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24 MATERIALS PERFORMANCE April 2011 NACE International, Vol. 50, No. 4

Corrosion Under Insulation— The Hidden Threat to Piping

and Equipment Integrity

Steel equipment in refineries and chemical processing plants, such as the vertical pipes (risers) on the front of the tower, is frequently insulated for personnel protection, energy conservation, or process stabilization. Photo courtesy of Hi-Temp Coatings Technology.

24 MATERIALS PERFORMANCE April 2011

NACE International, Vol. 50, No. 4 April 2011 MATERIALS PERFORMANCE 25

S P E C I A L F E A T U R E

NACE Standard SP0198 Includes Revised Guidelines for Protective Coatings Under InsulationKathy Riggs LaRsen, associate editoR

In refineries and chemi-

cal processing plants, steel

equipment is frequently in-

sulated for personnel protec-

tion, energy conservation,

or process stabilization, and there

is a risk that corrosion will occur

under the insulation material. This

corrosion mechanism, known as

corrosion under insulation (CUI),

occurs when water from the out-

side environment infiltrates an

insulation system and comes into

contact with the metal surface of

a pipe or piece of equipment. The

water may contain contaminants

from the surrounding atmosphere

as well as the insulation. As a re-

sult, the environment under the

insulation may be very aggressive,

and subsequent surface corrosion

is hidden underneath the insula-

tion system and undetectable

through visual inspections. NACE SP01981 outlines the current

technology and industry practices for mitigating corrosion under thermal insula-tion and fireproofing materials. Originally prepared in 1998 as RP0198, the standard has been revisited several times and was most recently revised and its designation changed to SP0198 in June 2010. (All NACE “recommended practices” are now being called “standard practices.”) The

CUI prevention and mitigation experience of many companies throughout the oil, gas, and chemical industries is incorporated into this new document.

“The recent changes we made to the present document were not substantive to the general structure of the standard, but rather we were seeking to fine tune it and add newer proven technologies that have been developed recently,” says NACE International member Murry Funderburg, senior staff engineer with Shell Oil Prod-ucts (Houston, Texas) and chair of NACE Task Group (TG) 325—CUI: Revision of NACE SP0198 (formerly RP0198), “The Control of Corrosion Under Thermal Insulation and Fireproofing Materials—A Systems Approach.” While many aspects of the standard remain virtually the same, there are modifications to the document that significantly impact the recommenda-tions for protective coatings to mitigate CUI. Some of these changes reflect tremen-

dous improvements made in the products and systems available to mitigate CUI, and the changes made to the document in 2010 bring the standard up to date.

“The problem with insulated equipment is that you really have no idea of what is go-ing on underneath the insulation and clad-ding, and it is very expensive to find out,” says NACE International member Peter Bock, a NACE-certified Coating Inspector Program (CIP) Level 1 Coating Inspector and a CUI specialist for Hi-Temp Coat-ings Technology (Houston, Texas). “There is that moment in many under-insulation repair projects when the maintenance per-sonnel remove some insulation to complete a minor repair job and find that the equip-ment under the insulation is extremely cor-roded. Quite often the degree of corrosion under the insulation is a surprise. NACE Standard SP0198 is the best guideline we have for mitigating CUI for both new construction projects and, to a very great

Severe corrosion was found under insulation that covered a vessel. Photo courtesy of Hi-Temp Coatings Technology.


S P E C I A L F E A T U R E Corrosion Under Insulation—The Hidden Threat to Piping and Equipment Integrity

extent, repairs on equipment that was insu-lated eight, 10, 15, and sometimes 20 years ago,” he adds. Bock, who is chair of NACE TG 425—State of the Art in CUI Coating Systems, was involved with updating NACE Standard SP0198. He comments that part of the evolution of controlling CUI is learn-ing how well the protective coatings systems applied in the past have survived and how compatible they are with the better coatings systems available today.

What is CUI?Insulation can instigate severe corrosion

problems, such as general corrosion and pit-ting in carbon steel (CS), and external stress corrosion cracking (ESCC) in austenitic and duplex stainless steel (SS). Insulation wicks or absorbs water that enters through breaks or degradation in the insulation system’s weatherproofing. Once it is wet, the insulation system’s weather barriers and sealants trap the water inside, so the insula-tion remains moist. Next to the equipment surface, the insulation forms an annular space or crevice that retains the water and other corrosive media, conditions that are conducive to corrosion. As corrosion occurs, the insulation hides the resulting corrosion damage from sight. Severe CUI has been responsible for major equipment outages, production losses, and unexpected mainte-nance costs, which are reasons why CUI is such a serious concern.

CUI of CS stems from wet metal expo-sure over a period of time and is possible under all types of insulation (calcium silicate [Ca2O4Si], expanded perlite, man-made mineral fibers, cellular glass, organic foams,

and ceramic fiber). The corrosion rate is affected mainly by contaminants present in the water and the metal temperature of the steel surface. Contaminants are generally chlorides and sulfates from sources such as cooling tower drift, acid rain, atmospheric emissions that deposit on the exterior of the insulation, and from the insulation itself. When the insulation is wetted, contami-nants are carried through the insulation by the water and deposited onto the equip-ment surface. As the water evaporates, chloride concentrations on the CS surface gradually increase. Industry recognizes that CS piping or equipment operating with a skin temperature within the range of 25 to 350 °F (–4 to 175 °C) are the most likely to experience CUI.

In austenitic and duplex SS, ESCC occurs when chlorides are transported by external water through the insulation to the hot surface of the SS, where they are concentrated when the water evaporates. The chloride concentration in the water doesn’t have to be high. ESCC failures occur when the metal skin temperature is between 120 to 350 °F (50 to 175 °C). For ESCC to develop, sufficient tensile stress must be present. An increase in tempera-ture increases the corrosion reaction and shortens the time required for initiation and propagation of ESCC. While ESCC most commonly occurs beneath all types of thermal insulation materials, the presence of insulation is not required.

According to NACE member Tim Hanratty, a NACE-certified CIP Level 1, Level 2, and Level 3—Peer Review Coating Inspector and corrosion special-ist and PetroChem business manager for The Sherwin-Williams Co. (Cleveland, Ohio), several factors over the years have contributed to CUI, such as the wrong coatings being specified, improper installa-tion of the insulation system, and the use of absorbent insulation materials. As water and contaminants infiltrated the insulation, the protective coating system was not capable of protecting the equipment, and corrosion and failure occurred.

The infiltration of external water can be reduced by changes in the insulation materi-als and the design of the equipment that is insulated; however, some amount of water

ingress into the insulation system eventually occurs. Also, condensation is a water source on piping that operates below the atmo-spheric dew point since insulation systems aren’t vapor tight. Because attempts to pre-vent water from entering insulated systems are not sufficiently reliable to prevent CUI, and corrosion protection techniques such as inhibitors and cathodic protection have been less effective than protective coatings in mitigating CUI, NACE SP0198 recom-mends the use of high-quality, immersion-grade protective coatings as a highly effec-tive method of protecting insulated CS and austenitic and duplex SS from corrosion. These barrier coatings prevent water and contaminants from penetrating the CS or SS substrate and initiating corrosion.

“Because water is trapped under the insulation, CUI is treated as an immersion condition. If the equipment is in a petro-chemical plant, any contaminants in the air will eventually get through the insula-tion with the water,” Hanratty explains. “So when we engineer a protective coating system to solve this corrosion issue, we use immersion-grade coatings as part of the solution because they can withstand these conditions. Since we can’t see CUI, it’s critical to get the protective coating system correct on the front end,” he emphasizes.

Hanratty, who writes specifications for protective coatings that mitigate CUI and participated in the NACE SP0198 review and update process, comments that coatings suppliers, owners, and engineering firms in the petrochemical industry do refer to NACE SP0198, specifically the coating tables, when designing protective coating systems to mitigate CUI.

Coating systems considered in the stan-dard have a history of successful use and include thin-film, liquid-applied coatings; fusion-bonded coatings; metalizing or ther-mal spray coatings; and wax-tape coatings. Other systems also may be satisfactory. For instance, aluminum foil wrapping may be used to prevent ESCC of austenitic and duplex SS under insulation.

A crucial consideration when determin-ing the appropriate protective coating to use under insulation is the service temperature of the equipment or piping. The coating should be selected based on the expected

Surface corrosion can be hidden underneath an insulation and cladding system and undetectable through visual inspections unless the insulation is removed. The exposed pipe section shows cor-rosion that occurred under the insulation. Photo courtesy of Hi-Temp Coatings Technology.


service temperature range if this range could allow moisture to occur on the substrate surface. This is especially true for processes using intermittent thermal cycling. Nor-mally the high end of a temperature range for equipment or piping is determined by the design temperature—the highest possible temperature that the equipment/piping is designed to withstand. Although typical operating temperatures for a piece

of equipment may not run at the high end of the temperature design, Bock explains, spikes in temperatures due to process varia-tions, maintenance cleaning during plant turnarounds, etc. must be considered when specifying a protective coating system. This is important, he points out, because one temperature excursion can damage a pro-tective coating system if it is not designed to withstand the higher temperature.

“A lot of coatings that we do use work better at one temperature range than an-other. One size doesn’t fit all,” Funderburg comments.

When looking at the process tempera-tures of insulated equipment, Hanratty notes that 300 °F (150 °C) used to be the norm for a process operating tempera-ture, but new processes in refineries and chemical plants are running at higher

TAbLE 1

Typical Protective Coating Systems for Austenitic and Duplex Stainless Steels Under Thermal Insulation (Reprinted from NACE SP0198, pp. 22-23.)

System Number

Temperature Range(A),(B)

Surface Preparation(C)

Surface Profile,µm (mil)(D)

Prime Coat,µm (mil)(E)

Finish Coat,µm (mil)(E)

SS-1 –45 to 60 °C (–50 to 140 °F)

SSPC-SP 1 and abrasive blast

50–75 (2–3) High-build epoxy, 125–175 (5–7)

N/A

SS-2 –45 to 150 °C (–50 to 300 °F)


50–75 (2–3) Epoxy phenolic, 100–150 (4–6)

Epoxy phenolic, 100–150 (4–6)

SS-3 –45 to 205 °C (–50 to 400 °F)


50–75 (2–3) Epoxy novolac, 100–200 (4–8)

Epoxy novolac, 100–200 (4–8)

SS-4 –45 to 540 °C (–50 to 1,000 °F)


15–25 (0.5–1.0) Air-dried silicone or modified silicone, 37–50 (1.5–2.0)

Air-dried silicone or modified silicone, 37–50 (1.5–2.0)

SS-5 –45 to 650 °C (–50 to 1,200 °F)


40–65 (1.5–2.5) Inorganic copolymer or coatings with an inert multipolymeric matrix,(F) 100–150 (4–6)

Inorganic copolymer or coatings with an inert multipolymeric matrix,(F) 100–150 (4–6)

SS-6 –45 to 595 °C (–50 to 1,100 °F)


50–100 (2–4) Thermal-sprayed aluminum (TSA) with minimum of 99% aluminum, 250–375(10–15)

Optional: sealer with either thinned epoxy-based or silicone coating (depending on max. service temperature) at approximately 40 (1.5)

SS-7 –45 to 540 °C (–50 to 1,000 °F)

SSPC-SP 1 N/A Aluminum foil wrap with min. thickness of 64 (2.5)

N/A

(A) The temperature range shown for a coating system is that over which the coating system is designed to maintain its integrity and capability to perform as specified when correctly applied. However, the owner may determine whether any coating system is required, based on corrosion resistance of austenitic and duplex stainless steels at certain temperatures. Temperature ranges are typical for the coating system; however, specifications and coating manufacturer’s recommendations should be followed. SS-4, SS-5, SS-6, and SS-7 may be used under frequent thermal cyclic conditions in accordance with manufacturer’s recommendations.

(B) Temperature range refers to the allowable temperature capabilities of the coating system, not service temperatures. An expe-rienced metallurgist should be consulted before exposing duplex stainless steel to temperatures greater than 300 °C (572 °F).

(C) To avoid surface contamination, austenitic and duplex stainless steels shall be blasted with nonmetallic grit such as silicon carbide, garnet, or virgin aluminum oxide. Because there are no specifications for the degree of cleanliness of abrasive blasted austenitic and duplex stainless steels, the owner should state the degree of cleanliness required after abrasive blasting, if applicable, and whether existing coatings are to be totally removed or whether tightly adhering coatings are acceptable.

(D) Typical minimum and maximum surface profile is given for each substrate. Acceptable surface profile range may vary, depend-ing on substrate and type of coating. Coating manufacturer’s recommendations should be followed.

(E) Coating thicknesses are typical dry film thickness (DFT) values, but the user should always check the manufacturer’s product data sheet for recommended coating thicknesses.

(F) Consult with the coating manufacturer for actual temperature limits of these coatings.


temperatures, up to 400 °F (205 °C). “In many specifications that we write today—in comparison to 2004, 2005, or even prior to that—we’re seeing that 400 °F is more common. This higher temperature has become the new benchmark versus 300 °F,” Hanratty says.

Changes in the NACE CUI Standard

One of the key modifications made to NACE SP0198 was an extensive revision of the tables that recommend coatings systems to protect the materials under the insulation, says Funderburg.

These updated tables reflect the revi-sions, which include the addition of new protective coating system technologies, the addition of metallic coating systems, the elimination of outdated coating sys-tems, and a modification of the recom-mendation for new pipe that is primed with an inorganic zinc (IOZ)-rich coating. The standard recommends the surface preparation, surface profile, and a coating system for particular operating tempera-ture ranges in Table 1, “Typical Protec-tive Coating Systems for Austenitic and Duplex Stainless Steels Under Thermal Insulation,” and Table 2, “Typical Pro-tective Coating Systems for Carbon Steels Under Thermal Insulation and Fireproof-ing,” which are reprinted in this article.

One significant change to the tables is the addition of thermal-sprayed alu-

minum (TSA) (with a minimum of 99% aluminum) to the coating choices, Fun-derburg remarks. TSA coatings have per-formed successfully in under-insulation onshore and marine environments. “The chemical process industries have found that TSA gives significantly longer life performance over traditional coatings,” he comments.

Another significant change, observes Bock, is the inclusion of high build, el-evated temperature coatings introduced in the 2000s—inorganic copolymers and coatings with an inert multipolymeric matrix—that can withstand higher op-erating temperatures, up to 1,200 °F (650 °C), depending on the product. These coatings can be applied as very thick films—typically 4 to 6 mils (100 to 150 µm) per coat. “These products allow you to build a thicker barrier coat, which provides longer life and better protec-tion,” he notes.

Hanratty mentions that the recom-mendation regarding bulk, shop-primed CS pipe with an IOZ coating is another important change to the standard. While it is a good temporary coating for protec-tion from mild atmospheric corrosion, an IOZ coating is not a preferred system for service temperatures in the CUI range up to 350 °F. Zinc provides inadequate corrosion resistance in closed, sometimes wet, environments. At elevated tempera-tures >~60 °C, the zinc may undergo a

galvanic reversal where the zinc becomes cathodic to the CS.

“For a new project, it’s very common in the petrochemical and refining indus-tries to use a shop-applied IOZ coating as a primer on all of the CS piping. It dries extremely fast and is cost efficient,” Hanratty comments. He explains that the shop-primed pipe is typically purchased in bulk for a project and then individual pieces of pipe are finish coated at the job site based on the service and operating temperature where they will be used.

According to the revised standard, an IOZ coating shall not be used by itself under thermal insulation in a service temperature range of 50 to 175 °C for long-term or cyclic service. In cases where pipe is previously primed with an IOZ coating, it should be topcoated to extend its life, and a coating manufacturer should be consulted for coating thickness and service temperature limits.

Hanratty says that industry has be-come more aware of the underlying causes of CUI and has taken steps to successfully address them, such as design-ing better insulation systems, noting the operating temperatures of the piping and equipment, and using NACE SP0198 as a guide for selecting protective coating systems. However, he adds, there are still many pieces of equipment and pip-ing that were insulated years ago that may be experiencing CUI, and several major companies have implemented a CUI initiative within the last few years to inspect equipment and pipe surfaces under insulation and take any necessary corrective action.

NACE SP0198 is available online for downloading. For NACE members, stan-dards can be downloaded at no cost. To download the standard, visit the NACE store at www.nace.org/store.

Reference

1 NACE SP0198-2010 (formerly RP0198), “Control of Corrosion Under Thermal Insulation and Fireproofing Materials—A Systems Approach” (Houston, TX: NACE International, 2010).

The thin lines of corrosion on the IOZ-coated pipe correspond to the spaces between the strips of insulation that provided a path for water to reach the equipment surface. Photo courtesy of Hi-Temp Coatings Technology.

S P E C I A L F E A T U R E Corrosion Under Insulation—The Hidden Threat to Piping and Equipment Integrity


TAbLE 2

Typical Protective Coating Systems for Carbon Steels Under Thermal Insulation and Fireproofing(Reprinted from NACE SP0198, pp. 25-26.)

System Number

Temperature Range(A),(B)

Surface Preparation

Surface Profile, µm (mil)(C)

Prime Coat, µm (mil)(D)

Finish Coat, µm (mil)(D)

CS-1 –45 to 60 °C (–50 to 140 °F)

NACE No. 2/ SSPC-SP 10

50–75 (2–3) High-build epoxy, 130 (5) Epoxy, 130 (5)

CS-2 (shop application only)

–45 to 60 °C (–50 to 140 °F)


50–75 (2–3) N/A Fusion-bonded epoxy (FBE), 300 (12)

CS-3 –45 to 150 °C (–50 to 300 °F)


50–75 (2–3) Epoxy phenolic, 100–150 (4–6)

Epoxy phenolic, 100–150 (4–6)

CS-4 –45 to 205 °C (–50 to 400 °F)


50–75 (2–3) Epoxy novolac or silicone hybrid, 100–200 (4–8)

Epoxy novolac or silicone hybrid, 100–200 (4–8)

CS-5 –45 to 595 °C (–50 to 1,100 °F)


50–100 (2–4) TSA, 250–375 (10–15) with minimum of 99% aluminum

Optional: Sealer with either a thinned epoxy-based or silicone coating (depending on maximum service temperature) at approximately 40 (1.5) thickness

CS-6 –45 to 650 °C (–50 to 1,200 °F)


40–65 (1.5–2.5) Inorganic copolymer or coatings with an inert multipolymeric matrix, 100–150 (4–6)

Inorganic copolymer or coatings with an inert multipolymeric matrix, 100–150 (4–6)

CS-7 60 °C (140 °F) maximum

SSPC-SP 216 or SSPC-SP 317

N/A Thin film of petrolatum or petroleum wax primer

Petrolatum or petroleum wax tape, 1–2 (40–80)

CS-8 Bulk or shop-primed pipe, coated with inorganic zinc

–45 to 400 °C (–50 to 750 °F)

Low-pressure water cleaning to 3,000 psi

(20 MPa) if necessary

N/A N/A Epoxy novolac, epoxy phenolic, silicone, modified silicone, in-organic copolymer, or a coating with an inert multipolymeric matrix, is typically applied in the field. Consult coat-ing manufacturer for thickness and service temperature limits(E)

CS-9 Carbon steel under fireproofing

Ambient NACE No. 2/ SSPC-SP 10

50–75 (2–3) Epoxy or epoxy pheno-lic, 100–150 (4–6)

Epoxy or epoxy pheno-lic, 100–150 (4–6)

CS-10 Galvanized steel under fireproofing

Ambient Galvanizing: sweep blast with fine, nonmetallic grit

25 (1) Epoxy or epoxy phenolic (for more informa-tion on coatings over galvanizing, see 4.3.3), 100–150 (4–6)

Epoxy or epoxy phenolic, 100–150 (4–6)

(A) The temperature range shown for a coating system (including thermal-cycling within this range) is that over which the coating system is designed to maintain its integrity and capability to perform as specified when correctly applied. However, the owner may determine whether any coating system is required, based on corrosion resistance of carbon steel at certain temperatures. Temperature ranges are typical for the coating system; however, not all coatings in a category are rated for the given minimum/maximum temperature. Specifications and coating manufacturer’s recommendations should be followed for a particular coating system.

(B) Temperature range refers to the allowable temperature capabilities of the coating system, not service temperatures. (C) Typical minimum and maximum surface profile is given for each substrate. Acceptable surface profile range may vary, depending

on substrate and type of coating. The coating manufacturer’s recommendations should be followed.(D) Coating thicknesses are typical DFT values, but the user should always check the manufacturer’s product data sheet for recom-

mended coating thicknesses. (E) If inorganic zinc-rich coating is applied in a shop and topcoat is applied in the field, proper cleaning of the inorganic zinc-rich

coating is required. The use of inorganic zinc-rich coating under insulation is not a preferred system for service temperatures in the CUI range up to approximately 175 °C (350 °F). However, bulk piping is often coated with inorganic zinc-rich coating in the shop and some owners purchase this piping for use under insulation. In these cases, the inorganic zinc-rich coating should be topcoated to extend its life.

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