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Corrosivity Factor
Without effective CP, bare steel exposed at coating faults can corrode, the rate of corrosion being determined by many factors in particular the corrosive nature of the soil in which the pipeline is buried. However, a fresh look at what happens has identified that a buried pipeline has two coatings not one at a fault area,. These coatings are classified as :-
1. The Organic Coating, typically Coal Tar, Tape, F B E etc.
2. The Inorganic Coating, a naturally formed coating covering the bare steel exposed at Organic Coating Faults. This film is composed predominantly of black magnetite and grey white calcareous deposit if the CP is effective or haemotite (red rust) if the CP is ineffective. The presence of these films concurs with steel electrochemical behaviour in aqueous electrolytes.
The Organic and Inorganic Coatings act in combination to protect the steel pipe from corrosion. CP control is easy when a well formed coherent magnetite film is formed in mildly alkaline environments to reduce the amount of bare steel exposed at an Organic coating fault. However, maintaining effective CP under acid soil conditions is difficult, as the passive magnetite film does not form on steel at low pH. The acid's strength in the soil and its local neutralisation by alkali generated by the cathodic reaction determines the actual pH at the steel / soil interface and what happens at that interface. In stronger acidic soils the pipe to soil interface pH probably will always be acidic with no films but in less acidic soils depending on the rate of the cathodic reaction the interface will become alkaline and a film of variable integrity can grow.
In practice measurement of steel / soil interface pH is impossible as it is not possible to correctly locate the electrodes at the interface of a buried pipeline. Bulk soil pH values are next to useless as it is the interface that matters. Our approach is not to measure the impossible, the interface pH but to assess the effects of competing influences of alkali generated by the CP and soil acidity etc. by measurements of the protective nature of the passive films on the steel surface. It is the integral quality of the black magnetite film that really matters. A new term has been introduced called the Corrosivity Factor see Pipeline & Gas Industry, March 2000 p25. The Corrosivity Factor, calculated from fundamental principles represents the contribution made to corrosion protection by the protective films on the steel exposed at a coating fault. If the Corrosivity Factor is high then there are poorly protective films and metal loss can occur more readily. These are early candidates for repair. As the Corrosivity Factor decreases, the passive film's protective nature increases and metal loss is less likely. To date, under one set of environmental conditions the use of the Corrosivity Factor to the prediction of metal loss has been 83% correct as confirmed by excavation.
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