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By Dan Zienty, Project Design Leader,
Short Elliott Hendrickson Inc.

While water storage tanks vary greatly in size, style and design, they share the same problem: coating failures. One of the most common and visible failures is rustbleed that occurs "Above the Waterline".

Evaluations conducted on numerous water storage tanks have revealed that areas above the waterline, such as along the edge of structural roof supports, between intermittent welds, around the compression ring, upper shell ring and lap joints are notorious locations for premature coating failures. In its initial stage, rustbleed is not so much a coating failure as it is a deficiency resulting from the coating process. The appearance of rustbleed is that of a reddish-orange discoloration and streaking along the surface or crevices. While not detrimental at first, rustbleed can lead to very costly maintenance repairs if left unchecked.

Rustbleed occurs when condensation settles in areas above the normal waterline. Water collects on areas like structural members and crevices where it's difficult to apply the coating system properly. Over time, these areas become a target for premature failure in the form of cracking and peeling. If corrective action is not taken, rustbleed can progress to a more serious corrosion problem. such as steel delamination, which may require structural repair or replacement.

In specifying the most appropriate solution for a particular structure, it is important to consider the following details:

• The total lineal length to be sealed
• The expected long- or short-term effectiveness of the method
• Mobilization (required rigging), ease of application and/or process involved

Cost

Prevention or elimination of rustbleed can be accomplished utilizing any of several accepted methods. The most common solutions include:

• Seal welding
• Caulking
• Seam sealing

While all three methods can be effective rustbleed solutions, each has its own advantages and disadvantages.

Seal Welding

The process of seal welding along lapped plates, at roof support connection points, or in lieu of bolted connections, provides an effective means of tackling potential long-term rustbleed problems. Seal welding permanently fills in small gaps permitting better surface-to-surface transition from edge to plate where rustbleed begins and where pull-back and peeling can occur. Completed welds can withstand the fluctuations brought about by thermal expansion and contraction, as well as plate stresses that accompany normal variations in tank water level.

On the other hand, seal welding could pose problems related to aesthetics and surface preparation. Since most tanks are constructed of 3/16 to 1/4 inch roof plates, the heat generated by complete seal welding across the surface may cause unattractive plate distortion or warpage. The severity of such distortion or warpage will also be affected by plate layout. In addition, welding in an overhead position is difficult and requires a skilled welder. On new construction, this is usually not a factor.

Surface preparation for seal welding is critical. On old tanks, the existing coating system must be sufficiently removed to prevent contamination of the weld. In both reconditioning and new construction, welding requires the added step of removing imperfections such as weld spatter prior to abrasive blasting and coating. Finally, the element of efficiency should be considered. The seal welding process is more time consuming as the installation rate is only around 25 feet per hour, and the cost runs about $18 to $24 per lineal foot.

Caulking

The use of caulk or elastomeric sealants is another method of treating typical joints and other points of contact. Sealants such as "Sikaflex-1a", manufactured by Sika Corporation, are single-component, polyurethane-based, high-performance adhesives. They are ANSI/NSF 61 certified for contact with potable water. High-performance adhesives such as these offer excellent aging and weathering characteristics, good resistance to water, are capable of +- 25% joint movement for high elasticity, and can be applied at temperatures ranging from 40 to 100 degrees F.

Caulks or sealants, depending on manufacturer, can typically be applied either before or after the finish coat. Application is made using standard sealed cartridges at a coverage rate of about 12 feet per cartridge, based on recommended maximum joint size. Installation costs will vary by region. Current application costs in the Midwest range from $4 to $10 per linear foot.

Limitations to the use of caulks and sealants depends on intended use, their compatibility with both the surface they are applied over and the coating system that may be used as a finish coat. Caulking is meant for situations where minimal gaps are present and where brush or spray application of the coating system may produce ineffective long-term protection. Adhesion testing may be necessary where sealants are top-coated with certain non-convertible or oil-based coatings used in repair of older tanks. Further, as sealants are not permanent, periodic evaluation is recommended to assess the need for maintenance involving possible reapplication.

Seam Sealing

The application of seam sealers, also known as epoxy fillers, may also be used in potable water. ANSI/NSF Standard 61 certification is not required as long as the surface filler is top-coated with a Standard 61 approved coating system. Epoxy filler/sealers are normally 100% solids, non-shrinking, modified amine epoxies. They exhibit high bonding strength and outstanding abrasion resistance. Being epoxies, these sealers are a two-component product. Application requires scarification of the substrate by brush blasting followed by a thorough cleaning. Epoxy sealers are normally troweled on to a thickness of up to 2 inches and can be applied at temperatures down to 35 degrees F. Installation costs are somewhat comparable to the elastomeric sealants at about $3 to $6 per linear foot.

Disadvantages to epoxy sealers compared to weld sealing and caulking includes their inflexibility, short pot life, and additional time required to complete larger installations. Because epoxy products are 100% solids, application along flexible lap plates could result in premature cracking. Additionally, since these fillers are applied by trowel or putty knife, getting the material into small crevices can be difficult and time-consuming. This difficulty in application, coupled with the fact that epoxies have a minimal pot life of approximately 15 to 30 minutes, can become a significant factor to the applicator if too much material is mixed at one time.

Conclusion

So, which is the better process? Which is the better product? Again, the distinct choices discussed here should center around the key conditions of: length of seal; effectiveness; ease of application; and cost. On new construction, the initial cost for rustbleed prevention is an insignificant factor in the total project cost. Up-front consideration affords no redundancy in process.

As a follow-up to the periodic maintenance evaluation of existing water storage tanks, consideration of rustbleed alternatives should be weighed based on the severity of the rustbleed problem and age of the structure. Since these areas are above the water line, progression of the corrosion can be quite slow.

Finally, does seal welding, caulking and seam sealing represent the only real solutions? The answer is definitely no! Design alternatives requiring fewer structural members do exist and should be discussed with the tank manufacturer and/or engineer during the planning stage. There is also the "do nothing" approach, which places reliance on the painting contractor to adequately coat the potential failure areas that do occur "Above the Waterline".

Biography

Daniel J. Zienty is an Associate/ Project Coordinator—Protective Coatings in the Water/Wastewater Practice Center for SEH. He has a Bachelor of Science Degree in Construction Technology from Purdue University, and is a participating member of the
National Association of Corrosion Engineers International (NACE) and Society for Protective Coatings (SSPC).

He is also certified as a Protective Coatings Specialist with SSPC, Coating Inspector with the National Association of Corrosion Engineers (NACE International) and as an Associate Welding Inspector with the American Welding Society (AWS).

Bibliography

• Sika, Construction Products Catalog, April 1996, p. 2.1.5
• Tnemec, Data Sheets, "Filler and Surfacer," September 1996.
• Association, American Water Works, ("Welded Steel Tanks for Water Storage," AWWA D100 -96), March 1, 1997.
• Coatings, Society of Protective, Good Painting Practice, Pittsburgh, Steel Structures Painting Council, 1989
• JPCL, Collected JPCL Issues June 1984-May 1994, 1989, February.