D AAUSTTEENNOFEE RRRIITTIICC UPUP STAAIINN · PDF fileNACE MR0175 ... Sulphide stress cracking...

version 01 07 December 2009

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Contents -------------------------------------------------------------------------- 2

Foreword ------------------------------------------------------------------------- 3

Main applications ---------------------------------------------------------------- 4

Analyses and reference standards --------------------------------------------- 6

1. Analyses ----------------------------------------------------------------------- 6

2. Reference standards --------------------------------------------------------- 7

2.1. Normative equivalences ---------------------------------------------------------- 7

2.2. Associated normative documents ------------------------------------------------ 7

Physical properties -------------------------------------------------------------- 8

Heat treatments and structure ------------------------------------------------- 9

Solution annealing ------------------------------------------------------------- 10

Structural transformations --------------------------------------------------- 10

Mechanical properties --------------------------------------------------------- 12

Forging -------------------------------------------------------------------------- 17

Machining ----------------------------------------------------------------------- 18

Welding ------------------------------------------------------------------------- 19

Corrosion resistance: examples of the use of duplex steels --------------- 20

1. Introduction to the corrosion resistance of duplex grades ------------ 20

2. Use of duplex steels in the chemical and paper manufacturing industries ----------------------------------------------------------------------- 22

3. Use of duplex steels in the building industry: for example concrete reinforcing bars ---------------------------------------------------------------- 29

4. Use of duplex steels in the petrochemical industry: stress corrosion problems. ----------------------------------------------------------------------- 30

Additional information -------------------------------------------------------- 32


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FFoorreewwoorrdd

Stainless steels with a high content of chromium and molybdenum, the major elements in corrosion resistance, are often recommended for use in certain very aggressive environments.

For several decades, the market share of austenoferritic or "duplex" stainless steels has been increasing.

Although, for many years, the use of duplex steels was almost exclusively restricted to the production of components which were cast and then forged, they are now available in an extensive range of long or flat laminated products.

Their outstanding characteristics, combining high mechanical properties with often exceptional corrosion resistance and their low cost – together with their low nickel content – make them attractive to industries that traditionally use high alloy grades:

Cellulose and paper pulp industry;

Oil industry;

Waste and effluent treatment;

Phosphoric and sulphuric derivative mineral chemical industry;

Building industries (see the special technical documentation), etc.

The purpose of this technical documentation is to help users to choose the right Duplex grades by giving them advice on how to proceed.

In order to be as comprehensive as possible, every effort has been made to compare these products with well-known reference stainless steels:

4404 (316L) and its improved machinability version UGIMA®.

4539 (904L), the "superaustenitic" grade, which is the reference grade for highly chlorinated environments (brine, sea-water treatment) where the risks of localised pitting or crevice corrosion are considerable.


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Although the majority of duplex steels are well known and widely used in the chemical and oil industries, their use is quickly spreading to the "green" industries associated with water and soil pollution control or waste processing.

Increasingly strict sanitary measures are being applied in the agri-food and health industry at the same time as stricter controls are being introduced into the chemical sanitisation environments: several occurrences of corrosion encountered on conventional grades (1.4307, 1.4404) can only be resolved by changing to "nobler", more appropriate solutions, such as Duplex grades.

Where long stainless steel products are concerned, such solutions will be of particular interest for fittings or mechanical components used in welded sheet metal assemblies, fluid systems or structures in the following fields:

Bolts and screws Cables and tie rods

Filters Handling hooks

Chains Mixers, blenders

Probe supports Various mechanical components

Valves and fittings Connections and flanges

Pump shafts

Building and civil works

Rams

Reinforcements, anchor bars

Duplex stainless steels are particularly recommended for use in the industries and applications listed below, although this list is by no means exhaustive:

PVC and chlorinated polymer synthesis;

Phosphoric acid and by-products (fertilizers, explosives);

Sulphuric acid and by-products;

Cellulose and paper pulp processing;

Textile fibre bleaching;

Boring and extraction;

Off-shore;

Refining;

Tidal power plant equipment;

Sea-water nuclear power stations;

Off-shore wind turbines;

Soft water production by desalination;

Chemical

Oil

Energy

Sea water


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Thalassotherapy equipment;

Fish farming;

Underwater work;

Nautical equipment;

Anchor bars;

Concrete reinforcement bars; (see the special documentation)

Dialysis equipment;

Thermalism;

Sanitisation and sterilisation;

Water treatment;

Waste and effluent treatment;

Brines (cheeses and cooked meats);

Mustard and vinegar;

Wine (sulphite treatment);

Building - civil works

Health

Environment

Agri-food


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1. Analyses

An analysis of Duplex grades is given in Table1; later in the document, the properties will be compared with those of the standard UGI 4404 (316L) or UGIMA® 4404 grades (improved machinability variant, considered to be the minimum requirement for harsh corrosive environments) and, at the top end of the scale, with those of UGI 4539 (904L), the superaustenitic grade.

Table1: Analyses

Grade C Si Mn Ni Cr Mo S P Cu N

UGI 4404 UGIMA® 4404

0.03 1 2 10 11

16.5 17.5

2 2.5

0.015 0.030

0.040 - -

UGIMA® 4460 0.03 0.75 1 4.5 5

26 27

1.3 1.8

0.005 0.025

0.035 - 0.05 0.2

UGI 4362 (UGI 35N)

0.03 1.0 2 3.5

5.5

22

24

0.1

0.6 0.015 0.035

0.1

0.6

0.05

0.2

UGI 4462 (UGI 45N)

0.03 0.75 1 2

5 6

22 23

2.5 3.5

0.01 0.035 - 0.11 0.22

UGI 4507 (UGI 52N+)

0.03 0.7 1.5 6 7

24.5 26

3.3 4

0.01 0.035 1.2 2

0.15 0.30

UGI 4539 (UGI 904L)

0.03 1 2 24 25

19 20

4 5

0.01 0.025 1.2 2

0.15


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2. Reference standards

2.1. Normative equivalences

UGI EN 10088

Numerical

EN 10088

Alphanumeric AISI

UNS and others

UGI / UGIMA® 4404 1.4404 X2 CrNiMo 17-12-2 316L UNS S31603

UGIMA® 4460 1.4460 X3 CrNiMoN 27-5-2 329 SUS 329J1

SIS 2324

UGI 4362 (35N) 1.4362 X2 CrNi 23-04 - UNS S32304

UGI 4462 (45N) 1.4462 X2 CrNiMoN 22-5-3 ASTM A 182-F51

UNS S31803

UNS S32205

SIS 2377

SUS 329532

UGI 4507 (52N+) 1.4507 X2 CrNiMoCuN 25-6-3 ASTM A

479

UNS S32550

SUS 39542

UGI 4539 (904L) 1.4539 X1 NiCrMoCu 25-20-5 904L UNS N08904

2.2. Associated normative documents

EN 10088-1 Stainless steels – List of stainless steels

EN 10088-3 Stainless steels – Semi-finished products, bars, wire rods, cold-drawn wires, profiles and cold-finished profiles in corrosion resistant steel for building and general use.

EN 10272 Stainless steel bars for pressure vessels

ASTM A276 Stainless and heat-resisting bars/shapes

ASTM A479 / ASME SA 479 Stainless steel bars for boilers and other pressure vessels

NACE MR0175

NF XP A 35-014

Sulphide stress cracking resistant material for oil field equipment

Steels for reinforced concrete: smooth stainless steel lock or print bars and coils


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Table 2: Physical properties

Symbol Unit Temperature Value

4404 4460 4362 4462 4507 4539

Density d No

dimensions 4°C 7.9 7.9 7.8 7.8 7.9 8.05

Specific heat c J.kg.°C 20°C 500 500 490 400 500 500

Thermal conductivity

k W/m.°C 20°C 15 15 17 16 17 14

Linear expansion ratio

10-6m/m.°C 20 to 100°C

20 to 300°C

19

20

13

13.5

13

14

13

14

12.5

13.5

15.1

16.8

Electrical resistivity

µ.cm 20°C 76 80 80 70 80 80

Longitudinal elasticity module

E MPa.103 20°C 200 200 200 200 205 205

Poisson coefficient

No

dimensions 20°C 0.30 0.30 0.30 0.30 0.28 0.28

A comparison of physical properties indicates the lower expansion ratio and higher thermal conductivity of Duplex steels.

Figure 1: Comparison of the thermal conductivity of austenitic stainless steels and duplex steels (comparison of average values)

10

12

14

16

18

20

22

24

0 100 200 300 400 500 600

Température °C

Co

nd

ucti

vté

th

erm

iqu

e e

n W

/m

.°C

Austénitique Duplex

Austenitic

Temperature

Th

erm

al

co

nd

ucti

vit

y i

n W

/m

. C

°


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All the grades referenced are used in the solution-annealed state under the conditions described in Figure 2a.

Figure 2a: Solution annealing values according to grade

940

960

980

1000

1020

1040

1060

1080

1100

1120

1140

1160

1180

Tem

pera

ture

(°C

)

T°C min 1025 1030 950 1030 1040 1075

T°C max 1100 1100 1050 1100 1120 1150

Ugine 4404 Ugima 4460 Ugine 4362 Ugine 4462 Ugine 4507 Ugine 4539

Solution annealing


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The structure of Duplex steels after annealing is two-phase ferrite + austenite, with a percentage of ferrite - appropriate for the optimisation of mechanical properties and corrosion resistance - of between 40 and 70%, depending on the grades.

The respective percentages of austenite and ferrite can vary according to the percentage of hot working and the temperature of the heat treatment.

Figure 3: Structures of austenitic and Duplex steels

Austenitic steel Duplex steel

Structural transformations

Compared with standard austenitic steels (1.4307, 1.4404), Duplex grades are liable to undergo various types of structural transformations depending on the temperature.

phase precipitation occurs when the steel is kept within a temperature range of

600 - 900°C. It causes embrittlement at ambient temperature and must therefore be avoided.

' phase precipitation can occur after the steel is kept at a temperature of between

350 and 550°C for a prolonged period. This embrittling phase weakens the resilience and reduces corrosion resistance.

phase

' phrase


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Figure 4: ' and phase precipitation TTT curves in Duplex steels

The and ' phases can easily be avoided if the thermal cycles (forging,

for example) are sufficiently controlled. The limit temperature at which duplex steels should be used is 300°C.

UGI 4362

UGI 4507

UGI 4462 Intergranular precipitates

Ferrite

sigma phase

chi phase

sigma phase

’ phase

’

phase

Time (h)

core

skin

sigma phase

carbides

’ phase


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A comparison of the mechanical properties between the different families of stainless steels – with the exception of martensitic steels whose behaviour is more similar to that of alloy steels - reveals that the best compromise between tensile strength and resilience is obtained with duplex steels.

Table 3 compares the typical mechanical properties for the different families of stainless steels (with the exception of martensitic stainless steels).

Table 3: Comparison of the mechanical properties of stainless steels

Type of steel Rm (Mpa) Rp0.2 (Mpa) KV (in J)

Ferritic 450 to 600 280 to 360 10 to 20

Austenitic 550 to 700 250 200

Duplex 650 to 750 480 150

The values shown in Table 4 refer to the annealed condition.

Table 4: Mechanical properties at ambient temperature

Grade Rm (Mpa) Rp0.2 (Mpa)

min A % min KV (J) min

UGIMA® 4404 460 – 660 185 40 150

UGIMA® 4460 620 – 880 450 20 85

UGI 4362 (35N) 600 - 830 400 25 100

UGI 4462 (45N) 660 - 860 450 25 100

UGI 4507 (52N+) 690 - 890 490 20 100

UGI 4539 (904L) 530 - 730 230 35 100

Once again, we strongly advise against "hardening" Duplex steels.

Mechanical properties at

ambient temperature


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The figures below illustrate the variations in Rm and Rp0.2 versus grades:

Figure 5: Resistance values Rm for various grades

400

500

600

700

800

900

1000R

m M

Pa

Rm Min MPa 460 620 600 660 690 530

Rm Max MPa 660 880 830 860 890 730

Ugine 4404 Ugima 4460 Ugine 4362 Ugine 4462 Ugine 4507 Ugine 4539

Figure 6: Yield strength value Rp0.2 for various grades

Ugine 4362

Ugine 4462

Ugine 4507

Ugine 4539

Ugima 4460

Ugima 4404

100

150

200

250

300

350

400

450

500

550

600

=

Rp0.2 Min 185 450 400 450 490 230

Ugima 4404 Ugima 4460 Ugine 4362 Ugine 4462 Ugine 4507 Ugine 4539

Rp0.2

- M

Pa


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In view of the structural transformations that are liable to alter their working properties, the use of Duplex grades above 300°C is not recommended.

Table 5: Mechanical properties at high temperature (minimum Rm and Rp0.2 values)

UGI and UGIMA® 4404

Test temperature

100°C 150°C 200°C 250°C 300°C 350°C

Rm MPa min 430 410 390 385 380 380

Rp0.2 MPa min 165 150 137 127 119 113

UGIMA® 4460

Test temperature

100°C 150°C 200°C 250°C 300°C 350°C

Rm MPa min 610 580 565 550 - -

Rp0.2 MPa min 360 335 310 295 - -

UGI 4362 (UGI 35N)

Test temperature

100°C 150°C 200°C 250°C 300°C 350°C

Rm MPa min 570 - 530 - 490 -

Rp0.2 MPa min 330 - 280 - 230 -

UGI 4462 (UGI 45N)

Test temperature

100°C 150°C 200°C 250°C 300°C 350°C

Rm MPa min 620 595 580 580 - -

Rp0.2 MPa min 360 340 320 310 - -

UGI 4507 (UGI 52N+)

Test temperature

100°C 150°C 200°C 250°C 300°C 350°C

Rm MPa min 680 655 640 640 - -

Rp0.2 MPa min 400 380 360 350 - -

UGI 4539 (UGI 904L)

Test temperature

100°C 150°C 200°C 250°C 300°C 350°C

Rm MPa min 500 480 460 450 440 440

Rp0.2 MPa min 205 190 175 160 145 140

The superaustenitic grade UGI 4539 (Ugine 904L) can be used up to about 600°C, but is not particularly useful from the technical point of view, nor is it at all economical compared with more conventional "refractory" grades at this temperature range. The figures below illustrate the variation in tensile strength versus temperature for the different grades:


high temperature


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Figure 6: Tensile strength versus temperature

300

350

400

450

500

550

600

650

700

100°C 150°C 200°C 250°C 300°C 350°C

Température °C

Rm

Mp

a

Ugima 4404

Ugine 4539

Ugima 4460

Ugine 4462

Ugine 4507

Ugine 4362

Figure 7: Yield strength versus temperature

100

150

200

250

300

350

400

450

500

100°C 150°C 200°C 250°C 300°C 350°C

Température °C

Rp

0.2

Mp

a

Ugima 4404

Ugine 4539

Ugima 4460

Ugine 4462

Ugine 4507

Ugine 4362

The "cryogenic" applications are not, strictly speaking, applications where corrosion is a serious problem (e.g.: transportation of liquified liquids); in fact, below a certain temperature threshold, the development kinetics of most forms of corrosion are much slower and the majority of "standard" austenitic grades (UGIMA® 4307, UGIMA® 4404) are perfectly suitable for most applications.

In some specific cases, if grades that can be used for a wide range of operating temperatures (for example + 100 to - 100°C) are required for use in highly corrosive environments, the choice of grade will be decided by the level of mechanical properties required for the components.

In the case of high stress at ambient temperature and ductility requirements at below -60°C, only a structurally hardened grade such as UGI 4944 (e.g. AFNOR


low temperature

Temperature °C

Temperature °C


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Z6NCTDV 25-15 B – ASTM grade 660) is an appropriate solution for a highly corrosive environment.

Table 6: Preferred parameters for cryogenic applications

Operating conditions Possible choice

Moderate tensile strength requirements (however, the tensile strength increases on austenitic and Duplex steels when the temperature decreases);

Operating temperature < -60°C;

Highly corrosive environment at ambient temperature.

UGIMA® 4404 possible, many corrosive environments become slightly aggressive at low temperature;

In the event of heat variations, where the top temperature may exceed 15 to 20°C, UGI grade 4539 should be used to ensure maximum safety.

Operating temperature above - 60°C;

Stricter requirements for tensile strength;

Highly corrosive environment at ambient temperature.

Duplex grade to be selected – according to the corrosive environment – from those recommended (their resilience transition temperature is in the region of –60°C)

Figure 8: Transition curves showing the resilience of UGI grades 4462 and 4507

0

50

100

150

200

250

300

Température °C

KV

(Jo

ule

s)

Ugine 4462 260 250 225 160 100 48

Ugine 4507 275 260 250 175 100 40

50 0 -20 -50 -75 -100

Température de transition -60°C

Temperature °C

Transition temperature -60 °C


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FFoorrggiinngg

The forgeability of these steels is acceptable between 1200 and 900°C, which is, however, less than that of current austenitic steels (1.4307, 1.4404).

Their deformability at high temperatures depends, for a given temperature, on the ferrite content, as a high ferrite content improves forgeability.

The forgeability of 1.4462 and 1.4507 grade steels is slightly lower below 1100°C, due to their nitrogen content.

Table 7: Duplex steel forging conditions

Preheating Forging Cooling

Direct oven loading at the forging temperature for small

components (~ 1200°C);

When forging large components, it can prove useful to preheat them at a temperature slightly above 850°C to ensure that their structure is properly

homogenised.

Between 1200 and 900°C

A reduction in forgeability may occur below 1100°C on grades with a high nitrogen content

(1.4507)

As quickly as possible below

900°C to prevent phase

formation

Duplex steels


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The corrosion resistance requirements of these grades considerably limit the possibilities of resulphurisation with a view to improving the machinability properties.

Although the S content "authorised" by the standards is less than 0.010%, in practice it only rarely exceeds 0.005%. Under such conditions, it is often very difficult to machine these grades.

The two-phase structure of these steels where each phase performs differently during machining makes them more difficult to machine than austenitic stainless steels. They put a great strain on the tools (risk of vibrations, coating chipping) if they are not machined under optimum cutting conditions and if the tools used are not of the correct quality. Furthermore, they require the use of coated carbide inserts and low cutting speeds, as opposed to austenitic stainless steels.

Fig 9: Machinability of duplex grades

0

50

100

Débit copeaux

(cm3/mn)

Tournage outil carbure revêtu

4404

4460

4462

0

1

2

3Débit copeaux

(cm3/mn)

perçage acier rapide

4404

4460

4462

Austenoferritic steels

Chip rate (cm3/mn

Coated carbide toll turning

Chip rate (cm3/mn

High-speed steel drilling


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WWeellddiinngg

The percentage of ferrite in the molten areas of these grades is higher when the composition of the filler metal is identical to that of the base metal. This should be taken into account when the optimum composition of the filler metal is determined. In addition, the areas affected by the heat are also liable to contain more ferrite than the base metal. To minimize this difference, high linear energy welding is recommended to reduce cooling times. However, only energy that does not cause phase formation should be used:

- for UGI 4362, there is very little risk - for UGI 4462, the energy must be less than 2kJ/cm - for UGI 4507, see the table below:

BUTT WELDING CORNER WELDING

Process Pulsed MIG TIG Pulsed MIG TIG

Gas Ar 95.5% + CO2 1.5% + N2 3% Ar + N2 4% Ar 95.5% + CO2 1.5% + N2 3%

Ar + N2 4%

Sheet metal

thickness (mm)

Min. weld energy

(KJ / mm)

Max. weld energy (KJ / mm)

Min. weld

energy (KJ / mm)

Max. weld

energy (KJ / mm)

Min. weld

energy (KJ / mm)

Max. weld

energy (KJ / mm)

Min. weld

energy (KJ / mm)

Max. weld

energy (KJ / mm)

4.76 0.38 0.47 0.60 0.80 0.60 0.77 1.00 1.30

6.35 0.55 0.65 0.90 1.10 0.73 1.05 1.24 1.73

7.93 0.65 0.87 1.10 1.45 0.80 1.22 1.60 2.05

9.50 0.73 1.05 1.24 1.75 0.85 1.30 1.60 2.15

12.00 0.94 1.15 1.60 1.95 0.97 1.35 1.60 2.20

16.00 0.95 1.30 1.60 2.20 0.97 1.35 1.60 2.20

19.00 0.97 1.32 1.60 2.20 0.97 1.35 1.60 2.20

26.00 0.97 1.35 1.60 2.20 0.97 1.35 1.60 2.20

A linear energy welding area where the two above-mentioned risks are minimised can therefore be determined. The thicker the components to be welded, the higher the energy in this area (i.e. rapid weld cooling).

It is not advisable to preheat the components prior to welding.

Components should not be heat treated after welding, but the solution annealing described in the "Structure heat treatment" section may be performed, if necessary.

The filler metals contain a higher nitrogen and/or Ni content than the base metal in order to optimise the ferrite content in the molten areas, see the table below:

Duplex grades

Filler wire

Base metal

ER2209

22.9.3NL

SMArc 45N

ER2553

Z3CND25-06-03Az

SMArc 52N

ER 309Lsi

23.12Lsi

SMArc 309LM

UGI 4362

UGI/UGIMA® 4460

UGI4462

UGI 4507


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1. Introduction to the corrosion resistance of duplex grades

Laboratory tests such as the "accelerated corrosion electrochemical test" have been widely used to study the corrosion of duplex or austenoferritic grades. A great deal has been written on this subject and we will simply mention grade classifications in a few laboratory electrochemical tests.

Figure 10 below compares 1.4462 / 45N duplex with standard austenitic grades 304L 1.4306 and 316L 1.4404 in an accelerated fatigue - corrosion test, which was performed in different environments: in air and for environments with pH values ranging from neutral to highly acid.

In all these cases, the duplex steel performed best.

Figure 10: Comparison of duplex 4462 with two standard austenitic stainless steels

Figure 11 shows the critical pitting temperature versus the mechanical strength for

two duplex steels (45N / 1.4462 and 52N / 1.4507) in comparison with two austenitic steels (316L / 1.4404 & 904L / 1.4539). This critical pitting temperature is determined according to the accelerated corrosion test (ASTM G48 Standard) in a 6% ferric chloride environment.

0

50

100

150

200

250

300

350

400

450

500

1.4306 (304L)

1.4404 (316L)

1.4462/ 45N (Duplex)

Limite de fatigue (MPa) à 20E+7 cycles

Air pH 7 pH 3 pH 1

Fatigue limit (MPa) at 20E+7 cycles


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Figure 11: Critical pitting temperature versus mechanical properties

Note the excellent behaviour of 4507 duplex with respect to its mechanical properties and pitting corrosion resistance.

Note: Example of UGIMA® 4460

As seen above, this grade is particularly useful, due to its machinability.

The following table compares the performances of UGI 4404 / 316L and UGIMA® 4460 / 329 stainless steels (for a more detailed comparison, sulphur contents of 0.02% were chosen).

Machinability

Overall general

corrosion in an H2SO4

acid environment

Stress corrosion

Localised crevice

corrosion

Localised pitting

corrosion

UGIMA®

4460 329

Reference "BASE 100" Much less sensitive

"BASE 100" Reference

UGI 4404 316L

Reference Much less effective (by 750%)

Sensitivity as for all austenitic grades

Slightly less effective (by approximately 10%)

Reference

Ugine 4507

Ugine 4462

Ugine 4539

Ugine 4404

0

10

20

30

40

50

60

70

80

0 100 200 300 400 500 600

Rp0,2 mini (MPa)

T° p

iqû

reP

itti

ng

T°

Rp0.2 min (Mpa)


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2. Use of duplex steels in the chemical and paper manufacturing industries

The overall general corrosion is marked by an even dissolution of the metal in contact with the corrosive environment. The results of this type of corrosion can be quantified in terms of loss of weight or thickness (mm/year, for example); a grade use limit of 0.2 mm / year is often acceptable.

The overall general corrosion on stainless steels is, for example, found in "strong" acids (sulphuric acid, phosphoric acid) and is a type of corrosion often encountered in the chemical industry; localised corrosion conditions may also be encountered (pitting, crevice, intergranular or stress corrosion), which is naturally far more difficult to quantify and anticipate).

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Phosphoric acid is a non-oxidising mineral acid. Industrial phosphoric acid is a very complex acid in terms of corrosion: it contains many impurities which can adversely affect corrosion resistance (hydrochloric acid; hydrofluoric acid; sulphuric acid), but it can also sometimes contain certain foreign bodies that have a beneficial effect on the corrosion resistance of the material in question (ferric and aluminium ions).

Recommendations for materials for the main stages in industrial phosphoric acid manufacturing:

ENVIRONMENTS and STAGES

MAIN

PROBLEM

Recommendations for a pure

environment

AGGRAVATING FACTORS

UGITECH RECOMMENDATIONS

Pure and aerated

phosphoric acid

Uniform surface attack

* Temperature < 90°C

UGIMA®4404

* Temperature 90-200°C

UGI4539

Natural phosphate attack

stage

(see Biblio 1 for further

information about this stage)

General attack

related to depassivation due to fluorinated ions, excessive amounts

of H2SO4, etc.

Grade 4404 not recommended

UGI4539 Considerable abrasion

phenomena

UGI 4507

(Behaves better than 4539 in attack tanks - stirrers with respect

to corrosion–abrasion

phenomena).

Slurry filtration stage

(see Biblio 2)

UGIMA 4404 or

UGI 4362 with no chlorides

Presence of chlorides

UGI 4539 up to

2000 ppm of chlorides.

UGI 4507 up to 3000 ppm of

chlorides.

Phosphoric acid


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Sulphuric acid is a non-oxidising mineral acid; it acts as a reducing agent if its concentration is less than 50% and as an oxidising agent if its concentration exceeds 80% (these limits vary according to the temperature). Its degree of ionisation is highest for a concentration of 30%, which leads to an irregularity in fields where grades with a maximum acid aggressivity of between 40 and 80% are used. The solubility of the oxygen is minimum at a concentration in the region of 70%.

ENVIRONMENTS

MAIN PROBLEM

Recommendations for a pure environment

AGGRAVATING FACTORS


Pure & aerated sulphuric acid (see Biblio 3)


*Concentration 40 to 85% UGI4539 (but for temperatures not exceeding 35°C at high concentrations) * Concentration < 40% or > 85% UGI4507

Chlorides in industrial acids (On the one hand, these ions act as a reducing agent and, on the other, they disturb passivity because they are adsorbed in place of hydroxide ions).

UGI 4539 (highly recommended with high percentages of chlorides up to 2000 ppm)

H2SO4 with oxidizing impurities (ferric ions, etc.) (see Biblio 4)


Factors that can have a beneficial effect on grade behaviour.

This iso-corrosion diagram was produced for pure acid: it is therefore limited with respect to the use of grades for low concentrations (<40%) compared with the diagram that might be produced if oxidising impurities were present. The operating limits are given for a maximum corrosion rate of 0.2 mm/year. They obviously apply to the use of stainless steel grades for welding, forming, etc. according to best practice.

Sulphuric acid

Note: The 4362 grade can be used instead of 4404 if the H2SO4 content is less than 13%


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RECOMMENDATIONS FOR USE IN HYDROCHLORIC ACID Hydrochloric acid is a mineral acid reducing agent. The problems mainly involve general corrosion due to very high acid activity and the fact that this reducing medium has an adverse effect on the passivation of stainless steel. The presence of oxidising agents therefore increases the dissolution rate of the steel.

The aggressive nature of hydrochloric solutions may therefore vary greatly for a given concentration, depending on whether or not the environment is in contact with the air, or whether or not it contains impurities that can be reduced.

Consequently, high carbon grades should not be used (this is not specific to hydrochloric acid, as dechromisation should also be avoided for many other environments) and steels with high chromium and molybdenum contents, which are required for concentrated solutions must be chosen very carefully, taking their structural stability into account.

There may, of course, also be a risk of localised pitting corrosion in certain cases, but only with the most corrosion resistant grades for which the overall general corrosion is low.

When the passive film is unable to form, i.e. in all cases except in highly diluted environments, the chromium no longer has a beneficial effect, since it dissolves in the solution; the beneficial elements are therefore nickel (reduction in H2

overpotential) and molybdenum, which is stable (does not dissolve).

ENVIRONMENTS MAIN

PROBLEM

Recommendation for a pure

environment

AGGRAVATING FACTORS


Pure and de-aerated hydrochloric acid

General attack on the surface, as the environment is detrimental to passivation Grade 4307 should not be used.

* Temperature < 60°C and concentration < 2% UGI4507

* Temperature < 20°C and concentration < 3% UGI4539 UGI4507

Presence of oxidizing agents (chlorine or iron chlorides) or aeration (dissolved oxygen, etc.)

625 or 2.4856 For transfer or storage facilities at ambient temperature.

Temperature < 20°C and concentration < 2% or Temperature < 50°C and concentration < 1% UGIMA®4404 UGI4362

High temperature risk of stress

corrosion

UGI 4507

UGI 4539 In certain processes where condensates enriched with small quantities of HCl are formed and cracks occur in grades 4307 and 4404, 904L or 52N+ may prove to be corrosion resistant (PVC manufacturing, for example)

Hydrochloric acid


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RECOMMENDATIONS FOR USE IN A BASE such as sodium hydroxide NaOH or potash KOH The soda and potash solutions are only slightly corrosive for stainless steels, whatever the concentrations, when the temperature does not exceed 100°C.

ENVIRONMENTS MAIN

PROBLEM


environment

AGGRAVATING FACTORS

UGITECH RECOMMEND-

ATIONS Basic environment Sodium carbonate or pure potash

General surface attack

For temperatures < 90°C UGIMA®4307 (see BIBLIO 5 for more information)

High temperature > 100°C risk of stress corrosion and general corrosion.

Up to 120°C UGI 4539 UGI 4507

UGI 4462

Industrial sodium carbonate or potash

Grade 304L not recomm-ended

Presence of chlorides and chlorates risk of

stress corrosion

Up to 80°C: UGI 4362 Up to 100°C: UGI 4462 Up to 140°C: UGI 4507

Industrial soda solutions are produced by sodium chloride electrolysis and are polluted by chlorides and chlorates whose concentrations vary from one manufacturing unit to another and also according to the soda concentration.

Typical solution: 50% NaOH; 1 to 5% NaCl and 0.1 to 1% NaClO3. For low chlorate contents, there is little effect on the uniform corrosion of materials, even at 150°C. However, the risk of stress corrosion cracking is considerably increased by the presence of such pollution. This type of corrosion is highly complex, and a transgranular cracking mechanism with high concentrations of chlorides and chlorates is observed in these basic environments, as in neutral chlorinated or acidified environments. It must be emphasised that the limit temperature at which cracking appears can vary considerably, depending on the level of local stress on the material, as well as on the aeration and concentration of the manufacturing environment.

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In this case, sulphides are a corrosion aggravating factor. The processes used to manufacture cellulose from wood (KRAFT process) involve attacking the wood chips at 170°C, under pressure, with a liquor composed of 20% soda, to which sodium sulphide Na2S, sodium carbonate Na2CO3 and traces of sodium thiosulphate Na2S2O3 have been added.

During curing cycles at a temperature between 70°C and 170°C, there is a change in the chemical composition of the environment, which then contains organic impurities in addition to polysulphides.

Grades with added Molybdenum and Nickel are not recommended for these environments; nickel, in particular, has an adverse effect in the presence of sulphur compounds, as it forms complexes.

Soda and potash

Paper manufacturing

industry


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Selecting materials for the paper pulp industry:

CHIP PREPARATION (ABRASION PROBLEMS) Digester; conveyors and storage; crusher; screens; defibrators

UGI 4462 and UGI 4362

DELIGNIFICATION USING THE KRAFT PROCESS Vapour (localised corrosion + stress corrosion)

UGI 4462; UGI 4507

Preheating; reactor (overall general corrosion + stress corrosion) UGI 4462; UGI 4507 and UGI 4539.

Impregnation; reactor (localised corrosion + stress corrosion + corrosion – abrasion) UGI 4462 and UGI 4362

Storage of black and green liquor (overall general corrosion) UGI 4362 and UGI 4404

BLEACHING Washing and filtration (pitting corrosion)

UGI 4462; UGI 4362; UGI 4301 and UGI 4404

High density storage and reactor (pitting corrosion) UGI 4462 and UGI 4404

Washing and filtration (pitting corrosion) UGI 4462; UGI 4362; UGI 4301 and UGI 4404

Chlorine bleaching: tower (very severe pitting corrosion) Ti; diffuser or washer and filtration tanks UGI 4507

Bleaching / sodium carbonate treatment (pitting corrosion) UGI 4462, UGI 4507 and UGI 4404,317L

Bleaching / hypochlorite (pitting corrosion); tower UGI 4539

washing and filtration (pitting corrosion) UGI 4539; UGI 4462; 317L; UGI 4507

Hydrogen peroxide bleaching: tower; washing and filtration (pitting corrosion) UGI 4362; UGI 4462; 317L; UGI 4507

PAPERMAKING High density storage (corrosion)

UGI 4362 and UGI 4404

Pulps and hydropulps (fatigue corrosion / abrasion corrosion) UGI 4362 UGI 4462, 317L

Drive head (localised corrosion) UGI 4462, UGI 4507

Cylinders (fatigue corrosion / stress corrosion) UGI 4507, UGI 4462

Pneumatic conveyor (corrosion abrasion) UGI 4462, UGI 4362


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RRREEECCCOOOMMMMMMEEENNNDDDAAATTTIIIOOONNNSSS FFFOOORRR UUUSSSEEE IIINNN AAACCCEEETTTIIICCC AAACCCIIIDDD The acidity in an aqueous solution rapidly increases in line with an increase in the concentration, making this product relatively aggressive.

ENVIRONMENTS MAIN

PROBLEM


environment

AGGRAVATING FACTORS


Pure acetic acid


* For temperatures < 80°C, and for all concentrations. UGIMA®4307

At 120°C, UGIMA® 4307 can be used up to concentrations of 20%. * In boiling condition and for a concentration of 50%: UGI4362

* In boiling condition and for all concentrations: UGI4539 UGI4507

UGI4462

Acetaldehyde oxidation process Presence of acetic anhydride impurities in the separation column and in particular at the bottom (at 150°C) Presence of byproducts at 200°C of the hydrocarbon chain oxidation reaction Grade 4404 not recommended

* bottom part of the column at 150°C: stainless steel not recommended. * median part of the separation column: UGI 4539

* top part of the column UGIMA® 4404 or UGI 4362

* for the hydrocarbon chain oxidation process in a liquid environment UGI 4539

Acid transport and storage (exchangers, heating coils)

UGIMA®4404 or UGI4362

The operating limits are given for a maximum corrosion rate of 0.1 mm/year. They obviously apply to the use of stainless steel grades for welding, forming, etc. according to best practice.

Acetic acid


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Formic acid is far more aggressive than acetic acid due to its high rate of dissociation in water.

ENVIRONMENTS MAIN

PROBLEM Recommendation for a pure environment

Formic acid


* At ambient temperature UGIMA®4307

* For concentrations < 1% or in 100% concentrated acid at high temperature. UGIMA®4307 * At a temperature < 80°C, whatever the concentration: UGIMA®4404

* At temperatures > 80°C and < 95°C, whatever the concentration: UGI4362

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There are two types of processes:

- Physical processes: RO (reverse osmosis) for large units, - Thermal processes: MSF (multi stage flash) which represents 90% of the

market. The thermal process involves the risks of pitting and crevice corrosion (if the dissolved oxygen content exceeds 1 ppm) and stress corrosion cracking (SCC) if O2 > a few ppb.

Grade 4404 is not sufficient to withstand pitting and crevice corrosion in the event of an increase in the percentage of oxygen; similarly, if brine residues have been deposited on the walls, 316L will not be sufficient to withstand pitting and crevice corrosion.

ENVIRONMENT MAIN

PROBLEM Basic

recommendations AGGRAVATING

FACTORS

UGITECH RECOMMEND-

ATIONS

Sea water desalination

Pitting corrosion

UGIMA®4404 Or UGI 4362

Increase in the percentage of oxygen

UGI 4462

Multistage process

Crevice corrosion

UGIMA®4404 Or UGI 4362

Brine deposits UGI 4462

Multi stage flash (MSF)

Stress corrosion

Temperature up to 120°C in an aerated environment

UGI 4462

Formic acid

Sea water desalination


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3. Use of duplex steels in the building industry: for example concrete reinforcing bars

This section deals with pitting corrosion in concrete reinforcing stainless steel bars.

OUR ACCELERATED CORROSION ELECTROCHEMICAL TEST CONDITIONS (simulation test developed at the CRU in collaboration with the CEA)

Determination of the pitting potential by electrochemical tests in environments simulating the "concrete" solution in contact with the steel reinforcement: the more positive the value obtained, the better the corrosion resistance.

Consideration of the change in composition due to changes in this environment

over time: carbonation; the pH decreases from 12 to 8; presence of chlorides.

The presence of chlorides is highly exaggerated in our experimental conditions: the concrete would be so cracked that sea water would be able to penetrate through to the steel reinforcement!

Figure 13: examples of the measurement of pitting potentials in a "concrete" environment after 50 years of exposure in a marine environment


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-600

-400

-200

0

200

400

600

800

1.4597 1.4301 1.4404 1.4462 acier 1.4362

pitting potential in mV/ ECS in a carbonate-bearing environment Na2CO3 + NaCl to 21 g/l

in chlorides and at pH=10

Figure 14: examples of the measurement of pitting potentials in a "concrete" environment after 25 years of exposure in a marine environment

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Duplex steels are recommended for reinforcing bars, as their higher mechanical properties allow the amount of reinforcement required to be considerably reduced, leading to a consequent reduction in building costs.

UGIGRIP 4362 is better than 1.4404 in this environment and under these test conditions.

UGIGRIP 4462 is the most corrosion resistant and is recommended for concrete structures in highly aggressive environments such as: bridge piers in the sea, etc.

4. Use of duplex steels in the petrochemical industry: stress corrosion problems.

The stress corrosion resistance of Duplex grades is excellent, due, on the one hand, to their high mechanical properties and, on the other hand, to the fact that it is difficult for cracks to propagate in a two-phase austenite–ferrite structure.

This type of corrosion is generated by acid chloride environments which often contain hydrogen sulphide (H2S) pollutants; they are mainly encountered in the oil industry and off-shore ("acid pits").

Duplex grades again appear to be far superior to austenitic grades in this field and are particularly attractive in terms of cost.


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It should be noted that in the long product sector, which mainly manufactures mechanical components with varying degrees of stress, resistance to this type of corrosion is more appreciated by users.

An H2S temperature versus pressure diagram showing the scope of use of the various grades (to the left of the thick line) is illustrated below.

UGI 4462 and UGI 4507 are used far more widely than grade 4401.

Figure 15: corrosion resistance in an H2S "sour gas" environment

-30

20

70

120

170

220

0,01 0,1 1 10

Temperature in °C

Pressure in H2S

in bars

UGIMA® 4404

UGINE 4462

UGINE 4507

UGINE 4539


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BIBLIO 1: Phosphoric acid - Natural phosphate attack stage:

The most widely used method is the wet process which involves attacking the natural tricalcium phosphates Ca3(PO4)2 with a concentrated solution of sulphuric acid at temperatures of between 80 and 110°C; the process is carried out when calcium fluoride CaF2, silica SiO2, and CaCl2 and NaCl chloride impurities are present.

This stage produces a slurry consisting of:

- 30% P2O5

- polluted by fluorinated compounds in the form of HF (0 to 0.2%) and/or hydrofluosilicic acid H2SiF6 (0 to 1.5%)

- polluted by chlorides (500 to 3000 ppm)

- containing solids non-reactive silica (quartz) and calcium sulphate.

Corrosion problems at this stage of the process with respect to the stirrers and pumps used to transfer the slurry: The entire surface may become highly corroded, due to depassivation of the materials in the presence of fluorinated compounds, excessive amounts of H2SO4 and chlorides. In addition, there are considerable abrasion phenomena.

BIBLIO 2: Phosphoric acid - Slurry filtration stage

This stage takes place at a temperature below 50°C and with no abrasion phenomenon. Its purpose is to remove the calcium sulphate.

BIBLIO 3: Sulphuric acid manufacturing process and the corrosion problems encountered: The most widely used method is the "contact" process which uses vanadium pentoxide as a catalyst to obtain concentrated acid.

Stainless steel is mainly used for hot concentrated acid transfer lines (60 to 110°C), drying and absorption columns and acid coolers; the converters are therefore made of stainless steel, sometimes with in-built gas/gas exchangers.

The main corrosion problem is a uniform or general attack over the entire surface. A pitting corrosion phenomenon may be encountered in acids containing chloride impurities.

BIBLIO 4: Sulphuric acid with oxidising impurities

Such oxidising impurities can have a beneficial or an adverse effect on corrosion. In fact, if the steel is in the active domain (which may be the case even if the corrosion rates are low, for example at low temperature) the presence of an oxidising agent increases the dissolution rate. The action of the oxidising agent is therefore beneficial if (and only if) the material is in, or is brought into the passive domain.

The presence of reducible compounds in the solution results in the passivation of corrosion resistant materials.

Example: in the zinc hydrometallurgical industry, certain stages in the process produce sulphuric solutions with a concentration of 10 or 20%, at temperatures in the region of 100°C. Under such conditions, no grade can have the correct resistance; UGI 4539, UGI


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4507 or UGI 4462 can nevertheless be used, as they contain traces of ferrous and ferric ions.

BIBLIO 5 / Sodium carbonate or pure potash:

For concentrations less than 30%, UGIMA® 4307/304L is appropriate in pure soda or potash; for example at 30% (or 20%) a temperature of 120°C (or 150°C) is possible. However, for concentrations greater than 30%, the use of UGI 4539 / 904L and duplex grades would appear mandatory if the temperature exceeds 90°C.

BIBLIO 6 / RECOMMENDATIONS FOR USE IN AMMONIUM CARBAMATE AND UREA

The first stage in the manufacture of urea is to synthesise the ammonium carbamate by reaction between CO2 and NH3 at high pressure. In the second stage, the carbamate is converted into urea by dehydration at high temperature (150 to 200°C).

The liquid ammonium carbamate and the urea aqueous solutions are very aggressive, all the more so as the temperature is always very high.

Summary of the effects of the different components:

Chromium

Very beneficial for corrosion resistance. Optimum between 19 and 25%

Molybdenum Beneficial. Aim at a content > 2.5%.

Nickel Adverse effect in this environment. However, it prevents localised dechromisations following the sigma phase formation due to traces of ferrite during instrument welding operations.

Copper Uncertain effect Never adverse for a content < 2%

Manganese Not decisive. Not adverse if Cr>19% and Mo>2.5% and Ni<6%.

Nitrogen < 0.2%. Prevents the precipitation of harmful intermetallic phases.

The main components are molybdenum and more particularly chromium, which explains why a type 1.4592 grade is economically optimum.

However, this type of grade is not necessarily easily available in any product. There are two alternatives:

A completely austenitic grade, type 25-22-2 / 1.4466, with a high chromium content

Or a superduplex.

Superduplex poses a problem with respect to pressure vessels (in this case reactors) where stress must be relieved at the end of the manufacturing process, owing to the demixing of the ferrite in the sigma phase and chromium carbides at the conventional temperatures used (550-580°C). That is why 25-22-2 / 1.4466 is recommended in this case.

Our recommendation: UGI 4507

Ferritic grades with a high chromium content (20 to 25%) and a high molybdenum content (2.5%) such as type 4592 may possibly be used.

Urea

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