Corrosion Protective Coatings: Rehabilitation of Buried Steel Pipelines

Corrosion-protective coatings have to provide a primary protection, achieved by covering the entire metal surface with a material that prevents the condensation of water on the steel surface. Suitable materials for covering and sealing the whole surface are permanently plastic compounds (petrolatum, butyl rubber) as well as rigid compounds (polyurethane, epoxy resins), which also prevent the interdiffusion or penetration of water and oxygen.

Coal tar and bitumen-based coatings

Coal tar and bitumen coatings are considered early methods, but are still in use in some countries such as India. These coatings can become brittle, resulting in the formation of crevices and cracks, and a significant decrease in adhesion to the steel surface occurs, risking corrosion. Protective current voltages for old coal-tar or bituminous coatings often exceed what is acceptable for an economically efficient cathodic-protection system. High voltage protective currents result in the formation of H2 and therefore increase corrosion.

Two-ply tape systems (PVC and PE tapes)

In most of the cases where tape coatings on pipelines require rehabilitation, two-ply tape systems are the cause. The main reasons for their failure are low-quality material properties, as well as incorrect application.

As PVC is a rather brittle material, tapes made of PVC contain plasticiser agents. During the lifetime of a pipe coating, these plasticisers diffuse out of the PVC carrier film. This results in an embrittlement of the carrier film and a decrease of adhesion when the plasticisers accumulate in the interface between the adhesive and the steel surface. Due to this effect only minor residues of the tape remain on the pipe surface when the pipe is excavated after years of service, and the mechanical and corrosive protection is no longer in place.

PE and butyl rubber-based two-ply tapes, which generally did not suffer from such material drawbacks, failed as well. Two-ply tapes generally contain a carrier film that is coated with an adhesive on only one side. Due to this structure, two-ply tapes can provide certain primary protection against corrosion because of their adhesion to the steel surface, when supported by primer paint. But the sealing properties in the overlapping areas of two-ply tape systems cannot completely prevent the penetration of corrosive agents. In the remaining and clearly defined interface between the layers of a two-ply tape system, micro-channels may exist. This facilitates a possible penetration path for water and oxygen, increasing the probability of corrosion.

Factory or field?

Factory coatings are intended to provide a maximum of mechanical and corrosive protection, while the method of application is not the main focus. Factory coatings can be applied independent of environmental influences and human factors. Field coatings, however, must match the same mechanical and corrosive protection of the corresponding factory coating, and ensure a similar application even under changing conditions on site.

Due to the versatility of application for field-coating systems, worldwide standards for all materials do not exist. German and European standards for corrosion materials are among the highest in the world and applied in several countries. The relevant standards DIN 30672 and EN 12068 for field-coating materials differentiate into three mechanical stress classes (A, B, and C) and three operating temperature classes (up to 30°C, up to 50°C, and high temperatures).

The solution: three-ply tapes

There exists a clear distinction between two-ply tapes and co-extruded self-amalgamating three-ply tapes. The latter contain a carrier film of stabilised PE, which is coated with a butyl-rubber adhesive on both sides. Carrier films of co-extruded three-ply tapes are manufactured with intermediate adhesive layers, ensuring that no clearly defined interface remains between carrier film and adhesive layer. When three-ply tapes are wrapped spirally around a pipe, the adhesive layers self-amalgamate in the overlap areas, forming a homogenous sleeve-type coating without any remaining interface.

Compared to two-ply tapes, no interface or penetration paths remain in the overlapping zones of high-performance, co-extruded three-ply tapes. The outstanding feature of co-extruded three-ply tapes and their layer of butyl rubber is their ability to self-amalgamate in the overlap areas, resulting in a completely sealed, impermeable, and sleeve-type coating.

The test of time

In November 2012, WINGAS – now Gascade Gastransport, a subsidiary of Wintershall and Gazprom – excavated two pipe sections of the 900 mm diameter Sachsen-Thüringen-Erdgas-Anbindungsleitung (STEGAL) transmission pipeline, which was constructed from 1991–1992 in the rocky soils of the Erz mountains in Saxony, Germany.

After 20 years of operation, the joints – covered by a real co-extruded three-ply tape – were still in excellent shape and exceeded the requested values of the guideline EN 12068 stress-class C50. Peel test on sites were taken and values measured up to maximum of 59 N/cm – the corresponding requested value according to EN 12068 stress-class C50 is 10 N/cm.

After the execution of the peel test, a cohesive break in the layers was noticed and the remaining layer showed a thickness of 344 microns.

The tape system used at the STEGAL pipeline was co-extruded three-ply tape DENSOLEN® AS 40 Plus, which was one of the first asymmetrical tape types with a thicker 0.43 mm inner butyl layer to cover the steel substrate better than the symmetrical predecessors.

Coat for your unique project

There are many pipeline rehabilitation systems available in the market of corrosion prevention materials, and all of them may have their advantages and disadvantages. The evaluation of an appropriate rehabilitation system depends on the unique project conditions, as well as the material and on-site requirements, especially taking into account an easy and economical way of applying the system.

For standard temperatures up to +50°C, state-of-the-art co-extruded three-ply tape systems offer the widest range to meet the diverse conditions on site. Those systems can be applied at ambient temperatures from -35°C to +60°C, even under difficult site conditions.

The newly developed system DENSOLEN AS50/R20HT is designed to combine ease of application and economical pricing with excellent mechanical resistance and outstanding corrosion prevention.

Sumber : Schad, Michael; GMBH, Denso; Leverkusen. "Corrosion Protective Coatings: Rehabilitation of Buried Steel Pipelines". 30 Januari 2014. http://pipelinesinternational.com/news/corrosion_protective_coatings_rehabilitation_of_buried_steel_pipelines/083226/