64 HYDROCARBON ASIA, JUL-SEP 2011 Visit our website at: http://www.safan.comCorrosion

TECHNOLOGY

Corrosion under Insulation

G lobally, companies such as ExxonMobiland Shell are turning to Thermal SprayAluminum (TSA) as the solution to CUI.The technology, which is more than a hun-

dred years old, has proven itself as corrosion underinsulation (CUI) mitigation coating.

Simple to apply, cheap, and robust, these coatingsprovide a 20-year CUI inspection cycle at ExxonMobil,and 40 years at Shell. TSA is unaffected by CUIconditions, even in severe cyclic operation withmineral wool. ExxonMobil has a vessel operating inthese conditions for over 50 years and it still has itsoriginal 1958 TSA coating on it, without any mainte-nance or touch-up done. It is pretty impressive, anda good reason to look at TSA. Shell has launched aglobal TSA implementation program to mitigate thedamage caused by CUI.

It is simply aluminum wire that is heated, atom-ized, and sprayed onto the blasted surface until thecorrect thickness is achieved. It is an old, simplesolution without many opportunities for peopleto make money on it; besides the applicators, theowners also save due to reduced maintenance costs.

It has been part of “NACE SP0-198-2010 Controlof Corrosion under Insulation and Fireproofing” foryears and peers from all industries (insulation, paint,cladding, applicators, owners, etc.) who have re-viewed this document all agree that TSA is an impor-tant CUI mitigation technology. TSA is also animportant part of “EFC WP 13 and WP15 Corrosionin the Refinery Industry CORROSION UNDERINSULATION (CUI) GUIDELINE”. The people atExxonMobil were very much involved with thisdocument, especially the TSA parts. Overall, bothExxonMobil and Shell consider TSA a CUI mitiga-tion method and freely say so.

In 2011, at the Houston coating society, Shell GlobalSolutions reaffirmed their global commitment to TSAand presented ~4 years worth of lab testing with TSAunder all common insulations in constant wet/drycycling and in boiling saltwater. TSA (1100 alloy) wasnot affected by mineral wool, pearlite, or any otherinsulation. Shell also presented this data and reaffir-

Corrosion under insulation is hard to detect but its effects can be cataclysmic.This article talks about corrosion under insulation, and how coating can prevent this.

mation, and had their global CUI manger there to saythat under all CUI conditions, TSA coated equipmentwill have a 40-year inspection cycle. Shell bases thistime-frame on lab data and actual historical perform-ance, like ExxonMobil’s 50-year old mineral woolinsulated vessel.

One thing to understand that while TSA is anodic tocarbon steel, it is also a huge anode as compared toany holidays that might be present (excellent anodeto cathode ratio). Due to this, and the inert, firmlyadhered oxide layer that forms on the surface and inany pores in the TSA (1100 alloy!), the coating isconsidered first and foremost a barrier to electrolytes(water) thus removing one part of the corrosion cell.To make certain that the TSA is a barrier, it is applied2X thicker for use in CUI conditions than normallyrequired (most specifications are looking for around12 mils/300 microns for CUI mitigation). It is a veryeffective barrier at this thickness, especially when thecorrect aluminum alloy is chosen and any porosityinherent in TSA is closed up by oxides (this happenswhen the TSA gets wet).

The second way it protects is by being an anode.The entire carbon steel surface coated is now ca-thodic to the entire TSA coating, therefore remov-ing the possibility of carbon steel corrosion cellsuntil the TSA is all consumed. With such a hugeanode to cathode ratio, the expected holidays in thecoating would take an incalculably long time toconsume. Also, the aluminum ions that transfer tothe holidays tend to passivate the uncoated carbonsteel surfaces, slowing down the corrosion processto almost a standstill.

This has all been proven out time and time againin tremendous amounts of testing done and realworld use over the last 100 years or so. Shell didintentional TSA holiday testing under variousinsulations with wet cycling, saw how well it per-formed, and concluded that 40 years would be a goodtime to strip insulation and inspect. If it did not workso well, I do not think companies like Shell andExxonMobil would be using it. The 40-year inspec-tion cycle (not 40-year life, as TSA users feel that

Corrosion

HYDROCARBON ASIA, JUL-SEP 2011 65

after 40 years the coating will still be corrosion-free)is based on actual experience with TSA/CUI coat-ings applied for more than 40 years in the field. ForShell, it is also based on their own internal lab tests.

The surface does not have to be too perfect. Somepetrochemical company written specs are “near-white”, others are “white”, most are in the 3mil/75micron range. Residual chlorides are not much ofan issue with TSA. Like any other process, the appli-cator needs to know how to do it.

Once the surface is blasted clean (which realisti-cally needs to be done when applying any goodCUI coating system) and we look at the costs of theactual coatings, TSA is not really that expensive.Figure on the application speed to be in the 100square foot per hour range. Slower than paint forsure (but it is a low-build system (12mil/350 mi-cron range) that is fully applied in one pass withno drying time.

This publication thanks Mr. Bill Jordan,General Manager, MTM Metalizing forproviding this article. Bill has been withthe company since the formation of thecompany in 2006 in Singapore.

Prior to MTM Metalizing, Bill worked for Bell Heli-copter, and prior to that, had seventeen years in thepetrochemical and offshore Inspection and Construc-tion industry, in the Asia Pacific region with NipponSteel, Oceaneering International, Sub Sea Interna-tional (now Subsea Seven) and Comex (now Acergy).

Bill is responsible for the development and qualityof the coatings applied by MTM Metalizing, includingexotic metals, as well as educating the customers onthe benefits of Thermally Sprayed coatings in corro-sion control. Bill graduated from the University ofAuckland, New Zealand with a Bachelor of Engineer-ing in Chemicals and Material Engineering. That waslater followed with a MBA from the National Univer-sity of Singapore and he is an SSPC Certified CoatingInspector. He has over twenty years experience in thecoating, inspection and construction industry.

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