Page 64 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

42 INTRODUCTION AND FORMS OF CORROSION

1.6.14 Hydrocarbons
This term refers to crude oils, distillation, and cracking products (coal tar) and emul-
sions of these substances. All of these substances are microbially biodegradable.
Hydrocarbons are conducive to microorganisms, and microbial growth causes dam-
age to materials such as fuel tanks and pipelines (51).
Concrete can be damaged by acids, sulfates, ammonia, and other corrosives pro-
duced by microorganisms. Steel reinforcing bars in concrete corrode in the presence
of microorganisms as a result of corrosive agents. Hydrogen sulfide generated by
SRB can cause corrosion of the rebar in reinforced concrete structures. Thiobacillus
bacteria are responsible for the deterioration of concrete. Thiobacillus converts sul-
fur and its compounds into sulfuric acid, which reacts with calcium hydroxide and
calcium carbonate to form calcium sulfate. This is one of the modes of destruction of
concrete sewage pipelines (51).

1.6.15 Types of Corrosion of Metals and Alloys
Microorganisms are more likely to cause localized corrosion than general corro-
sion because of the differential oxygen cell. In most cases the localized attack was
observed beneath macrofouling layers. Corrosion of copper, steel, and aluminum
anodes occur when connected to cathodes on which biofilms grow. Unexpectedly,
rapid localized corrosion of steel bulkheads in marine harbor environments and of
ship hull plating of several tankers has been observed (22).
Biofilms on passive alloy surfaces can increase cathodic kinetics by way of
increasing the propagation rate of galvanic corrosion. Cathodic kinetics increased
during biofilm formation on passive alloy surfaces. Crevice corrosion initiation times
were reduced when natural biofilms were allowed to form on passive alloys S 30400
and S 31600.
Pitting corrosion of integral wing aluminum fuel tanks in aircraft, which use
kerosene-based fuels, has been a problem for six decades. The fuel becomes contam-
inated with water by vapor condensation. The attack occurs under microbial deposits
in the water phase and at the fuel–water interface. Cladosporium resinae is the
organism involved, and it produces a variety of organic acids of pH 3–4 or lower and
metabolizes fuel constituents. These organisms act in concert with slime-forming
pseudomonas to produce oxygen concentration cells under the deposit. Active SRB
have been detected under these deposits (56).
Hormoconis resinae poses a constant problem in fuel storage tanks and in alu-
minum integral fuel tanks of aircraft. Brown, slimy mats of Hormoconis resinae may
cover large areas of aluminum alloy, causing pitting, exfoliation, and intergranular
attack because of the organic acids produced by the microbes and the differential aer-
ation cells. The problem of fungal growth in fuel tanks of jet aircraft has diminished
as improved design to facilitate better drainage of condensed water and biocides such
as organoboranes are gaining acceptance as fuel additives (41).
Organisms having a high tolerance for copper such as Thiobacillus thiooxidans
can tolerate copper concentration as high as 2%. Localized corrosion of copper alloys