corrosion under insulation white paper.pdf

8/17/2019 Corrosion Under Insulation White Paper.pdf

1/4

Corrosion Under InsulationThe Hidden Problem

by Tim Hanratty, The Sherwin-Williams Company


2/4

Corrosion Under Insulation (CUI) is one of the costliestproblem s facing the petrochem ical industry today. According

to one speci er from a global oil com pany, prob lem s such as

m ajor equipm ent outages and unexpected m aintenance costs

stem m ing from C U I account for m ore unp lanned dow ntim e than

all other causes.

For a reliab ility engineer in a hydrocarbon processing

environm ent, the issue has the potential to be nightm arish.

A re nery’s steel piping, storage tanks, container vessels and

other process equipm ent are subject to tem perature uctuations.

Therm al insulation applied to the pipe or vessel m itigates theeffects, but the presence of seam s, gaps or other discontinuities

in the insulation layer m akes them suscep tible to in ltration by

outside m oisture or from the process environm ent itself.

The result of in ltration is m oisture on the coated pipe or vessel

surface or w ithin the insulation itself, w hich m ay be m ade of

calcium silicate, expanded perlite, m an-m ade m ineral bers,

cellular glass, organic foam s, or ceram ic ber. The insulation

and cladding hides the protective coating system , though,

so even w ith observation ports, less than 1 percent of

the surface is visible –and those areas are not likely to berep resentative of the w hole unit.

Typically, rem oval of insulation is done only on a 15 –20 year

cycle. W ith no intervention, over tim e the coating system that

w as intended to guard against corrosion can prem aturely fail.

M aintenance personnel w ho happen to rem ove som e insulation

to com plete a m inor repair job are likely to nd a degree of

corrosion that is an unpleasant surprise.

This paper exam ines corrosion m echanism s and discusses

the recently revised N AC E standard governing the current

technology and best industry practices for m itigating C U I

using protective coatings. N AC E Standard SP0198-2010,

“The C ontrol of C orrosion U nder Therm al Insulation and

Fireproo ng M aterials –A System s Approach,”now re ects

latest insights from the C U I prevention and m itigation experience

of the oil and gas industry, and im provem ents m ade to the

products and system s available to com bat C U I.

The opportunity is tw o pronged , (1) identifying and correcting

incipient problem s that m ay be lurking on existing pipes and

vessels that w ere insulated in the past w hen coatings, insulation

and re nery operating tem peratures w ere different than they

are today, and (2) properly specifying protective coatings for

new construction projects. This paper w ill focus on the second

of these.

Corrosion Under Insulation The Hidden Problem

CUI Exposures can occur on both carbon steel (C S) orausten itic and duplex stainless steel (SS ) substrates. In C S ,

C U I occurs in p iping or equipm ent w ith a skin tem perature

in the range o f 25 –350°F (-4 –175°C ), w here the m etal is

exposed to m oisture over a period of tim e und er any kind

of insulation. The rate of co rrosion varies w ith the speci c

contam inants in the m oisture and the tem perature of the

steel surface. W aterborne chlorides and sulfates concentrate

on the C S surface as the w ater evaporates. In austenitic and

dup lex S S , a phenom enon called external stress corrosion

cracking (ESC C ) occurs, but the tem perature threshold is

higher, betw een 120 –350°F (50 –175°C ). For ESC Cto d evelop, suf cient tensile strength m ust be present.

H ere again, w aterborne chlorides concentrate on the SS’s

hot surface as w ater evaporates.

A Hidden Threat To Surfaces . W hen the cost of energy

increased in the 1970s, the problem of C U I as a pervasive

issue w as set in m otion. P rior to this, little if any therm al

insulation w as applied to heated C S equipm ent and vessels

below 300°F (150°C ). W ith the energy cost increase, it

sud denly becam e m ore cost effective to apply therm al

insulation even at m uch low er operating tem peratures. A t thesam e tim e, new er processes cam e onstream op erating at

low er and often cyclic tem peratures, and austenitic S S pipe

and equipm ent becam e m ore com m onp lace as w ell. Together

these developm ents dram atically increased the am ount of

insulation used in the industry and set the stage for C U I.

C U I generally stem s from w ater and in ltration of

contam inants to an insulated system w hich w ill vary in its

w ater retention, perm eability and w etability characteristics.

These m edia m ay enter the system due to breaks in

the w aterproo ng, inadequate system design, incorrectinstallation, poor m aintenance practices or a com bination

of the above.

Insulation w icks or ab sorbs w ater that enters through b reaks

or degradation in the insulation system ’s w eatherproo ng .

O nce w et, the insulation system ’s w eather barriers and

sealan ts trap the w ater inside, so the insulation rem ains

m oist. N ext to the equipm ent surface, the insulation form s

an annular space or crevice that retains the w ater and other

corrosive m edia.

Sources of w ater include rainfall, cooling tow er drift, steam

discharge, w ash dow ns and , because insulation is no t vapor

tight, condensation. The w ater m ay or m ay not contain


3/4

additional contam inants; for exam ple, rainw ater m ay becom eacid rain. Further, chlorides and sulfates that m ay be native to

the insulation can leach into m oisture to form acids.

The Role of Protective Coatings . Though valiant efforts

to keep w ater out of insulated system s can be m ade using

different design m aterials and con gurations of the equipm ent

to be insulated , C U I is not usually kep t at bay on the strength

of those m easures alone. Industry guidance, provided by

the trade association N AC E, ho lds that im m ersion-grade

protective coatings are the best defense against C U I in b oth

insulated C S and austenitic and d up lex S S .

C U I is treated as an im m ersion cond ition because o f the

trap ped w ater under the insulation. C oatings and linings

form ulated for im m ersion service are ideal for C U I because

the contam inants that pass through the insulation along w ith

the w ater create an aggressive operating environm ent.

C oating system s incorporated into the standard have a

track record of success and includ e thin- lm , liquid-ap plied

coatings; fusion-bonded co ating s; m etalizing or therm al spray

coatings; and w ax-tap e coatings. A crucial consideration

w hen determ ining the appropriate protective coating system

to use under insulation is the expected service tem perature of

the eq uipm ent or piping , especially w hen interm ittent therm al

cycling is present. The coatings on the m arket are engineered

to w ork at various tem perature ranges because one size does

not t all. The m ost com m on system s are p henolic epoxies,

for tem peratures of -50 –300°F (-45 –150°C) and novolac

ep oxies, for tem peratures of -50 –400°F (-45 –205°C ).

Additionally, coatings that can be categorized as inert

m ultipolym eric m atrix have show n a tem perature range from

-50 –1 ,200°F (-45 –650°C).

Ten years ago, m axim um tem peratures that re nery

equipm ent and p iping w as designed to w ithstand, and

thus the typical process operating tem perature of insulated

eq uipm ent, w ere low er than they are today. M odern facilities

are running at tem peratures as high as 400°F (205°C),

w here 300°F (150°C ) w as m ore standard previously.

A lthoug h m ost equipm ent doesn’t run at the high end of the

tem perature design, spikes can occur for various reasons and

m ust be taken into account w hen sp ecifying the appropriate

coating system .

C oatings su itable for use in re nery environm ents m ust

dem onstrate superior resistance to w et/dry cycling. In fact,

product testing can involve as m any as six cycles 12 w eeks in

duration at 425°F (220°C ) dry and 150°F (65°C ) w et, to certifyresults w here rusting, blistering and coating disbondm ent do

no t occur. These coating system s can be used as tank linings

as w ell so they are im m ersion grad e by their nature.

A Revised Standard . Acknow ledging the changes in

op erating tem peratures and m od i cations to coating

technologies, a revised N AC E standard w as pub lished in

2010. O ne of the m ost useful resources w ithin the standard

itself is the coatings tables, w hich outline, for given coating

system tem perature ranges, the recom m ended surface

prep aration, surface pro le, and categories of prim e and

nish coats.

The tab les, “Typical P rotective C oating System s for Austenitic

and D uplex S tainless S teels U nder Therm al Insulation,”and

“Typical P rotective C oating System s for C arbon S teels U nder

Therm al Insulation and Firep roo ng,”include the addition of

new protective coating system technologies and elim ination

of outdated ones, the addition of m etallic coating system s,

and a m odi cation of the recom m endation for new bulk piping

that is prim ed w ith an inorganic zinc-rich (IO Z) coating.

Although it is com m on in the petrochem ical and re ning

ind ustries to use a shop-applied IO Z coating as a p rim er on

new C S piping because it dries quickly and is cost effective,

IO Z provides inadequate corrosion resistance in closed,

som etim es w et, environm ents. A t tem peratures greater than

140°F (60°C ), the zinc m ay undergo a galvanic reversal w here

the zinc becom es cathodic to the C S . Shop-prim ed pipe

w ill be nish-coated at the job site dep ending on the service

cond itions need ed.

The standard recom m ends top coating the IO Z to extend its

service life, and that it not be used by itself under therm al

insulation in service tem peratures up to 350°F (175°C ) for

long-term or cyclic service. In cases w here pipe is previously

prim ed w ith an IO Z coating , it should be topcoated to

extend its life. The C S coatings tab le in the N AC E docum ent

should b e referenced and the coating m anufacturer should

be consulted for the generic coating, dry lm thickness and

service tem perature lim its.

Fo r further details, N AC E S tandard SP0198 is availab le online

for dow nload ing. For NAC E m em bers, standards can be

dow nloaded at no cost. To d ow nload the standard, visit the

N AC E store at w w w .nace.org/store.


4/4

About the Author . Tim H anratty is Petrochem B usiness D evelopm ent M anager and a C orrosion Speci cation Specialist for

The S herw in-W illiam s C om pany (C leveland, O hio). H e is a N AC E-certi ed C IP Level 1, Level 2 and Level 3 –Peer Review

C oating Inspector. H e w as a m em ber of the com m ittee that develop ed the N AC E revision .

Corrosion Under InsulationThe Hidden Problem

To learn more, visit us atsherwin-williams.com/protectiveEMEA

©2013 The Sherwin-Williams CompanyProtective & Marine Coatings 8/13 EMEA0025/V02

Sherwin-Williams Protective & Marine Coatings

W hether providing passive re protection from the w orld classFIR ETEX® range, or high perform ance anti-corrosive coatings

solutions, Sherw in-W illiam s is com m itted to developing

and delivering innovative p rotective and m arine coatings,

unp aralleled service and expert speci cation support to

its custom ers.

The S herw in-W illiam s C om pany w as founded in 1866 in

C leveland, O hio by H enry S herw in and Edw ard W illiam s.

Today, the com pany is a global leader in the developm ent,

m anufacture and sale of coatings and industrial related

prod ucts w ith approxim ately 33,000 em ployees and businessin 1 16 countries. W ith annual sales of m ore than $9 billion,

S herw in-W illiam s is one of the largest coatings com panies in

the w orld. In 2011, Sherw in-W illiam s acquired Leighs Paints

based in B olton, U K w hich now trades und er the nam e

S herw in-W illiam s P rotective & M arine C oatings.

corrosion under insulation white paper.pdf

Documents