E-mail: sguan@canada.com

21ST CENTURY UNDERGROUND STEEL TANK PROTECTION TODAY

by Peter Schwartz, Business Development Manager, Madison Chemical Industries Inc., Milton, Ontario, Canada, L9T 3Y5

ABSTRACT

Breakthroughs in 100% solids polyurethane coating technology have produced higher performing, non-flammable polyurethane coatings with 2000+ PSI adhesion to steel without primers and zero VOC’s. These coatings have produced environmentally friendly solutions to anti-corrosion tank protection while meeting the requirements of Underwriters Laboratories standards UL 1746 Part II and Part IV. They offer high physical properties and substantial performance, safety and environmental advantages over older systems and existing Fiberglass Reinforced Polyester (FRP) systems. The paper briefly reviews the history of underground steel tank anti-corrosion protection and discusses Underwriters Laboratories UL 1746, Part I, Part II and Part IV standards. It highlights the newest standard, Underwriters Laboratories UL 1746, Part IV and the advantages of systems that meet this new standard.

Keywords: 100% solids polyurethane, coatings, underground steel tanks, anti-corrosion protection, Underwriters Laboratories

INTRODUCTION

Existing anti-corrosion coatings for underground steel tanks have many drawbacks. The high performance systems are environmentally unfriendly, hazardous to the applicator and the environment and contain carcinogens. In addition, due to the VOC's being released into the atmosphere and community objections, some manufacturing locations are having their production curtailed. The flammability of the existing system has resulted in fires that have destroyed manufacturing facilities as well. A better safer, faster and less polluting system with better performance is required by the industry.

The new generation of underground steel tank anti-corrosion coatings is 100% solids polyurethane. This reaction product of an isocyanate and a polyol plays a vital role in many industries and in our everyday lives.

Recent developments in polyurethane technology have resulted 100% solids, instant setting, two component, highly cross-linked polyurethane coatings for use as anti-corrosion coatings for underground steel tanks. The new 100% solids polyurethanes offer higher performance and greater safety than older technologies all the while assuring maximum anti-corrosion protection. The new 100% solids polyurethane anti-corrosion systems meet the most rigorous requirements of the Underwriters Laboratories UL standard, UL 1746, Part IV.

The new 100% solids polyurethanes are safer and more environmentally friendly than traditional anti-corrosion coatings. Also, because of the 100% solids polyurethanes’ unique properties, they reduce the time necessary to apply the coatings. They can be applied regardless of ambient temperatures and dramatically reduce the overall time required to coat the tank.

In addition to these benefits, they meet the Underwriters Laboratories (UL) and Steel Tank Industry (STI) standards and specifications for underground steel tank anti-corrosion protection and have proven themselves over the years as excellent anti-corrosion coatings.

By definition, the term ‘100% solids’ means that the system does not use any solvent to dissolve, carry or reduce any of the coating resins. After reacting during application, the resins, which are normally in a liquid state, become a 100% solid film.

WHAT IS A POLYURETHANE?

Polyurethane is a very versatile thermoset plastic that was invented by Otto Bayer in the late 1930’s¹. Polyurethanes are created by reacting an isocyanate and a polyol (Figure 1). By carefully compounding various types of polyurethanes designed for specific uses and applications can be produced.

POLYURETHANE REACTION

Isocyanate + Polyol = Polyurethane

FIGURE 1 - Polyurethane Reaction

The instant setting nature of the 100% solids polyurethanes means that any thickness of coating may be applied in a one coat, multi pass operation. 100% solids polyurethanes also have the unique property of failing almost immediately if they are incorrectly applied or if there is a problem with the surface preparation or mixing ratio.

The Steel Tank industry is governed by two standards organizations., Underwriters Laboratories (UL) and the Steel Tank Institute (STI). There are two areas that each standard addresses. The first is the steel tank construction, which will not be addressed in this paper. The second area is steel tank anti-corrosion protection and this is what will be addressed in this paper.

The standards that will be discussed in this paper are the Underwriters Laboratories standards since they apply to all tank manufacturers regardless of whether they are members of the Steel Tank Institute or not. However, it should be noted that Steel Tank Institute (STI) standards meet the Underwriters Laboratories (UL) standards as a minimum and may be more stringent in some areas.

REVIEW OF UNDERGROUND STEEL TANK COATING HISTORY

Underground steel storage tanks have been in use for many years and during this period have progressed from unprotected steel to the most modern coating systems that provide superior anti-corrosion protection while being environmentally friendly. To appreciate the present day systems, it is worth looking at the history of the Underground Steel Tank coatings. The systems that we will review are:

Bitumen

Tape coatings

Coal tar epoxy

Cathodic protection

Underwriters Laboratories UL 1746, Part I

Fiberglass Reinforced Polyester (FRP) coatings and Underwriters Laboratories UL 1746, Part II

100% Solids Polyurethane coatings

Underwriters Laboratories UL 1746, Part IV

 

BITUMEN

Bitumen was the first anti-corrosion coating and is still in use in some applications. However, today, its main use is for waterproofing in the construction industry for below grade structures. It is easy to apply and relatively inexpensive but is environmentally unfriendly and can be removed by casual contact.

TAPE COATINGS

Tape coatings were used to protect underground steel tanks in an effort to reduce the steel tank’s exposure to corrosion from the surrounding soil. Although the tape did offer protection, it was difficult to apply properly, labor intensive, easily damaged and did not offer long term protection. Although tape coatings are still used in the pipeline industry, they have disappeared from the underground steel tank industry because of their inherent disadvantages and the higher performing and superior polyurethane coatings which are in wide-spread use today.

COAL TAR EPOXY

Coal tar epoxy was the next great underground steel tank coating. The coating itself provided superior anti-corrosion protection compared to tape coatings. Other advantages include more efficient and less expensive application, speed of application and long term protection.

Amongst the disadvantages of coal tar epoxy are its environmental effects (coal tar is a known carcinogen), safety of the painter, its long term protection and the VOC's used in the coating process. One of coal tar epoxy’s greatest disadvantages is the length of time required for the coating to dry. This created bottlenecks and delays in production. The coating process itself is more complex than tape coating, uses more sophisticated equipment than tape coating and better operator training is required.

The use of coal tar epoxy also introduced a new test, cathodic disbondment. Because the coal tar epoxy is bonded to the steel tank and might have a pinhole or the surface might be broken and corrosion might begin, it was important to know how resistant the coating was to cathodic disbondment or undercutting. Poor cathodic disbondment resistance would result in serious undercutting of the protective coal tar epoxy coating, the potential for large areas of coating to fall off and exposure of these areas to corrosion.

Coal tar epoxy’s cathodic disbondment resistance is not nearly as good as the modern 100% solids plural component polyurethane coatings. Over time this test has become a major qualification for any new anti-corrosion coating or system because of the nature of modern anti-corrosion systems.

Coal tar epoxies have few if any advantages as shown in Table 1 and they are no longer used as a major anti-corrosion coating in the steel tank industry.

TABLE 1

COAL TAR EPOXY COATINGS ADVANTAGES AND DISADVANTAGES

CATHODIC PROTECTION

When coal tar epoxy was being widely used, a new type of protection for underground steel tanks was developed and implemented. Cathodic protection attacked the corrosion process at the basic level. Rather than attempt to protect the underground steel tank from the effects of corrosion, cathodic protection used the electrical process of corrosion itself to protect the steel tank.

By attaching a sacrificial anode made of zinc or magnesium, the electrical effects of corrosion act upon the sacrificial anode and not the cathodically protected steel tank.

The sacrificial anode/cathodic protection system has become a standard system of underground steel tank anti-corrosion protection used in conjunction with tank coatings. Today, cathodic protection is receiving strong competition from systems dependent on the complete encapsulation of the steel tank with superior anti-corrosion coatings. These systems do not require any monitoring and save the tank owner these costs.

The advantages and disadvantages of the Cathodic Protection system are shown in Table 2.

TABLE 2

CATHODIC PROTECTION ADVANTAGES AND DISADVANTAGES

UNDERWRITERS LABORATORIES UL 1746, PART I ANTI-CORROSION TANK COATING SYSTEM

The next major step in anti-corrosion protection systems for underground steel tanks came from two sources. Underwriters Laboratories developed the Underwriters Laboratories UL 1746, Part I standard and the Steel Tank Institute (STI) STI-P3 which consists of three major components.

1) The steel tank made to Underwriters Laboratories UL 58 standard.

2) The anti-corrosion coating which is applied to the tank.

3) The cathodic protection system.

These standards are based on the principle of the complete encapsulation of the steel tank backed up by the cathodic protection of the sacrificial anodes.

The theory of this system is that if the anti-corrosion coating is not applied in such a manner as to completely encapsulate the steel tank or the thin coating is damaged during shipping or installation, the sacrificial anodes will continue to protect the steel tank.

The Underwriters Laboratories UL 1746, Part I standard specifies approved anti-corrosion coatings that have qualified by virtue of their passing the required physical and performance tests. Typically, these coatings are applied at a thickness of 10-15 mils.

Originally, the coatings used were the coal tar epoxies mentioned earlier. Today most, if not all, tanks are coated with 100% solid polyurethane resins. These will be discussed in greater detail later.

The Underwriters Laboratories UL 1746, Part I standard is shown below in Table 3. Of specific interest are the Resistance to Environmental Fluids Tests, the Abrasion Test, the Cathodic Disbondment Test and the Holiday Test.

UNDERWRITERS LABORATORIES UL 1746, PART I STANDARD

TABLE 3

UNDERWRITERS LABORATORIES UL 1746, PART I STANDARD²

The advantages of the Underwriters Laboratories UL 1746, Part I standard outweighed its disadvantages as shown in Table 4 and the use of this system was a major step in the anti-corrosion protection of steel tanks and became the most widely used system of anti-corrosion protection in the industry.

TABLE 4

UNDERWRITERS LABORATORIES UL 1746, PART I ADVANTAGES AND DISADVANTAGES*

* For the purposes of this comparison, it was assumed that a 100% solids polyurethane was used as the anti-corrosion coating.

FIBERGLASS REINFORCED POLYESTER (FRP) COATINGS

The next major advance in steel tank anti-corrosion protection was the development of the Fiberglass Reinforced Polyester(FRP) - coating system. This system is defined in Underwriters Laboratories UL 1746. Part II. Its effectiveness depends upon on the complete encapsulation of the steel tank by the Fiberglass Reinforced Polyester (FRP) coating. This coating consists of polyester resin externally reinforced with chopped fiberglass.

The Underwriters Laboratories UL 1746, Part II standard is shown in Table 5 below. Because the Underwriters Laboratories UL 1746, Part II standard does not require a sacrificial anode, the performance of the coating is critical. Note the Resistance to Environmental Fluids tests, the Corrosion Evaluation test, the Abrasion test, the Impact test and the holiday test.

UNDERWRITERS LABORATORIES UL 1746, PART II STANDARD³

TABLE 5

UNDERWRITERS LABORATORIES UL 1746, PART II STANDARD

The coating is applied at a thickness of 100 mils and requires a lot of hand work. It is critical that no glass fibers be exposed on the coating surface as they will wick moisture to the steel surface of the tank and corrosion will commence. The fiberglass itself is hazardous since the fibers are small and harmful if inhaled.

The polyester resin is extremely hazardous from a number of perspectives. It contains amines, styrene, VOC's and other toxic and carcinogenic components. In addition, the resin is very flammable and a number of fires at steel tank manufacturing facilities have resulted from this flammability. As you can see, the Fiberglass Reinforced Polyester (FRP) components and manufacturing process are dangerous and hazardous.

Although the cured Fiberglass Reinforced Polyester (FRP) is strong and will protect the tank from corrosion when the tank is completely encapsulated and the coating is undamaged, it is also very brittle and relatively porous. The possibility of shipping or installation damage is very real and there is no sacrificial anode to back up the Fiberglass Reinforced Polyester (FRP) coating. Its porosity makes it vulnerable to water infiltration and subsequent corrosion of the steel tank. See Table 6 for more information on advantages and disadvantages of Fiberglass Reinforced Polyester (FRP) systems.

TABLE 6

FIBERGLASS REINFORCED POLYESTER (FRP) ADVANTAGES AND DISADVANTAGES

Recent developments in Underwriters Laboratories UL 1746 and the continuing development of 100% solids polyurethanes have resulted in the development of Fiberglass Reinforced Polyurethane tanks. These Fiberglass Reinforced Polyurethane tanks meet and exceed Underwriters Laboratories UL 1746, Part II standard without any of the disadvantages of the polyester resin used in Fiberglass Reinforced Polyester (FRP) tank coatings. However, fiberglass is still used in these coatings and it required the development of a new tank coating system that met Underwriters Laboratories UL 1746, Part IV standard to eliminate the fiberglass from the coating. This will be discussed later.

100% SOLIDS POLYURETHANE COATINGS

A major advance in underground steel tank anti-corrosion protection was the development of 100% solids polyurethane coatings. These plural component (Part A and Part B) coatings have been widely used in the protection of steel underground tanks as per the Underwriters Laboratories UL 1746, Part I and/or STI-P3 specification for the past 20 plus years. During this time, over 300,000 tanks were coated with 100% solids polyurethane coatings and to date, no tanks have failed due to external corrosion of the tanks. Manufacturers of the STI-P3 tank continue to offer a 30 year warranty against external corrosion. ⁵

As mentioned earlier, 100% solids polyurethanes are now being used in the production of Underwriters Laboratories UL 1746, Part II specification tanks. With the continuing testing and qualification of 100% solids polyurethane resins, they now meet the requirements of Underwriters Laboratories UL 1746, Part IV standard.

These coatings have numerous advantages over other options such as coal tar epoxy and Fiberglass Reinforced Polyester (FRP). 100% solids polyurethanes contain no amines, styrenes or VOC's. They contain no carcinogens and are non-hazardous waste when cured. They are non-flammable and considered a safe coating. They are resilient, not easily damaged and easy to inspect for damage. They are also self-inspecting. If the coating is not applied correctly or the surface is badly prepared, the 100% solids polyurethane will signal the problem before the tank is buried.

100% SOLIDS POLYURETHANE SAFETY FEATURES

· No Solvents

· No Flammable Materials

· No Amines

· No Styrenes

· No Carcinogens

· Self Inspecting

Figure 2^4,5,6 -100% Solids Polyurethane Safety Features

For the tank manufacturer, they offer speedy curing, curing in any temperature and non-temperature dependent cure assuring increased capacity of the coating department and speedier delivery of completed tanks.

100% solids polyurethane coatings require specific complex equipment for application and the applicator must be well trained. They can cover complex shapes with relative ease and have superior adhesion to steel, of the order of 2,000+ psi.

Their advantages and disadvantages are shown in Table 7.

TABLE 7 - 100% Solids Polyurethane Coating Advantages And Disadvantages

UNDERWRITERS LABORATORIES UL 1746, PART IV ANTI-CORROSION TANK COATING SYSTEM

The latest development in underground steel tank anti-corrosion systems are Underwriters Laboratories UL 1746, Part IV 100% solids polyurethane anti-corrosion coatings. They have all the benefits of the 100% solids polyurethane coatings as well as the advantages of the Underwriters Laboratories UL 1746, Part II coatings. The Underwriters Laboratories UL 1746, Part II and Part IV standards are shown in Table 8.

UNDERWRITERS LABORATORIES UL 1746, PART II AND PART IV STANDARDS⁷

TABLE 8 - Underwriters Laboratories UL 1746, Part II And Part IV Standards

The Underwriters Laboratories UL 1746, Part IV standard differs from Underwriters Laboratories UL 1746, Part II standard in two major ways.

First, the Underwriters Laboratories UL 1746, Part IV standard does not require any external fibreglass reinforcement. Because of the self reinforcing nature of the 100% solid polyurethane it does not require external reinforcement of any kind, yet exceeds the Underwriters Laboratories UL 1746, Part II standards by a considerable amount. This allows the manufacturer to produce a higher performance tank more safely and in a more environmentally friendly manner.

Second, the exceptional strength of the 100% solids polyurethane enables it to meet and exceed the Underwriters Laboratories UL 1746, Part II standard at a thickness of only 70 mils versus the Underwriters Laboratories UL 1746, Part II standard of 100 mils. This means less coating and lower cost to the tank purchaser without any decrease in performance.

A comparison of the Underwriters Laboratories UL 1746, Part II/Part IV standard and typical results of a coating system that meets Underwriters Laboratories UL 1746, Part IV are shown in Table 9. Of particular interest is the amount by which the FibreThane coating exceeds the minimum values of the standards, the high performance of the coating system and the fact that all this is accomplished at a thickness of 70 mils versus the 100 mils thickness of the Underwriters Laboratories UL 1746, Part II specification.

UNDERWRITERS LABORATORIES UL TEST RESULTS OF FIBRETHANE VS UNDERWRITERS LABORATORIES UL 1746, PART II FIBERGLASS REINFORCED POLYESTER (FRP) STANDARD⁸

TABLE 9 - Underwriters Laboratories UL Test Results Of FibreThane Vs Underwriters Laboratories UL 1746, Part II Fiberglass Reinforced Polyester (FRP) Standard

100% solids polyurethanes that meet Underwriters Laboratories UL 1746, Part IV have many advantages over the Fiberglass Reinforced Polyester (FRP) coatings that Underwriters Laboratories UL 1746, Part II as shown in table 10.

TABLE 10 - Comparison Of Underwriters Laboratories UL 1746, Part II Fiberglass Reinforced Polyester (FRP) Coating To Underwriters Laboratories UL 1746, Part IV 100% Solids Polyurethane Coating

As can be seen, Underwriters Laboratories UL 1746, Part IV 100% solids polyurethane anti-corrosion coating systems offer superior performance, safer and more environmentally friendly application at a lower mil thickness than the existing Underwriters Laboratories UL 1746, Part II Fiberglass Reinforced Polyester (FRP) coating systems and will replace them. They truly offer 21st Century underground steel tank anti-corrosion protection today.

REFERENCES

Woods, G., The ICI Polyurethanes Book, (John Wiley and Sons, New York, 1987), p 8.

Underwriters Laboratories Inc., UL 1746, Part I, Standard for External Corrosion Protection Systems for Steel Underground Storage Tanks, Section 10-11.2, pp. 14-17

Mobay Corporation, "Polyurethane Coatings - Performance, Quality, Safety", video, Pittsburgh, PA, 1989).

Dow Chemical, Material safety data Sheets (amine, MDI isocyanate, polyol) - Various, 1985-1991

Ontario Ministry of Labour, "Air Quality Testing Reports", measurements taken yearly at the Madison Chemical Industries Milton, Ontario plant, (Toronto, Canada, 1987-1996)

Kennedy, H., "Over 200,000 STI-P3 Tanks Installed", (Milton, ON: Madison Chemical Industries Inc.., 1994).

Underwriters Laboratories Inc., UL 1746, Part II, Standard for External Corrosion Protection Systems for Steel Underground Storage Tanks, Section 14--16.8, pp. 21-27

Underwriters Laboratories Inc., UL 1746, Standard for External Corrosion Protection Systems for Steel Underground Storage Tanks, Section 14--16.8, pp. 21-27

Guan, Dr. S., Comparison of UL Test Results Between Madison’s FibreThane and FRP and Underwriters Laboratories Inc., UL 1746, Standard for External Corrosion Protection Systems for Steel Underground Storage Tanks, Section 14--16.8, pp. 21-27