Combating Corrosion with Coatings

A look at condition assessments involving sttel tanks & structures and a case study

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by Tony Ippoliti

• A look at condition assessments involving sttel tanks & structures and a case study

It's always a challenge to manage steel process and wastewater tanks and structures to minimize the total cost of owning and operating them while delivering required service levels. But one significant contributor to taking the headaches out of asset management is the adoption of standard corrosion maintenance procedures for these tanks.

Establishing an annual or bi-annual inspection cycle early in a tank's life cycle is one way to help minimize future maintenance costs. Pair this simple strategy with a thorough coating condition assessment checklist, and corrosion maintenance for steel systems will be effective.

Coating Condition Assessment

Coating condition assessments begin by recording a structure's size, diameter and height along with the types of exposures to which it is subjected. Whether these conditions are acidic, high temperature, caustic or atmospheric in nature, it's important to identify every characteristic to ensure all factors are included.

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Polyurea coating is applied inside water tank at Indianapolis facility, operated by Veolia Water North America, in November 2006.
Click here to enlarge image

The next step is recording relevant data about the structure's appearance, coating integrity, adhesion and substrate condition, along with the operating environment and public view.

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Click here to enlarge image

Other variables to analyze with any coating condition assessment of a steel structure are the rust grade, presence of lead and extent of damage to the structure, if any. Thorough inspection for any defects such as mildew, alligatoring, pinholing, chalking and discoloration provide another safeguard in proactively monitoring corrosion and ensuring the upkeep of any structure. Determining the percentage of the structure with defects and recording extent of the exposure are two additional key factors essential in evaluating this data.

After the coating condition assessment is complete, the priority level of the asset can be determined and next steps will be decided. The structure may need extensive work and already have heavy corrosion with steel loss, or it could be considered out of service and require demolition. Another result of the corrosion survey is determination that the tank or equipment could simply require surfaces to be freshly painted, or that no additional work is required.

Regardless of how thorough a corrosion maintenance protocol may be, and how systematically the schedule is upheld, there are times when a structure's coating will need to be replaced. When this occurs, the owner and supplier should re-evaluate the tank's condition and consider new coating options to ensure the structure's future reliability.

A Case Study

Veolia Water North America LLC, which runs the water utility in Indianapolis, IN, had a tank losing 1,000 gallons weekly from its 350,000-gallon capacity. Though less than 10 years old, the water-storage function of this bolted tank obviously had failed. A plan of action had to be identified to repair the tank and prevent the problem from returning.

Tank Industrial Consultants, an independent engineering and inspection firm, was hired to assess the tank and devise a plan to stop the leak. In its evaluation, the consultants noticed the tank's thin steel shell was flexing when it was filled – and re-filled. The owner needed assurances the new coating system chosen could stop he leaking and be able to withstand flexing, without losing its effectiveness.

Before the coating system could be applied, crews first conducted abrasive blasting operations, in accordance with SSPC-SP 10/NACE 2 Near White Blast Cleaning, to remove the existing 6.0 mils of factory-applied powder coating. After the tank's original coating was removed and the surface was cleaned and profiled, the consultants verified the surface would be ready for its new treatment.

The product selected to fix the tank was Envirolastic AR520PW, a 100% pure polyurea from Sherwin-Williams Industrial & Marine Coatings. Polyurea coating technology has been available for nearly two decades, and still manages to provide more advanced capabilities than traditional coating systems.

One of the main benefits of polyurea is its fast cure. It can typically return an area to service many times faster than traditional industrial coatings, allowing the owner to put the facility back in use without days and weeks of revenue-losing down time. Most industrial use polyureas are spray applied, making them fast to apply and precisely proportioned and mixed.

Polyureas can be formulated to achieve an advantageous range of properties such as superior tensile strength to and high elongation. In addition, no-VOC polyureas are flexible and seamless, have low odor and permeability, are resistant to impact and abrasion, offer crack bridging capabilities and become tack free in as little as 45 seconds. For more information, see www.polyurea.com

Conclusion

The highly elastic, waterproof durability of the polyurea coating has kept this tank in service since its rehabilitation in November 2006.


About the Author: Tony Ippoliti is a senior corrosion specification specialist at The Sherwin-Williams Co., of Cleveland, OH. Contact: tippoliti@sherwin.com

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