Pitting and Crevice Corrosion of an Advanced Chromium Based Stainless Steel

Paper No.

444

PITTING CORROSION AND CREVICE CORROSION OF AN ADVANCED CHROMIUM-BASEDSTAINLESS STEEL

M. KohlerKrupp VDM GmbH

P.O. BOX 1820D-58778 Werdohl

ABSTRACT

Alloy 33 is a (wt. %) 33 Cr-32Fe-3 lNi-1 .6Mo-0.6CU-0.4N austenitic stainless steel combininghigh yield strength of min. 380 N/mmz (55 KSI) with high resistance to local corrosion and superiorresistance to stress corrosion cracking. Ranking the material according to its PRE (pitting resistanceequivalent) value, the new alloy fits in between the advanced 6 ‘Yo Mo superaustenitics and the nickel-base Alloy 625 but due to the balanced chemical composition the alloy shows a lot less sensitivity tosegregation in the base material as well as in welded structures. It is recommended to weld the materialwith matching filler. The critical pitting temperature of such joints in the 10 0/0 FeCIJ “ 6HZ0 solution isreduced by only 10“C in comparison to the base material. Corrosion tests in artificial seawater (20 g/1 Cl-) with additions of chloride up to 37 g/1 as well as in a NaC1-CaCl, solution with 62 g/1 Cl- revealed thatthe critical pitting temperature does not differentiate from the 6 % Mo austenitic steel Alloy 926. Withrespect to crevice corrosion the depassivation pH value has been determined in 1 M NaCl solutionaccording to Crolet 9 and again there was no difference between Alloy 33 and Alloy 926.

SCC tests performed on Alloy 33 in the solution annealed condition as well as after heavy coldwork up to RPO,Z= 1100 – 1200 Nhmn2 (160 – 174 KSI) indicate the high resistance to stress corrosioncracking in hot sodium chloride solutions.

Keywords: Alloy 33, UNS R20033, local corrosion resistance, stress corrosion resistance, chloride-bearing solutions, cold work, stress corrosion crack growth rate

Copyright01999 byNACE International.Requestsforpermissionto publishthismanuscriptinanyform,inpartor inwholemustbe made inwritingto NACEInternational,Conferences Division, P.O. Box 218340, Houston,Texas 77218-8340. The material presentedand the views expreaeed in thispaper are solelythose of the author(s) and are not necessarily endorsed by the Association.Printedin the U.S.A.Chih Wee Tan - Invoice INV-219340-F0RLFN, downloaded on 5/28/2009 7:04:14 PM - Single-user license only, copying and networking prohibited.

INTRODUCTION

In the industry there is always a demand for advanced materials, which show superior localcorrosion resistance and easy fabrication characteristics. At the same time both high strength is requiredto allow the design of light constructions and the necessity to prove that the new material in the annealedand cold worked condition exhibits high resistance to stress corrosion cracking in chloride-containingsolutions.

Today for the inner barrier material for disposal of nuclear waste in Yucca Mountains thenickel-base C-type alloys are under consideration. Alloy 33 which has been developed in cooperationwith the Bayer company shows an excellent corrosion resistance in a broad variety of aggressive mediarequired in the chemical industry ‘>2.With respect to its local corrosion behavior and its resistance tostress corrosion cracking a lot of data have been evaluated since the introduction of Alloy 33 in 1995.Therefore this paper summarizes the recent research results to support that Alloy 33 may be consideredas more economical material for this application.

CHARACTERISTICS OF ALLOY 33

Alloy 33 is an austenitic material with a chemical composition given in Table 1 in comparison tostandard stainless steels, advanced 6 ‘Yo Mo superaustenitics as well as the nickel-base Alloy 625.Although chromium stabilizes the ferritic structure in combination of 31 ?40 nickel together with0.4 ‘?40 nitrogen a stable austenitic structure could be achieved. It has to be mentioned that the manganesecontent of the alloy has not been increased above 0.6 0/0 and only the high chromium level together withthe balanced nickel and iron content increased the nitrogen volubility to an extent that a stable austeniticmicrostructure could be formed. To achieve the excellent corrosion resistance to many environmentsrequired in the chemical industry 1.6 0/0 molybdenum and 0.6 0/0 copper have been alloyed. The alloy ismelted as a LC (low carbon) version without any stabilizing element additions. Due to the nitrogenadditions and the requirement of a solution annealing temperature of only 1120”C, the material exhibitsa fine grained microstructure. The grain size of 40 mm thick plate has been determined to be only 65~m. The PRE (pitting resistance equivalent) values, given in Table 1, which have been calculatedaccording to the formula ‘A Cr + 3.3 ‘A Mo + 30 YO N indicate that Alloy 33 will fall with respect to thisranking criterion in its local corrosion resistance between Alloy 926 and Alloy 625.

The mechanical properties for Alloy 33 in comparison to standard stainless steels, thesuperaustenitic and Alloy 625 are given in Table 2. In comparison to the nitrogen alloyed 6 VO Mosuperaustenitic Alloy 926, Alloy 33 exhibits an advantage in strength in the solution annealed conditionby almost 30 YO and comes close to Alloy 625 in the soft annealed condition without any loss inductility. The material can be cold worked up to 2000 N/mmz (290 KSI), which has been shown byH.J.C. Speidel 3. To achieve a strength level of RPO,Z= 1100 to 1200 Nhmn2 (160 to 174 KSI) only 35 ‘%0

of cold deformation is necessary. Stress corrosion crack growth rates have been determined in thiscondition by R. Magdowski et. al. 4 as shown below.

With respect to phase stability the high chromium content of 33 YO and a nitrogen content of0.4 % are not in contradiction to one another. Aging in the temperature interval of 700”C to 900”Ccaused a small loss of impact strength due to precipitation of some o-phase. Nevertheless, even beingsensitized for 8 hours the impact strength at ambient temperature was well above 100 J. Furthermore,sensitization up to 1000 hours in the temperature range of 600”C to 1000”C and testing for 15 cycles of

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48 hours in boiling nitric acid (Huey Test) using the distillation method did not reveal any significantincrease in overall corrosion rate nor any sign of intergranular penetration 5.

The weldability of Alloy 33 is approved for the GTAW process using matching filler, the plasmaprocess as well as for laser welding. The mechanical strength of the matching weldment compares withthe base material. Due to the low molybdenum content in the alloy there is no microsegregation in theweld seam and it has been shown elsewhere that the corrosion resistance of welded samples is verysimilar to the base material 2’5.Figure 1 shows a typical microstructure of a longitudinal welded tube,welded without filler material using the GTAW process.

CORROSION RESISTANCE OF ALLOY 33

Pitting and Crevice Corrosion in Chloride-Bearing Media

The standard test procedure for ranking materials with respect to their local corrosion resistanceis the 10 ‘A FeClj c 6H20 test according to ASTM-G 48 ‘. The testing environment represents achloride-containing solution with 40,000 ppm chloride at a pH of about 1.5. The samples will beexposed for 24 hours in the solution and the testing temperature will be increased by 5 K till the samplesfail by pitting. To evaluate the critical crevice corrosion temperature teflon fissure blocks according toMTI specification 7 are screwed on the sample using a torque of 0.28 Nm. Again after 24 hours thetemperature is increased till crevice is detected at the surface of the sample. The results of these tests as afunction of the PRE values are given in Table 3. A critical pitting temperature of 85°C and a crevicetemperature of 40°C have been determined for Alloy 33. According to this ranking model Alloy 33follows the empirical rule of PRE calculation and its local corrosion resistance is equivalent to6 % Mo superaustenitics. The reduced tendency of microsegregation during welding can be concludedbased on the test results given in Table 4. The critical pitting temperature of a 5 mm plate weldedwithout filler by the PAW (plasma arc) process is reduced by 10“C only in comparison to the basematerial when tested in the 10 0/0 FeCl~ c 6H20 solution. Further tests on longitudinal welded tubes,where it is the standard praxis to weld without filler, confirmed this result whereas for 6 0/0

superaustenitics a reduction of 20”C to 30”C in their critical pitting temperature has to be taken intoaccount.

In Table 5 the results of potentiodynamic polarization curves in artificial seawater(ASTM- 11418, are given. The pitting potential U, did not drop up to 75°C and is well above the redoxpotential of artificial seawater at 95”C. Furthermore, the small hysteresis between the pitting potentialU~ and the repassivation potential URPof 10 mV at 95°C has to be considered.

Additional potentiostatic tests in artificial seawater, in artificial seawater with additions of0.5 M NaCl as well as in a modified NaC1-CaC12 solution with a fiu-ther increased chloride content havebeen performed. These tests allowed the evaluation of the critical pitting and critical crevice temperaturein the range of 20 g/1 up to 62 g/1 of chloride and a direct comparison of Alloy 33 with the 6 0/0 Mosuperaustenitic Alloy 926. The testing conditions and the test results are given in Table 6. No differencewith respect to the determined critical pitting temperature has been observed between Alloy 33 andAlloy 926. As a function of the increasing chloride content the pitting temperature was slightly reducedfrom ~ 85°C to 75°C in the 62 g/1 chloride-containing solution. Nevertheless, in artificial seawater with20 g/1 chloride the crevice temperature of Alloy 33 has been determined to be 10“C higher than forAlloy 926.


To evaluate the depassivation pH in a NaCl solution the procedure after Crolet 9 was applied.A crevice flee electrode in a N, deaerated acidic NaCl containing electrolyte was polarizedpotentiodynamically. As a fimction of the adjusted pH, the current density for passivation was measured.Finally the depassivation pH was evaluated by extrapolation of ~w, =f(pH) to the passivation currentdensity of iP,,, = 10 @/cm2. The formation of hydrogen was suppressed by adding 1.1 “ 104 M KSCN.A schematic sketch of the testing procedure is given in Figure 2. The background for this procedure isbased on the model for crevice corrosion horn Oldfield ‘0 according to which at a critical pH value thepassivity in the crevice is lost. The results of the depassivation pH in a 1 M NaCl solution as a filnctionof the PRE is given in Figure 3 for Alloy 33 and Alloy 926 in comparison to different CrNiMo standardaustenitic alloys, According to these results standard stainless steels will loose the passivity at a pH 2.8to 2.2 in a 1 M NaCl solution, whereas Alloy 33 and Alloy 926 will depassivate at the higher acidity ofpH = 0.8. Again the results show that there is no difference in the crevice corrosion resistance betweenboth Alloy 926 and Alloy 33 but a large improvement in comparison to standard stainless steels.

Stress Corrosion Resistance in Chloride-Bearing Media

The resistance to stress corrosion cracking behavior of Alloy 33 has been tested in addition to theenvironment of CaC12-solutions “z now in saturated MgClz at 50”C. First, 2 mm sheet samples ofsolution annealed material as well as after applying 5 0/0 and 20 0/0 cold work have been bent to aU-shaped sample and spots of a saturated MgClz-solution have been put on the samples in the tensionzone. At 50”C and 35 YO humidity of the air the samples have been exposed for up to 2,500 hours (Table7) and none of the samples cracked due to SCC attack. There was some discoloring after 500 hours andsome pitting on the surface of the samples afler 1,000 hours but nothing more. On 10 mm bar samplesadditional SCC tests have been performed under constant load conditions again with spots of saturatedMgCl,-solution at 50”C on the surface of the sample. The applied load was 75 % to 80 % of the 0.2 yieldstrength of the annealed and cold worked material. Table 8 gives the results for testing times up to 2,000hours. So fw none of the samples cracked due to SCC attack. The metallographic evaluation of thesamples revealed that only some pitting up to a depth of 130 pm could be identified.

Furthermore, the stress corrosion cracking susceptibility of 32 ‘%0 cold rolled samples wasmeasured by fracture mechanics 4. Fatigue precracked double cantilever beam specimens (DCB) were

prestressed to a defined stress intensity of 50 to 70 MPa fi and then immersed in the environment ofpure hot water as well as in a 22 % NaCLsolution. After the testing time of up to 5,000 hours thesamples have been broken to measure the stress corrosion crack growth rate. Figure 4 shows the resultsin comparison to another high strength austenitic material. So far up to 288°C in pure hot water and up

to 105°C in 22 YO NaCl Alloy 33 in the cold worked condition of RP0,2s 1100 – 1200 N/mrn2 did notshow any sensitivity to stress corrosion crack growth.

DISCUSSION

Alloy 33 is a new austenitic material which combines high strength, toughness and resistance tolocal as well as to stress corrosion cracking. Based on the results of standard laboratory test procedures itcould be shown that Alloy 33, although the molybdenum content is relatively low, fits with respect to itslocal corrosion resistance between the superaustenitics and the nickel-base Alloy 625 as regardsstrength, the material exhibits an advantage of about 30 0/0 over the superaustenitics. The corrosionresistance of welded structures is almost similar to that of the base material. An important feature is thefact that the alloy can be cold worked up to 2000 N/mmz. After cold working to a strength level of RP0,2


x 900 N/mm’ the tested samples indicated resistance to SCC under constant load conditions in saturated

MgC12-solution at 50°C and at a strength level of RPO,Z-1100-1200 N/rnm2 in hot sodium chloridesolutions the stress corrosion crack growth rate was almost nil. All these data support that Alloy 33provides a cost efficient alternative over the high alloyed nickel-base alloys. On the other hand, althoughAlloy 33 is a relatively new alloy, which is approved for pressure vessel application according to theGerman VdT~ data sheet 516 and for welded and unwelded constructions according to ASME Sect.VIII, Div. 1 it is believed that this advanced chromium-based material will respond to many challengesarising in the future, if high corrosion resistance and economical design are required in the industry.

CONCLUSIONS

1. Alloy 33 is a high strength material, alloyed with (wt. Yo) 33 Cr, 32 Fe, 31 Ni, 1.6 Mo, 0.6 Cu and0.4 N, which offers excellent corrosion resistance to chloride-bearing media.

2. Alloy 33 exhibits a minimum room temperature yield strength of 380 N/mm’ (55 KSI) incombination with excellent ductility, toughness and high thermal stability.

3. Alloy 33 can be cold worked up to 2000 N/rnm2 (290 KSI),

4. SCC corrosion tests performed on 20 YO cold worked material under constant load conditions(80 % R,O,,) did not indicate any sensitivity to stress corrosion cracking in saturated MgCl,-solution at50°c.

5. The stress corrosion crack growth rate evaluated on 32 YO cold worked material (RPO,Z= 1100 Nhmn2-1200 Nhnrn2) revealed no sensitivity to SCC in 22 YO NaCl at 105°C after 5,000 hours.

6. With respect to cost ratio, corrosion resistance and mechanical strength, Alloy 33 features a uniquecombination between the austenitic stainless steels and nickel-base alloys.

ACKNOWLEDGMENT

Some of the electrochemical corrosion tests have been conducted at the IKS Institut fiirKorrosionschutz Dresden GmbH and some of the stress corrosion tests have been performed at theETH-Swiss Federal Institute of Technology Zurich as well as at the BAM. The author would like toacknowledge the assistance and valuable advises of Dr. C. Voigt and Dr. G. Riedel (IKS) as well asProf. Dr. M.O. Speidel, Prof. P.J. Uggowitzer and Dr. Magdowski (ETH) and Dr. Mietz (BAM).


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REFERENCES

M. Kohler, U. Heubner, K.-W. Eichenhofer, M. Renner: “Alloy 33- A New Corrosion ResistantAustenitic Material for the Refinery Industry and Related Applications”,CORROSION /95, Paper NQ 338, NACE International, Houston Texas, 1995

M. Kohler, U. Heubner, K.-W. Eichenhofer, M. Renner: “Progress With Alloy 33 (UNS R20033),A New Corrosion Resistant Chromium-Based Austenitic Material”,CORROSION /96, Paper NQ. 428, NACE International, Houston Texas, 1996

H.J.C. Speidel, P.J. Uggowitzer, P. Ernst, M.O. Speidel: “Properties of Cold Worked High NitrogenChromium-Based Alloys”, HNS Espoo / Stockholm 24./25.08. 1998

R, Magdowski, M.O. Speidel, International Conference Fonterraud IV, SFEN, Sept. 1998

M. Kohler, U. Heubner, K.-W. Eichenhofer, M. Renner: “Alloy 33, A New Nitrogen-AlloyedChromium-Based Material For Many Corrosive Environments”,Proc. Int. Conf. Stainless Steels ’96, Verlag Stahleisen, Diisseldorf, 1996, 178-181

ASTM-G 48: “Standard Test Methods for Pitting and Crevice Corrosion Resistance of StainlessSteels and Relevated Alloys by the Use of Ferric Chloride Solution”Annual Book of ASTM Standards, Vol. 03.02, Philadelphia, PA, 1995

R.S. Treseder, Guideline information on newer wrought iron and nickel-base corrosion resistantalloys, MTI Manual No. 3, Appendix B, Materials Technology Institute of the Chemical ProcessIndustry, Columbus (Ohio), USA, 1980

ASTM-D 1141-90: “Specification for Substitute Ocean Water”,Annual Book of ASTM Standards, Vol. 11.02, Philadelphia, PA, 1995

J.L. Crolet, J.M. Defanoux, L. Seraphin, R. Tricot: “M&m. scientif, de la revue de metallurgic”,paris 71 (1974) 797,72 (1975) 937

J.W. Oldfield, W.H. Sutton: Brit. Corr. Journal 13 (1978) P. 104, 15 (980) P. 31


TABLE 1

NOMINAL CHEMICAL COMPOSITION AND PITTING RESISTANCEEQUIVALENT (PRE) OF i4LL0Y33 AND VARIOUS OTHER ALLOYS

Alloy Ni Cr Mo Fe Others PRE

316 Ti 11 17 2.2 Bal. 0.4 Ti 24

904 L 25 21 4.8 Bal. 1,5 Cu 37

926 25 21 6.5 450.9 Cu 470.2 N

33 31 33 1.6 320.6 Cu0.4 N

50

625 62 22 9 4 3.4Nb 51

Numbers indicate wt. %, PRE is given by Cr + 3.3 Mo + 30 N

TABLE 2

MIN. MECHANICAL PROPERTIES OF xLLMIY33 AND VARIOUS OTHER ALLOYSAT ROOM TEMPERATURE ACCORDING TO VDT~ DATASHEETS

Alloy R RPLO % ~

N/:&2 N/mrn2 N/mmz

316Ti* 210 245 500 35

904 L 220 250 520 40

926 300 340 600 40

33 380 420 720 40

625 380 --- 760 35

* According to DIN EN 10088


TABLE 3

CRITICAL PITTING / CREVICE TEMPERATURE (CPT / CCT) OFWHEN TESTED ACCORDING TO ASTM-G 48

VARIOUS ALLOYS6

Alloy CPT CCT PREOc ‘c316 Ti 15 <() 24

904 L 45 25 37

926 70 40 47

33 85 40 50

625 77.5 57.5 51

PRE=%Cr+ 3.3(% Mo)+30(%N)

TABLE 4

CRITICAL PITTING TEMPEIL4TURE (CPT) OF ALLOY 33WHEN TESTED ACCORDING TO ASTM-G 48 bFOR 24 HOURS

Condition CPT in ‘C

5 mm plate 85solution annealed

PAW wekhnent 75of 5 mm plate without filler as welded


TABLE 5

PITTING POTENTIALS OF AhXIY 33 WHEN TESTED IN ARTIFICIAL SEAWATER(ASTM-D 11418, POTENTIODYNAMICALLY

Temp. Pitting Repassivation Free Corrosion RedoxOc Potential Potential Potential Potential

U,, mV U.O, mV U., mV mV

50 ~ 959 --- -141 209

75 ~ 959 -.- -91 159

95 359 259 -41 109

Potential in mV versus saturated Kalomelelectrode

TABLE 6

TEST SOLUTIONS USED TO DETERMINE THE PITTING BEHAVIOR lNPOTENTIOSTATIC TESTS

Solution Chloride Content

Artificial seawater 20 gll 0.55 Maccording to DIN 50905-4Artificial seawater 37 #l 1.05 M+ 0.5 M NaCl

50 g/1 NaCl + 50 g/1 CaC12 62 gll 1.75 M

RESULTS OF PITTING AND CREVICE CORROSION TESTS PERFORMED IN MODIFIEDARTIFICIAL SEAWATER BY POTENTIOSTATIC MEANS AT U = 0.3 V (SCE)

Chloride Ccmtent M1Oy926 Alloy 33

20 gflCPT > 85°C CPT > 85°CCCT = 45°C CCT = 55°C

37 g/1 CPT = 85°C CPT = 85°C

62 g/1 I CPT = 75°C I CPT = 75°C


TABLE 7

RESULTS OF STRESS CORROSION CRACKING TESTS AT U-BEND SPECIMENS WITH SALTSPOTS IN THE TENSION ZONE,

MgCl, SATURATED AT 50°C AND 35 % REL. HUMIDITY OF THE AIR

Condition of % % & Testkg Time

Sample N/mm’ N/mm’ % h

Annealed 441 825 55 2500

5 YO cold worked 657 916 45 2500

20 YO cold worked 930 1085 23 2500

No SCC determined

TABLE 8

RESULTS OF STRESS CORROSION CRACKING TESTS WITH SATURATED SPOTS OF MgCl,AT 50”C AND 35 % REL. HUMIDITY OF THE AIR UNDER CONSTANT LOAD CONDITIONS

Condition of Sample

Annealed

5 % cold worked

20 YO cold worked

RPO.2 Load Load Testing TimeN/mmz N/mmz Yo of Rma h

421215 74.8 2000313 74.3 1000

676 567 83.8 2000550 81.4 1000

936 757 80.9 2000759 81.1 1000

No SCC determined


FIGURE 1: Microstructure of a weld seam of a GTAW longitudinal welded tube0 21.3x 1.6 mm, welded without filler material

Pafmivatim current Ck3nsihf

:. 1“’”‘-”z20-

. .... ....

i 2.3\~

.,

!.. ...... .... ,,.........,.

2:4 & ‘+ j)H

-700

-20 1

FIGURE 2:

-600 ~

Y

-50 -400 “300P~ dep Potential in mV@KE)

Schematic polarization curves to determine the depassivation – pH – valueaccording to Crolet 9


w

PHdep

1

2

3

IM NaCI + XHZSOA+ 1.1 R104M KSCN

/d“ t: CrNi 18-10

/ 4*!: 2: C%NiMO18-12-21

$3: CrNi 26w6

* xaqpn

4: CrNMO 17uI3=55: CrNiMo 23-&2

/

FIGURE 3: Depassivation – pH – values of various alloys in acidic 1 M NaC1-solution withadditions of KSCN as a function of PRE values


M@eratwi?, T, ~C]300 200 1O(I 60 4(I 20 (j -20~1”-”””d

$trm corrosion cracking of high strengthaustenitic material([email protected] 100 lWa ]

U ❑ WaiaiflgringstwdP 000, cold expamkd~ Cl@alloy X+,32%cold rolled

FIGURE 4:

1,6 2.0 2.4 2.8 3.2 3.6 4.0reciprocal temperature, 1(I’, [l/K] MI(P

Stress corrosion crack growth rate of cold worked austenitic alloys(R,,,, = 1100-1200 N/mmz) in chloride-bearing media according toR. Magdowski et al 4.


Documents

Pitting and Crevice Corrosion of an Advanced Chromium Based Stainless Steel