Ask the Expert

Hira S. Ahluwalia

September 1, 2007

Insulation Outlook’s new column features Dr. Hira S. Ahluwalia answering questions about CUI. Dr. Ahluwalia is president of Material Selection Resources (MSR), Inc., a materials engineering and corrosion consulting organization based in New Jersey that serves the chemical, pharmaceutical, fabrication, and metal industries. Dr. Ahluwalia has extensive experience in the field of corrosion and material science, and has published numerous technical papers. His areas of expertise include CUI prevention, material selection, corrosion testing, metallurgy, and failure analysis. He has a PhD in materials and corrosion engineering from Newcastle University in the United Kingdom. A 20-year ASM and NACE member, Ahluwalia is chairman of the NACE Annual Conference Program Committee. He is also a Nickel Institute consultant and Materials Technology Institute (MTI) consultant.

Q: How would you specify a reliable insulation system to avoid CUI if cost was not a factor?

A: For carbon steel systems that require insulation, the first line of defense would be to apply thermal spray aluminum (TSA) coating on the carbon steel. An organic sealer or topcoat would be applied over the TSA. Depending on the temperature range, closed cell cellular glass would be specified as the insulation material of choice. Aluminum or plastic sheathing would be specified, ensuring that the design minimizes entry of water through protrusions and seals. This type of specification will result in a cost-effective approach resulting in an inspection-free and maintenance- free approach with a service life of over 25 years.

For stainless steel systems that require insulation and are in the chloride stress corrosion cracking range, aluminum foil wrapping would be specified as a CUI preventive measure. Closed cell cellular glass would be specified as the insulation material of choice. Aluminum or plastic sheathing would be specified, ensuring that the design minimizes entry of water through protrusions and seals. For new systems, consideration would also be given to using the lean or conventional duplex stainless steel alloys.

Q: How would you specify an insulation system for carbon steel to minimize CUI if cost was a major factor?

A: Risk analysis would be conducted by considering factors such as safety, environment, and financial impact. If one assumes a low-risk scenario, an organic coating may be specified as the corrosion barrier, although a TSA coating would be encouraged. Depending on the temperature, thermal property requirements, and cost, any of the commercially available insulation types with low permeability would be specified. Aluminum or plastic sheathing would then be specified as the outer barrier. This type of specification requires high maintenance and the lifetime may be less than 10 years. It is important to consider life-cycle costing in specifying an insulation system rather than considering the up-front insulation system cost.

Q: What about mineralization conversion coatings?

A: Mineralization conversion coatings can be applied electrolytically or with the use of gels, tapes, and sprays. A review of case histories and relevant literature indicates that mineralized conversion coatings can be very effective in preventing general corrosion of carbon steel, and perhaps chloride stress corrosion cracking on stainless steel surfaces. The principle behind this system is to create a highly basic localized environment and introduce silicates that create a protective mineral layer on the metal surface.

Q: We have carbon steel piping systems that are insulated but were not coated. How do we assess their risk of CUI?

A: This can be done using a risk-based inspection (RBI) protocol to assess the potential of CUI. By assessing the consequence of failure in terms of health and safety, environment, and financial impact—and by determining the probability of failure for each corrosion circuit—one can develop a matrix for prioritizing where to deploy inspection and maintenance resources to mitigate CUI risks. More details of this technique will be provided in a future article.

Readers are encouraged to send case histories on other innovative techniques that have been used to prevent CUI to the author at hira@doctormetals.com. Readers are encouraged to submit their own insulation questions to the experts by e-mailing asktheexpert@insulation.org. Questions can be on any insulation topic. Business and legal issues will be featured next month, followed by general industry or insulation questions. Future topics will include green building, LEED, acoustics, bril, and refractory.

Disclaimer: Unless specifically noted in the beginning of the article, the content, calculations, and opinions expressed by the author, as in any article in Insulation Outlook, are those of the author, are based upon the limited information provided to the author in the question asked, and do not necessarily reflect the views of the National Insulation Association (NIA). The appearance of an article, advertisements, and/or product or service information in Insulation Outlook does not constitute an endorsement of such products or services by NIA.