Corrosion is generally thought of as a chemical process – so what have bacteria got to do with it? NCIMB’s Energy and Environmental Microbiology Services Manger, Michelle Robertson, gives an introduction to Microbiologically Influenced Corrosion
Microorganisms are ubiquitous, occurring in all kinds of environments and they can quickly colonise the surfaces of man made structures, forming biofilms. Biofilms can be highly complex communities of different species with the early colonizers creating the conditions that allow others to follow, but the composition of communities varies from site to site. The figure below illustrates the stages and mechanisms that can be involved in biofilm formation. It also highlights the production of the extracellular polymeric substance matrix – a key characteristic of biofilms that provides structure and protects the cells within from environmental stresses – contributing to the difficulty of eradicating a well established biofilm.
The metabolic activity of microorganisms within biofilms can accelerate corrosion reactions on the surfaces that they colonise. This phenomenon is microbiologically influenced corrosion – sometimes also referred to as microbially influenced, or induced, corrosion, or MIC.
Microbiologically influenced corrosion accounts for a significant proportion of corrosion failures, and these failures can have serious consequences, so it is important that structures are monitored to assess their risk of being impacted by this common phenomenon.
A number of different methods can be used for determining the presence of microorganisms that influence corrosion, and one of the simplest is a culture-based enumeration known as the most probable number method or MPN. This method is based on the use of selective growth media to assess the presence of key groups of microorganisms commonly associated with microbiologically influenced corrosion.
As these groups of organisms each have different growth requirements the use of different selective growth media indicates which are present. As the name of this method suggests, the growth media is used, not only to indicate the presence or absence of different groups of microorganisms, but also to give an indication of the numbers present. This is done by creating a dilution series of biofilm sample in the media. The dilution at which no growth occurs can be used to estimate the number of microorganisms present in the original sample – the most probable number.
If the presence of microorganisms known to influence corrosion is confirmed, then remedial action can be planned and its efficacy monitored with further sampling.
At NCIMB we supply speciality growth media for the main groups of organisms of concern with respect to microbiologically influenced corrosion. The range includes NACE-recommended recipes and our own proprietary formulations developed from our extensive experience of working with these groups of organisms. For more information visit our speciality media webpage or get in touch using the form below.