Materials Selection for Condensing Economizerscleanboiler.org/files/Resources/2008_Economizer_WS/... · 2016-09-26 · for material selection !!! Critical Crevice Temperature (CCT)

Outokumpu Stainless

Materials Selection for Condensing Economizers

Gary M. Carinci, Ph.D. Outokumpu Stainless

2 | 5.14.08 | Economizer Materials

Factors Influencing Materials Selection in Condensing Economizers

• Temperature • pH control• Sulfur content of combustion fuel• Chloride content of fuel• Fluoride content of fuel• System design


Design Considerations to Limit Corrosion from Evaporation

• Non-condensing portions of the heat exchanger:• Do not cool flue gas below 400˚F• Maintain elevated wall temperature

• Condensing heat exchanger:• Eliminate hot spots• Allow heat exchanger to heat up rapidly

• Condensate handling:• Condensate should flow towards a cooler region


Materials Selection for Condensing Economizers

• Dry Regions• Bare and coated carbon steels (aluminized)

• Wet Regions with Dilute Condensate• Stainless steels (304L, 316L, LDX 2101, 904L, 2205)• Aluminum alloys (1100, 3003, 12% Silicon, 6061)

• Wet Regions with Concentrated Condensate• High performance stainless steels (2507, 254 SMO, 4565)• Nickel alloys (625, 825, C-276, C-22, 686)


What is Stainless Steel?


Chromium Increases Corrosion Resistance

Chromium, Wt. Pct.

Cor

rosi

on R

ate,

mm

/y 0.200

0.175

0.150

0.125

0.100

0.075

0.050

0.025

0 2 4 6 8 10 12 14 16


How Stainless Steel Works

Iron +

StainlessStainlessSteelSteel

Passive Film

< 11% Chromium > 11% Chromium

RustRust

Carbon Carbon SteelSteel


Stainless Steel Families

AusteniticFerritic

Martensitic

Duplex(Austenite + Ferrite)


Stainless Steel – Chemical Composition

UNS Grade Cr Ni Mo N Others PRE30

S31603 316L Austenitic 17.2 10.2 2.1 24S31703 317L Austenitic 18.2 13.7 3.1 28S31726 317LMN Austenitic 17.8 12.7 4.1 0.14 35N08904 904L Austenitic 20 25 4.3 1.5Cu 35S31254 254 SMO Austenitic 20 18 6.1 0.20 0.7Cu 46S34565 4565 Austenitic 24 17 4.5 0.45 5.5Mn 52S32101 LDX 2101 Duplex 21.5 1.5 0.3 0.22 5Mn 29S32205 2205 Duplex 22 5.7 3.1 0.17 37S32750 2507 Duplex 25 7 4 0.27 46

Typical chemical composition, % by weight

PRE30 =%Cr+3.3*%Mo+30*%N


Corrosion in Condensing Economizers

• Uniform Attack• Low pH, acid condensing environments

• Pitting • PREn = %Cr + 3.3%Mo + 30%N

• Crevice Corrosion • Fabrication, Scale Deposits

• Stress Corrosion cracking • High Temperatures, Chlorides


PittingCorrosion


damage appearancedamage appearance

PittingCorrosion

Basic Mechanism

Cl Cl --; F; F--; Br; Br--

Oxide Sulfide


316 L Corroded Weld


Crevice Corrosion©

TM

R


damage appearance

CreviceCorrosion

Basic Mechanism

Cl Cl --; F; F--; Br; Br--Cl Cl --; F; F--; Br; Br--

ScratchScratch

BoltBoltDirt


Crevice Corrosion

• Like pitting except the anode is a crevice

• Occurs more easily than pitting

• Results from design (under bolts)

• Results from service (under deposits)

• Crevice geometry is critical • (deep and tight crevices are detrimental)


Pitting and Crevice Corrosion

• Alloy composition• Environment• Metal surface conditions

Are affected by:


Minimizing Localized Corrosion

• Reduce chlorides• Reduce temperature• Reduce oxygen• Reduce bioactivity• Avoid acid conditions• Avoid stagnant conditions


Critical Temperatures for Corrosionin Chloride Containing Environments

Avoid use of these temperatures for material selection !!!

Critical Crevice Temperature (CCT)Critical Pitting Temperature (CPT)

Below the critical temperature corrosion will not occur in a specific environment.


Stainless Steel – Pitting Corrosion

0

10

20

30

40

50

60

70

80

90

100

S31603 N08904 S31254 S34565 S32205 S32750

Austenitic DuplexSteel grade

Max. test temp.

Typical CPT values in 1M NaCl, 120 mesh surface, ASTM G150


Stainless Steel – Crevice Corrosion

0

10

20

30

40

S31603 S31726 N08904 S31254 S34565 S32205 S32750

Austenitic DuplexSteel grade

CC

T, °C

<0°C

Typical CCT Values, 6% FeCl3 + 1% HCl, #120 surface, ASTM G48(F)


Stainless Steel – Uniform CorrosionSULFURIC ACID

20

40

60

80

100

120

140

0 20 40 60 80 100

H2SO4, weight-%

Tem

pera

ture

, °C

S34565

S31254S31600

N08904

S32205

S32750S31600

S32750

S32205

0.1 mm/year


Stainless Steel – Uniform Corrosion

20

40

60

80

100

0 10 20 30 40 50 60H2SO4 Weight -%

S34565

+ Cl-

N08904

+ Cl-

+ Cl-

S32750

S31254

+ Cl-

Pure Sulfuric Acid & Sulfuric Acid + 2000 ppm Cl-


Materials Selection Economics

Corrosion Resistance ~ %Cr + 3.3%Mo + 30%N

• Nickel content of stainless steel ($14/lbs)• Molybdenum content ($33/lbs)• Chromium content ($2/lbs)• Nitrogen content - free• Select stainless steel with high Cr, N content and low

Ni and Mo content - Duplex stainless steel!


LDX 2101 Duplex Design Targets

• Corrosion resistance ~ 304L• SCC resistance = “Resistant”• Yield strength 65 – 80 ksi• Good impact toughness –40 °F to 600 °F • Availability – All product forms – Service

centers• Cost effective


Properties of LDX 2101®

60

316

”Corrosion Resistance”

Yiel

d st

reng

th, k

si

2205

2750 Duplex

254 SMO

304

Austenitic

30

80

904L

2304

LDX 2101


LDX 2101®, Nickel price influence on alloy surcharge

0200400600800

100012001400160018002000

8,000 $/ton 15,000 $/ton 25,000 $/ton

304LLDX 2101

Nickel LME

Sur

char

ge $

/ton


Alloy Surcharges for May, 2008Grade Cr Mo Ni Total

2507 $0.58 $1.46 $1.02 $3.16

2205 0.51 1.09 0.80 $2.51

LDX 2101® 0.50 0.11 0.22 $0.93

316L 0.37 0.73 1.46 $2.67

304L 0.42 0 1.17 $1.69


LDX 2101®

ADVANTAGES

• Stress Corrosion Cracking -Resistant

• Pitting Resistance - similar to 316L

• Twice as Strong as 316L

• Ease of Fabrication

• Wear Resistant

• Availability

• Cost

DISADVANTAGES

New


Conclusions

• Environments in wet condensing systems can be very aggressive - determine what is condensing out before makingmaterial decisions

• There can be variations in environments between different plants and material selection has to be done for each plant.

• The main risks for stainless steel are pitting and crevice corrosion in chloride containing environments.


Conclusions

• By choosing the correct stainless steel grade, it is possible to meet a wide range of demands on corrosion resistance.

• If the main risks are chloride pitting or crevice corrosion,the following ranking applies:

304L<316L,~LDX2101<317LMN~904L~2205<2507~254SMO<4565

• Duplex stainless steels are a cost effective choice forcorrosion resistance with high Cr, N and low Ni, Mo contents

Documents

Materials Selection for Condensing Economizerscleanboiler.org/files/Resources/2008_Economizer_WS/... · 2016-09-26 · for material selection !!! Critical Crevice Temperature (CCT)