Page 62 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

40 INTRODUCTION AND FORMS OF CORROSION

The influence of microbiological organisms can be the initiation of either general
or localized corrosion. This influence is because of the effect of organisms to change
variables such as pH, oxidizing power, flow velocity, and concentration of chemical
species at the metal/solution interface.
Corrosion can be influenced by microorganisms in many ways such as: (i) pro-
duction of differential aeration cell; (ii) production of biofilms; (iii) production of
sulfides; (iv) production of organic and inorganic acids; (v) production of corrosive
gases.

(i) Production of differential aeration cell: a scatter of barnacles on a stainless steel
surface creates oxygen concentration cells. The formation of biofilm generates
several critical conditions for corrosion initiation. Uncovered areas with access
to oxygen act as cathodes, and the covered areas act as anodes. Underdeposit cor-
rosion (crevice corrosion) or pitting corrosion can occur. The corrosion rate may
be accelerated depending on the oxidizing capacity of bacteria and the chloride
ion content.

Considering pit formation on the surface of iron, the anodic and cathodic reactions
are
2+ −
2Fe → 2Fe + 4e (anodic)
−
−
O + 2H O + 4e → 4OH (cathodic)
2 2
The insoluble Fe(OH) can help bacterial film to control the diffusion of oxygen to
2
the anodic sites in the pit thus forming a typical tubercle. In the presence of chlorides,
the pH of the solution trapped in the tubercle can become highly acidic because of the
autocatalytic propagation mechanism of localized corrosion because of the reaction
involving the
FeCl + 2H O → Fe(OH) + 2HCl
2 2 2

formation of iron hydroxide and hydrochloric acid.

1.6.8 Production of Biofilms

The bacteria implicated in corrosion may begin their lives on a metal surface as a scat-
ter of individual cells. As the biofilm matures, the organisms are found as individuals
or in colonies embedded in the matrix of a semicontinuous and highly heterogeneous
biofilm. Microorganisms begin as individual bacteria on the surface of the metal and
progress to thick semicontinuous films or colonies in the form of slime or polymer
that can influence corrosion.
Depending on the flow velocity, the thickness may vary from 10 to 100 mm and
may cover from less than 20% to more than 90% of the metal surface. Biofilms or
macrofouling in seawater can cause redox reactions that initiate or accelerate corro-
sion. Biofilms accumulate manganese and iron in concentrations far above those in
surrounding bulk water. Biofilms can also act as a diffusion barrier. Some bacteria can