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Testing Underground PropaneTank Coatings

For more information on this and other research projects,go to www.propaneresearch.com.

Assessing coating performance

Residential Use ofUnderground Tanks

Technology Fact Shee

Steel underground propane tanksfor residential use are growing inpopularity because they offerseveral advantages:

• Long life. 

• Flexibility to support indoorand outdoor applications. Tanks can fuel outdoor rooms(see below) and indoorappliances (such as waterheaters, furnaces, and clothesdryers) and provide standbypower for backup generation.

• Availability in a wide rangeof sizes. Residential tanksizes typically range from 150to 1,000 gallons toaccommodate varying needs.

teel underground tanks store safe, clean, reliable propane for awide range of residential needs. Discreetly installed out of sight,Sthe tanks are in constant contact with moisture, bacteria, and

dissolved minerals in the surrounding soil. Left unchecked, theseforces can wear away the surface of a tank, potentially graduallydamaging it.

To repel damaging elements, underground tanks have a mastic orthermoplastic outer coating. Used in conjunction with cathodicprotection, which counteracts the corrosive effects of natural electricalcurrents in the earth, tank coatings can help ensure that undergroundtanks are protected.

Proper tank surface preparation and coating application is essentialfor the tank coating to afford maximum corrosion protection. Factorsthat may affect the corrosion protection of a tank’s coating includepinholes, gaps in the coating, disbondments (loss of adhesion), andincomplete application (inadequate coating thickness, typicallybetween the tank and legs). Some coatings may also react differentlyto varying ground types and other environmental elements.

To select the best tank and coating for a specific user or application,propane marketers and retailers need information about theeffectiveness of different coating types under certain environmentalconditions and the effects of variation in tank surface preparation andcoating application.

To obtain meaningful information on the performance and applicationof various underground propane tank coatings, thePropane Education & Research Council (PERC)initiated Testing and Evaluation of Underground Propane Tank Coatings (Docket 12469). As part ofPERC's equipment comparison testing program, theproject's goal was to achieve a clearer understandingof how well coatings protect the exterior surfaces ofsteel underground storage tanks.

The study's objectives include the following:

• Identify which levels of surface preparation and

which coating systems offer the best protection.• Determine which areas on tanks are typically the

least protected, dividing the tanks into zones (top,bottom, sides, ends, legs, and welded seams).

• Understand the consequences of a weakenedcoating on the corrosion protection afforded bythe coating system.

Project Description

Underground ResidentialTank Installation

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Project Partn

KTA-TATOR, William D. Cor

Technical Services Mana115 Technology DPittsburgh, PA 15

412-788-1

Project Implementation

Project Completion: Key Conclusions

The project evaluated the performance of variouscoatings on steel underground storage tanks under avariety of service environments, simulated byweathering, chemical exposure, and thermal cycling.

Researchers purchased seven 500-gallon sample

tanks that represent the types of coatings used by fiveU.S. underground tank manufacturers: AmericanWelding & Tank, Trinity Industries, Quality Steel Corp.,Thompson Tanks, and Liberty Tank & Vessels.Researchers assessed the performance of the tank’scoatings through the following steps:

• Evaluation of the initial condition of tanks' coatingsusing low-voltage holiday (coating discontinuity)and pinhole detection, nondestructive coatingthickness measurements, and infraredspectroscopic analysis.

• Collection of coating test samples from differentplaces on each tank (see sidebar).

• Mechanical testing for Group 1 samples, includingimpact resistance, adhesion, pencil hardness, andabrasion resistance.

• Environmental exposure testing for Group 2samples, including accelerated weathering; 30-,60-, and 90-day immersions in five differentchemical solutions (nitric acid, sodium chloride,sodium hydroxide, sodium bicarbonate, anddistilled water); and thermal cycling. The samplesthen underwent mechanical testing similar to thatperformed on Group 1 samples.

Mechanical integrity tests were conducted to

determine whether different areas of the tankexperienced differences in the effectiveness of thecoatings. Testing of the Group 1 samples found noarea to be consistently inferior in performance.

However, when the Group 2 samples were exposed tochemical solutions, five of the seven tanksexperienced damage from the solutions on the weldedseam areas more than other areas.

Mechanical integrity tests conducted on these Group 2samples after environmental exposure also indicatedareas that performed consistently:

July 2

For More Information:

Propane Education & Research Counc ilGregory KerrDirector of Research and Development1140 Connecticut Ave. NWSuite 1075Washington, DC 20036202-452-8975

www.propaneresearch.comwww.usepropane.com

• On three of the seven tanks, the welded seamareas performed worst.

• On four of the seven tanks, side wall samplesperformed best.

Testing also compared the effectiveness of differentcoating types. Powder coatings consistentlyoutperformed liquid coatings; three of the fourtop-performing coating systems were powders.Performance inconsistencies across tanks may haveresulted from improper tank surface preparation andcoating application.

With the results of this study, buyers of undergroundsteel storage tanks will be more educated regardingthe types of coating systems for different serviceenvironments, and be more confident about thecorrosion protection they are purchasing for theirtank.

Collection of Test Samples

Group 1: Mechanical TestingFive 6-inch-by-6-inch samples were taken from the followingplaces on each tank: a leg attachment, the end, the top,the side, and a weld point. Oversized samples wereremoved and were then carefully cut to size using a bandsaw to ensure that the tank coatings were not damaged.

Group 2: Environmental and Mechanical TestingFrom each tank, 25 4-inch-by-6-inch samples were taken,as follows: 10 samples from the weld line, 10 samples fromthe side of the tank, and five samples from the areaswhere the tank legs are attached to the tank.

Legs

Legs

Side view of tank Bottom view of tank  

Side view of tank 

Weld line sample

Tank top sample

Leg attachment sample

End sample

Weld line sample

Tank side sample

Leg attachment sample

Tank side sample