Anaerobic biocorrosion

from Wikipedia, the free encyclopedia

The anaerobic biological corrosion designates the forms of bio-corrosion under the anaerobic arise conditions, ie in the absence of oxygen.

properties

A significant proportion of corrosion damage takes place with the complete exclusion of oxygen, e.g. B. in the lower area of ​​a filled storage tank or in piping systems. Anaerobic bio- corrosion caused by bacteria is known as bacterial anaerobic corrosion . It creates problems for various industries due to pipeline corrosion. Oxygen-independent bacteria cause by their metabolism, combined with the production of certain enzymes, a significantly higher rate of corrosion than without bacteria. Furthermore, toxic hydrogen sulfide can be produced.

Sulphate- reducing bacteria (SRB) and iron and manganese-oxidizing microorganisms are partly responsible for this anaerobic corrosion . These microorganisms accelerate the corrosion of steel in petroleum tanks and other technical systems that come into contact with both water and organic materials, compared to corrosion without the involvement of microorganisms by a factor of several powers of ten. If such areas are not cleaned at relatively short intervals, the strong and rapidly advancing colonization with various microorganisms, including slime- forming agents, will result in gel-like biofilms . This phenomenon associated with strong slime and odor formation is known as biofouling . It can be the precursor to biocorrosion caused by SRB.

Iron-containing metal alloys that have been corroded in this way have black spots of iron (II) sulfide on the surface. Under this black coating there are anodically formed depressions in the metal with a bare metal surface. The rapid progress of this corrosion leads to pitting corrosion and extensive damage.

Occurrence

Anaerobic biocorrosion occurs in various areas of technical systems. In addition to iron alloys, the following are required: anaerobic environment (exclusion of oxygen) and the presence of water and sulfate As a rule, the presence of organic substances is also required, since most of the microorganisms in question (sulfate-reducing bacteria) are heterotrophic, i.e. they depend on organic substances that can be used by them for their growth. Crude oil usually offers good conditions for this because, in addition to organic compounds, it contains larger amounts of surface water, which is itself microbially contaminated, and sulphate due to the extraction measures. However, good conditions for anaerobic biocorrosion can also exist in other technical systems. An example of this are the landside areas on water sheet piling, provided that there are microbially usable organic substances there.

Chemism

In hydrous crude oil, the SRB carry out a reduction of sulfur compounds, especially the reduction of sulfate to sulfide. The sulfur compounds represent the electron acceptors (receivers) for the redox reaction taking place. The adequate electron donors (electron donors) are normally molecular hydrogen and / or organic carbon compounds that form on the metal surfaces, such as lactate, pyruvate, malate, high molecular weight fatty acids, simple aromatics and unsaturated compounds Hydrocarbons, which are primarily oxidized to acetate. When surfaces of iron-containing metal alloys are colonized, the metallic iron can be oxidized to form iron 2+ ions, as will be described in detail below.

Initial colonization by oxygen-tolerant microorganisms

First of all, the surfaces are colonized by optionally anaerobic, oxygen-tolerant microorganisms, even in areas containing oxygen, but at a moderate rate. These microorganisms multiply best in places without convection. You can find such low-circulation niches in the lower areas of oil tanks and in cracks, crevices and pores of the coating. This means that every area of ​​storage tanks and even pipelines can be "first populated". These microorganisms produce oxygen-consuming enzymes (catalase and superoxide dismutase) and thus create an oxygen-free environment.

Tubercle formation and destruction of iron

In these anaerobic areas, obligate anaerobic sulfate-reducing bacteria (SRB) multiply particularly quickly; they carry out an oxidative energy metabolism in which sulfate serves as an oxidant and is reduced to hydrogen sulfide (H 2 S). The SRB settle primarily as tubercle-shaped colonies. In the vicinity of the ferrous surface of the tanks, there is now a change in the physico-chemical conditions. Here, the result of the are Autoprotolyse existing water hydrogen film on the iron surface, produced by the SRB enzyme hydrogenase and the resulting iron (II) sulfide (FeS) is decisive.

The enzyme hydrogenase breaks down the protective hydrogen film and makes the hydrogen available as an energy supplier and electron donor. As a result, the above-described RedOx process begins, whereby mainly iron is oxidized to iron-2+ and sulfate is reduced to sulfide. The iron 2+ ions are intercepted by the sulfide ions to form iron (II) sulfide and form a black crust around the rust tubercles.

The resulting iron (II) sulfide (FeS) becomes the cathode and the galvanic cell is built up; Iron / iron sulfide (conversion of chemical into electrical energy). Here the hydrogen could reduce the voltage of this cell. However, the enzyme hydrogenase also prevents this protective mechanism and constantly regenerates the iron (II) sulfide layer.

Concrete substrates are also massively attacked by the hydrogen sulfide produced by the SRB. Extreme washout of concrete occurs.

literature

  • Iwona B. Beech: Sulfate-reducing bacteria in biofilms on metallic materials and corrosion. In: Microbiology Today. Volume 30, No. 3, 2003, pp. 115-117 ( PDF ).
  • RD Bryant, W. Jansen et al. a .: Effect of hydrogenase and mixed sulfate-reducing bacterial populations on the corrosion of steel. In: Applied and environmental microbiology. Volume 57, Number 10, October 1991, pp. 2804-2809, ISSN  0099-2240 . PMID 16348560 . PMC 183878 (free full text).

Individual evidence

  1. ↑ Horst Briehl: Chemistry of materials . Springer-Verlag, 2014. ISBN 978-3-658-06225-5 . P. 118.
  2. ↑ Gerhard Hauser: Hygienic production technology . John Wiley & Sons, 2012. ISBN 978-3-527-66009-4 . Chapter 7.5.1.11.