Page 60 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

38 INTRODUCTION AND FORMS OF CORROSION

been removed by mechanical action, allowing the attachment of barnacles. The other
alternative, a continuous film of bacteria, algae, and slime can occur and provide a
barrier film and limiting corrosion. However, it should be noted that it is rare that a
film of microorganisms in the marine environment is continuous over a large area of
exposed surface (41).

1.6.5 Industries Affected

Many industries are affected by microbiological problems, which involve the material
and the features of the medium. The industries affected by MIC are chemical process-
ing, energy generation systems, pulp and paper, hydraulic systems, fire protection,
water treatment, sewage handling and treatment, highway maintenance, buildings
and stone works, aviation, underground pipeline and onshore, and offshore oil and
gas equipment.

1.6.6 Role of Some Microbiological Species in Corrosion
Some bacteria are involved directly in the oxidation or reduction of metal ions such
as iron and manganese. Some microbes produce organic acids such as formic and
succinic acid or mineral acids such as sulfuric acid. Some bacteria can oxidize sulphur
or sulfide to sulfate or reduce sulfates to hydrogen sulfide (H S) (41).
2
SRB are anaerobic bacteria that obtain the carbon from organic nutrients and their
energy from the reduction of sulfate to sulfide. Sulfate is abundant in fresh water,
seawater, and soils. Sulfide appears as dissolved or gaseous H S, bisulphide, or sul-
2
fide, metal sulfides, or a combination of these according to the conditions. Sulfides
are highly corrosive. SRB are anaerobic and facilitate the cathodic reaction, which
controls the corrosion rate in these media. In the case of buried steel in soil contain-
ing near neutral-pH solution, the presence of SRB accelerates the electrochemical
reactions of corrosion as given below:

−
4Fe → 4Fe 2+ + 8e (anodic)
+
−
8H + 8e → 8H (cathodic)
SRB 2−
2−
+
SO + 8H −−−−→ S + 4H O
4 2
Fe 2+ + S 2− → FeS (corrosion product)
−
Fe 2+ + 2(OH) → Fe(OH) (corrosion product)
2
+
2H + S 2− → H S (possible gas product)
2
The ferrous sulfide (FeS) is not continuous, and the base iron can corrode. H Smay
2
also be produced. The SRB have been found to contribute to corrosion of stainless
steels, copper, and aluminum alloys.
Most sulfate-reducing bacteria are obligate anaerobes, yet they accelerate corro-
sion in aerated media. This is possible when aerobic organisms form a film or colony