct regions or patches. 

These then form tubercles which help to further encourage concentration and protect the corrosive region from treatment and removal. 

While corrosion could spread to cover an entire surface given enough time, symptoms are often detected — or failures created — before reaching this point.

Biofilms: A Powerful Microbiologically Influenced Corrosion Force

In many cases, microbial corrosion involves different bacteria supporting one another. 

This can make the prevention and combat of microbial corrosion difficult, with early detection and treatment comprising a large part of the risk and damage mitigation process.

The formation of biofilms during biocorrosion is an excellent example of multiple bacteria combining to accelerate corrosion. 

Consider the impact of a mixed microbial slime consisting of multiple layers. 

With an outer layer consisting of bacteria with high oxygen demands, the inner layers begin to collect and concentrate more anaerobic bacteria — such as sulphate-reducing bacteria.

In turn, this leads to the creation of hydrogen sulphide, further accelerating corrosion progression.  

Biofilm formation can result in drastic changes to the ion concentrations, pH values, and oxidation reduction potential of a given environment.

Parameters affecting the development of biofilms include:

  • Effectiveness of biofouling remedial measures.
  • Nutrient availability
  • The pH of water in the system 
  • The temperature of the system or ambient temperature
  • Water flow rate past the surface
  • The surface of the substratum

Once formed, the various layers of the biofilm can also increase resistance to biocides, chemical treatments, and even temperature treatments, further complicating the treatment process and making it harder to slow corrosion progression and eliminate microbes from a piping process or system.

SANDVIK highlights how delicate this balance can be, noting, “… The immersion of stainless steels, as well as any kind of material, in natural seawater induces the development of a microbial film known as biofilm and usually established after 1–3 weeks.”

Stainless Steel and Microbial Corrosion Risk

Stainless steel’s passivation layer makes it one of the most corrosion resistant alloys available. 

This increased resistance carries over to microbial corrosion as well. 

The passivation layer both prohibits microbes from attaching to the underlying steel and — depending on the finish — creates a surface that is easier to clean and less prone to developing biofilms. 

Certain grades of stainless steel — including duplex and austenitic alloys — can offer enhanced resistance to microbial corrosion when needed.

Of course, stagnant water or damaged surfaces can change this. 

With enough exposure or abuse, even stainless steel will succumb to the attack of microbes. 

This makes proper care and maintenance of stainless steel components a critical aspect in fending off microbial corrosion.

In most cases, the result will be a spotty attack with pits or tubercles forming in areas with a damaged passivation layer. 

Once the passivation layer is compromised, the corrosion can spread into the metal. This leads to loss of material and holes in the pipe, sheet, or component. 

With enough time, it can even result in complete structural failure.

Industries Impacted by Microbial Corrosion

Microbial corrosion is prevalent in a range of industries. 

However, industries that either heavily use saltwater, freshwater, or chemicals in their operations — or operate in environments rich in them — are most prone to biocorrosion risks.

The following are a selection of commonly impacted industries:

  • Onshore and offshore oil and gas industries: Special care should be taken by those operating in sulphate-reducing bacteria-rich environments.
  • Underground pipelines: Clay-type soils and near-neutral pH levels can create concerns when water saturates the area and mixes with decaying organic matter.
  • Water treatment industry: Heat exchangers and piping are particularly susceptible.
  • Water treatment industry: Particularly those involving natural river or well waters. Stainless steel tanks, pipelines and flange joints are all potential microbial corrosion targets — especially near welds.
  • Nuclear power generation: Increased risk during construction, hydrotest, and outage periods. Susceptible components and structures include carbon and stainless steel piping and tanks. Other metals, such as copper-nickel, brass, and aluminum bronze, may also exhibit corrosion signs and should be monitored.
  • Metalworking industry: The presence of biofilms and changes in local environments can increase wear due to the breakdown of machining oils and emulsions.

Microbial corrosion isn’t exclusive to stainless steel pipes or structural carbon steel either. 

It can impact virtually any material with the proper conditions. Examples of industries facing microbial corrosion with other metals and materials include:

  • Marine and shipping industry: Left unchecked, microbial corrosion can significantly accelerate damage to ships and barges — especially when fouling prevention measures are also low or ineffective.
  • Sewage handling and treatment industry: Concrete and reinforced concrete structures are particularly susceptible to biocorrosion — both within bricks or concrete and any joining materials.
  • Aviation industry: Monitoring aluminum wing tanks and fuel storage tanks — both prone to microbially induced corrosion — is essential for safe operation.

Preventing Microbial Corrosion

Given the sheer number of variables involved in the development and spread of microbial corrosion, exact methods for prevention and treatment will vary depending on the environment, processes, and materials involved.

However, these four considerations offer excellent foundations for any microbial corrosion prevention and control plan:

  1. Always consider materials: Choosing the ideal alloys to avoid the types of corrosion common in your industry or operating environment is essential. For example, hyper-duplex, super-duplex, and high-alloy austenitic stainless steel grades offer exceptional microbiologically influenced corrosion resistance in seawater operations.
  2. Mechanically clean when possible: While exact methods should account for any industry requirements, the finish of your piping or containment products, and environmental and worker safety, mechanical cleaning of all surfaces can drastically reduce the chance of biofilm formation and prevent the onset of microbial corrosion.
  3. Implement biocide treatment routines: Controlling bacterial populations is also essential in preventing microbial corrosion. All treatments should consider the influence in resistance levels created by biofilms, pockets of concentration, and shifting environmental variables.
  4. Drain and store components dry when possible: While not always the most critical factor in initiating a microbial corrosion attack, nearly all scenarios involve moisture. When feasible, emptying and storing pipe, flanges, containment vessels, and other carbon steel or stainless steel components in dry environments can prevent potential bacterial corrosion concerns. 

Key Takeaways

  • Microbial corrosion doesn’t describe the type of corrosion so much as it describes how corrosion types are initiated. Microbial corrosion types may include pitting corrosion, crevice corrosion, galvanic corrosion, intergranular corrosion, stress corrosion cracking, uniform corrosion, and dealloying or selective leaching.
  • Microbes involved in microbial corrosion often fit in one of four categories: sulphate-reducing bacteria, slime-forming bacteria, iron-oxidizing bacteria, and sulphur-oxidizing bacteria.
  • Microbially induced corrosion (MIC) progresses in three stages: microbe attachment, growth of the initial pit and nodule, and maturation of nodule and pit.
  • Industries impacted by microbial corrosion include onshore and offshore oil and gas industries, water treatment industries, and nuclear power generation.
  • Consideration of materials, establishing effective mechanical cleaning routines, utilizing biocides, and eliminating moisture and stagnation can reduce microbial corrosion risks.

Canada’s industries have trusted Unified Alloys with their stainless steel needs for more than four decades. From stainless steel pipes and fittings to bar and plate products and supports, our selection of high-quality stainless steel products ensures we have a solution to help with your next project or process. Contact us today to speak with an expert sales analyst.

References

  1. Frontiers in Materials: Polymers for Combating Biocorrosion
  2. The International Journal of Advanced Manufacturing Technology: Microbiological Corrosion: Mechanism, Control and Impact—a Review
  3. Digital Refining: Microbiological Causes of Corrosion
  4. The International Society for Microbial Ecology Journal: The Dual Role of Microbes in Corrosion
  5. Corrosion Doctors: Microbial Corrosion Cells
  6. American Society for Microbiology Applied and Environmental Microbiology Department: Corrosion of Iron by Sulfate-Reducing Bacteria: New Views of an Old Problem
  7. ScienceDirect: Microbiological Corrosion
  8. Corrosionpedia: Microbial Corrosion
  9. Microbe Wiki: Microbial corrosion
  10. Corrosion Clinic: Microbiologically Influenced Corrosion (MIC)
  11. SANDVIK: Microbiologically Influenced Corrosion (MIC)
  12. Environmental Safety Technologies: Microbiological Influenced Corrosion Identification, Testing and Control
  13. Wikipedia: Microbial Corrosion