Corrosion protection evaluation of polyurea as anti corrosion coating in transmission lines

It will not be too long that polyurea will be the preferred alternative to current organic polymeric materials used for pipe coatings. This is due to their good corrosion resistance, imperviousness to water/air, resistant to salinity, immunity to large variations in pH and chemical and physical stabilities at moderately high temperatures and exceptional performance in combination with Cathodic protection specially in buried conditions This article discusses polyurea coating s and its fitness for purpose as a long term anti corrosion protective system in different operating conditions and environments both in exposed, buried or immersion services.

Every decision made on a pipeline, whether driven by cost or technical considerations will have an impact on the long-term integrity of a pipeline. This article assists decision makers and provides them an argument as to why Nukote Polyurea is the right choice as an anticorrosion system particularly as a pipe-coating product. An ideal coating will stop pipeline corrosion.  The minimum requirement is that a coating should stop corrosion for the design life of the pipeline but a more realistic objective is that the coating should stop corrosion for as long as the pipeline remains in service.  Most pipelines are operated well beyond the original design life.  Inevitably coatings get damaged by external forces or by a number of long-term degradation processes that affect the constituents of the coating.  Typically this results in coating defects that expose the pipe steel to the environment around the pipeline and this corrosion risk is and can be controlled by cathodic protection. 

Nukote manufactures and distributes two principal coating product lines for use in pipe coating applications. These product types include plural component polyureas and ceramic metal polymers. The two systems are different in regards to price and performance aspects. Polyurea polymers are designed to compete with tri ­laminate products such as FBE, 3LPP and 3LPE, providing higher performance coatings at competitive costs. Ceramic Metal Polymers are designed to compete with high-end alloy lined pipes for use in extreme corrosive and/or high temperature environments. Both product lines can be applied internally or externally.

The application processes required are simple and efficient and the equipment systems utilized are inexpensive when compared to the costs of systems used in the application of FBE, Tri-Laminate and similar coating technologies and not to mention about the field joints which is compatible on all of the above with added advantage of cost, speed and minimum back to service time.

Polyurea will provide effective corrosion protection for the design life of the pipeline, and beyond and following are few key arguments to substantiate our claims. (see table for the physical properties)

Damage control is probably the major argument used in the comparison of the performance of 3LPE and FBE coatings. FBE requires more careful handling during transportation and construction because the coating is more susceptible to mechanical damage than 3LPE. Polyurea Scores well in this matter due to its high impact resistance.

Polyurea is characterized by many essential properties which make it an effective corrosion resistant coating. Resistance to water is perhaps the most important characteristic since water a universal solvent in combination with other materials can take various form and can create very corrosive environment and have a damaging effect on the substrates. Less than 1% water absorption and a Moisture vapour transfer rate of less than 0.02 perms is an essential feature which makes it an effective coating barrier which very few coating can match.

The dielectric strength is a key property of polyurea which helps in breaking the electric  circuit set up during corrosion reaction and makes it suitable to be used as a corrosion resistant coating  by resisting passage of electron . Polyurea has a dielectric strength of > 16 KV and this combined with low moisture absorption makes it an ideal anti corrosion coating.

Polyurea coating’s strong resistant to ionic passage is a desired coating characteristic and prevents transfer or passage of chlorides, sulphides, or similar ions which accelerates corrosion. Resistance to ionic passage is a contributing factor to chemical resistance and polyurea is good for chemicals between Ph 12 and Ph 4 particularly the alkalis.

Polyurea adheres well to a well-prepared, primed substrate and the tensile adhesion is in the range of 15 Mpa. The strong adhesion prevents the problem caused by temperature gradient, osmosis and electroendosmosis and retains its integrity for longer time and thus enhancing the life of the coating. A strong adhesion also prevents undercutting.

A well-formulated elastomeric Polyurea has a good resistance to abrasion, impact and scouring and the loss is < 6 mg on C17 wheel comparable if not better than most of the Anti corrosive coatings being patronized to day.

One of the characteristics which make Polyurea unique is its retention of physical property on ageing which makes it weather and Age resistant and make it suitable as a coating for long term protection. A well-formulated Polyurea retains more than 90 % of its physical even after ageing.

Resistance to cathodic disbonding is an essential feature which makes polyurea the ideal coating to be used along with Cathodic protection. A polyurea coated substrate will have a lower demand of current. 

The universal strategy for external corrosion protection on buried or sub-sea pipelines accepts that coating damage will occur and CP is built into the pipeline design to prevent metal loss at these sites of coating damage. If the failed coating does not impede the flow of CP current onto the pipe steel then normal CP monitoring will ensure continued pipeline integrity. When a PE based coating loses adhesion from the metal substrate then a condition of CP shielding can be created. The corrosion risk created by CP shielding can only be eliminated by excavation and re-coating. Coatings that can fail and create conditions of CP shielding include cold applied tapes, heat shrink materials (particularly mastic backed heat shrink), 2LPE, 3LPE.

Extensive adhesion problems have been seen with 3LPE coatings over a period so the risk of CP shielding must be assumed to apply to these coatings. Nukote HT has been formulated in to take care of the deficiency which leads to the Cathodic Shielding.

A very good external corrosion control on steel pipelines is achieved by the combined use of a Nukote HT and cathodic protection. The coating provides the first, and major, defence against corrosion and the cathodic protection serves to prevent pitting or general corrosion where the pipe steel is exposed by damage to the coating. Coating damage is to be expected on all pipelines using traditional coatings and the degree of damage with Polyurea is minimal or none unless these are very roughly handled and hence the degree of risk is very less.

An FBE coating is assumed to lie toward the bottom end of the scale of costs and 3LPE toward the top end. Nukote HT polyurea is right there with FBE on cost and a proven performance better than 3 LPE on all projects that has been completed with reduced inspection, survey or monitoring cost.

About Kishore Kumar

Kishore Kumar has 20 years of progressive experience in techno-commercial and business development of advanced and high performance coating, anticorrosion solutions and services, waterproofing and construction chemicals.
He graduated in Applied Science and Polymers with an additional law degree and a postgraduate diploma in Business Management. Mr Kumar also qualified as a Coating Inspector certified by FROSIO & the council for Norwegian standard NS 476.
He has worked with international paints, such as AKZO-Permoglaze, Fosroc, Jotun and Leighs paints prior to the assignment at Nukote Coating Systems.

Authored couple of technical papers and have conducted workshop on high performance coatings particularly on polyurea as pipeline coating.