Remaining Life of a Pipeline

3.2. Corrosion Model
Corrosion is the destructive attack of a metal by chemical or electrochemical reaction with its
environment. The major classifications of corrosion given by Fontana ( Reference # 9) are:
Erosion corrosion
Crevice corrosion
Galvanic corrosion
Stress corrosion cracking
Uniform attack
Selective leaching
Pitting corrosion
Intergranular corrosion
Cavitation
Corrosion is such a complex failure mechanism, which can damage a metal surface in many different
ways. The work developed by M. Ahammed is particularly concern with those types of corrosion
which produce macroscopic defects due to material losses, because these defects results in the
reduction of the metal cross section with its correspondent reduction of the pipeline strength (Sp strength .
Such corrosion types are erosion, pitting corrosion and uniform attack.
This approach is not because of simplicity, but because of the experience in analyzing pipeline
failures, which has shown that for most of the common industrial applications, pipeline failures aren’t
due to other types of corrosion than the ones mentioned. Nevertheless, for complex applications, as
nuclear applications or operating conditions with extremely high stress solicitude, a more extensive
analysis is required.
The Ahammed’s considerations about corrosion are focused in the growth rate of macro-defects.
The growth of macro defects is directly related with the exposure period and depends on the
characteristics of the pipeline material, properties of the fluid being transported and the surrounding
environment. It has been found that this rate is high during an initial period and then gradually
decrease to finally reach a steady-state rate.
The researches cited by Ahammed (Southwell et al. Reference # 10) shown that the initial period of
relatively high corrosion rate, (about a year in average), is not of much concern because during this
period the defects are usually small and hence do not pose much threat to pipeline integrity.
As the exposure period increases, the growth rate decreases, but the overall size of corrosion defect
increases and becomes a greater risk for the pipeline integrity. By this time, the steady-state growth
rate is a good approximation.
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