1822

3. 1822

3. s Steels 1823

3. 1824

3. 1824

3. 1825

3. 1826

3.

1804.8.1 Passivity

04.8.2 Contribution of Main Alloy Elements to Passivation

04.8.3 General Electrochemical Considerations in Corrosion of Stainles

04.8.4 Breakdown of Passivity

04.8.5 Localized Corrosion Pitting and Crevice Corrosion

04.8.5.1 Influence of alloy composition on localized corrosion

04.8.5.2 Pitting corrosion3.04.8 Corrosion Properties of Stainless Ste

3.3.04 Aqueous Corrosion of Stainless SteelsA. IversenOutokumpu Stainless AB, PO Box 74, SE 774 22 Avesta, Sweden

B. LefflerOutokumpu Stainless, Hot Rolled Plate, SE 693 81 Degerfors, Sweden

2010 Elsevier B.V. All rights reserved.

3.04.1 Introduction 1806

3.04.2 The use of Stainless Steels 1807

3.04.3 Definition of Stainless Steels, Alloying Elements, and Microstructure 1808

3.04.3.1 Classification of Stainless Steels 1808

3.04.3.2 Alloying Elements and Microstructure 1809

3.04.3.2.1 Chromium (Cr) 1809

3.04.3.2.2 Nickel (Ni) 1809

3.04.3.2.3 Molybdenum (Mo) 1809

3.04.3.2.4 Copper (Cu) 1809

3.04.3.2.5 Manganese (Mn) 1810

3.04.3.2.6 Silicon (Si) 1810

3.04.3.2.7 Carbon (C) 1810

3.04.3.2.8 Nitrogen (N) 1810

3.04.3.2.9 Titanium (Ti) 1810

3.04.3.2.10 Niobium (Nb) 1811

3.04.3.2.11 Aluminum (Al) 1811

3.04.3.2.12 Cobalt (Co) 1811

3.04.3.2.13 Vanadium (V) 1811

3.04.3.2.14 Sulfur (S) 1811

3.04.3.2.15 Cerium (Ce) 1811

3.04.4 Mechanical Properties 1812

3.04.4.1 Mechanical Properties at Room Temperature 1812

3.04.4.2 The Effect of Cold Work 1815

3.04.4.3 Toughness 1815

3.04.4.4 Fatigue Properties 1816

3.04.5 Precipitation and Embrittlement 1817

3.04.5.1 Embrittlement at 475C 18173.04.5.2 Carbide and Nitride Precipitation 1817

3.04.5.3 Intermetallic Phases 1817

3.04.5.4 Carburization 1818

3.04.5.5 Heat Treatment 1818

3.04.5.5.1 Solution annealing 1818

3.04.5.5.2 Quenching, tempering, and ageing 1818

3.04.5.5.3 Stabilization annealing 1819

3.04.6 Physical Properties 1819

3.04.7 Property Relationships for Stainless Steels 1820

els 182104.8.5.3 Crevice corrosion 1829

02

Aqueous Corrosion of Stainless Steels 18033.04.8.6 Stress Corrosion Cracking 1830

3.04.8.6.1 SCC mechanisms 1831

3.04.8.6.2 Impact of mechanical stress on corrosion: stress intensity factor and crack rate 1832

3.04.8.6.3 Chloride-induced SCC 1832

3.04.8.6.4 Caustic SCC 1833

3.04.8.6.5 Sulfide stress cracking (SSC) by hydrogen sulfide 1833

3.04.8.6.6 Hydrogen-induced stress cracking (HISC) using cathodic protection 1833

3.04.8.6.7 SCC in atmospheric environments 1834

3.04.8.6.8 SCC of martensitic stainless steels 1835

3.04.8.6.9 SCC of ferritic stainless steels 1835

3.04.8.6.10 SCC of austenitic stainless steels 1835

3.04.8.6.11 SCC of duplex stainless steels 1836

3.04.8.7 Corrosion Fatigue 1836

3.04.8.8 Corrosion on Stainless Steels Related to Welding Procedures 1836

3.04.8.8.1 Ferritic stainless steels 1837

3.04.8.8.2 Duplex stainless steels 1837

3.04.8.8.3 Austenitic stainless steels 1837

3.04.8.8.4 Postweld treatment 1837

3.04.8.9 General Corrosion 1838

3.04.8.9.1 Sulfuric acid 1838

3.04.8.9.2 Hydrochloric acid 1840

3.04.8.9.3 Phosphoric acid 1841

3.04.8.9.4 Nitric acid 1842

3.04.8.9.5 Organic acids 1842

3.04.8.9.6 Alkaline solutions 1843

3.04.8.10 Galvanic Corrosion 1844

3.04.8.11 Intergranular Corrosion 1845

3.04.8.12 Erosion Corrosion 1846

3.04.8.13 Common Test Procedures and Standards for Stainless Steels 1846

3.04.8.14 Localized Corrosion Testing of Stainless Steels using Electrochemical Methods 1846

3.04.8.15 Different Stainless Steel Grades and their Resistance to Pitting and

Crevice Corrosion 1847

3.04.8.16 Screening of General Corrosion Properties of Stainless Steel Grades 1849

3.04.8.17 Testing Stress Corrosion Cracking of Stainless Steels in Environments Containing

Hydrogen Sulfide Under Acidic Conditions 1849

3.04.8.18 Laboratory Tests of SCC 1850

3.04.9 Stainless Steels in Natural Wet Environments 1851

3.04.9.1 Microbially Influenced Corrosion 1851

3.04.9.1.1 Chlorination 1852

3.04.9.2 River Waters and Freshwater 1853

3.04.9.2.1 Drinking water32 1853

3.04.9.2.2 Freshwater 1853

3.04.9.3 Seawater 1854

3.04.9.3.1 Material selection 1854

3.04.9.3.2 Polluted seawater 1855

3.04.9.3.3 Cathodic protection and hydrogen embrittlement 1856

3.04.9.3.4 Seawater exposures 1856

3.04.9.3.5 Anaerobic seawater environments 1857

3.04.10 Stainless Steels Performance in Atmospheric Environments32 1858

3.04.10.1 Types of Atmosphere, Corrosivity, and Material Selection 1858

3.04.10.1.1 Indoor, heated, nonheated. Outdoor, arid, low pollution, deserts and arctic areas(Rural), C1C2 1859

3. astal are

derate p

3. emical

Pollute

t depos

3. orrosio

3. ignifica

3.

3.

3.

3.

3. d paper

3.

3. duction3

3. atment 3

3.

3.

3.

3.

3.

3.

R

Active Describes a metal which corrodes in the

negative direction of electrode potential.

caused by repeated or fluctuating stresses in

a corrosive environment.

1804 Ferrous Metals and AlloysActivation polarization Corrosion reaction

determined kinetically by the participating

electrode reactions.

Anaerobic Free of air or oxygen.

Anode The electrode of an electrolyte cell at which

oxidation occurs.

Anodic polarization The change in the electrode

potential in the noble positive direction.

Austenite A face-centered cubic crystalline phase

of iron-base alloy.

Cathode The electrode of an electrolytic cell at

which reduction is the principal reaction.

Cathodic polarization The change of the electrode

potential in the active negative direction.

Cathodic protection Reduction of corrosion

rate by shifting the corrosion potential of

the electrode towards a less oxidizing

potential by applying an external

electromotive force.

Cold work The operation of shaping metals at

temperatures below their recrystallization

temperatures so as to produce

strain-hardening.

Corrosion potential, Ecorr The potential of a

corroding surface in an electrolyte, relative to

a reference electrode, also called open

circuit potential.

Double layer The interface between an electrode

or a suspended particle and an electrolyte

created by charge-charge interaction

leading to an alignment of oppositely

charged ions at the surface of the electrode

or particle.

Ductility Ability of materials to be deformed by

working process and to retain strength and

freedom from cracks when their shape is

altered.

Electrolyte A chemical substance or mixture,

usually liquid, containing ions that migrate in

an electric field.

Elongation The percentage plastic extension

produced in a tensile test.

Embrittlement Loss of ductility of a material

resulting from a chemical or physical change.

Equilibrium potential The potential of an electrode

in an electrolytic solution when the forward04.10.1.2 Indoor, humid, low pollution. Co

and industrialized areas with mo

04.10.1.3 Indoor with volatile aggressive ch

load in swimming pool buildings.

Coastal areas with moderate sal

04.10.2 Factors Influencing Atmospheric C

04.11 Application Areas of Commercial S

04.11.1 Domestic Kitchenware

04.11.2 Process Industry

04.11.2.1 Hydrometallurgy32

04.11.2.2 Desalination32

04.11.2.3 Stainless steel within the pulp an

04.11.2.4 Architecture Art 32

04.11.2.5 Stainless steel in oil and gas pro

04.11.2.6 Stainless steel in wastewater tre

04.12 High Temperature Corrosion32

04.12.1 Oxidation

04.12.2 Sulfur Attack

04.12.3 Halogen Gas Corrosion

04.12.4 Molten Salt corrosion

04.12.5 Molten Metal Corrosion

eferences

Glossaryas with low deposits of salt. Urban

ollution, C3 1859

compounds, roof parts with mechanical

d urban and industrialized atmosphere.

its, C4C5 1859

n on Stainless Steels 1860

nce 1860

1860

1861

1861

1863

industry 1865

18662 18672,49 1870

1873

1875

1876

1876

1876

1877

1877

Corrosion fatigue Fatigue type cracking of metal

Aqueous Corrosion of Stainless Steels 1805rate of a given reaction is exactly equal to the

reverse rate.

ErosionDestruction of materials by abrasive action

of moving fluids, usually accelerated by the

presence of solid particles.

Fatigue The phenomenon leading to fracture under

repeated or fluctuating stresses having a

maximum value less than the tensile strength

of the material.

Ferrite A body-centered cubic crystalline phase of

iron-base alloys.

Galvanic corrosion Corrosion associated with the

current resulting from the electrical coupling

of dissimilar electrodes in an electrolyte.

General corrosion A form of deterioration that is

distribute more or less uniformly over a

surface.

Hardening The process of making steel hard by

cooling from above the critical range at a rate

that prevents the formation of ferrite and

pearlite and results in the formation of

martensite.

Heat affected zone (HAZ) That portion of the base

metal that was not melted by welding but

whose microstructure and properties were

altered by the heat of the welding process.

Inclusion A nonmetallic phase such as an oxide,

sulfide or silicate particle in a metal.

Martensite Metastable body-centered phase of

iron super-saturated with carbon, produced

from austentite by shear transformation

during quenching or deformation.

Mixed potential A potential resulting from

two or more electrochemical reactions

occurring simultaneously on one metal

surface.

Noble The positive direction of electrode potential.

Open circuit potential The potential of an

electrode measured with respect to a

reference electrode when no current flows to

or from it (see also corrosion potential).

Passivation A reduction of the anodic reaction rate

of a metal.

Passive A metal corroding under the control of a

surface reaction product.

Passivity The state of being passive.

Pits, pitting Localized corrosion of a metal surface

that is confined to a small area and takes to

form of cavities.

Polarization The deviation from the open circuitpotential of an electrode.Polarization curve or polarization diagram A plot

of current density versus electrode potential

for a specific electrode-electrolyte

combination.

Potentiodynamic The technique for varying the

potential of an electrode in a continuous

manner at a present rate.

Potentiostate An instrument for automatically

maintaining an electrode at a constant

potential or controlled potential with respect

to a reference electrode.

Potentiostatic The technique for maintaining a

constant electrode potential.

Precipitation hardening Improving the strength of

solid solutions alloys by controlling the

formation of precipitates on a crystal lattice

scale.

Reference electrode A reversible electrode used

for measuring the potentials of other

electrodes.

Relative humidity, RH The ratio, expressed

as a percentage, of the amount of

water vapor present in a given volume of air

at a given temperature to the amount

required to saturate the air at that

temperature.

Scanning electron microscope (SEM) An

electron optical device that images

topographical details with maximum

contrast and depth of field by the detection,

amplification and display of secondary

electrons.

Sensitizing heat treatment A heat treatment,

which causes precipitation of constituents at

grain boundaries.

Solution heat treatment Heating a metal to a

suitable temperature and holding at that

temperature long enough for one or more

constituents to enter into solid solution, then

cooling rapidly enough to retain the

constituents in solution.

Tempering The reheating of hardened steel at any

temperature below the critical range, in order

to decrease the hardness. Sometimes

drawing.

Toughness Condition intermediate between

brittleness and softness, as indicated in

tensile tests by high ultimate tensile stress

and low to moderate elongation and

reduction in area, or by high values of energyabsorbed by impact tests. More precisely it

CPT Critical pitting temperature (C)FCC Face centered cubic structure

A5 Elongation or permanent extension of the gauge

length after fracture, as expressed as a

Ms Martensite temperature, the starting

temperature for martensite transformation

n Charge number

3

T -na dm -re s.A -tio fca oth tpr tan rpr dga -tr eau sS4 -

1806 Ferrous Metals and Alloyspercentage of the original gauge length.

ba Anodic Tafel slope

Cs Concentration of metal ion

d Diffusion length (m)

D Diffusion coefficient of component i (m2 s1)E Potential (V)GS Grain size

HISC Hydrogen-induced stress cracking

LT-MED Low temperature multi effect, desalination

plant

Me Metal

MIC Microbially influenced corrosion

MIG Gas metal arc welding

MSF Multistage flash

OCP Open circuit potential (V)

PRE Pitting resistance equivalent

REM Rare earth metals

RH Relative humidity (%)

RO Reverse osmosis

RT Room temperature

SCC Stress corrosion cracking

SCE Saturated calomel electrode

SEM Scanning electron microscope

SHE Standard hydrogen electrode

SSC Sulfide stress cracking

SWRO Seawater reverse osmosis

TIG Gas tungsten welding

TTS Time temperature sensitization

WPA Wet process phosphoric acid

Symbolsa Body centered cubic ferrite rich in iron

a0 Body centered cubic ferrite rich in chromiumis the value of the critical strain energy

release rate.

Transpassive The noble region of potential where

an electrode exhibits a current density higher

than passive current density.

AbbreviationsBCC Body centered cubic structure

CCT Critical crevice corrosion temperature (C)Et Transition potential (V) teN Number of cycles

ws Ohmic potential drop in a pit cavity

R Stress ratio

Rm Tensile strength (MPa)

Rp0.2 Proof strength at which thematerial undergoes

a 0.2% nonproportional (permanent)

extension during a tensile test (MPa)

Rp1.0 Proof Strength at which thematerial undergoes

a 1.0% nonproportional (permanent)

extension during a tensile test (MPa)

S Stress amplitue (MPa)

.04.1 Introduction

he corrosion resistance of stainless steels, in combition with their good mechanical properties ananufacturing characteristics, makes them an extmely valuable and flexible material for designerlthough the usage of stainless steel may have tradinally been relatively low compared with that orbon steels, growth has been steady, in contrast te growth of structural steels. The most dominanoduct form for stainless steels is cold rolled sheed the major application areas include consumeoducts and plant and equipment for the oil ans, chemical process, and food and beverage indusies. The most widely used stainless steels are thstenitic grade S30400 and ferritic grades such a1000, followed by the molybdenum-alloyed ausEcorr Corrosion potential (V)

Ep Pitting Potential (V)

Epp Passivation Potential (V)

Erp Repassivation Potential (V)

Etr Transpassive Potential (V)

F Faradays constant (A s mol1)I Current (A)

Icrit Critical current density for passivation

(A cm2)iL Anodic limiting current density (A cm

2)K Equilibrium constant

KISCC Threshold stress intensity factor

(MPa m1/2)

Md30 The temperature at which martensite will form

at a strain of 30%

Md The temperature below where martensite will

formnitic grades, notably S31600.

sion-resistant alloys. The first patent for stainless steel

hot rolled plate and sheet, bar, tube and pipe, indi-vidually account for only a third or less of the totalvolumes of cold rolled sheet produced.

The use of stainless steels can be divided into afew major areas, with consumer products, industrialequipment, transport, and construction being thebiggest. Table 1 gives a breakdown of the use ofstainless steel by end-user segment.

The most widely used stainless grades are austen-itic steels, which typically contain 18% chromium and8% nickel, that is, S30400/304L. These steels accountfor more than 50% of the global production of stain-less steel. The next most widely used grades are fer-ritic steels, for example, S41000 (EN 1.4000), followedby molybdenum-alloyed austenitic steels, for example,S31600/316L (EN 1.4401/1.4404). Together, thesegrades account for over 70% of the total tonnage ofstainless steels. However, the use of duplex stainlesssteels, such as UNS S32205 (EN 1.4462), has beengrowing considerably recently and this group of stain-less steels now accounts for a considerable proportionof the stainless market. The use of duplex grades willcertainly increase in future. Figure 1 shows an

Table 1 Use of stainless steel divided into applicationcategories

End-user segment %

Aqueous Corrosion of Stainless Steels 1807was registered in the United Kingdom in 1912 and thedevelopment of stainless steels was then sparked offsimultaneously in the United Kingdom, the UnitedStates, and Germany. Even if most of the stainlesssteel types currently in use had been available in the1930s, it was not until the 1960s that developmentsin process metallurgy gave rise to the growth andwidespread use of modern stainless steels.

Early research found that increasing the chromiumcontent of iron to about 1014% produced a massivedrop in corrosion rate in many diverse environments.In time it was also realized that increasing the chro-mium content above 1820% improved the corrosionresistance by decreasing the corrosion rate even fur-ther. This is the reason that even today many commonstainless steel grades contain about 18% chromium. Itis the resistance to many common corrosive environ-ments, in combination with good mechanical andfabrication properties, that makes stainless steels uni-versally useful whenever corrosion resistance andlong-lasting endurance are required.

3.04.2 The use of Stainless Steels

Steel is the predominant metal used in industry. Theglobal production of steel is around 1 billion metrictons a year, of which stainless steel accounts for about2.5%.

The use and production of stainless steels have formany years been dominated by the industrializedWestern nations and Japan, but in recent years, Asiancountries, such as India and China, as well as someSouth American countries, have emerged as importantproducers and consumers of stainless steels.

The most common product form for stainlessIn terms of their durability and corrosion resistance,iron and most iron-alloyed steels are relatively poormaterials, since they easily corrode in air and acidenvironments unless protected by some external coat-ing and scale in furnace atmospheres. Stainless steels,however, although also belonging to the category ofiron-base alloys, offer superior corrosion resistanceproperties and durability in such diverse environmentsas seawater, diluted and concentrated acids, and hightemperature environments up to 1100C.

It was the discovery, made about 100 years ago, ofthe effect of chromium on the resistance of iron inmany environments that triggered the development ofwhat is currently the most common group of corro-steels is cold rolled sheet. Other products, such asCatering & household 32Industrial equipment 26

Transport 16

Construction 15

Tubular products 5Other 6

Figure 1 White liquor tank from the pulp and paperindustry built in a recently developed stainless steel grade,

S32101 (1.4162).

example of the use of a recently developed duplexstainless steel grade, S32101 (1.4162), in a white liquortank for the pulp- and paper industry.

3.04.3 Definition of Stainless Steels,Alloying Elements, and Microstructure

3.04.3.1 Classification of Stainless Steels

Stainless steels are generally defined as iron-basedalloys containing at least 10.5% chromium (byweight) and a maximum of 1.2% carbon (by weight).This is the definition given in the European stan-

categories of stainless steel are refined further fromthese basic criteria, either as a single-phase micro-structure or as combinations of these structures, toresult in five categories of stainless steel, as follows:

ferritic stainless steels austenitic stainless steels ferriticaustenitic (duplex) stainless steels martensitic stainless steels precipitation-hardening (PH) steelsThe most common alloying elements used in stain-less steels are chromium, nickel, molybdenum, car-bon, nitrogen, silicon, and manganese. Silicon and

sta

Precipitation hardening 0.030.20

0.050.15

1808 Ferrous Metals and AlloysFerritic

Figures 2 and 3 show examples of a normalmicrostructure for austenitic and duplex stainlesssteel.

For commercial stainless steel grades, severalinternational and national standards describe andspecify the composition and required properties ofstainless steel products. One of the most comprehen-sive catalogs of steel grades is the Unified NumberingSystem (UNS). This system does not contain speci-fications but provides a unified list of alloys that havecompositions specified in standards or elsewhere. Allstainless steels have the letter S as a prefix in theUNS number. European stainless steel grades arelisted along with their compositions in EN 10088-1.1

American steel grades and their compositions arenormally listed in the material and product standardspublished by ASTM or ASME, for example, ASTMA 240 or ASME SA 240. Table 3 shows the typicalchemical composition for a number of commercialstainless steels of various types based mainly onAmerican and European standards.

steel grade will thus be determined by the combined

3.04.3.2.2 Nickel (Ni)

the strength of the steel.

Aqueous Corrosion of Stainless Steels 1809Figure 3 Typical microstructure of a duplex stainlesssteel.

Figure 2 Typical microstructure of an austenitic stainlesssteel. 3.04.3.2.3 Molybdenum (Mo)

Molybdenum substantially enhances the resistance toboth general and localized corrosion and increasesthe mechanical strength of steels. In addition to pro-moting a ferritic structure, molybdenum promotesthe formation of secondary phases in ferritic, duplex,and austenitic steels. In martensitic steels, it willincrease the hardness at higher tempering tempera-tures because of its effect on carbide precipitation.

3.04.3.2.4 Copper (Cu)Copper enhances the corrosion resistance of steels incertain acid environments and promotes an austeniticstructure. In precipitation-hardening steels, copper isused to form the intermetallic compounds to increaseThe main reason for adding nickel is to promotean austenitic structure. Nickel generally increasesductility and toughness. It also lowers the corrosionrate and can thus be used to good effect in acidenvironments. In precipitation-hardening steels,nickel is also used to form the intermetallic com-pounds to increase the strength of the steel.3.04.3.2.1 Chromium (Cr)

This is the most important alloying element as it pro-vides stainless steels with their basic corrosion resis-tance. Generally speaking, the higher the chromiumcontent, the better the corrosion resistance. Chromiumalso enhances the steels resistance to oxidation at hightemperatures and promotes a ferritic structure.effect of the alloying and trace elements in thatspecific grade. A brief overview of the alloying ele-ments and their effects on the structure and proper-ties of the steel are given in the following sectionstogether with an explanation of why various elementsare added to certain grades.2 It should also be notedthat the effect of the alloying elements differs in someaspects between the hardenable and the nonharden-able stainless steels.3.04.3.2 Alloying Elements andMicrostructure

Stainless steels contain a number of different alloyingelements, each of which has a specific effect on theproperties of the steel. The properties of a specific

erc

1810 Ferrous Metals and AlloysTable 3 Typical chemical compositions for some comm

Steel grade Microstructure

UNS ASTM EN

S41000 410 1.4006 Martensitic

S41600 416 1.4005 Martensitic

S42000 420 1.4021 MartensiticS43000 430 1.4016 Ferritic

S43400 434 1.4113 Ferritic

S44400 444 1.4521 Ferritic

S20100 201 1.4372 AusteniticS20400 204 Austenitic

S30100 301 1.4310 Austenitic

S30100 301LN 1.4318 Austenitic

S30300 303 1.4305 AusteniticS30400 304 1.4301 Austenitic

S30403 304L 1.4307 Austenitic

S30500 305 1.4303 Austenitic

S34700 347 1.4550 AusteniticS31600 316 1.4401 Austenitic

S31603 316L 1.4404 Austenitic

S31635 316Ti 1.4571 AusteniticS31653 316LN 1.4406 Austenitic

S31600 316 1.4436 Austenitic

S31703 S31703 1.4438 Austenitic3.04.3.2.5 Manganese (Mn)

Manganese is generally used in stainless steels toimprove hot ductility. Its effect on the ferrite/austen-ite balance varies with temperature; at low tempera-tures, manganese is an austenite stabilizer, whereas athigh temperatures, it will stabilize ferrite. Manganeseincreases the solubility of nitrogen and is used toobtain high nitrogen contents in austenitic steels.

3.04.3.2.6 Silicon (Si)

Silicon increases the resistance to oxidation, bothat high temperatures and in strongly oxidizing solu-tions at lower temperatures. It promotes a ferriticstructure.

3.04.3.2.7 Carbon (C)

Carbon is a strong austenite former and stronglypromotes an austenitic structure. It also substantiallyincreases the mechanical strength. Increasing carboncontent reduces the resistance to intergranular cor-rosion. In ferritic stainless steels carbon will stronglyreduce both toughness and corrosion resistance.

N08904 N08904 1.4539 AusteniticS31254 S31254 1.4547 Austenitic

S34565 S34565 1.4565 Austenitic

S32101 S32101 1.4162 Duplex

S32304 S32304 1.4362 DuplexS32205 S32205 1.4462 Duplex

S32750 S32750 1.4410 Duplexial stainless steel grades

Typical chemical composition (wt%)

Cr Ni Mo N Other

12 0.04

13 0.04 S

1316

17 1

17 2

17 5 0.15 Mn(7.08)16 2.2 0.17 Mn(9.07)

17 7

17.7 6.5 0.14

17.3 8.218.1 8.3

17.5 8

17.7 12.5

18 9.5 0.04 Nb,Mn(2.0)17.2 10.2 2.1

17.2 10.2 2.1

16.8 10.9 2.1 Ti17.2 10.3 2.1 0.14

16.9 10.7 2.6

18.2 13.7 3.1In the martensitic and martensiticaustenitic steels,carbon increases hardness and strength. In the mar-tensitic steels, an increase in hardness and strength isgenerally accompanied by a decrease in toughnessand in this way carbon reduces the toughness ofthese steels.

3.04.3.2.8 Nitrogen (N)

Nitrogen is a very strong austenite former andpromotes an austenitic structure. It also substantiallyincreases the mechanical strength of steel andenhances its resistance to localized corrosion, espe-cially when used in combination with molybdenum.In ferritic stainless steels, nitrogen will lead toa significant reduction in toughness and corrosionresistance. In martensitic and martensiticausteniticsteels, the addition of nitrogen will increase hardnessand strength but reduce the toughness of the steel.

3.04.3.2.9 Titanium (Ti)

Titanium is a strong ferrite former and a strongcarbide former, and, as such, helps lower the effective

20 25 4.5 Cu(1.5)20 18 6.1 0.20 Cu

24 17 4.5 0.45 5Mn(5.5)

21.5 1.5 0.3 0.22

22 3.5 0.1 0.1022 5.5 3 0.17

24 6 3 0.27

high temperature steels and alloys in order to increasethe resistance to oxidation and high temperaturecorrosion.

Since it is the combined effect of the alloyingelements that decide both the microstructure andthe properties of a certain grade, the effect of thealloying elements can be summarized in various ways.One such summary of the effect of alloying elementson the microstructure is the SchaefflerDelong dia-gram presented in Figure 4. A guide to the composi-tion of the stainless steels presented in Figure 4 isgiven in Table 4.

The diagram is based on the fact that the alloyingelements can be divided into ferrite-stabilizers andaustenite-stabilizers. This means that they favor theformation of either ferrite or austenite in the micro-structure of the steel. Assuming that the ability ofaustenite-stabilizers to promote the formation of aus-tenite is related to the nickel content, and that theability of ferrite-stabilizers is related to the chromiumcontent, it becomes possible to calculate the totalferrite and austenite stabilizing effect of the alloyingelements in the steel, and thereby obtain the

310 S

904L

5%FFerriticaustenitic

Austenitic

Ni-equivalent = %Ni + 30 (%C + %N) + 0.5 (%Mn + %Cu + %Co)

24

26

Aqueous Corrosion of Stainless Steels 1811carbon content and promote a ferritic structure intwo ways. In austenitic steels, it is added to improvethe resistance to intergranular corrosion, but it alsoenhances the mechanical properties of the steel athigh temperatures. In ferritic stainless steels, titaniumis added to improve toughness and corrosion resis-tance by lowering the amount of interstitials in solidsolution. In martensitic steels, titanium reduces themartensite hardness and increases the temperingresistance. In precipitation-hardening steels titaniumis used to form the intermetallic compounds used toincrease the strength of the steel.

3.04.3.2.10 Niobium (Nb)Niobium is both a strong ferrite and carbide former.Like titanium, it promotes a ferritic structure. In austen-itic steels, it is added to improve the resistance to inter-granular corrosion, but it also enhances mechanicalproperties at high temperatures. In martensitic steels,niobium reduces the hardness and increases the temper-ing resistance. It is also referred to as columbium (Cb).

3.04.3.2.11 Aluminum (Al)

Aluminum improves the oxidation resistance, if addedin substantial amounts. It is used in certain heat resistantalloys for this purpose. In precipitation-hardeningsteels, aluminum is used to form the intermetallic com-pounds that increase the strength in the aged condition.

3.04.3.2.12 Cobalt (Co)

Cobalt is only used as an alloying element in marten-sitic steels in which it increases the hardness and tem-pering resistance, especially at higher temperatures.

3.04.3.2.13 Vanadium (V)

Vanadium increases the hardness of martensitic steelsdue to its effect on the type of carbide present. Italso increases tempering resistance. Vanadium stabi-lizes ferrite and will, at high contents, promote fer-rite in the structure. It is only used in hardenablestainless steels.

3.04.3.2.14 Sulfur (S)

Sulfur is added to certain stainless steels in order toincrease the machinability. At the levels present inthese grades, sulfur will substantially reduce corro-sion resistance, ductility, and fabrication properties,such as weldability and formability.

3.04.3.2.15 Cerium (Ce)

Cerium is one of the rare earth metals (REMs) and

is added in small amounts to certain heat-resistant,405430

A

A+F

10%F

40%F

20%F

80%F

60%F2205

2507

444

F18 - 2 FM

Ferritic

316 high Mo

316 low Mo

304

317L

316LN

304LN

Martensitic

Martensiticaustenitic

2304

0% ferrite in wrought,annealed material

410

420L

M

M+F

18

16

14

12

10

8

6

4

2

012 14 16 18 20 22 24 26 28 30

20

22

Cr-equivalent = %Cr + 1.5%Si + %Mo

A+M

100%F

Figure 4 The SchaefflerDelong diagram. A guideline to

the steel grades is presented in Table 4.

s p

6

1

61

1

11

6

6

89 20 25 4.5 Cu(1.5)

2 22 3.5 0.1 0.10

2

05

1812 Ferrous Metals and Alloyschromium and nickel equivalents in the SchaefflerDelong diagram. It is thus possible to take intoaccount the combined effect of alloying elements onthe microstructure of a steel grade.

The SchaefflerDelong diagram was originallydeveloped for weld metal, that is, it describes thestructure after melting and rapid cooling, but thediagram has also been found to give a useful pictureof the effect of the alloying elements for wrought and

Table 4 Guide to the compositions of the stainless steel

Note in Figure 4 Steel grade

UNS ASTM EN

410 S41000 410 1.400

420L S42000 420 1.402

430 S43000 430 1.401444 S44400 444 1.452

304 S30400 304 1.430

304LN S30451 304LN 1.431316LowMo S31600 316 1.440

316LN S31653 316LN 1.440

316HiMo S31600 316 1.443

317L S31703 317L 1.443904L N08904 N08904 1.453

2304 S32304 S32304 1.436

2205 S32205 S32205 1.446

2507 S32750 S32750 1.441310S S31008 310S 1.484heat-treated material. However, in practice, wroughtor heat-treated steels with ferrite contents in therange 05% according to the diagram in fact containsmaller amounts of ferrite than that predicted by thediagram. It should be noted that the SchaefflerDelong diagram is not the only diagram for assessingthe ferrite content and microstructure of stainlesssteels. Several other diagrams have been published,all with slightly different equivalents, phase limits, orgeneral layout.

Precipitation-hardening stainless steels can bedivided into three subcategories: martensitic,semiaustenitic, and austenitic grades. The commondenominator here is not the microstructure but thehardening mechanism, precipitation hardening.This involves the formation of second-phase parti-cles from a supersaturated solution which inducesan internal strain in the microstructure and thusincreases the strength of the material. The elementsmost commonly used to induce precipitation hard-ening, either individually or in combination, arealuminum, titanium, and copper.3.04.4 Mechanical Properties

The difference in the mechanical properties of dif-ferent stainless steels is perhaps seen most clearlyin the stressstrain curves in Figure 5. The highyield and tensile strengths but low ductility of themartensitic steels are apparent, as are the low yieldstrength and excellent ductility of the austeniticgrades. Duplex and ferritic steels both lie somewhere

22 5.5 3 0.17

24 6 3 0.2725 20resented in Figure 4

Typical chemical composition (wt%)

Cr Ni Mo N Other

12 0.04

13

1617 2

18.1 8.3

18.5 10.5 0.1417.2 10.2 2.1

17.2 10.3 2.1 0.14

16.9 10.7 2.6

18.2 13.7 3.1between these two extremes. Ferritic steels generallyhave a slightly higher yield strength than austeniticsteels, whereas duplex steels have an appreciablyhigher yield strength than both austenitic and ferriticsteels. The ductility of ferritic and duplex steels is ofthe same order of magnitude, even if the latter aresomewhat superior in this respect.

3.04.4.1 Mechanical Properties at RoomTemperature

In terms of their mechanical properties, stainlesssteels can be roughly divided into four groups: mar-tensitic, ferritic, duplex, and austenitic, and the prop-erties within each group are relatively similar.Table 5 shows the typical mechanical properties atroom temperature for a number of stainless steels.

Stress values are given as the nearest 10 MPa.Standard deviations are normally about 20 MPa forproof strengths, Rp0.2, Rp1.0 and tensile strength, Rm;and 3 wt% for the elongation, A5. More detailedinformation can be found in Nordberg et al.4

Martensitic steels are characterized by their highstrength and the fact that this strength is stronglyaffected by heat treatment. Martensitic steels are usu-ally used in the hardened and tempered condition.

In this condition, the strength of the steel improvesin relation to the carbon content. Steels with morethan 13 wt% chromium and a carbon content above0.15 wt% are completely martensitic after hardening.A reduction in the carbon content causes an increasein the ferrite content and thus has an adverse effect onthe strength of the steel. The ductility of martensiticsteels is relatively low. Low carbon martensitic grades,often alloyed with nickel, have high strength in thehardened and tempered condition and good ductility.The mechanical properties of martensitic stainlesssteels are heavily influenced by the heat treatmentstowhich the steels are subjected. A brief description ofthe general heat treatment of martensitic stainlesssteels and the effect on the mechanical properties isgiven as follows.5,6

To obtain useful properties, martensitic stainlesssteels are normally used in the hardened and tem-pered condition. The hardening treatment consists ofheating to a high temperature in order to produce anaustenitic structure with carbon in solid solution fol-lowed by quenching. The austenitizing temperatureis generally in the range 9251070C. The effect of

Table 5 Typical mechanical properties for stainless steels at room temperature for hot rolled plate

Micro structure Steel grade Rp0.2 (MPa) Rp1.0 (MPa) Rm (MPa) A5 (%)

EN UNS(ASTM)

Martensitic 1.4006 S410081.4021 S42003

S43100

ASTM 248 SV

Ferritic ASTM4461.4521 S44400

Duplex (ferriticaustenitic) 1.4162 S32101

1.4362 S32304

1.4462 S318031.4410 S32750

Austenitic 1.4301 S30400

1.4307 S304031.4311 S30453

S30451

1.4541 S32100

1.4404 S316031.4571 S31635

1.4401 S31600

1.4432 S31603

1.4438 S317031.4439 S31726

1.4529 N08904

Ferritic (444Ti) Austenitic (S31600)

Duplex (S32205)

Martensitic (S41500), quenched and tempered

Martensitic (S42000), quenched and tempered1250

1000

750

500

250

0 10 20 30 40 50 60 70Strain (%)

Str

ess

(MP

a)

Figure 5 Stressstrain curves for some stainless steels.Reproduced from Leffler, B. Stainless Stainless Steels andTheir Properties; Avesta Sheffield AB Research Foundation,

Stockholm, Sweden, 1996.

Aqueous Corrosion of Stainless Steels 18131.4547 S31254Austenitic (heat-resistant steels) 1.4845 S31008

1.4818 S30415

1.4835 S30815

1.4854 S35315540 690 20780 980 16

690 900 16

790 840 930 18

340 540 25390 560 30

450 650 30

470 540 730 36

500 590 770 36600 670 850 35

310 350 620 57

290 340 590 56340 380 650 52

350 400 670 54

280 320 590 54

310 350 600 54290 330 580 54

320 360 620 54

300 340 590 54

300 350 610 53320 360 650 52

260 310 600 49

340 380 690 50290 330 620 50

380 410 700 50

410 440 720 52

360 400 720 50

austenitizing temperature and time on hardness andstrength varies with the composition of the steel,especially the carbon content. In general, the hardnesswill increase in relation to the austenitizing tempera-ture up to a maximum and then decrease. The effectof increased time at the austenitizing temperature isnormally a slow reduction in hardness with increasedtime. Quenching, after austenitizing, is done in air, oil,or water depending on the steel grade. On coolingbelow the Ms temperature, the starting temperaturefor the martensite transformation, the austenite trans-forms to martensite. The Ms temperature lies in therange 70300C and the transformation is usuallycompleted at about 150(200248)C below the Mstemperature. Almost all alloying elements will lowerthe Ms temperature, with carbon having the greatesteffect. This means that in higher alloyed martensiticgrades, the microstructure will contain retained aus-tenite due to the low temperature (below ambient)

elongation, and yield strength are more or less unaf-fected. Above this temperature, there will be a moreor less pronounced increase in yield strength, tensilestrength, and hardness because of the secondary hard-ening peak, around 450500C. In the temperaturerange around the secondary hardening peak, there isgenerally a dip in the impact toughness curve. Aboveabout 500C, there is a rapid reduction in strengthand hardness, and a corresponding increase in duc-tility and toughness. Tempering at temperaturesabove the AC1 temperature (780C for the steel inFigure 6) will result in partial austenitizising and thepossible presence of nontempered martensite aftercooling to room temperature.7

Ferritic steels have relatively low yield strength andthe work hardening is limited. The strength increases asthe carbon content is increased, but the effect ofincreased chromium content is negligible. However,ductility decreases at high chromium levels and good

200

up

mp

al p

1814 Ferrous Metals and Alloysneeded to finish the transformation of the austeniteinto martensite.

In the hardened condition, the strength and hard-ness are high but the ductility and toughness is low.To obtain useful engineering properties, martensiticstainless steels are normally tempered. The temperingtemperature used has a large influence on the finalproperties of the steel. The effect of tempering tem-perature on the mechanical properties of a martensi-tic stainless steel (Type 431) is shown in Figure 6.Normally, increasing tempering temperatures above400C will lead to a small decrease in tensile strengthand an increase in reduction of area while hardness,

100

0

100

200

300

400

500

600

700

800

900

Austenitic (S31603)

Ferritic (S43000)D

Te

Rp

0.2

(Mp

a)

Figure 6 Effect of tempering temperature on the mechanicductility requires very low levels of carbon andnitrogen.Duplex (ferriticaustenitic) steels have a high

yield strength, which increases with higher carbonand nitrogen levels. Increased ferrite content will,within limits, also increase the strength of duplexsteels. Their ductility is good and they exhibit strongwork hardening properties.

Austenitic steels generally have a relatively lowyield strength and are characterized by strong workhardening properties. The strength of austeniticsteels increases with higher levels of carbon, nitrogen,and, to a certain extent, also molybdenum. The det-rimental effect of carbon on corrosion resistance

400300

Ferritic (S44700)

Martensitic (S41000)

Martensitic (S41500)

lex

erature (c)

roperties (R 0.2% proof strength).p0.2

means that this element cannot be used for increasingstrength. Austenitic steels exhibit very high ductility;they have a high elongation and are very tough.

Some austenitic stainless steels with a low overallcontent of alloying elements, for example, type 301and 304 steels, can be metastable and may formmartensite, either due to cooling below ambienttemperatures or through cold deformation, or acombination of both. The formation of martensitewill cause a considerable increase in strength, asillustrated in Figure 7. The Md temperature isdefined as the temperature below which martensite

rather simple expressions, the actual strengthening

decrease the elongation. Figure 8 shows cold hard-

Aqueous Corrosion of Stainless Steels 1815will form. The stability of the austenite depends onthe composition of the steel; the higher the contentof alloying elements, the more stable the austenite.A common equation for relating austenite stabilityto alloy composition is the Md30, which is defined asthe temperature at which martensite will form at astrain of 30%:

Md30 551 462CN 9:2Si 8:1Mn13:7Cr 29Ni Cu 18:5Mo 68Nb 1:42 GS 8:0C

where GS is the grain size, ASTM grain sizenumber.

This type of equation gives a good idea of thebehavior of lean austenitic stainless steels, but itmust be noted that it is only approximate as interac-tions between the alloying elements are not takeninto account.

The effects of alloying elements and structure onthe strength of austenitic and duplex steels have beendiscussed over the years and several regression equa-tions have been proposed for identifying the effectsof the various alloying elements in stainless steels.Most of the regression equations proposed apply toaustenitic stainless steels but some have also includedduplex stainless steels in the equations.8,9,10 These

100

80

60

40

20

0

2500

2000

1500

1000

500

00 0.2 0.4 0.6 0.8 1

Martensite

Stress

True

str

ess

(MP

a)

Mar

tens

ite c

onte

nt (%

)

True strain

Figure 7 The effect of strain on martensite and yieldstrength of AISI 301. Reproduced from Peckner, B.Handbook of Stainless Steel; McGraw-Hill, 1977.ening curves for some stainless steels.The work hardening is greater for austenitic steels

than for ferritic steels. The addition of nitrogen inaustenitic steels makes these grades particularly hardand strong: compare S31603 and S31653. The strongwork hardening of the austenitic steels means thatlarge forces are required for forming operations eventhough the yield strength is low. Work hardening can,however, also be deliberately used to increase thestrength of a component.

3.04.4.3 Toughness

The toughness of the different types of stainless steelshows considerable variation, ranging from excellenttoughness at all temperatures for austenitic steels tothe relative brittleness of martensitic steels.

Toughness is dependent on temperature andgenerally increases with increasing temperature.mechanism might be more complex. At chromiumcontents over 20%, austenitic steel with 10% Niwill contain d-ferrite, which in turn causes a smal-ler grain size, and this will enhance both yieldstrength and tensile strength. Nitrogen has a strongstrengthening effect but is also a powerful austenitestabilizer. In duplex stainless steels, the strengthen-ing effect of nitrogen is, to a certain extent, coun-tered by the increased austenite content caused bythe addition of nitrogen.

3.04.4.2 The Effect of Cold Work

Stainless steels will harden during deformation andthe mechanical properties of stainless steels arestrongly influenced by cold deformation. The amountof hardening depends on both the composition andthe type of steel. The work hardening of austeniticand duplex steels in particular causes considerablechanges in properties after cold forming operations.The general effect of cold work is to increase theyield and tensile strengths and at the same timeequations may be used to estimate the strength ofaustenitic and duplex steel.

In contrast to the constructional steels, austeniticsteels do not exhibit a clear yield stress but beginto deform plastically at a stress around 40% of thetensile strength, Rp0.2.

It should be noted that although the differentelements are included in the equation throughOne measure of toughness is impact toughness, that

is, the toughness measured on rapid loading. Figure 9shows the impact toughness for different categoriesof stainless steel at temperatures ranging from 200to +100C. It is apparent from the diagram that thereis a fundamental difference at low temperaturesbetween austenitic steels on the one hand and mar-tensitic, ferritic, and duplex steels on the other.

Themartensitic, ferritic, andduplex steels are char-acterized by a transition in toughness, from tough tobrittle behavior, at a certain temperature, the transitiontemperature. For the ferritic steel, the transition tem-perature increaseswith increasing carbon and nitrogencontent, that is, the steel becomes brittle at successivelyhigher temperatures. For the duplex steels, increasedferrite content gives a higher transition temperature,

that is, more brittle behavior. Martensitic stainlesssteels have transition temperatures around or slightlybelow room temperature, while those for the ferriticand duplex steels are in the range 060C, with theferritic steels in the upper part of this range.

The austenitic steels do not exhibit a toughnesstransition as the other steel types but have excellenttoughness at all temperatures. Austenitic steels arethus preferable for low-temperature applications.

3.04.4.4 Fatigue Properties

During cyclic loading, stainless steels, like other mate-rials, will fail at stress levels considerably lower thanthe tensile strength measured during tensile testing.The number of load cycles the material can withstandis dependent on the stress amplitude.Figure 10 shows

300amp

S32205

S32205

316LN

316LN

316L

316L

1250

1000

750

500

250

00 30252015105 35

Str

ess

(MP

a)

RmRp0.2 A5 60

50

40

30

20

10

0

Elo

ngat

ion

(%)

Strain (%)

Figure 8 Effect of cold work on some stainless steels.

1816 Ferrous Metals and Alloys50100150200 0 500

50

100

150

200

250

300

350

Ab

sorb

ed e

nerg

y (J

)

S43000

S44635S44400S41000S31603S31653

S32205

S32304

Figure 9 Impact toughness for different types of stainless

Reproduced from Leffler, B. Stainless Steels and Their

Properties, 2nd revised ed.; Outokumpu Stainless Research

Foundation, Stockholm, 1998.steels.10 000 0001 000 000100 00010 000Number of cycles, N

200

Str

ess

Figure 10 SN curve (Wohler curve) for an austeniticstainless steel of Type S31600 (hMo) in air. S stress

amplitude, N number of cycles. Reproduced from Leffler,

B. Stainless Steels and Their Properties, 2nd revised ed.;Outokumpu Stainless Research Foundation, Stockholm,how the lifetime, that is, the number of cycles tofailure, increases with decreasing load amplitudeuntil a certain amplitude is reached, below which nofailure occurs. This stress level is called the fatiguelimit. In many cases, there is no fatigue limit, but thestress amplitude shows a slow decrease with anincreasing number of cycles. In these cases, the fatiguestrength, that is, the maximum stress amplitude for acertain time to failure (number of cycles) is called thefatigue strength and it is always given in relation to acertain number of cycles.

The fatigue properties of ferriticaustenitic andaustenitic stainless steels with a fatigue limit at alifetime of 106107 load cycles can be described bythe Wohler curve or SN curve and related to theirtensile strength, as shown in Table 6. The relationbetween the fatigue limit and tensile strength is also

500

400

F = 90 Hz Rm= 620 MPa

litud

e, S

(MP

a)1998.

phenomenon is encountered in grades containing

Aqueous Corrosion of Stainless Steels 18173.04.5.2 Carbide and Nitride Precipitation

If ferritic steels are heated to temperatures aboveapproximately 950C, they suffer precipitation ofchromium carbides and chromium nitrides duringmore than 1518% chromium and the origin of thisembrittlement is the decomposition of the ferrite intotwo phases of body-centered cubic, bcc, structure,a and a0. The former is very rich in iron and thelatter very rich in chromium. This type of embrittle-ment is usually called 475C embrittlement after themidpoint of the temperature range.3.04.5 Precipitation andEmbrittlement

Under various circumstances, the different stainlesssteel types can suffer undesirable precipitation reac-tions, which can cause a decrease in both corrosionresistance and toughness.

3.04.5.1 Embrittlement at 475C

If martensitic, ferritic, or duplex steels are exposedto temperatures in the range 350550C, a seriousdecrease in toughness will occur after some time. Thedependent on the type of load, which is the stress ratio(R). The stress ratio is the ratio of the minimum stressto the maximum stress during the loading cycle. Thecompressive stresses are defined as negative.

Table 6 Fatigue properties of stainless steels, relationbetween tensile strength and fatigue strength

Microstructure So/Rm, Stress ratio Maximum stress

R=1 R=0

Ferritic 0.7 0.47 Yield strength

Austenitic 0.45 0.3 Yield strengthDuplex 0.55 0.35 Yield strengththe subsequent cooling, which cause a decrease inboth toughness and corrosion resistance. This typeof precipitation can be reduced or eliminated bydecreasing the levels of carbon and nitrogen to verylow levels and/or stabilizing the steel by additions oftitanium as in 18Cr2MoTi.

Carbide and nitride precipitation in austeniticand duplex steels occurs in the temperature range550800C. Chromium-rich precipitates form in thegrain boundaries and can cause intergranular

mation occur throughout the material. Nitridation,

that is, nitride formation, causes chromium depletionand reduced oxidation resistance in the same way ascarburization. This can lead to catastrophically highoxidation rates on the outer surface of equipment,which is subjected to a nitriding atmosphere oncorrosion and, in extreme cases, even a decrease intoughness. However, during the short times in thecritical temperature range experienced in the heat-affected zone adjacent to welds, the risk of deleteriousprecipitation is very small for the low-carbon steels.

3.04.5.3 Intermetallic Phases

In the temperature range 700900C, iron alloys witha chromium content above about 17% form interme-tallic phases such as s phase, w phase, and Lavesphase. These phases all have high chromium contentand are brittle and consequently large amounts ofthese phases in the microstructure will lead to adrop in toughness and a decrease in resistance tocertain types of corrosion.

Chromium, molybdenum, and silicon promote theformation of intermetallic phases, so the majority offerritic, duplex, and austenitic steels show some pro-pensity to form these phases. Intermetallic phases formmost readily from highly alloyed ferrite. In ferritic andduplex steels, intermetallic phases therefore form read-ily but are, on the other hand, relatively easy to dissolveon annealing. In the austenitic steels, it is the highlyalloyed grades that are particularly susceptible to inter-metallic phase formation. The low chromium contentand low molybdenum grades are considerably lesssensitive to the precipitation of these phases.

Heat treatment may remove all types of pre-cipitates by redissolving them. Renewed heat treat-ment of martensitic steels and solution annealing andquenching ferritic, duplex, or austenitic steels restorethe microstructure. Relatively long times or hightemperatures may be required for the dissolution ofintermetallic phases in highly alloyed grades.

Stainless steels can pick up nitrogen if exposedto nitrogen-containing atmospheres such as nitrogen,nitrogen mixtures, and cracked ammonia. Duringnitrogen pick-up, nitrides and other brittle com-pounds of chromium, molybdenum, titanium, vana-dium, and aluminum can form. Atmospheric oxygen,even at relatively low levels, reduces the risk fornitridation. At temperatures between 400 and 600C,a layer of nitrides is formed at the steel surface; athigher temperatures, nitrogen uptake and nitride for-the inside for example, the muffles in annealing

steels, that is, the types of stainless steel in whichthe mechanical properties cannot be set by heattreatment. Instead the aim of the heat treatment isto restore the microstructure by allowing recrystalli-zation to occur and deleterious phases, such as car-bides and intermetallic phases, to be dissolved.

During solution annealing the material is heated toa temperature where detrimental phases will be dis-solved, held at temperature for a time long enough toallow the unwanted phases to dissolve, and then rap-

1818 Ferrous Metals and Alloysfurnaces. Nitrogen pick-up can also cause embrittle-ment because of to surface or internal nitride forma-tion. Nickel is the alloying element, which providesthe greatest protection against nitridation, as nickeldoes not form stable nitrides.

3.04.5.4 Carburization

If a material is exposed to gases containing carbon,for example, in the form of CO, CO2, or CH4, it canpick up carbon. The degree of carburization is gov-erned by the levels of carbon and oxygen in the gas,also the temperature and steel composition. The car-bon, which is picked up by the steel will largely formcarbides, primarily chromium carbides.

The formation of a large amount of chromiumcarbides causes chromium depletion and thus areduced resistance to oxidization, because carbides,or even a network of carbides, form in the grainboundaries as well as within the grains. The resis-tance to thermal cycling is reduced and, since car-burization leads to an increase in volume, there is adanger of cracks developing in the material. Carbonpick-up can occur even at relatively low tempera-tures (400800C) in purely reducingcarburizingatmospheres and gives rise to catastrophic carburiza-tion or metal dusting. Attack is severe and character-ized by powdering of the steel surface because of thebreakdown of the protective oxide layer and inwarddiffusion of carbon, which forms grain boundary car-bides. The increase in volume on carbide formationmeans that grains are rapidly broken away from thesteel surface, giving rapid and serious attack.

Chromium, nickel, and silicon are the alloying ele-ments, which most improve resistance to carburization.

3.04.5.5 Heat Treatment

The aim of heat treatment of stainless steels is torestore the microstructure after forming or other fab-rication and production operations thereby removingor at least minimizing any possible negative effects.However, in the case of the hardenable stainlesssteels, that is, martensitic and precipitation-hardeninggrades, heat treatment is used to set the mechanicalproperties at the required level. In other cases, that is,stress relief heat treatment, the aim is to reduce thelevel of residual stresses caused by fabrication opera-tions such as cold forming and welding.

3.04.5.5.1 Solution annealing

Solution annealing is the most common heat treat-

ment for ferritic, austenitic, and duplex stainlessidly cooled or quenched. Solution annealing is nor-mally performed on austenitic and duplex stainlesssteels at temperatures above 1020C. A higher alloy-ing content will normally require a higher solu-tion annealing temperature in order to produce aprecipitate-free microstructure. For the stabilizedaustenitic grades, the temperature used should allowchromium carbides and other unwanted phases todissolve but should be low enough to retain the tita-nium or niobium carbides used to stabilize the steel.Ferritic stainless steels are normally only annealedusing temperatures below 1000C. Table 7 presentstypical heat treatment as solution annealing orannealing temperatures for stainless steels.

Rapid cooling after heat treatment is normallyrequired to ensure that unwanted reactions in themicrostructure do not occur. Whether air- or water-cooling is required depends on parameters such assection thickness and the type of steel. It is not thecooling rate per se that is important, but the time spentin the temperature range in which precipitation orother unwanted reactions may occur. Higher alloyedgrades and thicker sections will thus generally requirewater quenching rather than air-cooling.

3.04.5.5.2 Quenching, tempering, and ageing

Martensitic grades

Heat treatment of the martensitic stainless steels isessentially the same as the heat treatment of otherhardenable steels. The difference is that the high

Table 7 Heat treatment temperatures for ferritic, aus-tenitic and duplex stainless steels

Category Temperature (C) Quenching

Ferritic 700850 Water/forced air

Ferritic (highalloyed)

750950 Water/forced air

Austenitic 10201100 Water/forced air

Austenitic (high

alloyed)

10801200 Water

Duplex 10201150 Water

in all environments.

Aqueous Corrosion of Stainless Steels 1819alloy content slows the transformation reactions andincreases hardenability.

The normal heat treatment cycle of martensiticstainless steels is

1. hardening by austenitizing at temperatures of 9501050C followed by quenching in oil or water, and

2. tempering at temperatures of 300700C to setfinal properties.

Quenched hardness will increase with increasingaustenitizing temperature in the lower end of the tem-perature range, but austenitizing in the high end of thetemperature rangewill lower the as-hardened hardness.

Tempering is designed to allow the material toreach an optimum between strength and ductility.The tempering temperature is thus dependent onthe strength level specified and these may normallybe found in the appropriate product standards.

Precipitation hardening Grades

These steels can be divided into three subcategories:martensitic, semiaustenitic, and austenitic grades. Allof the precipitations hardening grades depend on aprecipitation reaction to induce the strengtheningduring aging. The precipitates may differ from gradeto grade, but the principle is that a solution annealis used to put some alloying elements into solutionat high temperature followed by quenching to lowtemperature at which a supersaturated solution isobtained. Ageing at an elevated temperature willthen cause the precipitation and hardening resultingin an increased strength. The heat treatment cycles ofthe different types of precipitation-hardening steelscan be summarized as follows:

1. Martensitic gradesa. solution anneal in the austenite region

(10201050C)b. quench to room temperaturec. age at 470630C to produce precipitation and

hardening2. Semiaustenitic grades

a. solution anneal in the austenite region(10201050C)

b. quench to room temperature followed by sub-zero cooling or tempering at about 750C

c. age at 470570C to produce precipitation andhardening

3. Austenitic gradesa. solution anneal (10001100C)b. quench to room temperaturec. age at 700800C to produce precipitation and

hardening3.04.6 Physical Properties

The physical properties of the different stainlesssteels are dependent on both the microstructure(crystal structure) and the amount of alloying ele-ments added. In many cases the physical propertiescannot be manipulated by heat treatment or fabrica-tion practices and are more or less locked in theatomic arrangement of the steel. Again, the groupingof the property values follows the division of steelgrades into the main stainless categories.

Ferromagnetism is a characteristic property offerrite and martensite while austenite is not ferro-magnetic. This means the ferritic and martensiticstainless steels are strongly magnetic while thefully austenitic stainless steels are nonmagnetic.However, since many of the more common andlower alloyed austenitic grades contain smallamounts of ferrite the might show a weak magneticbehavior. The duplex steels containing about 4060% ferrite will naturally be magnetic even if theirmagnetism is weaker that that of the ferritic ormartensitic stainless steels.

Regarding the other physical properties it may be3.04.5.5.3 Stabilization annealingA stabilization heat treatment is applied to titanium-or niobium-stabilized grades in order to enhance theresistance to intergranular corrosion, that is, to makesensitization more difficult. The aim of this treatmentis to ensure that the carbon dissolved in the matrixis forced to combine with the stabilizing element,for example, titanium or niobium, thus becomingsecurely locked up in as titanium or niobium carbidesand therefore not unavailable for chromium carbideformation.

Titanium and niobium both form more stable car-bides than do chromium and these carbides precipitateat higher temperatures than do chromium carbides. Thestabilizing treatment therefore consists of heat treatmentat a temperature slightly above the temperature rangefor chromium carbide precipitation. The temperatureselected is normally in the range as low as possible inorder to obtain maximum driving force for the precipi-tation. Stabilization heat treatment is usually performedat temperatures in the range 850980C on materialthat has previously been solution annealed. The lowerpart of the temperature range should be usedwith somecaution, as this type of treatment is not equally effectivenoted that the thermal expansion is strongly related to

the microstructure and this gives the austenitic steelsa thermal expansion that is about 50% higher thanthat of the ferritic and martensitic stainless steels. Thethermal conductivity is lower for austenitic steelscomparedwith that of the ferritic or martensitic steelsand within each category the thermal conductivitydecreases with increasing alloying content. Otherproperties such as thermal capacity and the modulusof elasticity show relatively little variation across thedifferent stainless steel categories. The most highlyalloyed austenitic grades have a somewhat lowermodulus compared with other stainless steels.

Typical values of the physical properties for thesome stainless steels from the different categories areshown in Table 8.11,12

3.04.7 Property Relationships forStainless Steels

the nickel content also increases toughness. In con-trast to the martensitic steels, the martensiticaustenitic steels do not have to be welded at elevatedtemperatures except in thick sections; even then onlylimited preheating is required.

The areas of use of martensitic and martensiticaustenitic steels are naturally those in which the highstrength is an advantage and the corrosion require-ments are relatively low. The martensitic steels withlow carbon contents and the martensiticausteniticsteels are often used as stainless constructional mate-rials. The martensitic steels with high carbon contentare used for springs, surgical instruments, and forsharp-edged tools such as knives and scissors.

The ferritic steels are characterized by goodcorrosion properties, very good resistance to chloride-induced stress corrosion cracking (SCC), andmoderatetoughness. The toughness of ferritic stainless steels isgenerally not particularly high. Lower carbon andnitrogen levels give a considerable improvement in

eels

grade (kg dm3) (GPa) expansion(106 C1)

1820 Ferrous Metals and AlloysRT RT RT400 C

Ferritc

S43000 7.7 220 10.5

MartensiticS42000 7.7 215 12.0

Duplex

S32205 7.8 200 14.5

AusteniticS30100 7.9 200 18.0

S20100 7.8 200 17.5

S30403 7.9 200 18.0

S31603 8.0 200 17.5N08904 8.0 195 16.9

S31254 8.0 195 18.0

S34565 8.0 190 16.8

RT, room temperature.Martensitic and martensiticaustenitic stainlesssteels are characterized by their high strength butlimited corrosion resistance. An increased carboncontent increases strength, but at the expense oflower toughness and considerable degradation ofweldability. The martensitic 13% chromium steels,with higher carbon contents, are not designed to bewelded, even though it is possible under special cir-cumstances. In order to increase high temperaturestrength, alloying with strong carbide formers suchas vanadium and tungsten are used. An increase in

Table 8 Typical physical properties of some stainless st

Stainless steel Density Modulus Thermalboth toughness and weldability, although toughness islimited for thicker dimensions. Consequently ferriticsteels are usually only produced and used in thinnerdimensions. Ferritic stainless steels are used in house-hold products and in applications with fairly lowdemands of the corrosion resistance of the material incombination with aesthetic reasons. Examples of suchapplication areas are cookware lids, washing machinedrums, refrigerator doors etc. They have stressstraindata similar to carbon steel; generally have higher yieldstrength than the austenitic stainless steels. Their

1

Thermalconductivity(W m1 C1)

Thermal capacity(J kg1 C1)

Electricalresistivity ( m)

RT RT RT

25 460 0.60

30 460 0.65

15 500 0.80

15 500 0.73

15 500 0.70

15 500 0.73

15 500 0.7512 450 1.00

14 500 0.85

12 450 0.92

chloride-induced SCC; only the highly alloyed steelssuch as N08904 (1.4529), and S31254 (1.4547) exhibit

Aqueous Corrosion of Stainless Steels 1821thermal conductivity is high and they transmit heatefficiently, which is one of the reasons to frequent usein electric irons and heat exchangers. They have fur-ther a low thermal expansion coefficient, lower than forthe austenitic stainless steels, which give them lessdistortion when heated. Ferritic stainless steels arewidely used in large tonnage all over the world.

The modern duplex steels span the same widerange of corrosion resistance as the austenitic steelsdepending on the alloy composition. Duplex equiva-lents can be found to both the ordinary austeniticgrades, such as S31600 (1.4401), and to the high-alloyed austenitic grades, such as S31254 (1.4547).The corrosion resistance of S32304 (1.4362) typeduplex is similar to that of S31600 (1.4401) whileS32205 (1.4462) is similar to N08904L (1.4539) andS32750 (1.4410) is similar to the high-alloyed austen-itic grades with 6% molybdenum, such as S31254(1.4547). High strength, good toughness, and verygood corrosion resistance characterize the duplexsteels in general and excellent resistance to chlo-ride-induced SCC and corrosion fatigue in particu-lar. An increased level of chromium, molybdenum,and nitrogen increases corrosion resistance, while thehigher nitrogen level also contributes to a furtherincrease in strength above that associated with theduplex structure. Applications of duplex steels aretypically those requiring high strength, good corro-sion resistance and low susceptibility to SCC orcombinations of these properties. The lower alloyedS32304 (1.4362) is used for applications requiringcorrosion resistance similar to S31600 (1.4401) orlower and where strength is an advantage. Someexamples of such applications are: hot water tanksin the breweries, pulp storage towers in the pulp andpaper industry, tanks for storage of chemicals in thechemical process industry, and tank farms in tankterminals in the transportation industry. The higheralloyed S32205 (1.4462) is, for example, used in pulpdigesters and storage towers in the pulp and paperindustry where it is rapidly becoming a standardgrade. It is also used in piping systems, heat exchan-gers, tanks and vessels for chloride-containing mediain the chemical industry, in piping and process equip-ment for the oil and gas industry, in cargo tanksin ships for transport of chemicals, and in shafts,fans, and other equipment which require resistanceto corrosion fatigue. High alloyed grades, for exam-ple, S32750 (1.4410), are used in piping and processequipment for the offshore industry, that is, oil andgas and in equipment for environments containing

high chloride concentrations, such as seawater.good resistance to this type of corrosion.The austenitic stainless steels are used in almost

all types of applications and industries. Typical areasof use include piping systems, heat exchangers, tanksand process vessels for the food, chemical, pharma-ceutical, pulp and paper, and other process industries.Nonmolybdenum alloyed grades, for example,S30400, are normally not used in chloride-containingmedia but are often used where demands are placedon cleanliness or in applications in which equipmentmust not contaminate the product. The molybde-num-alloyed steels are used in chloride-containingenvironment with the higher alloyed steels, N08904,S31254, being chosen for higher chloride contentsand temperatures. S31254 is used to handle seawaterat moderate or elevated temperatures. Applicationsinclude heat exchangers, piping, tanks, process ves-sels, etc. within the offshore, power, chemical andpulp- and paper industries. The low-alloyed grades,especially S30400 and 31600, are used in equipmentfor cryogenic applications. Examples are tanks,heaters, evaporator, and other equipment for thehandling of condensed gases such as liquid nitrogen.Finally it is worth mentioning that austenitic stainlesssteels are often used in applications requiring non-magnetic materials since they are the only nonmag-netic steels.

3.04.8 Corrosion Properties ofStainless Steels

Stainless steels are widely used throughout the worldin a variety of applications in both industrial anddomestic environments, for example, as a construc-tion material, in the manufacture of everyday uten-sils. The use of stainless steels has been growingsteadily and new areas of application, often indemanding service environments, are constantlyVery good corrosion resistance, very good tough-ness, and very good weldability characterize theaustenitic steels. They are also the most utilizedstainless steels. Resistance to general corrosion,pitting, and crevice corrosion generally increaseswith increasing levels of chromium and molybdenum.The low-carbon grades exhibit good resistance tointergranular corrosion and consequently the higheralloyed steels are only available with low carbon con-tents. Austenitic steels are generally susceptible tobeing developed. The serviceability of stainless steels

in many of these applications is determined by thematerial properties of the steels and how they per-form when exposed to different service environ-ments. Historically, the success stories of stainlesssteel usage have been widely accounted for in corro-sion engineering. Extensive research has been carriedout with the aim of reducing further the risk ofvarious types of corrosion by choosing appropriatestainless steel grades for specific service environ-ments. The following chapter is intended as a guidefor explaining the different types of corrosionmechanisms that can affect stainless steel and theircauses. This section also aims to increase the knowl-edge of stainless steels and their corrosion resistancein different service environments. Depending on theservice environment involved, several types of corro-sion may affect stainless steels.

Figure 11 shows part of a stainless steel pipe afterservice in a very severe corrosive environment con-sisting of high amounts of chlorides, water, and oxy-gen in combination with high temperatures underevaporative conditions. The pipe suffered from sev-

3.04.8.1 Passivity

surface film, so called passive film or passive layerin oxygen containing environments. The passivefilm is mainly a chromium oxide, which protects thesteel from corrosion attack in an aggressive environ-ment. For a passive film to form on a stainless steelsurface, a certain amount of chromium is required inthe steel as an alloying element. As chromium isadded to steel, a rapid reduction in corrosion rate isobserved around 10 wt% because of the formationof this protective layer or passive film, as illustrated inFigure 12.

The commercial alloys of austenitic stainlesssteels typically contain between 16 and 28 wt% ofchromium, while the chromium content of ferriticstainless steels ranges from 10.5 to 30 wt%. The

layer of chromium hydroxide. This film has self-healing properties when damaged in the presence of

1822 Ferrous Metals and AlloysThe reason for the good corrosion resistance of stain-less steels is that they form a very thin, invisible

Figure 11 SCC, crevice corrosion, and pitting corrosionon a stainless steel pipe, after service in a very

aggressive environment consisting of high amounts of

chlorides, water, and oxygen in combination with hightemperatures under evaporative conditions. Photo:

Outokumpu Stainless.eral corrosion forms that affect stainless steels, nota-bly pitting corrosion, crevice corrosion and SCC.oxygen and a repair of the passive film can easilyform after a scratch or other surface damage.

The thickness of a passive film is commonly con-sidered to be in the range of 13 nm, depending onthe service environment and the steel grade, but italso depends on pH values and the electrochemicalproperties in contact with a surrounding solution.

3.04.8.2 Contribution of Main AlloyElements to Passivation

As already indicated, chromium is the most impor-tant alloying element for corrosion resistance instainless steels as it helps to form a passive film on

00

0.05

0.1

0.15

0.2

0.25

5 10 15% chromium

Cor

rosi

on r

ate

(mm

yea

r1 )

Figure 12 The effect of chromium content on passivity.Reproduced from Design Guidelines for the selection anduse of stainless steel, specialty steel Industry of the Unitedchromium content varies in martensitic stainlesssteels, from 11.5 to 18 wt% and in duplex stainlesssteels; it is usually between 21 and 29 wt%. A passivefilm on a stainless steel surface consists of an innerlayer of mixed iron/chromium oxides and an outerStates, Washington, DC, USA.

austenitic stainless steels. As the nitrogen stays sol-uble in the stainless steel, without any precipitation,

Aqueous Corrosion of Stainless Steels 1823the localized corrosion resistance is improved forrather high amounts. However, if nitrides are formedin the stainless steel the corrosion resistance is dras-tically decreased. More about nitride precipitationcan be read in Section 3.04.5.

Nickel contributes to improved corrosion proper-ties by assisting the repassivation process and helpingreduce the rate of corrosion, for example, in strongacid solutions.

3.04.8.3 General ElectrochemicalConsiderations in Corrosion of StainlessSteels

The basic electrochemical reactions for corrosionare that the metal, Me, is defined as the anodeand balances the oxygen reduction attributed to thecathode.

Me! Men ne

In the classical electrochemical oxygen cathodicreaction there are two pathways accepted for thereduction according to.

O2 2H2O 4e ! 4OH

neutral or alkaline environmentO2 4H 4e ! 2H2O acid environment

The oxygen reduction is, however, a known complexprocess with many proposed pathway, for example:the surface. The spontaneous formation of intactchromium oxide on the surface acts as a barrierlayer and the surface electrochemistry is changed asa result of equilibrium reactions between the passivefilm and any surrounding solution.

It is also well-accepted and proved that the addi-tion of molybdenum, as an alloying element, offers anefficient method for preventing corrosion in stainlesssteel by improving the passive film properties. Themolybdenum content in stainless steels can be asmuch as 8%. However, the exact mechanism ofdetailed chemistry of molybdenum in a passive filmwith respect to passivity, interaction, and formationof compounds is the subject of extensive discussion,since molybdenum as an alloying element shows acomplex oxide chemistry in the passive film withmany oxidation stages.

The addition of nitrogen contributes to enhancingthe resistance to pitting and crevice corrosion on

14Four electron pathway:

O2 4H 4e ! 2H2O E01 1:229V=SHE

Two electron pathway:

O2 2H 2e ! H2O2 E02 0:695V=SHEH2O2 2H 2e ! 2H2O

The reduction pathway is influenced by many factors,for example, the surface composition of the electrodes.

In strongly reducing environments, reduction ofhydrogen ions is often the cathodic reaction to bal-ance the metal dissolution:

2H 2e ! H2 E 0V=SHEThe oxidation of a metal, that is, corrosion, requiresalways a counter balance by a reduction.

A common test procedure for the investigation ofstainless steels resistance to corrosion in wet environ-ments is to perform dynamic anodic polarizationmeasurements in a conductive solution to obtain apolarization curve. Figure 13 shows a schematic polar-ization curve undertaken for a stainless steel presentingthe different potential area and the current densityresponse.

The high current densities shown in Figure 13represent a corrosion process and the very low cur-rent response represents the passivity of a stainlesssteel. At low cathodic potentials, a line of high currentdensities represents the cathodic reaction. At thepotential, when the cathodic reaction and the anodicreaction meet in the active area on occurring corro-sion is denoted the corrosion potential, Ecorr.

At the higher anodic potential, the passivity isrepresented by a decrease in current densities cor-responding to the passivation current to stabilize inthe passive potential area to a very low current density,the passive current density. Pitting corrosion or crevicecorrosion is typically represented by a steep increasein the current response at an even higher potential,the pitting potential, Ep. Transpassive corrosion isrepresented by an increase in current density responsein a rather extended potential area at even higheranodic potentials, associated with the transpassivepotential, Etr. In order to investigate the ability ofa stainless steel to repassivate in, for example, chlorideenvironment, a reverse scan is commonly performedwhile performing polarizationmeasurements. The cur-rent response on the repassivation process is a cleardecrease in the passive current and the potential forthe steep decrease is noted repassivation potential, Erp.

Pas

el.

1824 Ferrous Metals and Alloys3.04.8.4 Breakdown of Passivity

All corrosion forms on stainless steels are related toany permanent damage of the passive film, either as acomplete breakdown of the film causing uniformcorrosion or locally as in pitting and crevice corro-sion. Intergranular corrosion occurs along grainboundaries due to local breakdown of the passivefilm where chromium has been depleted. Once thepassivity of stainless steel is broken down, completelyor locally, and repassivation is not promoted by the

IPassivation

IPassive

Cathodicreaction

Activearea

+

log I (A cm2)

EppEcorr

Figure 13 Schematic polarization curve for a stainless steaggressiveness of the surrounding solution, activecorrosion occurs.

Local breakdown such as pitting and crevice cor-rosion is commonly initiated in neutral solutions, butcan also occur in solutions with a low pH. Thegeneral aspect to consider in pitting and crevicecorrosion is the very fast corrosion rate that cancause penetration through the steel in a short timeand may lead to catastrophic failure.

Complete breakdown of the passive film causinguniform corrosion may occur in solutions of eitherlow or high pH. Uniform corrosion or generalcorrosion occurs when the passive layer on a stain-less steel surface breaks down partly or completely.The corrosion then propagates at a rate deter-mined by a corrosive environment and the alloycomposition in combination. Uniform corrosioncauses a corrosion rate that can be expressed as amean value of the attacked surface, making it pos-sible to calculate a material loss, from weight lossdeterminations.An even worse type of corrosion from the con-struction service point of view is SCC, which ischaracterized by the cracking of materials that aresubjected to both a tensile stress and a corrosiveenvironment. Most reported failures due to SCCoccur in the standard stainless steel grades S30400and S31600 with tensile stress in aqueous solutions con-taining chlorides at elevated temperatures, >60C.However, solutions containing chlorides are not theonly environments that cause SCC in stainless steels.Similar cracking can also occur in hot caustic solutions

sivity PittingTranspassivepotential area

Oxygenevolution

E(V)EtrEpErp

+and in environments containing impurities such ashydrogen sulfide.

3.04.8.5 Localized Corrosion Pitting andCrevice Corrosion

Smialowska recently reviewed the tremendous workperformed on pitting and crevice corrosion and theuse of electrochemistry to characterize stainless steelregarding pitting and crevice corrosion.15 Detailedinformation about pitting and crevice corrosion andthe historical research can be read in this review.

Defects in the passive film enhance the risk ofpitting and crevice corrosion. Manganese sulfidesare identified to be initiation points to pitting andcrevice corrosion. However, in modern commercialstainless steels, the sulfur content is normally so lowthat there are only few recent reports about sulfurcausing pitting corrosion.

Environments, which represent the greatest riskof pitting and crevice corrosion of stainless steels,

include seawater and process solutions in which thereis a high concentration of chlorides sometimes also incombination with an increased temperature. Highhalide concentration, commonly chlorides, low pH,and high temperature increase the probabilityof pitting and crevice corrosion on stainless steels.Pitting corrosion may also be caused by the presenceof thiosulphate in combination with chloride ions.The risk of pitting corrosion increases when therelative levels of thiosulfate, sulfate, and chloridereach a certain amount, but can be avoided bycontrolling the amount of thiosulfate.

Different types of formulas have been proposed torank the resistance of stainless steels to pitting corro-sion and to compare a steel grade among other gradesby the influence of main alloying elements. Thepitting resistance equivalent (PRE) of a specificsteel grade can be estimated by formulas in whichthe relative influence of a few elements, that is, chro-mium, molybdenum, and nitrogen, are considered.The higher the PRE value of a stainless steel, thebetter the resistance to pitting corrosion in neutral

There are also other formulas with different alloyingelements included, such as manganese, wolfram,sulfur, and carbon.

Alfonsson and Qvarfort investigated some formu-las for PRE, based on the chromium, molybdenum,and nitrogen content and compared the PRE valueswith critical pitting temperatures (CPT) measuredpotentiodynamically, using the Avesta cell.16 Theyfound an acceptable linearity between the CPT andPRE values for any of the PRE formulas studied.

3.04.8.5.1 Influence of alloy composition on

localized corrosion

The increase in the chromium content and molybde-num content enhances the passive potential area tomore anodic potentials and consequently increasesthe stainless steels ability to passivate in moreaggressive solutions. The higher alloyed stainlesssteel grades, for example, those containing 6% ofmolybdenum, do not often show any pitting potentialdue to pitting corrosion in polarization measure-ments at room temperature, and elevated tempera-

rs f

che

Aqueous Corrosion of Stainless Steels 1825chloride containing solutions. Table 9 shows thecompositions of some commercial stainless steelsand calculated PRE, indicating their relative pittingcorrosion resistance.

One frequently used expression is:

PRE %Cr 3:3%Mo 16%N

Table 9 Typical chemical compositions and PRE numbe

Steel grade Typical

UNS EN Microstructure Cr

S41000 1.4006 Martensitic 11.5

S42000 1.4021 Martensitic 12

S43000 1.4016 Ferritic 16S44400 1.4521 Ferritic 17

S20100 1.4372 Austenitic 17

S20400 Austenitic 16S30100 1.4310 Austenitic 17

S30100 1.4318 Austenitic 17.7

S34700 1.4550 Austenitic 18

S30400 1.4307 Austenitic 17.5S31600 1.4404 Austenitic 17.2

S31600 1.4571 Austenitic 16.5

N08904 1.4539 Austenitic 20

S31254 1.4547 Austenitic 20S32101 1.4162 Duplex 21.5

S32304 1.4362 Duplex 22

S32205 1.4462 Duplex 22S327