Biocorrosion

Corrosion caused or promoted by micro-organisms, usually chemotrophs is called Biocorrosion or Microbially Influenced Corrosion (MIC). It can apply to both metals and non-metallic materials.

It refers to the influence of micro-organisms on the kinetics of corrosion processes of metals, caused by micro-organisms adhering to biofilms. Prerequisites for MIC is the presence of micro-organisms. If the corrosion is influenced by their activity, further requirements are : an energy source, carbon source, electron donor, electron acceptor, and water.

ROLE OF BIOFILMS IN BIOCORROSION

In principle, corrosion is an interfacial process. The kinetics of corrosion are determined by the physio-chemical environment at the interface, e.g., by the concentration of oxygen, salts, pH value, redox potential and conductivity. The organisms can attach to the surfaces, embed themselves in slime, so-called Extra cellular Polymeric Substances (EPS) and form layers called "biofilms".

These biofilms can be very thin (monolayers) but can reach the thickness of centimeters, as it is the case in microbial mats. Biofilms are characterized by a strong heterogeneity. It is well known that the metabolic activity of clusters of biofilm organisms can change the pH value for more than three units locally. This mean that directly at the interface, where the corrosion process is actually taking place, the pH value can differ significantly from that in the water phase. Thus, water sample values do not reflect such effects.

MICRO-ORGANISMS INVOLVED IN BIOCORROSION

Micro-organisms implicated in biocorrosion of metals such as iron, copper and aluminium and their alloys are physiologically diverse. Bacteria involved in metal corrosion have frequently been grouped by their metabolic demand for different respiratory substrates or electron acceptors. The capability of many bacteria to substitute oxygen with alternative oxidizable compounds as terminal electron acceptors in respiration, when oxygen becomes depleted in the environment, permits them to be active over a wide range of conditions conducive for corrosion of metals. The ability to produce a wide spectrum of corrosive metabolic by-products over a wide range of environmental conditions makes micro-organisms a real threat to the stability of metals that have been engineered for corrosion resistance.

The main types of bacteria associated with corrosion failures of cast iron, mild and stainless steel structures are sulphate reducing bacteria, sulphur oxidizing bacteria, iron oxidizing/reducing bacteria, manganese oxidizing bacteria, as well as bacteria secreting organic acids and Extracellular Polymeric Substances (EPS) or slime.

Some sulphate reducing bacteria produces hydrogen sulphide, which can cause sulphide stress cracking. Acidithiobacillus bacteria produce sulphuric acid.

Acidithiobacillus thiooxidans frequently damages sewer pipes. Ferrobacillus ferrooxidans directly oxidizes iron to iron oxides and iron hydroxides, the rusticles forming on RMS Titanic wreck are caused by bacterial activity. Other bacteria produce various acids both organic and mineral.

In the presence of oxygen, aerobic bacteria like Acidithiobacillus thiooxidans, Thiobacillus thioparus, and Thiobacillus concretivorus, all three widely present in the environment, are the common corrosion causing factors resulting in biogenic sulphide corrosion.

Without presence of oxygen, anaerobic bacteria, especially Desulfovibrio and Desulfotomaculum, are common. Desulfovibrio Salixigens requires atleast 2.5% of NaCl for growth, but, D.vulgaris and D.desulfuricans can grow in both fresh and salt water.

   

   

Desulfotomaculum genus comprises of sulphate reducing spore-forming bacteria. Dtm. orientis and Dtm. nigrificans are involved in the corrosion process. Sulphate reducers

requires reducing environment, an electrode potential lower than -100mV is required for them to thrive. However, even a small amount of produced hydrogen sulphide can achieve this shift, so the growth, once started, tends to accelerate.

Layers of anaerobic bacteria can exist in the inner parts of the corrosion deposits, while the outer parts are inhabited by aerobic bacteria. Some bacteria are able to utilize hydrogen formed during cathodic corrosion processes. Bacterial colonies and deposits can form concentration cells, causing and enhancing Galvanic Corrosion.


PREVENTION AND PROTECTION

Chemical treatments applied to control biofilms involve the use of biocides and other products such as penetrating or dispersive agents (which enhance the efficacy of the treatment). The classical criteria governing the selection of an effective biocide have been generally summarized as follows:-

1. Proven efficacy against a broad spectrum of micro-organisms.
2. Ability to penetrate and disperse microbial slime.
3. Chemical and Physical compatibility with other products and the environment.
4. Easy use and safe storage.
5. Appropriate biodegradability.
6. Cost effectiveness.

Unfortunately, biocides are inherently toxic and frequently are difficult to degrade being persistent in the natural environment or able to accumulate in a variety of matrices causing contamination of areas distant from the site of treatment. Thus, biocides may have a very negative impact on the environment if they are applied without a proper environmental risk assessment.

Ozone has attracted special interest in recent years as an effective and non-polluting biocide for cooling water systems. Taking into account environmental concerns, the use of ozone offers several advantages over other biocides:-

1. Minimal on-site chemical inventory.
2. Non-toxicant discharge.
3. Potential for water conservation.

The unique combination of high toxicity during treatment with non-toxicant discharge could make ozone the leading choice biocide in the near future, if an appropriate balance between positive effects, problems caused and cost, is reached.

Two innovative attempts to replace toxic biocides through the use organic film-forming corrosion inhibitors have been reported. Some of the advantages of this approach are:-

1. Lower operational costs due to the lower concentration of chemicals used.
2. Less frequent dosages and environmental control measures.
3. Simultaneous action on corrosion inhibition and bacterial adhesion.

There have been reports that some quaternary amines form a film on a substratum and inhibit the bacterial attachment onto it, thus inhibiting the formation of biofilm and subsequently retarding the corrosion rate of that metal. Today, in addition to quaternary amines, a wide variety of corrosion inhibitor formulations are available.

Microbial adhesion is widely accepted as the main stage prior to the induction or initiation of biocorrosion, therefore, various innovative substances are introduced to prevent bacterial adhesion and biofilm formation. For example, An innovative method for preventing biocorrosion through microbial adhesion inhibition has been proposed recently by forming an immunoglobulin film on metal surfaces. This procedure has been effective to prevent the formation of P. fluorescens biofilms on two different types of stainless steel. The bacterial adhesion inhibitory action of IgA was found to be concentration dependent. IgA is one of the five major classes of immunoglobulins in the human body. It is present selectively in seromucous secretions as a dimer stabilized against proteolysis by combination with an other protein (the secretory component) that has a single peptide chain of molecular weight 60.