January 2013, Vol. 25, No.1

Extra

Better coatings standards protect welded-steel tanks

Paul Trautmann

Welded-steel water storage tanks have been in use for more than 100 years. The oldest registered and functioning tank, located in Whitewater, Wis., was built in 1889. Coatings always have been considered an essential part of the corrosion control process to protect steel surfaces, such as those in water storage tanks.

Corrosion in steel water storage tanks, ship tanks, and petrochemical tanks has been studied and corrosion control measures developed for the interior, underside, and exterior surfaces of these tanks. The main causes of early corrosion on surfaces that have been coated include inadequate installation, insufficient dry-film thickness (DFT), poor film thickness on sharp edges or irregular surfaces, pinholes, voids, “holidays” (discontinuities, such as holes), or incompatibility with cathodic protection.

In October 2011, the American Water Works Association (Denver) put into effect the latest version of its D102 standard for coating welded-steel water storage tanks after a lengthy consensus-based industry evaluation of coating technologies. The standard establishes minimum requirements for steel surface preparation and generic classifications of outside coating systems and inside coating systems (ICS).

New coating standard

Most notable among the items that changed was the revision to ICS No. 3 (ICS-3). The ultrahigh solids, medium-film coatings now recognized by ICS-3 have advanced design features meant to significantly mitigate early corrosion failures. The attributes of these coatings include edge-retentive properties, higher solids and lower solvent content, high flash point, medium film thickness of 20 to 50 mm, 24-hour curing cycles, and NSF/ANSI 61 approvals. The D102 revision demonstrates acceptance by the water storage industry to advance its technique of preventing corrosion in welded-steel water storage tanks in the most sustainable manner possible.

Until the latest revision, the D102 standard encompassed only thin-film epoxy lining systems, generally two- or three-coat systems that utilize moisture-cure, zinc-rich primers or epoxy primers followed by epoxy intermediate and topcoats. The minimum total thickness of thin-film systems is 8 to 12 mm. Now, ICS-3 provides for the use of epoxy coatings with 96% or greater volume solids to protect the interior surfaces of steel water storage tanks where the minimum film thickness is 20 mm DFT.

Learning from other industries

For many years, the U.S. Navy was plagued with early corrosion and coating failures while trying to maintain chemical holding tanks, water storage tanks, ballast and fuel tanks, and other critical ship structures in immersion service. These structures are full of hard edges, weld seams, irregular surfaces, and hard-to-access surfaces, much like the roof and rafter systems typically seen in steel water storage tanks.

In 1995, the Navy began a 5-year program to improve its materials and practices for preserving tanks with coatings. At that time, the Navy’s traditional solventborne thin-film tank-coating systems had a life expectancy of 2 to 10 years. The goal of the program was to extend the service life of coating systems beyond 20 years.

The 2001 article, “The U.S. Navy’s Advances in Coating Ship Tanks” in the Journal of Protective Coatings and Linings, concluded that adequately protecting sharp edges and weld defects, as well as controlling the preparation and application of coatings, are the keys to ensuring performance. According to the article, a “thorough review of Navy tank coating failures revealed failure modes consistent with edge failure — especially in sharp-edged stiffeners, failure initiation at weld defects and weld spatter, premature failures at previously repaired areas, insufficient paint thickness, inadequate surface preparation and residual contamination leading to blistering and delamination, and inappropriate environmental conditions during surface preparation and coating application.”

The Navy also identified several solvent-free epoxy coatings to address this issue, including edge-retentive formulations designed to be applied at 20 to 40 mm DFT. These products were intended to prevent early failures at edges and irregular surfaces where traditional epoxy shrinks during the curing phase and was providing inadequate performance. A medium film thickness of 20 to 40 mm was identified as the best-performing DFT. The ultrahigh-solids, edge-retentive formulations generally improved the thickness and performance of the anticorrosive barrier on these irregular surfaces.

Seeing the promise in these medium-film coating systems, the Navy in 2003 adopted Performance Specification Coating Systems for Ship Structures (MIL-PRF-23236C) — most recently updated in 2009 as MIL-PRF-23236D — which directly addresses the importance of edge-retention performance by specifying that “the retained percent average coating retention on a 90 degree outside edge of no less than three specimens shall be an average minimum of 70% of the measured dry film thickness on the flat areas of the test specimen.”

Some manufacturers began to develop solvent-free coatings to meet this standard, and compliant coatings were issued a qualified products list (QPL) number. By 2006, the Navy was seeing results. New studies showed that approved coatings increased the practical service life in a chemical holding tank from 2 years to 10 years and in a ballast tank from 3 years to 20 years.

Applying it right

When identifying the interior tank space, it is common to picture it divided into two parts — interior normally wet and interior normally dry. Interior wet spaces are subject to constant immersion and typically include the wall and floor areas of the water tank from the overflow down. These areas include the interior ceiling or roof, support rafters, girders, and crow’s nest, all of which are located above the top capacity level but still within the tank’s vapor zone.

The vapor zone is a harsh environment and an area of concern, especially for maintaining structural integrity. Coating systems must perform without the support of cathodic protection. Roof and rafter spaces contain many hard-to-reach areas, hard edges not properly ground smooth, and multiple angles to paint. This vapor space continues to cycle from wet to dry and hot to cold and is ventilated minimally.

Due to the difficulty of accessing and coating these areas, two-coat systems typically are selected — often multicoat systems that use proven organic-zinc rich primers or epoxy primers with edge-retentive characteristics. However, ultrahigh-solids, medium-film-finish coat formulations meeting MIL 23236 QPL-approval improve the thickness and performance of the anticorrosive barrier coating while using the same number of application steps.

From the overflow down, walls and columns can be accessed with traveling scissor lifts or scaffolding. The floors are also more accessible. Applicators can take advantage of these open spaces to apply a single coat of 96% volume solids or greater epoxy at 20 to 50 mm DFT and achieve a holiday-free surface. This single-finish coat applied directly to prepared steel surfaces provides improved process efficiencies for applying, holiday inspecting, and curing of potable-water storage tank linings for interior wet spaces.

Application equipment for ultrahigh-solids, medium-film coatings also has advanced. ICS-3 epoxy coatings typically are applied by brush or spray. Stripe coats are applied by brush over weld seams, edges, or irregular surfaces and can be immediately coated “wet over wet” with spray-applied material.

Ultrahigh-solids epoxy coating with greater than 96% volume solids cure rapidly but also have an extremely short pot life. Due to the limited amount of time available for application after the coating system components are mixed, this type of coating (depending on the manufacturer’s formulation) is best applied with “mix-on-demand” heated plural component equipment.

Contractors working in the water storage industry are becoming increasingly familiar with these systems, often having been trained by The Society for Protective Coatings (Pittsburgh) with courses on airless spray application, abrasive blasting, and plural component basics to elevate their knowledge-based skills. The society provides these and quality management system certifications to contractors and also provides a means to qualify contractors’ performance abilities and track records.

Calculating financial effect

Evaluating the financial feasibility of a coating system and comparing one system versus another requires certain assumptions about initial costs and total service life of the installed system. Initial fees associated with installing a coating system include the cost of paint on a project, surface preparation, application, inspection, ventilation/dehumidification, repairs, curing, and disinfection. After installation, factors associated with the coating system’s longevity also should be considered in looking at long-term performance of a coating system.

The study, “Expected Service Life and Cost Considerations for Maintenance and New Construction Protective Coating Work,” provides a model to calculate a particular coating system’s total service life. J. Helsel, R. Lanterman, and K. Wissmar presented this study at the 2008 NACE International Corrosion Conference and Expo. The total life of a coating system, according to the model, starts with the practical service life expectancy combined with routine maintenance painting sequences:

  • The practical life, P, is considered to be the time until 5% to 10% coating breakdown occurs (SSPC-Vis Grad 4) and active rusting of the substrate is present.
  • Typical maintenance painting sequence includes touch-up painting that occurs at P service life, as defined.
  • Maintenance repaint occurs at P life + 33% (P × 1.33).
  • Full repaint occurs at Year of Maintenance Repaint = 50% of P life (P × 1.50).
  • By making these calculations for each of the system’s painting operations, the true cost of painting operations can be compared and the coating selection made on a comparable basis, according to the authors.

Three full cycles of apply, maintain, and repaint were chosen using the model’s calculation points so that the results could provide insight into the maintenance cycle and total service life of each system, as well as the cost values for a period of more than 50 years. The table (above) shows the results.

In this evaluation, three cycles of ICS-1, which refers to a thin-film epoxy method, provided only 44 years of operation at a 2012 present value cost of $246.50/m2 ($22.90/ft2). Three cycles of ICS-2, which refers to a separate thin-film epoxy method, provided 71.5 years of operation at a 2012 present value cost of $192.90/m2 ($17.92/ft2). Finally, three coating cycles using ICS-3 produced 93.5 years of operation at a 2012 present value cost of $177.18/m2 ($16.46/ft2).

Lasting longer

The chemical formulation of ultrahigh solids, medium-film coating systems translates to low volatile organic compound content because of low- or no-solvent composition. Low solvent content in the coating system provides a better method to pass extraction tests, mitigate taste and odor issues, and ensure water quality for customer distribution.

These coatings now provide a longer proven track record of performance compared to reformulated thin-film epoxy and a safer means to comply with continued rule changes from air quality management districts.

As was the case with Southern California’s South Coast Air Quality Management District in 2006 and the Ozone Transport Commission (a multistate commission that advises the U.S. Environmental Protection Agency on ozone transport and control issues) in 2009, restrictions on volatile organic compounds for industrial maintenance coatings — in this case, pertaining to tank linings for water storage — are becoming more restrictive.

Improved application efficiencies are gained when using the ultrahigh-solids, medium-film coatings. These efficiencies effectively reduce installation costs.

The design features of these systems include single-coat applications capable of 24-hour cure-to-service. Single-coat applications reduce the extra time and cost involved in applying multiple-coat, thin-film systems, which normally require 5 to 12 days of controlled ventilated curing before a tank is disinfected and subsequently filled.

Also reduced are the time and costs associated with rental and power fees for dehumidification and heating units within the tank work space. Shorter curing cycles reduce stagnant job activity and return the asset to an income-generating condition more quickly.

 

Paul Trautmann is a corrosion specification specialist and a NACE International (Houston)-certified inspector at The Sherwin–Williams Co. (Cleveland).