Basic Microbiologyand Bacterial

Metal Corrosion

Jonathan Beresford BSc (Hons) MScAKVO Ltd

www.akvo.co.uk

Bacteria

Bacteria are Unicellular (Single Cell) Have a range of shapes: Sphere, Rod, Spiral A Few micrometers in length Will grow in a wide range of environments Approximately 5x1030 bacteria on Earth Only about half the known species of bacteria

can be grown in the laboratory

Bacteria

Approximately 10 times more bacteria associated with a person than there are human cells

The most common fatal bacterial infections are associated with respiratory infections

TB kills around 2 million people per year in sub Saharan Africa

Bacteria

Bacterial life developed around 4 billion years ago

Multicellular organisms began to develop around 800 million years ago

Bacteria form bio-films or bio-mats on surfaces Bio-films can range in depth from a few

micrometers to half a meter and can be made up of multiple bacterial species

Bacteria

Some bacteria can detect surrounding cells and then move toward them (quorum sensing)

Bio-films can be very complex with different bacteria doing different jobs e.g. form spores at the surface to protect the film from drying out

Bacteria

2 general types of bacteria:

Gram +ve and Gram -ve Gram +ve have a thick

cell wall surrounding the membrane

Gram -ve have a thin cell wall surrounding the membrane

Bacteria

Some Gram +ve bacteria such as Clostridium can form spores (endospores)

When a bacteria forms and endospore it becomes dormant. In this form it is highly resistant to the effects of UV light, Gamma Radiation, Disinfectants, Heat, Pressure and Drying

Can stay dormant for thousands of years and survive in space

Bacteria

Bacteria grow by cell division in a process known as binary fission

Growth of cell numbers is exponential

Bacteria - Legionella

Legionella is a Gram -ve non-capsulated rod Often found growing within amoebae In humans they infect alveolar macrophages Can be treated with specialist antibiotics as

growth is intra-cellular

Bacteria - Pseudomonas

Gram -ve rod present in water systems and plant seeds

Motile due to flagella Aerobic non-spore forming bacteria Often forms bio-films Manufacture Exopolysaccharides which cause

slime layers to be produced making them difficult to remove

Bacteria - Pseudomonas

Only require a basic Carbon source for growth and so can use a wide range of nutrients

This leads to growth in unexpected places including in antiseptics

Common food spoilage bacteria especially in the dairy industry

Bacterial Corrosion of Metals

Some species of sulphate reducing bacteria produce hydrogen sulphide, which can lead to sulphide stress cracking

Acidithiobacillus produce sulphuric acid

Bacterial Corrosion of Metals

Ferrobacillus ferroxidans directly oxidises Iron to Iron Oxides and Hydroxides

This is seen in the “Rusticles” present on the Titanic

Bacterial Corrosion of Metals

In aerobic conditions species of Thiobacillus are the common corrosion causing bacteria

All species of Thiobacillus are relatively common

In anaerobic conditions other species are the main corrosion causes

They are usually spore forming sulphate reducers

Bacterial Corrosion of Metals

This process requires a reducing environment with an electrode potential of at least -100 mV

However small amounts of hydrogen sulphide can cause these conditions in the local environment, so once growth has started it accelerates as does the corrosion

Several species of bacteria can be found around areas of corrosion with some consuming the released hydrogen causing galvanic corrosion

Bacterial Corrosion of Metals

Bacterial corrosion of metals usually appears as pitting corrosion, however in some cases it can cause the metal to go brittle by stripping the Iron out and leaving a graphite skeleton

Bacterial Corrosion of Metals

In bio-films aerobic bacteria can grow and consume oxygen at different rates causing varying oxygen concentrations

Two different oxygen concentrations on a metals surface cause a difference in electrical potential and subsequently corrosion currents form

The area under the colony becomes anodic and the area surrounding it cathodic

FOR FURTHER INFORMATIONand Corrosion Inhibition

Strategies

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