by Bengt Wallén, Avesta Sheffield AB, Research & Development, SE-774 80 Avesta, Sweden

Introduction Duplex stainless steels were developed in the early thirties in Sweden and in France. The driving force to the development was the sensitivity to intergranular corrosion of existing austenitic steels which often contained 0.08-0.10% carbon. The duplex steels had the same carbon content but proved much less sensitive to this type of corrosion. Already before the end of 1930 Avesta Jernverk, now part of Avesta Sheffield, had developed the grades 453 and 453S and in 1932 these steels constituted 6.5% of the total production (1). The typical compositions of these steels were 25Cr, 5Ni with 0 and 1.5% molybdenum respectively, i.e. the latter steel was a forerunner of AISI 329. The steels were first produced as cast products but also as plate. The main application area was in the sulphite industry. The French correspondence to 453S was Uranus 50 containing 21Cr, 8Ni, 2.5Mo and 1.5Cu (2). The first duplex steels had about 65% of ferrite in the solution annealed condition and the high ferrite content resulted in rather bad mechanical and corrosion properties in the heat affected zone after welding. When it was discovered, in the beginning of the fifties, that duplex steels have a good resistance to stress corrosion cracking (3, 4) the development of steels with a better weldability started.

One of the first new steels was UNS 31500 (3RE60 = 17Cr, 4.5.Ni, 2.7Mo, 0.030 which was later followed by UNS 31803 (2205 = 22Cr, 5.5Ni, 3Mo, 0.03C, N). However, none of the duplex steels existing in the early seventies were resistant enough for a general seawater use and not until the so-called superduplex steels were introduced did seawater resistant duplex steels become available. These steels all contain at least 25% chromium and have increased levels of molybdenum and nitrogen. Table 1 summarizes various superduplex steels mentioned in the approximate order they were announced in the literature. The list includes steels which are produced in most product forms and have a great use in seawater applications as well as steels which seem to have been used very little so far. Two of the first superduplex steels were UNS 31260 and UNS 32550 which were introduced to the market already in the seventies. In the eighties a number of new superduplex steels were introduced, all containing 25-27% chromium, 3-4% molybdenum, 0.15-0.30% nitrogen and some with copper and tungsten additions. In the following a review over most types of corrosion occurring in seawater applications is given. With just a few exceptions, only tests using real seawater have been taken into consideration. Whenever possible, the behaviour of superduplex steels is compared with that of superaustenitic steels. The superaustenitic steels that are included in any of the succeeding tests are shown in Table 2.

acom

Corrosion of Duplex Stainless Steels in Seawater

1-1998

AVESTA SHEFFIELD CORROSION MANAGEMENT AND APPLICATION ENGINEERING

acom No. 1-98 Table 1. Superduplex stainless steels. Steel grade UNS Trade name

Typical composition, % Cr Ni Mo

S31260 S32550

DP 3 Ferralium 255 Alloy 381

1 2 3

25 25 25

7 6 7

3.0 3.0 3.9

S31200 S32550 S32760

UR 47N UR 52N Zeron 100

4 4 5

25 25 25

7 7 7

3.0 3.0 3.5

S32750

SAF 2507 Fermanel Atlas 958 DPS 28

6 2 7 1

25 27 25 27

7 8.5 7 7.5

4.0 3.1 4.5 3.8

4 8 1 9 10

25 26 25 25 25

6.5 7.5 7 7.5 7

3.4 4.7 3.0 3.9 4.0

S32520 (1.4469) S39274 S39277 (1.4501)

UR 52N+ Märker G-4469 DP 3W DTS 25.7 NWCu A911

Steel producers:

1 2 3 4 5

Sumitomo Metal Langley Alloys Climax Molybdenum Creusot Loire Weir Materials

Seawater as a corrosive medium Cu

N

W

Ref.

0.5 2.5

>0.10 >0.10 0.15

0.4

5 6 7

1.5 0.7

1.0 0.3 1.5

1.7 0.6

0.18 0.18 0.25

8 8 9

0.7

0.30 0.23 0.18 0.30

10 11 12 13

0.24 0.27 0.30 0.28 0.23

14 15 16 17 18

2.0 1.0 0.7

6 Sandvik Steel 7 Atlas Foundry 8 Schmidt & Clemens 9 CSM 10 Böhler Edelstahl

Table 2. Superaustenitic stainless steels used as reference material. Steel grade UNS

Trade name

S31254 S32654 S34565 N08926 N08031 N08926 N08932 N08926 N08367 -

254 SMO 654 SMO Alloy 24 1925 hMo Alloy 31 UR B26 UR SB8 25-6Mo AL 6XN K-C 32

Steel producers:

1 1 2 2 2 3 3 4 5 6

1 Avesta Sheffield 2 KruppVDM 3 Creusot Loire

Typical composition, % Cr Ni Mo

Cu

N

20 24 24 20 27 20 25 20 20 26

0.7 0.5 1.0 0.9 1.5 0.9 1.5 0.9 0.2 1.0

0.20 0.50 0.40 0.20 0.20 0.20 0.20 0.20 0.20 0.20

18 22 18 24 31 24 25 24 24 37

6.1 7.3 4.5 6.2 6.5 6.2 4.7 6.2 6.2 5.2

4 Inco Alloys 5 Allegheny Ludlum 6 Schmidt & Clemens

2

Natural seawater In the mid-seventies Mollica, et al, noticed that stainless steels, independent of their composition, have surprisingly noble potentials in natural seawater (19). Mollica attributed this observation to a microbial slime layer, the biofilm, which is quickly formed on an inert surface. He showed that the biofilm has a catalytic effect on the cathodic reaction in the corrosion process, i.e. the oxygen reduction. After this discovery a very large number of investigations have been carried out to study the nature and effects of the biofilm. The noble corrosion potentials, normally in the 300 to 350 mV SCE range, mean that the risk for initiation of localized corrosion such as crevice and pitting corrosion is greater in natural, living seawater than in sterile solutions like artificial seawater or sodium chloride solutions, where the potentials are at least a couple of hundred mV lower. Also due to the biofilm the rate of the oxygen reduction is higher in the natural water. At a potential corresponding to that of a stainless steel specimen attacked by crevice corrosion (≤ 0 mV SCE), the reduction current is about two orders of magnitude greater than in a sterile chloride solution (20). This means that also the propagation rate of any localized corrosion is higher in the natural water. Even if an active biofilm seems to exist in all seawaters, independent of the temperature, heating of the water will kill the biofilm and stop its catalytic ability. In northern Atlantic this happens when the water is heated to around 30°C (21), while in the Mediterranean the activity is not completely lost until at 40°C (22). This means that natural seawater has its highest corrosivity at temperatures slightly below the temperature at which the biofilm is killed, e.g. at 2530°C in the northern Atlantic.

acom No. 1-98

Microbially induced corrosion (MIC) other than that caused by the biofilm has been much discussed but seems not to be a problem with the supersteels recommended for seawater applications (23). One exception, however, is the sulphate reducing bacteria, the effect of which is discussed in section "Sulphite polluted water", page 7.

Chlorinated seawater In most seawater applications the stainless steels will handle water that has been chlorinated in order to avoid fouling problems. Chlorine/ hypochlorite is a strong oxidant which displaces the corrosion potential of stainless steels in the noble direction. The high potentials, which are in the order of 500 to 600 mV SCE, are consequently more positive than those measured in natural water and make the risk for initiation of crevice and pitting corrosion very great. On the other hand, chlorine kills the biofilm which then loses its ability to catalyse the oxygen reduction. Therefore the cathodic reaction, i.e. reduction of oxygen plus chlorine, is between two and three orders of magnitude slower than in natural seawater in the presence of a biofilm (24). This means in practice that the propagation rate of any localized corrosion is much lower in a chlorinated water as long as the effective cathodic area is not very large. Since there is no temperature-sensitive biofilm in a chlorinated water a higher water temperature will always increase the corrosivity of the water, and so will an increase in the chlorine concentration. In practice, the chlorine additions are often higher than necessary. It has been reported that a residual chlorine level of 0.1-0.2 ppm is sufficient to reduce the microbial activity on a stainless steel surface to close to zero (25, 26). If the chlorine demand of most seawaters is added, addition of less than 1 ppm at the injection point is normally sufficient. If intermittent chlorination is used, a residual level of 1 ppm used during 30 minutes per day seems enough to stop the microbial activity (26).

Seawater tests When stainless steels are used for handling seawater the main corrosion risks are crevice corrosion and sometimes pitting corrosion in weld areas. Stress corrosion cracking seldom occurs at the water temperatures normally encountered. One exception, however, is when seawater evaporates on a hot wall creating a very concentrated chloride solution. This type of corrosion will be treated in section "External stress corrosion cracking", page 8. In this section just crevice and pitting corrosion tests will be treated. The most frequent way of testing the crevice corrosion resistance is to apply some kind of crevice formers on the surface of the specimens and then immerse them in the water. Pitting corrosion may occur on the creviced specimens but, since crevice corrosion is normally the dominant type, pitting is often studied on welded specimens without using intentional crevices. Tests of this kind are valuable for the screening of stainless steels in that they indicate the probability of localized corrosion initiation. However, the most reliable (and most expensive) way of evaluating the crevice and pitting corrosion resistance of a material is to perform tests with prototype systems composed of real components. In the following both types of tests will be treated.

Immersion tests — natural seawater A great number of immersion tests have been reported in the literature and in Table 3 a compilation of some tests reported in recent years is shown. The table includes the superduplex and superaustenitic steels tested. The results are presented as "no corrosion" = 0, "crevice corrosion" = cc or as "pitting" = p. No attempt has been made to classify the degree of attack. Table 3 does not include lower alloyed steels like S31803 or N08904. However, when these steels have been tested they have almost always corroded. In most cases the superduplex and the superaustenitic steels 3

behave similarly and are mostly not attacked at all. In the tests reported in ref. 34 however, most specimens have been attacked by pitting corrosion beneath salt deposits which formed on the test plates above the waterline. The crevice corrosion reported in ref. 36 was very mild and only occurred as end grain attack in the holes through which the retaining bolt was passing.

Immersion tests — chlorinated seawater As can be seen in Table 4 most of these tests have been performed at elevated and controlled temperatures, and they involve only a few of all superduplex steels available. The superaustenitic steel UNS S31254 has almost always been included as a reference material. UNS S32750 and S31254 seem to have practically the same corrosion resistance. When the water is chlorinated to 2 ppm, crevice corrosion did not occur at 35°C but only at 45°C. Welded specimens, not equipped with crevices, were not attacked at 45°C, however (33, 39). The second generation superaustenitic steel UNS S32654 resisted both crevice and pitting corrosion at 45°C (38). A comparison between UNS S32760 and S31254 is reported in ref. 34. As in the preceding tests the superduplex and the superaustenitic steels have about the same corrosion resistance. The only exception is at the lowest temperature (30°C) and 3 ppm of chlorine where only the superaustenitic steel was resistant. It should be observed that in these tests pitting corrosion often occurred beneath salt deposits above the water-line. These attacks will probably lower the potential of the specimens and consequently make the crevice corrosion results too positive. It is not likely that the steels are resistant to crevice corrosion at 70°C and 1.5 ppm of chlorine.

acom No. 1-98 Table 3. Results of immersion tests in natural seawater. Test conditions Temp. Duration °C months

Type of specimens

Results

Amb. Amb. Amb.

12 24 3

crevice crevice crevice

30 35 35 35 30 40 70 35 Amb. 40 60 Amb.

6 3 3 12 3 3 3 6 1 6 6 3

crevice crevice welded crevice crevice crevice crevice crevice crevice crevice crevice crevice

S31260

Superduplex steels S31200 S32550 S32760

S39274

S32750

S39277

Superaust. steels

o o

o o

o

S08932(o) S31254(o) S08932 (o) N08926 (o)

o

o

o S31254(o)

o o

S31254 (cc) S31254 (o/o) S31254 (o/p) S31254 (o/o) S31254 (cc) S31254 (o)

o o/o o/p o/p cc

o cc

o o

o o

cc

o

(cc)

S31254 (cc) N08031 (cc)

Ref. 5 8, 27-30 31

32 33 13 34 16 35 17 36

o = no corrosion p = pitting corrosion cc = crevice corrosion

Table 4. Results of immersion tests in chlorinated seawater. Test conditions Cl2 Temp. ppm °C 2 2 2 2 1 1 1 10 2 2 * * 1.5 1.5 1.5 3.0 3.0 3.0 2

Results Duration months

35 35 45 45 30 40 Amb. 45 55 55 55 55 30 40 70 30 40 70 Amb.

3 3 3 3 5 5 1 3 3 3 3 3 3 3 3 3 3 3 1

Type of specimens crevice welded crevice welded butt welded tubes butt welded tubes crevice crevice crevice welded crevice welded crevice crevice crevice crevice crevice crevice crevice

S32750

Superduplex steels S39274 S32760

Superaustenitic steels S32550

Ref.

o o

S31254 (o)

cc

S31254 (cc)

o o o o cc cc p cc p o/o o/p o/p cc/p cc/p cc/o

cc

S31254 (o) S31254 (o) S31254 (o) S31254 (cc), S32654 (o) S31254 (cc), S32654 (cc) S31254 (p), S32654 (o) S31254 (cc), S32654 (o) S31254 (p), S32654 (o) S31254 (o/o) S31254 (o/o) S31254 (o/p) S31254 (o/o) S31254 (cc/p) S31254 (cc/p)

o

* Intermittent chlorination (2 ppm, 1h/dl during 1 month followed by continuous chlorination (2 ppm) during 2 months. o = no corrosion p = pitting corrosion cc = crevice corrosion

4

33 33 33 33 33 33 35 37 38 38 38 38 34 34 34 34 34 34 16

acom No. 1-98 Table 5. Environments used in prototype tests in natural and chlorinated seawater. Type of test

Test conditions Cl2 Temperature ppm °C

Steels tested Duration months

Heat exchanger Heat exchanger Heat exchanger Piping system Heat exchanger Piping system

0.5-0.8 0.5-0.8 0.5

≤90 Amb. →35 50→60 Amb. Amb.→≤50 30

12-24 12 6 6-12 3 3

Piping system

1.5

30

3

Prototype tests A summary of tests performed with real components is shown in Table 5. Results from testing of heat exchangers and piping systems have been reported. Heat exchangers: In ref. 40 and 41, tubes of the superduplex steel UNS S31260 have been roller expanded into a S31260 tube plate forming a model heat exchanger that has been tested at different temperatures. It is concluded that S31260 tubes allow a maximum process fluid temperature of up to 100°C with a minimum seawater flow rate of 0.5 m/s. This corresponds to a maximum skin temperature of 80°C. In the same type of heat exchanger a comparison has been made between the superduplex steels S31260 and S39274 (16). In one test ambient temperature seawater was heated by steam (125°C) to 35°C at a flow rate of 1 m/s. S31260 suffered crevice corrosion in the tube-tube plate joint while S39274 was resistant. In a second test the water (2 m/s) was heated from 50° to 60° by 125°C steam. None of the steels corroded. However, since the tube plate was made of 316, cathodic protection of the tube plate was employed. This might explain why S31260 did not corrode in this case. A similar test is described in ref. 31. Chlorinated, ambient temperature seawater was used to cool steam having a temperature of 50-60°C. Since the water velocity was very low (0.1 m/s) one might assume that the outgoing water had a temperature close to that of the steam. Using

Ref. Superduplex S31260 S31260, S39274 S31260, S39274 S32760, S32550 S32550 S32550, S32760, S32750, (1.4469) S32550, S32760, S32750, (1.4469)

Superaustenitic 40,41 16 16 31 31

S31254 S31254 S31254, S08932, N08367, N08926 S31254, S08932, N08367, N08926

15, 42, 43 15,42,43

Table 6. Results of comparative testing in parallel loops. Chlorinated seawater, 30°C, 85 days (15, 42, 43) Steel grade

0.5 ppm Cl2 Attacked (out of 12)

1.5 ppm Cl2 Attacked (out of 12)

UNS

Type

Flanges

Welds

Flanges

Welds

S31254 N08367 S08932 N08926 N08926 K-C 32

Superaustenitic Superaustenitic Superaustenitic Superaustenitic Superaustenitic Superaustenitic

0 1 3 8 0 5

0 0 0 0 0 0

3 2 1 8 3 7

0 0 0 0 0 0

S32550 (1.4469) S32760 S32750 S31803

Superduplex Superduplex Superduplex Superduplex Duplex

3 0 0 0 7

6 0 2 1 0

2 0 3 4 6

7 0 0 1 0

S31254 tube plates, tubes made of S32550, S31254, and titanium were tested. Titanium and S31254 showed no signs of corrosion and only one pit was evident on the S32550 tubes. Pitting corrosion under salt deposits was detected, however, on the S31254 tube plate just above the waterline. Piping systems: In a very extensive programme different piping system components were tested for 6-12 months in natural and chlorinated (0.5-0.8 ppm) seawater of ambient temperature (31). The programme included several test loops containing flanged pipes, valves, pipe branching and deadlegs. Pumps were tested in separate non-metallic systems. In all tests, components made of 316L and S31803 were attacked by severe crevice corrosion after a short time. The highly alloyed steels S32760 and S31254 performed in an

5

excellent way, and so did titanium and S32550 which, however, were only tested as pump and valve respectively. In another programme eleven 2 inch pipe loops, composed of different stainless steels, were tested in parallel and thus being exposed to exactly the same environment (15, 42, 43). Each loop consisted of six 1 m long pipes onto which flanges had been welded. Two tests were performed, one where the seawater was chlorinated to 0.5 ppm residual chlorine and one where 1.5 ppm was used. Since both tests were performed at 30°C the environments were considerably more corrosive than in the preceding test. The results of the tests are shown in Table 6 where the number of flanges attacked by crevice corrosion, and the number of flange to pipe welds attacked by pitting corrosion, are given.

acom No. 1-98 As expected, most of the duplex S31803 flanges exhibit crevice corrosion. The superduplex steels behave very differently. While S32550 is attacked by crevice corrosion in both tests, S32750 and S32760 only corrode at the higher chlorine level. The very highly alloyed cast material 1.4469 was resistant in both tests. Also the superaustenitic steels behave differently and only S31254 and one N08926 grade resist crevice corrosion at the lowest chlorination level. The results are quite different when considering the joint welds. The superaustenitic steels were never attacked by pitting corrosion but only two of the duplex steels were resistant even at the lowest chlorine concentration. The fact that S31803 was not attacked is due to the crevice attacks, which exist on all six pipes, and which brings cathodic protection to the rest of the pipes. Samples from the same steel grades as those tested in the loops were used for determining CCT (plate) and CPT (butt welded tube) in the laboratory (44, 45). The results do not always show a good relation with the results of the loop tests and this is especially true for the CCT ranking. This emphasizes the importance of qualifying a material by testing real components under realistic conditions.

Repassivation In practice, stainless steels which are used within proven application limits may still be attacked during unforeseen temporary service upsets, e.g. by temperature excursions. Once initiated the attack may continue to propagate even when normal service conditions are re-established. The conditions for repassivation of crevice corrosion, initiated in seawater at 75°C and 300 mV SCE, have been investigated in some detail (46, 47). The stainless steels studied were superduplex S32750 and superaustenitic S31254, both in the form of plate, welded plate and cast material. The repassivation was studied at three temperatures under conditions simulating a chlorinated water. The results are shown in Table 7.

Table 7. Effect of repassivation conditions on maximum repassivation time and maximum crevice corrosion depth (46) Steel grade UNS

Repassivation conditions Potential Temp. mV SCE °C

Product

Results Time days

Attack depth mm

S31254

rolled welded cast

0

40

49 35 36

0.046 0.035 0.042

S32750

rolled welded cast

0

40

59 59 >182

0.010 0.060 2.510

S31254

rolled welded cast

600

40

11 16 17

0.378 1.211 1.242

S32750

rolled welded cast

600

40

25 26 17

0.501 0.598 0.930

S31254

rolled welded cast

0

15

31 68 45

0.006 0.013 0.011

S32750

rolled welded cast

0

15

129 >131 >131

0.023 0.034 0.093

S31254

rolled welded cast

600

15