RESICOAT Powder Coatings for pipelines

By Akzo Nobel

Pipelines are the most important medium for the transportation of oil, gas, petrochemicals and water. It is vitally important that pipelines operate safely throughout their lifetime.

The general properties typically required of the coating are:

• Strong adhesion of the coating to the surface of the pipe in order to avoid delamination. Surface pre-treatment and proper curing of the coating influence this property.
• The coating needs to withstand various mechanical forces during transportation, handling, commissioning and operation and must maintain good mechanical strength even at higher temperatures.
• A high degree of flexibility at lower temperatures.
• Cathodic disbonding must meet the requirements of recognized international standards.
• The coating must resist chemical and bacterial attack.

Three-Layer systems, stand-alone Single Layer FBE and Dual-Layer Systems for corrosion protection are all well known in the pipeline industry.

Over the past 30 years the development of Epoxy powder coatings with improved properties has continued together with application process improvements and an increase in technical requirements. FBE powders are generally based on epoxy resin, curing agent, fillers, pigments and additives.

What are the current trends?

Three Layer Systems
Three Layer systems, based on epoxy powder coating primer, adhesive and PE topcoat, are well-known in the pipe coating industry and have been available commercially for many years.

Many specifications for Three-Layer systems require a layer thickness of only 50-100µm. Experience has shown that the possibility of blistering of an epoxy film increases at these lower film thicknesses. We recommend that standard epoxy primers are applied at a film thickness range of 150-300µm. This means that epoxy primers have to be flexible enough to avoid micro-cracking during handling and transportation which may result in the onset of the corrosion process.

Developments in Three-Layer systems have been made to improve the long term performance of the system in response to issues arising from adhesion failure in operation. Changes include a move to higher density PE or even PP in order to reduce water permeation and to provide a tougher coat.

Epoxy powder coating primers for Three-Layer systems generally have a high filler content and therefore show only limited flexibility. The simplest solution to increasing corrosion protection is to increase the thickness of the epoxy layer to 200-300µm. However, if the epoxy primer has limited flexibility then micro-cracking can occur during handling and transportation, resulting in the start of the corrosion process.

A very high standard of surface preparation is required in order to provide the optimum adhesion of the epoxy primer and hence to protect against delamination of the coating system under severe conditions, as in certain thermal cycling tests. In addition increasing adhesion will minimise the potential corrosion effects resulting from shielding of the impressed cathodic protection current by the coating system.

The above-mentioned points can be eliminated by using a high-flexible universal epoxy system such as Resicoat ® R-726 as a primer in Three Layer systems at a recommended film thickness of 200-300µm.

Single Layer FBE Coatings
Resicoat ® R-726 can also be used as a stand-alone 1-Layer FBE coating at film thicknesses up to 500 µm. Resicoat ® R-726 has excellent flexibility properties even at 500µm. Flexibility is measured according the CAN/CSA specification at -30°C and fulfils the 3° bending requirement.

Dual-Layer Systems
Dual Layer FBE coatings were originally developed to provide greater damage resistance in directional drilling applications.

In this coating system, two stand-alone FBE coatings are applied successively by electrostatic spray in one spray booth and the overspray of both layers can be recycled and applied immediately after the first layer and immediately before the second. Through this combination, the resulting film performance characteristics are better than the sum of the individual coatings.

The first layer Resicoat ® R-726 has the excellent adhesion and corrosion-resistance properties of a Single Layer FBE coating. The second layer typically is deposited during the melt stage of the first layer and results in an intimate chemical bond between the two layers. The key to the superior properties of the Dual Layer coating system is the special formulation of the second layer. We recommend a combined total film thickness of 600-1200µm.

Dual Layer Systems Product Range

Gouge resisting system
Pipes can be exposed to various mechanical forces that cause damage to the coating. With a gouge resistant Resicoat ® FBE topcoat, this system ensures optimal protection while laying, handling during backfill and during horizontal drilling.

Flexible system
This system ensures excellent flexibility even at film thicknesses up to 1100µm. It even exceeds the requirement of CSA Z245.20-06 12.11 at -30° C with bending of 3.0°.

Anti-Slip System
During pipe laying from a lay barge it is important to prevent the pipe slipping out of the concrete weight coating. This can be achieved by applying a rough FBE coating as a second layer which increases the bond strength to concrete by a factor of 4. Pipe handling is improved through increased friction. In addition, the anti-slip properties of the pipe surface make it safer for walking on wet pipe.

UV-resistance
A polyester powder on top of a Dual Layer system will ensure good resistance to sunlight exposure. The polyester layer absorbs the UV radiation that causes degradation and erosion of the FBE coating. This provides long life-time protection for above-ground pipelines.

New Trends in Pipe Coating – Coatings with higher glass transition temperature
Oil and gas reserves are becoming increasingly difficult to locate and exploit, therefore oil and gas companies are developing pipelines that will transport products at higher temperatures. Deeper drilling and more efficient transportation requirements result in the need for pipelines operating at increasingly higher temperature and pressure.

The challenge now is to formulate FBE coatings with higher glass transition temperature in order to meet the higher specifications required now and in the future for successful operation in these demanding environments.

A key aspect of coating performance at high temperature is to maintain adhesion. If the adhesion is lost, cathodic protection systems may fail to protect the pipeline. For high operating temperature performance one of the most fundamental properties in selection of the FBE coating is its glass transition temperature. In addition the coating must withstand a host of stresses which a hot environment produces and which derive particularly from thermal cycling. It must also possess excellent oxidative stability and still deliver the anticorrosion performance expected of a fusion bonded epoxy.

Akzo Nobel has developed Resicoat products that are based on specially designed polymers and are available in different Tg levels.

What is the answer?
Resicoat ® R-556 is suitable for use in combination with commonly-accepted PP for higher operating service temperatures and formulated with a Tg2 of 130° C ± 5. This technology is also suitable for Single Layer application with an applied thickness of 300-500µm and it exhibits excellent flexibility.

Resicoat ® Middle-Tg products are based on specially designed polymers and are available in different Tg grades: for example Resicoat ® R-556 which is suitable in combination with widely accepted polypropylene for higher operating service temperatures. This unique technology is also suitable as a stand-alone FBE-product in a layer thickness up to 500µm.

About Akzo Nobel
Akzo Nobel is the world’s largest manufacturer of coatings and is the world leader in supply of powder coating technology. It has over 27 manufacturing sites worldwide, providing coverage across all continents.

Akzo Nobel through the Resicoat ® functional powder coating division, with its global headquarters near Stuttgart, Germany, has been a supplier of fusion bonded epoxy (FBE) powder coatings since the early 1960s. The coatings have been used to provide long term corrosion protection of pipelines, valves, fittings and rebar in an extensive range of environments for above and below ground applications.

For further information please contact us at info@resicoat.com or visit www.resicoat.com.