Research on corrosion resistance of super austenitic stainless steels such as 254SMO, 904l, AL-6XN

Introduction to Super Austenitic Stainless Steel

About Super Austenitic Stainless Steel

The basic metallographic structure of super austenitic stainless steel is typical, 100% austenite. However, due to the high content of chromium and molybdenum, some metal mesophases are likely to appear. These metal mesophases often appear in the center of the plate. However, if the heat treatment is correct, the formation of these metal mesophases will be avoided, resulting in nearly 100% austenite. The metallographic structure of 254SMO does not have any other metal mesophase. The structure is obtained after heat treatment at a temperature of 1150 to 1200 degrees. 

In the actual use process, if a small amount of metal mesophase appears, they will not have a great impact on the mechanical properties and corrosion resistance of the surface. But try to avoid the temperature range of 600 ~ 1000 degrees, especially during welding and hot processing. 

Mechanical properties 

Austenitic structure generally has medium strength and high forgeabilitAfter adding a certain amount of nitrogen, in addition to improving the corrosion resistance, while maintaining the forgeability and toughness of austenitic stainless steelhigh nitrogen super austenitic stainless steel also has high mechanical strengthIts yield strength is 50 to 100% higher than ordinary austenitic stainless steel. The effects of nitrogen on mechanical properties at room temperature and higher temperatures are shown in Table 1 and Table 2, respectively.

Table 1 Mechanical properties of high-alloy austenitic stainless steel at +20℃ 

alloy

Steel grade

Nitrogen content

Yield Strength

tensile strength

Elongation

ASTM

EN

GB

%

Rp0.2MPa

RmMPa

As%

316L

316L

1.4404

 

0.06

220

520

45

904L

NO8904

1.4539

00Cr20Ni25Mo4.5Cu

0.06

220

520

35

317LMN

317LMN

1.4439

 

0.15

270

580

40

254SMO

S31254

1.4547

00Cr20Ni18Mo6CuN

0.20

300

650

40

654SMO

S32654

1.4652

 

0.50

430

750

40

Table 2 Yield strength of high alloy austenitic stainless steel at high temperature (Rp0.2MPa) 

alloy

ASTM

EN*

GB

Nitrogen content%

100℃

200℃

400℃

316L

316L

1.4404

 

0.06

166

137

108

904L

N08904

1.4539

00Cr20Ni25Mo4.5Cu

0.06

225

175

125

317LMN

317LMN

1.4439

 

0.15

225

185

150

254SMO

S31254

1.4547

00Cr20Ni18Mo6CuN

0.20

230

190

160

654SMO

S32654

1.4652

 

0.50

350

315

295

As shown in Table 1 and Table 2, the mechanical strength increases with increasing nitrogen content at all temperatures. Despite the increased strength, the elongation of super austenitic stainless steel is still very high. It is even higher than the elongation of many low alloy steels. This is mainly due to its high nitrogen content and another characteristic associated with it-high work hardening rate. Therefore, parts formed by cold working can obtain high strength. Applications that can take advantage of this feature include pipes and bolts in deeper wells. Like ordinary austenitic stainless steels, the low-temperature performance of super austenitic stainless steels is also very good. The impact resistance and fracture resistance of super austenitic stainless steel are very high, and only slightly decrease when it reaches as low as -196℃. 

Physical properties

Physical properties mainly depend on the austenite structure, and also partly depend on the chemical composition of the material. That is to say, super austenitic stainless steel is not much different from ordinary austenitic stainless steel, such as type 304 or 316, in terms of physical properties. Table 3 lists some typical physical properties of different alloys.

Table 3 Physical properties of some stainless steels and a nickel-based alloy 

alloy

Steel grade

density

Modulus of elasticity 
KN/mm2

Coefficient of thermal expansion×10-6/℃

Thermal conductivity W/m℃

ASTM

EN*

GB

kg/dm3

20℃

400℃

20℃

400℃

20℃

400℃

2205

S31803

1.4462

 

7.8

200

172

13.0

14.5

15

20

304

304

1.4301

 

7.9

200

172

16.0

17.5

15

20

254SMO

S31254

1.4547

00Cr20Ni18Mo6CuN

8.0

195

166

16.5

18.0

14

18

Alloy 625

N06625

2.4856

 

8.4

200

180

12.0

13.5

10

16

The super-austenitic stainless steel containing 6 molybdenum has a greater thermal expansion than the duplex stainless steel 2205, so some deformation may occur at the joint during welding. Although the thermal expansion of nickel-based alloys is generally low, its poor thermal conductivity just offsets this advantage. These physical properties are of great significance, especially when designing parts made of stainless steel or connecting stainless steel with other alloys.

Corrosion resistance of super austenitic stainless steel

To a large extent, the development of austenitic stainless steel is to meet the requirements for corrosion resistance in various environments. Many alloys were designed to be used in a specific environment, and then their application range has become more and more extensive. Therefore, the selection of super austenitic stainless steel, its corrosion resistance is an important basis. Here mainly introduces uniform corrosion, pitting corrosion, crevice corrosion and stress corrosion cracking. 

Uniform corrosion

The most important alloying elements to improve the stability of stainless steel are chromium and molybdenum. The content of these components in super austenitic stainless steel is relatively high, so it shows good corrosion resistance in various solutions. In some environments, the addition of elements such as silicon, copper, and tungsten can further improve the corrosion resistance of the material. 
Figure 1 shows the isocorrosion rate curve of some austenitic stainless steels in pure sulfuric acid. It can be seen that stainless steel with higher alloy content, such as 904L, 254SMO and 654 SMO, has better corrosion resistance than ordinary austenitic stainless steels such as 304 and 316 in a larger concentration and temperature range. The figure also shows that the high-silicon stainless steel SX has a very strong ability to resist concentrated sulfuric acid.

Fig.1 The curve of equal corrosion rate of some austenitic stainless steels in pure sulfuric acid, the corrosion rate is 0.1 mm/year

904L  AL 6XN clip image001 - Research on corrosion resistance of super austenitic stainless steels such as 254SMO, 904l, AL-6XN

Another way to illustrate the ability to resist uniform corrosion in a specific environment is to measure the temperature that causes a corrosion rate of 0.1 mm per year (or 0.5 ml per year). Table 4 lists a series of chemical solutions with different concentrations. These solutions are relatively common in chemical production, and the critical temperature at which the corrosion rate of different steel grades in these solutions is 0.1 mm/year is also given. It can be seen that the critical temperature increases with increasing alloy content. In all solutions, the critical temperature of super austenitic stainless steels such as 254SMO and 654 SMO is the highest, which fully demonstrates its excellent uniform corrosion resistance. 
Table 4 also includes two common wet industrial phosphoric acids, WPA 1 and WPA 2 whose main components are given in Table 5. 

Table 4 Critical temperature leading to corrosion rate of 0.1 mm/year in different chemicals ℃ 

Solution

654SMO

254SMO

317L

2205

1%HCl

95

70

50

85

10%H2SO4+0.33%NaCl+SO2, saturated

75

60

50

<10

96%H2SO4

30

20

35

25

85%H3PO4

90

110

120

50

83%H3PO4+2%HF

85

90

120

50

WPA1

95

80

50

45

WPA2

80

60

35

60

5%CH3COOH+50%(CH3CO)2O

>126*

126*

>126*

100

50%NaOH

135

115

144*

90

Table 5 Main chemical components of WPA 1 and WPA 2, weight percentage

WPAMNo

H3PO4

Cl-

F-

H2SO4

Fe2O3

Al2OS

SiO2

CaO

MgO

1

75

0.20

0.5

4

0.3

0.2

0.1

0.2

0.7

2

75

0.02

2.0

4

0.3

0.2

0.1

0.2

0.7

The order of different alloys varies with the working conditions. A good example is 2205 duplex stainless steel. The performance of this steel in some environments is even better than that of some high-alloy austenitic stainless steels. But in some environments, its performance is not very good. Another example is 904L stainless steel. In pure phosphoric acid, this stainless steel is the best among all steels, but in wet industrial phosphoric acid, it is not comparable to the other two super austenitic stainless steels. In a mixed liquid WPA 2, its corrosion resistance is the worst, see Table 5. 
Therefore, we must be very cautious when recommending the most suitable stainless steel for manufacturing equipment, such as reactors, pipes, and storage tanks. It is best to have specific data about the working conditions. 

Pitting corrosion and crevice corrosion

Pitting corrosion and crevice corrosion are two closely related types of corrosion, both of which are localized corrosion. Its main production condition is an environment containing chloride ions. But temperature and pH (PH value) also play a very important role. When stainless steel is in a chlorine-containing environment, pitting corrosion occurs at a certain temperature. As we all know, the increase in chromium and molybdenum content helps to enhance the resistance of stainless steel to local corrosion. The combined effect of chromium, molybdenum and nitrogen on the ability to resist local corrosion is often expressed by the empirical formula WS (Wirksumme). 

  • WS(PRE)=% chromium+3.3×% molybdenum+16×% nitrogen 

The WS value in the formula is generally called the “pit corrosion resistance index (PRE)”. Therefore, it is often expressed by PRE. The coefficient of nitrogen given by the formula 16 is the most frequently used. However, according to literature reports, other coefficients are also used. For example, Dr. Herbsleb of the Mannesmann Institute recommends the use of 30. Other ingredients such as tungsten also have a positive effect on corrosion resistance. Calculated by weight percent algorithm, the effect is about half of molybdenum. For comparison, use 16 and 30 as the coefficient of nitrogen in the PRE formula to calculate the PRE value for some steel grades in Table 1.

Table 6 PRE value and critical pitting temperature and critical crevice corrosion temperature of some high alloy stainless steels 

Alloy

ASTM

EN*

PRE(16)

PRE(30)

CPT℃**

CPT℃**

2205

S31803

1.4462

34

36

53

35

317LMN

317LMN

1.4439

33

35

53

904L

NO8904

1.4539

36

37

61

15

Sanicro28

1.4563

39

40

AL-6XN

41

41

254SMO

S31254

1.4547

43

46

90

60

654SM0

S32654

1.4652

56

63

>100

100

  • *European unified standard, **in 1 molar NaCl solution, ***in 3.5% NaCl solution, the corrosion potential is 700mV SCE

It can be seen that PRE(16) and PRE(30) are not very different for many steel types. The most important thing is that the two coefficients have no effect on the arrangement of different stainless steels. 
Table 6 also shows the critical pitting temperature (CPT) and critical crevice corrosion temperature (CCT) of some stainless steels. These two critical temperatures are often used to measure the resistance of stainless steel to local corrosion. A lot of research work and practical experience show that the PRE value is proportional to the ability of the stainless steel to resist local corrosion, such as CPT and CCT values. 317LMN, 904L two austenitic stainless steels and 2205 duplex stainless steels have approximately the same PRE value, and their pitting and crevice corrosion resistance should also be the same. The recorded usage data shows that the resistance of 904L stainless steel to pitting corrosion is slightly better than other steel grades, while the resistance to crevice corrosion of 2205 is stronger, which is consistent with the actual use. 
Super austenitic stainless steels containing 6% molybdenum and 7% molybdenum, such as 254SMO and 654 SMO, have higher PRE values and CPT/CCT values, see Table 6. Denotes its superior ability to resist local corrosion. Therefore, the super austenitic stainless steel family has also been widely used in applications requiring high pitting corrosion resistance, such as components used in seawater treatment equipment, pulp bleaching and flue gas desulfurization devices. The material is immersed in a solution saturated with sulfur dioxide and containing acidic (PH value 1) chloride at a temperature of 80°C. The test results of some candidate materials are shown in Table 8.

Table 7 The critical chlorine content that can cause crevice corrosion in a simulated desulfurization tower environment at a temperature of 80°C 

alloy

ASTM

EN*

Cl-ppm

316L

316L

1.4404

50

904L

N08904

1.4539

500

254SMO

S31254

1.4547

5000

654SMO

S32654

1.4652

12500

Alloy 625

NO6625

2.4856

4000-5000**

 *Unified European standards, **For samples with poor metallographic structure, problems have also occurred when the chloride ion concentration is as low as 4000 ppm.

It can be seen that in this very harsh environment, the corrosion resistance of super austenitic stainless steel is on a level with that of nickel-based alloys. 

Stress corrosion cracking

Ordinary austenitic stainless steel is more prone to stress corrosion cracking caused by chloride than ferritic stainless steel and duplex stainless steelHowever, super austenitic stainless steel has a very high resistance to stress corrosion cracking, and in many cases its effect is better than the ability of duplex stainless steel to resist stress corrosion cracking. Table 9 shows the critical stresses that cause stress corrosion cracking under evaporation (determined by spot test). The test time is 500 hours. 
It can be clearly seen that compared with ordinary stainless steel, super austenitic stainless steel has very excellent resistance to stress corrosion cracking.

Table 8 Critical stress causing cracks 

alloy

ASTM

EN*

Critical yield strength at 200℃

316

316

1.4401

<10

2205

S31803

1.4462

40

904L

N08904

1.4539

70

254SMO

S31254

1.4547

90

654SMO

S32654

1.4652

>100

  •  *Uniform European Standard

The presence of hydrogen sulfide (often found in oil and gas wells) increases the risk of stress corrosion cracking. Due to the hydrogen embrittlement of the ferrite phase, duplex stainless steels, especially deep-processed parts, are more prone to cracking. In the presence of hydrogen sulfide and chloride ions, the risk of stress corrosion cracking of stainless steel is greater. Super austenitic stainless steel has strong resistance to stress corrosion cracking in such “acid” environment. NACE MR0175-95 is a standard specially formulated for the selection of materials for sulfide stress corrosion cracking in oil and gas production. This standard includes 254SMO, but also includes the annealing and cold working conditions. The maximum allowable hardness value (35 HRC) is also much higher than that of ordinary austenitic stainless steel (22 HRC). From this point of view, super austenitic stainless steel is the best material choice in the harshest oil and gas environment containing a large amount of hydrogen sulfide. 

Corrosion in seawater

The most common environment that causes pitting corrosion, crevice corrosion and stress corrosion cracking of stainless steel is in water, especially in seawater. Because the chloride ion content of seawater is very high. Since the critical pitting corrosion temperature and critical crevice corrosion temperature of super austenitic stainless steel are very high, see Table 7, which shows that its ability to resist local corrosion in seawater is also very strong. Therefore, super austenitic stainless steels containing 6% molybdenum and 7% molybdenum have been widely used in seawater like nickel-based alloys. Because the actual situation is very different, the reported results are also very different. Some are still in good condition after being used for a few years, and some have serious corrosion problems within a year. As with all stainless steels that come into contact with chloride-containing water, the decisive factor is still oxides and tiny gaps due to welding, and the residual chlorine content is also a very important factor. 
Chlorine added to sea water to kill marine microorganisms is a very strong oxidant, which can easily make the corrosion potential of stainless steel exceed its critical pitting and crevice corrosion potential.
In the case of less than 50 ℃, there should be no pitting corrosion on the surface of the clean 6 molybdenum super austenitic stainless steel. But in some practical applications, there are also examples of 6 molybdenum super austenitic stainless steel with better performance at higher working temperatures. The most restrictive factor is crevice corrosion. If the gap is severe, corrosion will occur even at a temperature of 20-30°C. However, at least at temperatures up to 30°C and a residual chlorine content of about 0.5 parts per million, this type of stainless steel is generally acceptable. When the gap is very serious (such as will be found on some types of plate heat exchangers), even if the temperature has been kept below 25 ℃, 6 molybdenum super austenitic stainless steel is generally not used for this type use. In applications where the gap is severe but no chlorine is added, the use of 6-molybdenum super austenitic stainless steel has been very successful at least at a temperature of 35°C.

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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