Comparative Evaluation of Low Nickel and Nickel Free ... - Springer Link

Trans Indian Inst Met (February 2013) 66(1):25–31 DOI 10.1007/s12666-012-0164-3

TECHNICAL PAPER

TP 2641

Comparative Evaluation of Low Nickel and Nickel Free Lean Duplex Stainless Steels with 316L in a Variety of Corrosive Media Lokesh Kumar Singhal • Prashant Thimmappa Poojary Alok Kumar

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Received: 4 November 2011 / Accepted: 8 August 2012 / Published online: 23 November 2012 Ó Indian Institute of Metals 2012

Abstract For many applications duplex stainless steels with their superior strength coupled with lower raw material cost have emerged as an attractive alternative to austenitic stainless steels. With emphasis on conservation of scarce resources like nickel and molybdenum there is continuing endeavour to develop essentially molybdenum free lean duplex stainless steels with low nickel content such as 2304 (23Cr–4Ni), 2202 (22Cr–2Ni), 2101 (21Cr–1.5Ni). This paper compares the corrosion behaviour of a low nickel duplex (21Cr–1.5Ni) and a nickel free duplex (21Cr–1.5Cu) with 316L stainless steel in several corrosive media. All the three alloys exhibit similar excellent corrosion resistance under boiling conditions in less aggressive organic acids such as 20 % acetic acid, 25 % lactic acid, 25 % citric acid. However, in stronger organic acids such as 5 % formic acid, 5 % oxalic acid, and mixture of formic and acetic acid, the duplex grades exhibit superior corrosion resistance. This edge over 316L continues on addition of chloride ions in these acids. In boiling 50 % nitric acid solution, the corrosion resistance of these nickel free and low nickel duplex is slightly better than 316L grade. Since 304L grade is generally used in nitric acid plants, tests were also conducted on 304L and these duplex grades were found to be more resistant. Similarly in 50 % phosphoric acid also, the duplex grades exhibit superior corrosion resistance compared to 316L grade. Alloying with nickel and molybdenum is known to give rise to significant improvement in corrosion resistance in this acid. However, even in the absence of these elements, the beneficial effect of higher chromium content is evident. Of all the inorganic acids, sulfuric acid is used in largest volume in the industries. Boiling tests in dilute 1 and L. K. Singhal P. T. Poojary (&) A. Kumar JSL Stainless Ltd, Hissar, Haryana, India e-mail: [email protected]

5 % H2SO4 indicate that nickel free copper bearing duplex is more resistant than low nickel duplex grade and vastly superior to 316L Thus nickel-free and low-nickel duplex stainless steels offer a very attractive combination of high corrosion resistance coupled with cost effectiveness in a wide variety of corrosive media.

1 Introduction Duplex stainless steels, on account of their high yield strength and good resistance to corrosion and stress corrosion cracking, are attractive materials for corrosive environment. Due to high costs of nickel, there has been focus on lowering nickel content of duplex stainless steels. Whereas grade UNS 32304, with 4 % nickel was developed more than two decades back, recent years have witnessed several grades with lower nickel content around 1–2 % such as UNS 32100, S32101, S32202 and 2102. This paper compares corrosion behavior of a low nickel and a nickel free duplex stainless steel both having nearly 21 % chromium with 316L grade. Low nickel duplex tested conforms to UNS S 32101 whereas in nickel free duplex, copper was added to replace nickel. The chemical composition and PREN of these alloys are given in Table 1.

2 Experimental Work Heats of nickel free lean duplex alloy were made in 100 kg induction melting furnace whereas low nickel duplex as per UNS S 32101 and UNS 31603 were produced through EAF-AOD-LF-CC route as 50 ton commercial heats. Microstructures of 21Cr–1.5Cu and 21Cr–1.5Ni duplexes in annealed condition etched by boiling Murakami reagent

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Trans Indian Inst Met (February 2013) 66(1):25–31

Table 1 Typical composition of the alloys C

Mn

Cr

UNS S 32101 (21Cr–1.5Ni)

0.030

4.90

20.82

21Cr–1.5Cu

0.030

4.97

21.40

316L

0.021

0.97

16.20

Ni

Cu

Mo

N

S

P

PREN

1.58

0.23

0.29

0.24

0.001

0.024

25.62

0.17

1.65

0.02

0.23

0.002

0.032

25.15

0.38

2.05

0.04

0.003

0.033

23.60

10.0

are given in Figs. 1 and 2. The percentage of austenite in nickel free duplex 21Cr–1.5Cu and 21Cr–1.5Ni as per ASTM E 562 is 48 and 49 % respectively. Weight loss measurements were carried out after boiling tests for 48 h in various organic and inorganic acids with or without salt ions. Potentiodynamic tests were carried out in flat cell model Gill AC, ACM instruments. The tests were carried out at 25 °C and saturated calomel electrode (SCE) was used as reference electrode. The tests were carried out, after subjecting the sample to free potential for 1 h, at scan rate of 60 mv/min from -500 mV until current reached 1,000 lA/cm2. Double loop electrochemical potentiokinetic reactivation (DL-EPR) were carried out on samples sensitized at 680 °C for 1 h, as per ASTM G108 in a round cell using Gill AC, ACM instruments in 0.5 M H2SO4 ? 0.01 KSCN solution at 30 °C. Several potentiodynamic curves were plotted with pH decreasing from 3 to 1.2 in order to measure imax. The tests were carried out, after de-aerating 2 M NaCl (pH adjusted by HCL) for 1 h by argon gas and subjecting the sample to free potential for 1 h, followed by curve plotted in anodic direction at scan rate of 60 mv/min from -500 mV until pitting potential. 2M NaCl corresponds to the generally accepted chloride concentration inside a crevice. Mechanical properties of the three alloys have been compared and behavior of nickel free duplex on aging was also examined.

Welding of copper bearing duplex was carried out by TIG and MIG using ER 2209 filler. Microstructures were studied and toughness at different temperatures was evaluated.

Fig. 1 Microstructure of 21Cr–1.5Cu grade duplex

Fig. 2 Microstructure of 21Cr–1.5Ni grade duplex

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3 Mechanical Properties The tensile test results at room temperature as per ASTM A 370-03a are tabulated in Table 2. It is noted that nickel free and low nickel duplex are characterized by similar high proof strength which is nearly double of 316L grade. Charpy impact tests were carried out at different temperatures which are tabulated in Table 3. The variation of hardness of both duplex grades with ageing time at 475 °C is plotted in Fig. 3:

4 Boiling Tests in Various Acids The results of weight loss in boiling tests in various organic and inorganic acids are given in Table 4. In weak inorganic acids such as 20 % acetic, 25 % lactic and 25 % citric, all the three grades exhibit outstanding behavior. In stronger organic acids such as 5 % oxalic, 5 % formic and 5 % formic ? 5 % acetic acid, the superiority of duplexes over 316L is seen.


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Table 2 Tensile properties at room temperature Grade

0.2 % Yield strength

Ultimate tensile strength

% Elongation

UNS S 32101 (21Cr–1.5Ni)

501

644

38

21Cr–1.5Cu

562

686

42

316L

221

685

63

Table 3 Charpy impact strength of base metal 23 (°C)

0 (°C)

-40 (°C)

UNS S 32101 (21Cr–1.5Ni)

320*

280*

30*

21Cr–1.5Cu

398*

255*

32*

316L

195*

194*

192*

*Impact values were calculated by using sub-size samples (10 9 7.5 9 55 mm)

In such strong organic acids with salt addition, the corrosion resistance of 21Cr–1.5Cu grade is observed to be higher than the other two grades. In boiling inorganic acids like 5 % sulphuric, 50 %orthophosphoric and 50 % nitric acid, the corrosion resistance of duplex grades is better.

5 Potentiodynamic Tests Potentiodynamic tests at 25 °C in 1 % sulfuric acid are given in Fig. 4. All the three alloys are characterized by similar passivation current and breakdown potential of

Fig. 3 Variation of hardness with ageing time at 475 °C

nearly 950 mV. In 5 % sulphuric acid solution at 25 °C, while the breakdown potential is similar the passivation current of nickel free duplex is marginally higher which can be attributed to dissolution of Cu from the surface. Beneficial effect of copper on resistance to sulphuric acid has been the subject of a large number of studies in austenitics [1–6], ferritics [7] and duplex stainless steels [8]. Analysis of passive film of copper containing austenitic stainless steels have revealed presence of copper in the film [9,10] in addition to enrichment of chromium and nitrogen. Rapid formation of such film in copper containing duplex steel was observed during boiling tests. The beneficial effect of copper in boiling sulphuric acid and in contrast slightly higher ipass in the passive region is in conformity with observation of Ujiro et al. [11]. They noted that the deposited Copper suppresses anodic dissolution of steel in general corrosion whereas negative effect of copper can be seen at noble potential range where deposited copper cannot exist. In 50 % phosphoric acid at 25 °C, both duplex stainless steels exhibit similar i-pass which is lower than that of 316L. The breakdown potential of the duplex grades is also slightly higher in this medium (Fig. 5). Alloying with nickel and molybdenum is known to improve corrosion resistance in this acid. However, even in absence of these elements, the beneficial effect of higher chromium content is evident. In 50 % nitric acid, all three grades have similar Breakdown potential, i-pass (passivation currents) with different E-corr values (Fig. 6). The values of duplexes are more ‘noble’ than 316L indicating better corrosion properties. The behavior with regard to passivation current and breakdown potential is very similar in 50 % formic acid (Fig. 7), 20 % acetic acid and 10 % oxalic acid.

Ni free duplex 2101

102

Hardness(rockwell)

100

98

96

94 0

100

200

300

400

500

600

Time (minutes)

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28 Table 4 Boiling test results in various acids


Acid

Duplex (21Cr–1.5Ni) Duplex (21Cr–1.5Cu) 316L (mmpy) (mmpy) (mmpy)

Mild organic acids 20 % Acetic acid

0.004

0.003

0.005

25 % Lactic acid

0.005

0.002

0.016

25 % Citric acid

0.001

0.003

0.006

Strong organic acids 5 % Oxalic acid

0.063

0.010

0.105

5 % Formic acid

0.015

0.002

0.124

5 % Formic ? 5 % acetic

0.004

0.004

0.105

0.442

0.534

1.250

5 % Formic acid ? 1,000 ppm chloride 0.035 5 % Formic ? 5 % acetic ? 1,000 ppm chloride 0.042

0.015 0.015

0.405 0.277 0.100

Organic acids containing chloride ions 5 % Oxalic acid ? 1,000 ppm chloride

Inorganic acids 50 %Nitric acid

0.094

0.060

50 %Orthophosphoric acid

0.008

0.008

0.035

5 %Sulphuric acid

2.357

1.508

17.251

Fig. 4 Polarization curves in 1 % sulphuric acid at 25 °C

Ni free Duplex 316L 2101

1000 800

Potential(mV)

600 400 200 0 -200 -400

0.01

0.1

1

10

100

1000

2

Current ( μ A /cm )

The duplexes and 316L grades were sensitized at 680 °C for 1 h and the DL EPR tests were carried out. 316L grade is immune to sensitization after this treatment. The presence of peak during reverse scan in duplexes implied the susceptibility of duplexes due to ferrite phase in duplex stainless steel (Fig. 8).

propagation steps respectively. Graphs were plotted using pH and maximum active current density imax, which were determined by various potentiodynamic tests. Depassivation pH is defined as pH at which maximum active current density is 10 lA/cm2. Crevice corrosion resistance of Cu bearing duplex is comparable with 2101 and 316L for pH greater than two. Inferior behavior of 21Cr–1.5Cu as compared to 2101 and 316L at pH less than two can be attributed to importance of Ni and Mo in crevice corrosion resistance [12].

7 Crevice Corrosion Resistance

8 Weldability

Crevice Corrosion is governed by two steps viz initiation and propagation. The depassivation pH and maximum active current density imax, are characteristics of initiation and

Weldability of 21Cr–1.5Ni and 21Cr–1.5Cu duplex grades has been studied by Metal Inert Gas (MIG) and Tungsten Inert Gas (TIG) welding techniques. The plates of these

6 Double Loop Electrochemical Potentiokinetic Reactivation Tests (DL EPR)

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Trans Indian Inst Met (February 2013) 66(1):25–31 Fig. 5 Polarization curves in 50 % Phosphoric acid at 25 °C

29 Ni Free Duplex 316L 2101

1400 1200 1000

Potential(mV)

800 600 400 200 0 -200 -400 0.1

1

10

100

1000

10000

2

Current (μ A /cm )

Fig. 6 Polarization curves in 50 % nitric acid at 25 °C

Ni free Duplex 316L 2101

1400

Potential(mV)

1200 1000 800 600 400 200 0.01

0.1

1

10

100

1000

10000

100000

2

Current ( μ A /cm )

Fig. 7 Polarization curves in 50 % formic acid at 25 °C

Ni Free Duplex 316L 2101

1200 1000

Potential(mV)

800 600 400 200 0 -200 -400 0.01

0.1

1

10

100

1000

2

Current ( μ A/cm )

123

30


800 700

Current(μΑ)

600

Ni free duplex 2101 316L

500 400 300 200 100 0 1.2

1.4

1.6

1.8

2.0

2.2

2.4

pH

Fig. 8 Crevice corrosion resistance in 2M NaCl solution at 23 °C

Fig. 9 Fusion line of 21Cr–1.5Ni

two grades of 12 mm thickness having V-groove were butt welded using ER 2209 filler electrode. The plates of 21Cr– 1.5Ni and 21Cr–1.5Cu grades were found to be easy to weld using the above filler electrode with practice similar to duplex grade 2205 [13]. 8.1 TIG welding of 21Cr–1.5Ni and 21Cr–1.5Cu The ferrite content of Weld zone in 21Cr–1.5Ni and 21Cr– 1.5Cu was measured to be 35 and 37 % respectively. The ferrite content in the HAZ of these grades was nearly 55 % each. Whereas the base metal ferrite content for these grades were 51 and 52 % respectively. 8.2 MIG welding of 21Cr–1.5Ni and 21Cr–1.5Cu Fig. 10 Fusion line of 21Cr–1.5Cu

The ferrite content of MIG weld zone in 21Cr–1.5Ni and 21Cr–1.5Cu was measured to be 38 and 39 % respectively. The ferrite content in the HAZ of these grades was nearly 54 and 55 % respectively and corresponding base metal ferrite content for these grades were 51 and 52 %. As seen in the Figs. 9 and 10 and the measured ferrite, presence of lower ferrite content in the weld can be attributed to high Ni content in the filler metal ER 2009 (7.5–9.5 % Ni) and due to the presence of austenite stabilizer N2 in shielding gas. The toughness of MIG and TIG welded samples (in Joules) at different temperatures (in °C) is shown in Fig. 11. From this study of welding of the two duplex stainless steels, it is evident that on account of high nitrogen content in two duplex stainless steels, there is rapid reformation of austenite in HAZ in both the duplexes. Both duplex stainless steels exhibit a narrower HAZ in comparison to 316L, due to low heat input welding processes and higher thermal conductivity of these materials.

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Fig. 11 Impact energy of the MIG and TIG welded samples

9 Conclusion Both Duplexe steels and 316L steel exhibit similar outstanding corrosion resistance in boiling tests in less aggressive organic acids such as 20 % acetic, 25 % lactic and 25 % citric acid. However, in stronger organic acids,


such as formic, oxalic acids and mixture of formic and acetic acid, the duplex grades exhibit superior corrosion resistance. Similarly in 50 % phosphoric acid, the duplex grades exhibit superior corrosion resistance compared to 316L. In boiling 5 % sulphuric acid solutions, the duplex grades show vastly superior behavior compared to 316L and copper bearing duplex steel is more resistant than low nickel duplex. Crevice corrosion resistance of duplex stainless steel in 2 M NaCl solution is comparable to 316L steel up to pH value two. However 316L is better than duplexe steels for pH less than two. Both the duplexe steels exhibit good weldability with standard TIG and MIG welding practices.

References

31 2. Hermas A A, Ogura K, Takagi S, and Adachi T, Corosion, 51 (1995) 3. 3. Lin H T, Tsai W T, Lee J T, and Huang C S, Corros Sci, 33 (1992) 691. 4. Hultquist G, Seo M, Leitner T, Leygraf C, and Sato N, Corros Sci, 27 (1987) 937. 5. Fedrizzi L, Molinari A, Defiorian F, and Tiziani A, Br Corros J, 26 (1991) 46. 6. Cortie M B, Welbeloved D, Kincer M, and Lula R A, in High Manganese High Nitrogen Austenitic Steels, (ed) Lula R A, ASM, Novelty (1992), p 177. 7. Seo M, Hultquist G, Leygraf C, and Sato N, Corros Sci, 26 (1986) 949. 8. Scoppio L, and Nembrini I, 6th World Duplex Conference, Vol 12, 18–20 October 2000, Stainless Steel World, Italy (2000). 9. Rao V S, Singhal L K, ISIJ Int, 49 (2009) 1902. 10. Kim S T, and Park Y S, Corrosion, 63 (2007) 114. 11. Ujiro T, Satoh S, Staehle R W, and Smyrl W H, Corros Sci, 43 (2001) 2185. 12. Peultier J, Chauveau E, Jacques S, and Mantel M, 6th European Stainless Steel Conference, (2008), p 605. 13. Bergstrom D, Botti C A, and Dunn J J, SSW Conference, SSW, Maastricht (2009), PS 9029.

1. Streicher M A , Met Prog, 128 (1985) 29.

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