Understanding and preventing corrosion on pipeline

Corrosion causes billions of dollars worth of damage each year. As a result of this chemical-physical process, pipeline sections often have to be taken out of service and replaced.

Corrosion can be caused by ineffective pipeline coatings, soil conditions and the circumstances under which a pipeline’s coating is rehabilitated onsite. In practice, surface preparation and the application circumstances appear to be critical in creating a long-lasting, high-performance coating.

The chemistry of corrosion

Corrosion can also be described as oxidation because the process involves the formation of bonds between steel and oxygen. Oxygen, however, is not solely responsible for the oxidation process. In dry environments, many materials, including steel, do not rust. The cause of oxidation is to be found in the presence of water.

Coating types

In order to prevent corrosion, the bare substrate can be protected by means of several available coatings including:

Factory coatings

• Fusion-bonded epoxy (FBE), high-density polyethylene (HDPE) and urethanes.

Field-applied coatings

• Spray coatings like epoxy, urethane and zinc;
• Residues of refinery like waxes and petrolatum;
• Bitumen-based coatings; and,
• Single or multiple-layer PE/butyl tapes.

The selection of the coating depends on various factors such as:

• The estimated lifetime of substrate;
• Environment;
• Material, shape, and position of the substrate; and,
• Application and repair costs.

Phenomena and problems facing pipe coating

There are a number of phenomena that contribute to the corrosion process and that should be considered when discussing the application of pipeline coatings and the corrosion process.

Salts and osmosis

The presence of salt plays an important role in a corrosion mechanism because salt particles are present in most situations and are difficult to remove. Rinsing a blasted pipeline coating with clean water will not remove the salt particles and other contaminations in the voids of the pipe. As many pipeline coatings are not 100 per cent water resistant, the presence of salt is always a risk because it attracts water. When water and salt are present, the phenomenon of osmosis occurs.

Water permeability

If the slightest permeability for water exists, corrosion will occur. Regardless of how well a coating has been applied in the factory, practice has revealed that disbondment due to the presence of water may still occur. While corrosion is always caused by a combination of factors, permeability for water should be viewed as a serious hazard to pipeline health.

Adhesion problems

Any pipeline coating must have good adhesion to the substrate. To obtain an effective adhesion is not easy because application circumstances must be taken into consideration, and many coatings require a perfect surface preparation. The difference between tensions of the surface and coating material also play an important factor in adhesion failures.

Surface preparation

Studies have shown that bad surface preparation appears to be a main cause of corrosion problems. Very often field-applied coatings need a well-prepared surface in order to secure excellent adhesion and sandblasting is often required. However, remaining pollutions in the voids of the blasted surface and salt particles can create problems, and rapid disbondment may occur.

Microbiologically-influenced corrosion

Microbiologically-influenced corrosion (MIC) is a phenomenon in which corrosion is initiated or accelerated by the activities of micro-organisms. The first case of MIC was discovered in 1934 in which sulphate-reducing bacteria were responsible for the corrosion failure of cast iron pipe.

MIC is responsible for a large portion of corrosion problems experienced in the pipeline industry. During the metabolic process, sulphate is reduced to sulphide, which reacts with hydrogen to produce hydrogen sulphide. Hydrogen sulphide is very corrosive to ferrous metals and further reacts with dissolved iron to form an iron sulphide film over the metal pipe.

Overcoming these problems

In order to prevent corrosion, steel parts must be protected from contact with water. This can be achieved through the application of a protective coating. This coating must be 100 per cent water-repellent, with the capability to perfectly-match the surface of the pipeline. It should have a perfect adhesion and reduce the risk for MIC.

Source :

http://pipelinesinternational.com/news/understanding_and_preventing_corrosion/43736