study project report-siddharth, sarvesh

7/24/2019 Study Project Report-siddharth, Sarvesh

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BIRLA INSTITUTE OF TECHNOLOGY AND

SCIENCE PILANI

PRACTICE SCHOOL I

KUDANKULAM NUCLEAR POWER

PROJECT

STUDY PROJECT REPORT

ON

CORROSION OF STAINLESS STEEL IN THE

PRESENCE OF SULPHURIC ACID

SIDDHARTH PANDEY [2013A1PS742P PILANI CAMPUS]

SARVESH SHUKLA []


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INDEX

I. In!"

II. A#$n%&'!(!)!n*III. In*+%,#*-%nIV. /-# % C%++%-%nV. !/-%,+ % */-n'! *!!'

VI. S,',+-# /#- /n'-n( 5*!) -n

K,/n$,'/) N,#'!/+ P%&!+ P'/n*VII. P+%%! ,((!*-%nVIII. C%n#',-%nI6. -'-%(+/5


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ACKNOWLEDGEMENT

We(Siddharth and Sarvesh) would like to take this

opportunity to thank the Senior Staf at Kudankulam

Nuclear Power Project, NPC! "ecause they have

#ranted us the chance to learn directly at the

industrial scale "y visitin# this sa$e and secure $acility%

Secondly, we would like to thank the &rs% !akshmi and

&r% Shashikanth $or continuously #uidin# us, orientin#

us with the $unctionalities, operation and maintenance

o$ the plant% 'hey also arran#ed an ecellent

orientation pro#ram $or the students o$ 'S*P!+N

alon# with various project opportunities%

'his study report on the Corrosion of stainless

steel in the presence of sulphuric acid was allotted to

us "y our mentor in Kudankulam nuclear power

project, &r% Naveen alla% t is with his insi#ht and

systematic #uidance that we have "een a"le to compile

this report $or the a"ove stated topic%

!astly, we would like to show appreciation towards

our institute, irla nstitute o$ 'echnolo#y and Science,

Pilani and its Practice School -ivision $or directin# us

towards industrial eposure and helpin# us tackle real

pro"lems in our .elds o$ interest as part o$ the

Practice School pro#ram% +lso, all our work has "een

completed under the #uidance and supervision o$ our


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Practice School Pro$essor, -r% &icheal +lphonse,

whose help has "een without dou"t, irreplacea"le in

our eperience o$ the Practice School%

INTRODUCTION

Sulphuric acid is a chemical that is used in various

industrial processes, includin# ones in Nuclear Power

plants and speci.cally at Kudankulam Nuclear Power

Project% t is produced $rom sulphur dioide, whichmay "e #enerated "y "urnin# sulphur, it may "e a "y*

product o$ a metallur#ical smeltin# process, or it may

"e produced "y thermal decomposition (re#eneration)

o$ spent acid% 'he sulphur dioide is reacted with

oy#en over a catalyst at /0123 to 4153C to $orm

Sulphur trioide% 'he latter #as then reacts with water

in the a"sor"in# towers to $orm sulphuric acid% 'his

process is eothermic and the acid can reach

temperatures as hi#h as 6723 to 1223C% &ost o$ this

ener#y is recovered "y a ran#e o$ means to minimi8e

ener#y consumption% 9sually the acid is then cooled

$rom around 6223C to close to am"ient $or stora#e%

'his process is used in many plants $or the production

o$ sulphuric acid, and su"se:uently its stora#e%


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n a nuclear power plant, handlin# o$ this

corrosive sulphuric acid re:uires well desi#ned pipin#

systems inclusive o$ special valves, stora#e tanks and

miers% ;ach o$ these components re:uires speci.cmaterials to len#then their lives to reverse the efects

o$ their continuous contact with sulphuric acid% 'he

most commonly used material $or this is stainless steel,

a class o$ steel with sli#htly varyin# compositions

manu$actured to withstand oidation or reduction

reactions with ions in the passin# rea#ents% 'here are

hundreds o$ #rades and su" #rades in the stainlesssteel $amily, each desi#ned $or a special application%

Chromium is the ma#ic element that trans$orms iron

into stainless steel% Stainless steel must contain at

least 62%5< chromium to provide ade:uate resistance

to rustin#, and the more chromium the alloy contains,

the "etter the corrosion resistance "ecomes% 'here is,

however an upper limit to the amount o$ chromium theiron can hold% 'here$ore additional alloyin# elements

are necessary to develop corrosion resistance to

speci.c medias%

'hus in this report, a study on the usa#e o$

stainless steel is made, alon# with some other

su"stitute materials, and their speci.c compositions

and usa#e% 'his report also studies the phenomenon o$

corrosion etensively%

BASICS OF CORROSION


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What is c!!si"#

'echnically, corrosion is the tendency o$ any

metal to return to its most sta"le thermodynamic state%Namely, that is the state with the most ne#ative $ree

ener#y o$ $ormation% &ore simply stated, it is a

chemical reaction o$ the metal with the environment to

$orm an oide, car"onate, sul$ate, or other sta"le

compound% n most cases, usin# a diferent alloy,

material, proper coatin#, or impressed current can

prevent corrosion pro"lems% When a metal part $ails inservice, it is essential to determine the cause o$ the

$ailure so that the replacement part can "e

manu$actured $rom the proper alloy to prevent $uture

$ailure% &any times a $ailed part is replaced with the

same alloy%

Corrosion can "e "roadly classi.ed in two $orms=

(1) chemical dissolution of the metal and

(2) galvanic, or electrically driven%

+"rasion, $rettin# and erosion sometimes are classi.ed

as corrosion mechanisms, "ut technically they are a

mechanical metal removal process as compared to a

chemical removal process% Chemical reaction mayaccompany the mechanical removal process to speed

up the dissolution, "ut the chemical reaction will .t

into the two "asic $orms% Some authorities list other

types o$ corrosion, "ut the other types #enerally are

modi.cations o$ one o$ the eistin# corrosion $orms% +

correct alloy choice $or one type o$ corrosion

mechanism may "e entirely the wron# choice $oranother% 'here$ore, a proper dia#nosis o$ the $ailure is


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essential to make the correct material choice% Within

these two "asic classi.cations there are .ve types o$

corrosion=

1.General or uniform corrosion

9ni$orm corrosion occurs over lar#e areas o$ the metal

sur$ace% 'his is the most common $orm o$ corrosion

with steel and copper% t is the easiest $orm o$

corrosion to measure, and service li$etime is easy to

calculate% 'his is the only $orm o$ corrosion that may

"e accurately calculated $or li$etime "e$ore $ailure andthe only corrosion mechanism in which increased

section thickness #ives lon#er li$e% 'his type o$

corrosion is measured "y corrosion rate, usually

reported as mpy (mils per year), mm>y (millimeters per

year), ipm (inches per month), or m#>sdm>yr

(milli#rams per s:uare decimeter per year)% 'his type

o$ corrosion may "e minimi8ed in the active metals "ypaintin# the sur$ace, and unepected $ailures can "e

avoided "y periodic inspections% +cid cleanin# o$

metals is an ea##erated eample o$ #eneral corrosion%

;very time a copper or car"on steel sur$ace is acid

cleaned, the metal walls are thinned due to uni$orm

corrosion% Stainless steel is su"ject to #eneral

corrosion in many acids and some salt solutions% 'heyare not su"ject to #eneral corrosion in water?

there$ore, no data is availa"le%

9ni$orm corrosion can "e reduced or even prevented

"y proper selection o$ materials that are resistant to

the corrosive environment% Certain elements make the

alloy more resistant to diferent media% @or eample,

hi#h chromium content imparts oidation resistance%


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'here$ore, look $or hi#h chromium $or use with nitric

acid, the hi#her the "etter% Ai#h chromium is use$ul $or

hi#h temperature oidation resistance? so, any

stainless steel is "etter than car"on steel in elevatedtemperature applications% Ai#h copper content in

stainless steel imparts resistance to sulphuric acid%

Ai#h nickel content #ives resistance to reducin# acids

and produces a ti#htly adherin# oide .lm in hi#h

temperature oidation%

2. Inter granular corrosion

+ll metals are composed o$ small #rains that are

normally oriented in a random $ashion% 'hese #rains

are each composed o$ orderly arrays o$ atoms with the

same spacin# "etween the atoms in every #rain%

ecause o$ the random orientation o$ the #rains, there

is a mismatch "etween the atomic layers where the

#rains meet% 'his mismatch is called a B#rain

"oundary%B n a typical stainless steel product, there

are a"out 6,222 #rain "oundaries that intersect a one*

inch (15 mm) line drawn on the sur$ace%

rain "oundaries are re#ions o$ hi#h*ener#y

concentration% 'here$ore, chemical or metallur#icalreactions usually occur at #rain "oundaries "e$ore they

occur within the #rains% 'he most common reaction is

$ormation o$ chromium car"ide in the heat*afected

8one (A+D) durin# weldin#% 'hese car"ides, $ormed

alon# the #rain "oundaries, are called Bsensiti8ation%B

ecause the car"ides re:uire more chromium than is

locally availa"le, the car"on pulls chromium $rom the


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area around the car"on% 'his leaves a low chromium

#rain "oundary 8one and creates a new low chromium

alloy in that re#ion% Now there is a mismatch in

#alvanic potential "etween the "ase metal and the#rain "oundary? so, #alvanic corrosion "e#ins% 'he

#rain "oundaries corrode, allowin# the central #rain

and the chromium car"ides to drop out as i$ particles

o$ rusty sand%

3. Galvanic corrosion including pitting and

crevice corrosion

alvanic corrosion occurs whenever two

electrically diferent metals are connected in a circuit

and are in an electrically conductive solution% 'his type

o$ corrosion re:uires three conditions= two metals that

difer in the #alvanic or electromotive series, an

electrically conductive path "etween the metals and

"oth metals su"mer#ed in a conductive solution%

+ variation o$ #alvanic corrosion can occur with

passive .lm metals% $ the alloy loses the passive .lm in

one spot, then it "ecomes active in that area% Now the

metal has "oth passive and active sites on the same

sur$ace% 'his is the mechanism $or pittin# and crevice

corrosion% 'he $ollowin# ta"le is a list o$ materials and

their relative position in the #alvanic series% 'his ta"le

allows selection o$ metal pairs that are #alvanically

compati"le% n #eneral, when an anode, $or eample

aluminum, is connected to a cathode or no"le metal in

salt water, the anode will corrode and the cathode will

"e unafected%

Metal Volt orresponding alloy


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s&a#nesium *

6%55

&a#nesium

+luminum *6%EE +luminum

Dinc *

2%F4

Dinc

ron *

2%00

Steel, ron, stainless steel(active),

alloy C(active)Nickel *

2%1E

Nickel (active), +lloy 422 (active),

+lloy *1Aydro#en 2%22

Copper G2%

E0

rass, copper, &onel, Nickel

(passive), alloy 422 (passive),

stainless steel (passive)Silver G2%

72

Silver

old G6%

E4

old

Aowever, there is another $actor called Barea

relationshipB? i$ the anode is very lar#e, such as a

vessel wall, and the cathode is small like a "olt head,

the #alvanic action is sli#ht% ut, i$ the anode is small

and the cathode is lar#e, the anode will corrode very

rapidly%

!itting corrosionis a $orm o$ #alvanic corrosion

in which the chromium in the passive layer is dissolved

leavin# only the corrosion prone iron% 'he volta#e

diference "etween the passive and active layer on an

austenitic stainless steel is G2%F7 volts% +cid chlorides

are the most common cause o$ pittin# in stainless


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steel% Chlorides react with chromium to $orm the very

solu"le chromium chloride (CrClE)% 'hus, chromium is

removed $rom the passive layer leavin# only the active

iron% +s the chromium is dissolved, the electricallydriven chlorides "ore into the stainless steel creatin# a

spherical, smooth wall pit% 'he residual solution in the

pit is $erric chloride (@eClE), which is very corrosive to

stainless steel% 'his is the reason $erric chloride is

used in so many o$ the corrosion tests $or stainless

steel% When moly"denum and>or nitro#en is used as an

alloyin# element in stainless steel, the pittin#corrosion resistance improves% n an attempt to

:uanti$y the efect o$ alloyin# elements, a relationship

o$ the various elements responsi"le $or corrosion

resistance was developed% 'he resultin# e:uation is

called the pittin# resistance e:uivalent num"er, or

PH;N%

PH;N I


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Crevice corrosion is usually the .rst to occur and is

predicta"le as to when and where it will take place%

!ike pittin#, a conductive solution must "e present?

and, the presence o$ chlorides makes the reactionproceed at a $ast rate% Crevice corrosion depends on

the environmental temperature, alloy content and

metallur#ical cate#ory o$ the alloy% +lso, there is a

relationship "etween the ti#htness o$ the crevice and

the onset time and severity o$ corrosion% 'here is a

Bcritical crevice corrosion temperatureB (CCC') "elow


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which corrosion will not occur%

SOURCE : www.stainless-steel-world.net reports

archive

'he #reater the diference "etween the CCC'

and the operatin# temperature, the #reater the

pro"a"ility that crevice corrosion will occur% 'his chart

is very use$ul in determinin# the efect o$ temperature

on corrosion "y indicatin# the approimate

temperature at which pittin# corrosion "e#ins% 'he
http://www.stainless-steel-world.net/http://www.stainless-steel-world.net/


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efect o$ temperature on pittin# corrosion is not as

clear as that $or crevice corrosion, "ut "y addin#

approimately 6223 @ (423 C) to the CCC', the

approimate temperature at which pittin# starts can"e determined%

". #tress corrosion crac$ing

Stress corrosion crackin# (SCC) is one o$ the

most common and dan#erous $orms o$ corrosion%

9sually it is associated with other types o$ corrosion

that create a stress concentrator that leads to crackin#$ailure% Nickel containin# stainless steel is especially

suscepti"le to chloride induced SCC%Stress corrosion

crackin# (SCC) has three components= alloy

composition, environment and the presence o$ tensile

stress% +ll metals are suscepti"le to stress corrosion

crackin#%


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'he stress component is more su"tle% @irst the stress

must "e tensile, and it must eceed the yield stren#tho$ the component% 'his sounds simple enou#h, "ut any

time a component is "ent or strai#htened, or when any

physical eertion is made to place the material into a

.ed shape, the yield stren#th is eceeded% Net,

matters can "e complicated "y stress multiplication

$actors% $ a pit or other sharp notch is present, the

residual stress is multiplied several times resultin# in astress $ar in ecess o$ the tensile yield stren#th% 'hus,

SCC usually starts with pittin# or crevice corrosion as

a precursor to $ormin# a stress concentrator%

%. Micro&iologically induced corrosion (MI).

n some cases, the meta"olic "yproducts react

with the environmental solution to create a very


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corrosive media% +n eample is the reaction o$ chlorine

in water with the man#anese dioide "yproduct $rom

#allionella "acteria on the sur$ace o$ the stainless

steel% 'his reaction #enerates hydrochloric acid, whichcauses rapid pittin# o$ many common #rades o$

stainless steel% ne o$ the most common $orms o$ &C

is the meta"olic "yproduct o$ the sul$ur*.in# "acteria

that produces sul$urous or sul$uric acid% 'hese

"acteria cause rapid corrosion o$ the lower alloy

stainless steels, like 'ypes E20! or E64!, resultin# in

throu#h wall crevice corrosion under the "acteriacolonies%

&any times, a metal starts to corrode "y one

mechanism, $or eample pittin# corrosion, and then

$ails "y a second mechanism, stress corrosion

crackin#%


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BEHA$IOUR OF STAINLESS STEEL

C%))%n */-n'! *!!' #%)%-*-%n

'ame e r 'i Mo ' u #iE20 al 67 7 * * * * *E64 al 6F 62 1 * * * *E62 al 15 12 * * * * *

+lloy 12 al 12 17 1%5 * E%5 * *

Deron622

al 15 F E%5 2%15 2%F 2%F 2%4

152F al 15 F E%5 2%15 * * 2%4S*1E al 67 67 * * * * 5S*15 al 67 67 6 * 1 5S*SL al 67 67 6 * 1 5DeCor al 60 65 6 * 6 * 4

'raditionally materials such as acid*"rick lined steel

were used $or vessels, and ductile irons, such as

&ondiM or low alloy austenitic stainless steels such as

E64 $or pipin#, within a limited temperature and acid

concentration ran#e% Aowever, the development o$

modern, hi#h alloy stainless steels, with improved

resistance to hot concentrated acid has chan#ed thematerials selection options% 'a"le 6 shows the

composition o$ some stainless steels that are used with

sulphuric acid% E20 and E64 are the common austenitic

#rades that are widely used "y the chemical and

process industry% +lloy E62 is a hi#h chromium, nickel

austenitic alloy that has superior acid corrosion

resistance compared with E20 and E64% D;HN622


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and 152F are superduple stainless steels with an

approimate 52>52 austenite>$errite phase "alance%

'his structure #ives a much hi#her stren#th (/1

times) than that o$ the austenitic alloys and ofers thepossi"ility o$ wall thickness savin#s $or applications

involvin# hi#h pressures and>or temperatures%

Saramet, Sandvik SL and DeCor are all proprietary

austenitic stainless steels containin# / 5< silicon,

which improves the corrosion resistance in hot stron#

acid% Saramet comes in two variants, with sli#htly

diferent compositions% DeCor is leaner in chromiumand nickel than the other two proprietary alloys "ut it

contains more silicon, an element known to promote

corrosion resistance in hot, stron# acid% 'he namin#

and compositions (within 2%25


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@i#ure 6 shows the iso*corrosion curves $or some

common alloys in sulphuric acid% t can "e seen that

the superduple alloys are superior to E64!% D;HN

622 is also superior to 152F, which is "elieved to "e

due to the deli"erate additions o$ tun#sten and copper

to D;HN 622% +lloy 12 is commonly used in sulphuric

acid and $rom a"out 52< to O2< acid it is superior to

D;HN 622% Aowever, in stron# acid (O2


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@i#ure 1 compares the iso*corrosion curves $or

the three proprietary alloys containin# silicon and

D;HN 622% 'here are clearly diferences "etween the

alloys, with the silicon*containin# alloys showin#improved corrosion resistance in more dilute acid%

@i#ure E shows the iso*corrosion curves $or E20, E62

and Saramet 1E in very stron# acid 6,1% t can "e seen

that there is an increase in the corrosion resistance o$

"oth E62 and Saramet in the temperature ran#e 672Q

to 122QC% t is assumed that SL and DeCor show


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similar "ehaviour% 'his means that these alloys can "e

used in the hi#her temperature parts o$ acid plants%

@i#ure 0 shows the corrosion rate o$ some

stainless steels in stron# sulphuric acid at 6623C taken$rom the manu$acturersR pu"lished data% t can "e seen

that the corrosion resistance o$ Saramet 1E decreases

with increasin# acid concentration unlike the other

alloys% +t acid concentrations #reater than 622< there

is ecess sulphur trioide and the miture is then

known as oleum% 'his is known to "e more corrosive to

alloys like Saramet than to D;HN 622 and alloy E62%


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+lthou#h there is no pu"licly availa"le data on

E62 stainless in very stron# acid, there is a sin#le data

point% +t an acid concentration o$ OO< and a

temperature o$ 662QC, the corrosion rate o$ E62 was2%6mm>y6% 'his shows the improved resistance o$

D;HN 622 over E62 stainless at this temperature

(@i#ure 0)% D;HN 622 also has similar corrosion

resistance to DeCor and Sandvik SL in stron#er acid,

OF wt


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@i#ure 5 shows the efect o$ 5ppm o$ iron on the

corrosion rate o$ D;HN 622 at 6623C% t can "e seen

that, within eperimental error, there was no

si#ni.cant efect o$ iron on corrosion%

*+ect of velocity of o-

ecause stainless steels are o$ten active (as

opposed to passive) in hot, concentrated sulphuric

acid, the corrosion rate is a $unction o$ velocity% t is

commonly recommended that alloys such as E64 andE62 "e restricted to a maimum Jow velocity o$


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6%5m>sec 1% elocity tests have "een conducted in

aerated O5 wt< sulphuric acid at F23C usin# rotatin#

cylindrical samples%

'he corrosion rate o$ D;HN 622 was hi#h $or

the .rst two or three days% 'herea$ter the corrosion

rate was less than 2%6mm>year% 'he hi#h initial rate o$

corrosion was associated with the $ormation o$ a thin

"lack .lm on the metal sur$ace% 'he .lm appears to

con$er corrosion resistance as shown "y the

su"se:uent low metal loss rate% 'hese results show

that D;HN 622 can "e used at hi#her temperatures

and velocities than E64! in stron# sulphuric acid% 'ests

in stron#er acid showed even lower corrosion rates%

Silicon additions tend to remove the velocity sensitivity

o$ stainless steels to corrosion in hot, stron# sulphuric

acid% Sandvik reported etremely low corrosion rates

(T2%26mm>y) $or SL in O4< acid at F23C and 15m>sec

in the alloy data sheet% 'hey o"tained a similar

corrosion rate in O7%5< acid at 6653C and 62m>sec

Jow velocity% Saramet E5 showed similar very low

corrosion rates in O7%5< acid at 6123C at O and 15 m>s

velocity0% +lthou#h there is no data pu"lished $or

DeCor at hi#h velocities, it is presumed that it is also

superior to the E20 and E64 #rades(most commonly

used #rades o$ stainless steels)%

+$ter conductin# multiple tests on most samples,

it was $ound that since sulphuric acid density increases

sharply with concentration alon# with its viscosity, it is

advisa"le $or plant desi#ners to maintain a low velocity

in acid pipin# systems to prevent the corrosion o$ the

passive layer that protects the inner lyin# steel% + low


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temperature .eld $urther aids the lon#evity o$ the

pipin# systems%

study project report-siddharth, sarvesh

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