Coatings trade-off triangle

Coatings trade-off triangle

Bob Buchanan, Seal For Life Industries, Country, weighs up the choice between FBE and liquid epoxy coatings.

“Fast, cheap, and good” is a concept known as the cost-quality tradeoff triangle which can be applied in a number of situations. I once heard about it in terms of automobiles with a salesman saying, pick any two, “Fast, Cheap or Good”. This is also commonly applied to project management or software development, but one can think of many other examples.  In writing this article, I started thinking of it in terms of coatings for pipelines that are intended for corrosion protection and/or mechanical protection if the pipe is destined to be directionally drilled.  For directional drilling, the coating, or coating system, must protect against corrosion but also possess resistance to mechanical damage, be it impact, abrasion or gouging.  Taking the concept to coating performance, I came up with a Performance triangle which includes Hard, Tough and Thick, and perhaps the pick two idea will work here.

Over the years I have had many conversations with spec writers and some believe that if a coating is thick enough then it will survive the directional drill and still perform as a corrosion layer. So thick & tough or thick & hard are the options.  However, how does that impact the corrosion protection aspect?  Good corrosion coatings tend to balance hardness with ductility so the trade-off maybe be difficult to attain.  Thickness without one of the other two is a problem.

Seal for Life Industries is a company that manufactures a number of types of coatings for pipelines and has specialty products designed for both corrosion and mechanical protection. In some cases, the products would be direct-to-metal corrosion protection, others designed for corrosion and mechanical protection and still more for mechanical protection alone.  Based on that experience and my viewpoint on the industry, I’ll walk through some of the options.

 

Popular main line coatings

The most common types of mainline coatings used around the world are multi-layer polyethylene and polypropylene. These coatings generally have a primary layer of epoxy and are considered as thick coatings of up to 3mm / 120 mils thickness or greater.  These may be used “as-is” for directional drilling depending on the conditions, but how will the field joint be treated is often the question.  Heat-shrinkable sleeves are globally the most widely used technology for pipeline field joint protection since they can closely replicate the mainline coating, but tend to have softer adhesive layers due to coating cutback step-downs, weld bead configurations and the need for field application versus plant application.  In these situations, there are a few ways to make a heat-shrink sleeve field joint suitable for directional drilling and different manufacturers will offer their own solutions.  One premier product that has many years of success is Seal For Life’s Covalence Dirax, which combines a primary corrosion protective epoxy layer applied to the steel, and a fiber reinforced heat-shrinkable sleeve that is extremely tough.  Thick and Tough would be the keys to performance, with the toughness coming from the reinforced backing and very high shear strength adhesive.  Other ways to augment a standard sleeve would be to add sacrificial wear cones or coat the complete sleeved joint with an epoxy or fiberglass reinforced wrap, hence, Thick and Hard.  Again, Seal For Life have options in these types of systems.

 

Fusion bonded epoxy

Another type of mainline coating is fusion bonded epoxy, or FBE, which is common in certain parts of the world and highly favored in the North America, Mexico and selectively around the world. These coatings tend to have preference in areas where good transportation infrastructure and construction methods will minimize the impact on fusion bonded epoxy mainline coatings since they are susceptible to mechanical damage.

This is a thin-film coating since it is typically applied at ~350 microns / 13-15 mils, so how the pipe coating would survive the damage from directional drilling is the question. Typically it won’t (not thick, not especially tough).  One method of protecting the pipe is to put on a second layer of FBE that essentially provides thickness and hardness.  Dual-layer FBE consists of a second layer of powder epoxy applied immediately after the corrosion layer.  This adds thickness and the outer layer is formulated to be more resistant to abrasion and gouging, i.e. Hard.

 

Liquid epoxy

Another method is to apply a thick layer of liquid epoxy. The trick with adding the layer of liquid epoxy is choosing which liquid epoxy. Seal For Life’s Powercrete brand of liquid epoxy products has a long history and continues to be a strong player in the competitive landscape of protective coatings for pipelines. Although it’s a brand under Berry’s Seal For Life division, it was developed about 30 years ago with the name’s first trademark application in 1990 by Power Lone Star Inc.  In 2001, the company was acquired by Tyco Adhesives and incorporated into their line of pipeline corrosion protection products.  Tyco eventually sold the pipeline corrosion protection business and today we are a division of Berry Global Inc. with a suite of pipeline products, including Covalence mentioned earlier and Powercrete.

In looking at the liquid epoxy market, there are many products on the market that are designed as pure corrosion protection so it may not be enough to simply add thickness if the product does not have sufficient hardness or toughness. For everyday use, commodity epoxies have found a niche due to low cost and high volume sellers, although one must be cautious in terms of performance.  Powercrete products on the other hand have been viewed as higher performance products and largely used for directional drilling applications.  However, Powercrete is more than one product, it’s a series of products that includes;

  • Powercrete DD, an abrasion resistant overcoat (ARO) commonly applied to FBE coated pipe in a coating plant
  • Powercrete J & R60, field applied joint coating products commonly used with DD coated pipe
  • Powercrete R65 F1, a fast cure, field applied direct to metal corrosion and abrasion resistant coating
  • Powercrete R95 & R150, higher temperature resistant products for direct to metal corrosion and abrasion resistance
  • Powershield XT, a specialty polyurethane based outer wrap for severe service mechanical protection

Why so many products? Perhaps it comes back to the performance triangle, but in many cases it comes down to user preference, whether that is a specifier who has history of performance or a contractor who likes the handling characteristics of one product versus another.  With Powercrete’s long history of performance, some customers may use the commodity epoxies for low risk applications and Powercrete when there is any concern of potential for damage or when performance really matters.

Powercrete DD liquid epoxy which can be applied at some point after the pipe has been coated with the FBE corrosion resistant layer. Often this is done by the same pipe coater who applied the FBE but in a different location of the plant since it is spray applied using specialized equipment.

The application of liquid epoxy does tend to be more expensive than powder epoxy, but specifiers and end users who have many years of success on critical projects will choose the option simply due to history of performance. Maybe this gets to the cost-quality tradeoff triangle I mentioned at the start of this article. Aside from the directional drilling applications, direct to metal corrosion protection on field joints or special sections coated in a plant or in the field are common applications for Powercrete.

As a final comment ending where I started, maybe ‘Good’ is most critical when choosing a coating for a pipeline that must survive underground for many decades.