Pipeline Corrossion

Degradation of pipelines is the result of the persistent attack by the environment on pipeline materials (coatings, welds, pipe, etc.). Buried pipelines are located within ever changing environmental conditions that may lead to a corrosive environment. Factors that may prevent or contribute to the initiation and attack on buried pipelines include the following. Additional information can be obtained from the report published by the National Energy Board in 1996 entitled “Stress Corrosion Cracking on Canadian Oil and Gas Pipelines”.

Pipe Coatings Buried pipe is coated to offer protection from the surrounding environment. A breakdown in the coating will result in pipeline metal being exposed. The material used for coating pipes varied over the years as technology evolved. For example; in the 1940’s and 50’s coal tar, wax, and vinyl tape were used; in the 1960’s asphalts were used; and in the 1970’s to present day fusion bond epoxy was and is being used. Polyethylene tape and extruded polyethylene jacket material was also used from the early 1950’s to the present day.

Cathodic protectionThe introduction of an electrical current on a buried pipe such that the electrode potential of the buried pipe is lowered creates an environment where metal loss is reduced. Soil conditions, such as moisture content and mineralogy influence the effectiveness of the cathodic protection, as does the type of coating on the pipe. For example, pipe coated with polyethylene material is shielded from cathodic protection more than pipes coated with asphalts.

Soil conditionsSoil structure and conditions will not only impact the effectiveness of the cathodic protection but also may contribute to the creation of a corrosive environment. Factors such as soil type, drainage, temperature, CO2concentration, and electrical conductivity all contribute to the environment surrounding the pipe.
TemperatureThe temperature of the soil as well as the temperature of the pipe may create favorable conditions for attack on pipeline materials. Liquid and gas lines have slightly different operating temperature characteristics but both are still susceptible. For example, with gas pipelines both the pipe and surrounding ground can vary from a high of 40oC upon leaving the compressor station down to 5oC at distances from the station.

Stresses (residual and others)Stresses in the pipe may lead to premature degradation of the pipeline strength. Stresses acting on the pipe include:

1. residual stress from the manufacturing process,
2. external stress such as those incurred due to bending, welding, mechanical gouges, and corrosion, and
3. secondary stresses due to soil settlement or movement.

Pipe pressureCorrosion, in particular cracking, is related to the pressures exerted on the pipe. As the pressures within the pipe are increased, the growth rates for cracks also increase. The circumferential stress (hoop stress) generated by the pipeline operating pressure is usually the highest stress component that exists.

Cyclic loading effectsConditions where the pipe is under cyclic loads may result in increased crack growth rates. Operating pressures for large diameter pipe can measure up to 8700kPa (1250psi). The pipeline pressure continually fluctuates due to loading and unloading of product and is influenced by pump activity. This applies to both gas and liquid lines but has greater influence in liquid systems.

Source:http://pipabawahlaut.wordpress.com/2014/01/30/pipeline-corrosion-and-cracking-and-the-associated-calibration-considerations-for-same-side-sizing-applications/