Corrosion Properties of Enhanced Duplex Steel UNS S32304

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1 A CORROSION MANAGEMENT AND APPLICATIONS ENGINEERING MAGAZINE FROM OUTOKUMPU Corrosion Properties of Enhanced Duplex Steel UNS S3234 2/215

2 2/215 2 Corrosion Properties of Enhanced Duplex Steel UNS S3234 Lena Wegrelius and Sukanya Hägg Mameng Outokumpu Stainless AB, Avesta, Sweden Abstract The enhanced duplex stainless steel S3234 is a recently developed duplex steel grade with enhanced properties; increased strength and improved corrosion resistance. The improvement has been made possible by a combination of modern production technology and a change of the chemical composition within the chemical range of the UNS S3234 standard. The enhanced duplex steel grade S3234 has higher contents of chromium, molybdenum and nitrogen than the traditional S3234. The PREN (Pitting Resistance Equivalent) value is guaranteed to be 28, as compared to only 26 for the standard S3234 grade. The production process has been verified according to the NORSOK M-65 framework. This paper describes the results of corrosion properties in the terms of localized corrosion testing according to ASTM G48 (in FeCl 3 ) and G15 (in NaCl) and stress corrosion cracking testing according to G36 (in MgCl 2 ) and G123 (in NaCl). Uniform corrosion testing in different media is illustrated with the critical temperatures, evaluated according to the MTI-1 (G157) method. The enhanced duplex steel S3234 is an excellent choice in many applications where S3225 is over-specified in regards to corrosion resistance. The improved properties make the enhanced S3234 well suited for optimal designs with respect to strength, reduced maintenance, durability and long-term service. Potential applications for this grade are within structural design, e.g. in the oil and gas industry. Key words: Duplex stainless steel, lean duplex, corrosion resistance, welding Introduction The enhanced duplex stainless steel UNS (1) S3234, is a recent development of the traditional UNS S3234 grade for the offshore industry s needs. The industry needed a competitive alternative that would have higher corrosion resistance than S3163 (316L) and would be stronger than the traditional S3234 grade. The improvement has been made possible by a combination of modern production technology and a change of the chemical composition, still within the chemical range of the UNS S3234 standard. The enhanced duplex steel grade S3234 has higher contents of chromium, molybdenum and nitrogen than the traditional S3234. The PREN (Pitting Resistance Equivalent Number) is guaranteed to be 28, as compared to only 26 for the traditional S3234 grade. Furthermore, proof strength has been increased from 4 to 5 MPa for thicknesses up to and including 13 mm. The production process has been verified according to the NORSOK M-65 framework [1]. The enhanced duplex steel S3234 is an excellent choice in many applications where S3225 is over-specified in regards to corrosion resistance. The improved properties make the enhanced S3234 well suited for optimal design with respect to strength, reduced maintenance, durability and long-term service. Potential applications for this grade are within structural design, e.g. offshore topside structural components in the oil and gas industry. Figure 1 illustrates the positioning of the enhanced S3234 grade in relation to other duplex and austenitic stainless steel grades. The objective of this work was to investigate and document the corrosion properties of the enhanced duplex grade S3234. Some welded material was included to illustrate the influence of microstructure and structural stability on the corrosion resistance. The result is compared with data from traditional S3234 grade and the standard austenitic grade S3163. Strength MPa, Min values, hot rolled coil (ASTM A 24) S3275 Enhanced S3234 S82441 S3211 S3225 S34565 S3234 S31254 S31726 N894 S3173 S343 S Critical pitting temperature ( C) ASTM G 15 Figure 1 Positioning of the enhanced S3234 grade. (1) Unified Numbering System for Metals and Alloys (UNS).

3 2/215 3 Experimental Procedure Material The chemical composition of the stainless steel grades included in this study is given in Table 1. Because a number of different heats were used for the various tests, the typical compositions are given. The austenitic stainless steel grade UNS S3163 is includes as reference. Also included in Table 1 is the Pitting Resistance Equivalent Number (PREN). PREN is in this study defined as PREN = %Cr + 3.3(%Mo) + 16(%N). Corrosion testing Several corrosion tests were performed to verify the improved corrosion properties of the enhanced grade S3234. Most of the tests were performed on 6 8 mm thick un-welded material but also some tests were performed on 2 mm thick welded material. Pitting and crevice corrosion testing The pitting resistance was evaluated using either ASTM (2) G 48 [2] method E or ASTM G15 [3] and the crevice corrosion resistance was evaluated using ASTM G48 method F. The G48 methods involve immersion of the test specimens in 6% FeCl 3 + 1% HCl for 24 hours at different temperatures with an increment of 5 C for evaluation of critical pitting temperature (CPT) and critical crevice temperature (CCT). The specimens were cut to approximate dimensions of 5 x 25 mm. The material investigated was dry ground to 12 grit finish including the cut edges. New specimens and test solutions were used for each test temperature. The electrochemical ASTM G15 determines the critical pitting temperature in 1M NaCl solution by a potentiostatic technique which uses a temperature scan and a flushed port cell (the Avesta Cell [4]) that eliminates the occurrence of crevice corrosion. All specimens, except those with weld, were wet ground to 32 grit at least 18 hours prior corrosion testing. Pitting potential (E pit ) measurements were performed in 1 M sodium chloride solution at different temperatures ranging from 2 ± 1 C up to 5 ± 1 C. Specimens of size 3 x 3 x t mm were used and were wet ground to 32 grit at least 18 hours before testing, then immediately before the experiment cleaned with ethanol. The flush port cell was used in all the electrochemical experiments. Polarization measurements were made using a Solartron 1287 potentiostat. The potential of the working electrode was measured versus a saturated calomel (SCE) reference electrode. The auxiliary electrode was platinum (Pt) wire. Dissolved oxygen was maintained at a low level by bubbling nitrogen through the sodium chloride solution during the whole test time. Before each polarization measurement, the open circuit potential (E OCP ) was measured for 1 minutes in the test solution. The polarisation started at -3 mv SCE and the scan rate was 2 mv min -1. E pit was defined at the point where the current exceeds 1 μa cm -2 and remains above this level for at least 1 minute. Duplicate samples were tested and gave in most cases a difference of <1 mv SCE. If larger differences were observed, additional measurements were performed. Uniform corrosion testing The uniform corrosion tests were carried out according to MTI (3) -1 test procedure [5] (ASTM G 157 [6]). This involves determination of the critical temperature, at which the corrosion rate exceeds.127 mm/year. Specimens of each steel grade were cut to the dimension 5 x 2 x t mm. The surfaces were dry ground to 12 grit finish, degreased in acetone. Before testing the specimens were allowed to passivate in air for at least 18 hours. 6 ml solution was either cooled or heated to a suitable start temperature and duplicate specimens were thereafter immersed in the test solution. Nitrogen was used to deaerate the solutions during the 96 hours test period. If one or both of the test specimens had a corrosion rate higher than.127 mm/year the temperature was reduced by 5 C during the next test period, otherwise the temperature was increased 5 C, until the critical temperature was reached and evaluated. Stress corrosion cracking testing Three different stress corrosion cracking test methods were performed; ASTM G 36 [7] (MgCl 2 ), ASTM G123 [8] (NaCl) and in concentrated CaCl 2 solution. U-bend specimens were prepared according to ASTM G3 [9]. Specimens with dimension 127 x 13 mm were cut from sheet parallel to the rolling direction. All specimens were dry ground to 12 grit. The specimens were stressed using a two-stage method around a mandrel with 25 mm diameter, and secured with nuts and bolts. Before the final stressing stage the specimens were degreased in acetone. The time between the two stressing stages, and between the final stressing stage and the start of the test, was kept as short as possible. In the final stage the specimens were adjusted to a correct U-shape with parallel legs. To maintain the load tightening bolts made of titanium were used together with Teflon washers for insulation. In the first test the U-bend specimens were exposed to 45% boiling magnesium chloride solution for 24 hours and the temperature was maintained at 155 C. Steel grade C N Cr Ni Mo PREN UNS S3234 traditional UNS S3234 enhanced UNS S Table 1 Typical composition of the tested steel grades [wt%]. (2) ASTM International, 1 Barr Harbor drive, PO Box C7, west Conshohocken, PA , USA. Trade name. (3) Materials Technology Institute of the Chemical Process Industries, Inc., 1215 Fern Ridge parkway, Suite 116, St. Louis, MO , USA.

4 2/215 4 In the second test the U-bend specimens were immersed in 25% boiling sodium chloride solution acidified to ph 1.5 with phosphoric acid. The boiling point of the solution is expected to be C. The exposure time was 1 hours (~ 6 weeks) and for each one-week period fresh solution was used. Finally, in the third test solution with 4% calcium chloride was prepared and the ph adjusted to 6 using a slurry of calcium oxide. The U-bend specimens were placed in the solution and the solution was heated to 1 C. Throughout the test period, the temperature was maintained at 1 C and the ph measured every second day and adjusted to 6 if necessary. The time to cracking was observed and if cracking occurred before the end of the test period of 5 hours the test was terminated. After exposure the specimens were evaluated by visual inspection and by optical light microscopy. Welding and microstructure To investigate the corrosion properties after welding 2 mm enhanced S3234 was mechanized TIG (4) welded using filler metal EN ISO: N and thereafter tested according to ASTM G15, critical pitting temperature in 1M NaCl. Two shielding gas types were used in the test; pure argon and argon with 2% nitrogen. The CPT ( C) Figure 2 Critical pitting temperatures according to ASTM G 48 E and ASTM G 15 respectively. Critical crevice temperature according to ASTM G 48 F. E pit, V (SCE) ,4 1,2 1,8,6,4,2 S3163 S3163 S3234 S3234 enhanced G48 E G48 F G15 S3234 S3234 enhanced backing gas was either pure argon or Formier 1 (9% N 2 + 1% H 2 ). All welded specimens were sand blasted and pickled in mix acid of 3.1M nitric acid and 2.8M hydrofluoric acid at 6 C for 5 minutes prior corrosion testing. To investigate the microstructure of the weld and the heat affected zone, cross sections of the welded specimens were ground and polished to 3μm finish prior to the color etching in modified Beraha (5) etchant at room temperature. Results & Discussion Corrosion properties of base material Pitting and crevice corrosion The results from ferric chloride immersion tests of the base material evaluated according to ASTM G48 method E and F and the electrochemical testing in sodium chloride according to ASTM G15 are illustrated in Figure 2. For the enhanced grade S3234, the ASTM G48 E and F test methods were repeated six and three times respectively. For the other grades, typical values are given. As can be seen in the figure, the enhanced S3234 grade performs significantly better than the traditional S3234 in all three tests. In fact, the enhanced S3234 grade was the only grade were it was possible to get a positive CCT value. For the other two grades, traditional S3234 and S3163, the CCT values were not possible to measure without cooling the solution below zero. This is also in agreement with the fact that the enhanced grade S3234 has a higher alloying content of chromium, molybdenum and nitrogen than the traditional grade S3234. The result from the pitting potential measurements is illustrated in Figure 3. At all investigated temperatures, the enhanced S3234 gave the highest break through potential in comparison to the other two investigated materials. In the tests performed at 2 and 3 C the enhanced S3234 was passive, without pitting, up to the transpassive region while the traditional S3234 and S3163 suffered from pitting corrosion and experienced pitting potentials at all test temperatures. Uniform corrosion The evaluated critical temperatures in the uniform corrosion test are presented in Table 2. The enhanced duplex stainless steel grade S3234 shows good result; equal or slightly better than the traditional S3234 in the tested solutions (the exception is WPA (6) -1, where the result for the traditional grade was somewhat better than for the enhanced S3234) Temperature ( C) Figure 3 Break through potentials versus temperature. (4) Tungsten Inert-Gas arc welding (TIG). ( ) Trade name. (5) 5 ml HCl + 1 ml H 2 O, 1 g/1 ml K 2 S 2 O 5.

5 2/215 5 Stress corrosion cracking In Table 3 the result from the stress corrosion cracking testing is summarized. All investigated steel grades failed in the most aggressive test, boiling magnesium chloride solution. In the two other tests, both the traditional and the enhanced S3234 performed well with no signs of cracking while the standard austenitic steel grade S3163 failed due to stress corrosion cracking. Properties of welded material The critical pitting temperature according to ASTM G15 was determined on welded and pickled specimens of enhanced grade S3234. For comparison, the CPT was determined for the base material of the same heat as the welded specimens. The result is illustrated in Figure 4. By using argon based shielding gas, the CPT was decreased significantly most probably due to nitrogen depletion. The microstructure of the TIG welded specimens shows somewhat higher austenite content when nitrogen was present in the shielding and backing gas compared to pure argon, as shown in Figure 5 a and b. This is also numerically shown in Table 4 which exemplifies the beneficial effect of using nitrogen based shielding and backing gas when TIG welding duplex stainless steels, especially higher nitrogen alloyed grades like the enhanced S3234 grade. The heat affected zone, HAZ, is relatively narrow and shows a level of ferrite close to that of the weld metal and even lower than the weld metal for argon based gas. Critical pitting temperature (CPT, C) a) b) S3234 enhanced Base material Argon Nitrogen Figure 4 Critical pitting temperatures according to ASTM G 15 on welded specimens of enhanced S3234. Figure 5 a) Argon based gas protection. b) Nitrogen based gas protection Test solution HCl H 2 SO 4 H 2 SO 4 H 3 PO 4 WPA 1 (7) HNO 3 NaOH HCOOH Conc. %wt S3234 traditional S3234 enhanced 75p (8) p Table 2 Critical temperatures in different solutions according to the MTI-1 procedure. ASTM G 36, 45% MgCl 2 ASTM G123, 25% NaCl, ph 1.5 4% CaCl 2, ph 6 Test method U-bend U-bend U-bend Tested Failed Tested Failed Tested Failed S3234 traditional S3234 enhanced S Table 3 Test result for the stress corrosion cracking (SCC) testing. ID Shielding gas Backing gas Rm, MPa Fracture location Bend test, 4xt, 18 Ferrite content G15, CPT, C WM (9) HAZ (1) BM (11) Ar Ar Ar 742 ±12 WM 4 OK 71 ±4 68 ±3 45 ±2 32 ±3.4 N 2 Ar + 2%N 2 9% N 2 + 1% H ±11 WM 4 OK 62 ±4 67 ±5 46 ±3 41 ±1.3 Table 4 Properties of enhanced S3234 welded with different shielding and backing gas. (6) Wet Phosphoric Acid (WPA) (7) WPA-1 = 75% H 3 PO 4,,2% Cl -,,5% F -, 4% H 2 SO 4,,3% Fe 2 O 3,,2% Al 2 O 3,,1% SiO 2,,2% CaO,,7% MgO (8) p = pitting corrosion. (9) WM = weld metal. (1) HAZ = heat affected zone. (11) BM = base metal.

6 2/215 6 Conclusions In this study the corrosion properties of enhanced duplex grade S3234 have been investigated by different corrosion test methods in order to evaluate the performance under different process conditions and environments. The result from these tests shows that: The increased alloying content of chromium, molybdenum and nitrogen in the enhanced duplex grade S3234 has a significant beneficial effect on the pitting and crevice corrosion resistance compared to the traditional S3234 grade. The resistance to uniform corrosion is equal or slightly better for the enhanced S3234 than for the traditional S3234. The resistance to stress corrosion cracking is equal for the enhanced S3234 and the traditional S3234. Both the enhanced and the traditional S3234 grades have significantly higher resistance to stress corrosion cracking (SCC) than the austenitic grade S3163. It is beneficial to use nitrogen based shielding and backing gas when TIG welding nitrogen alloyed duplex grades like the enhanced S3234. The enhanced duplex grade S3234 is an excellent choice in many applications where there is a need of higher corrosion resistance and higher mechanical strength than what the traditional S3234 and the standard austenitic grade S3163 can offer. Acknowledgements References [1] NORSOK STANDARD M-65:211, Qualification of manufacturers of special materials. [2] ASTM G48 Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution. [3] ASTM G15 Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels. [4] R. Qvarfort, New Electrochemical Cell for Pitting Corrosion Testing, Corrosion Science, Vol 28, No 2, 1988, pp [5] MTI-1 Corrosion resistance in Selected Media, MTI Publication No. 46, (1995). [6] ASTM G157 Standard Guide for Evaluating Corrosion Properties of Wrought Iron- and Nickel-Based Corrosion Resistant Alloys for chemical Process Industries. [7] ASTM G36 Standard Practice for Evaluating Stress- Corrosion-Cracking Resistance of Metals and Alloys in a Boiling Magnesium Chloride Solution. [8] ASTM G123 Standard test Method for Evaluating Stress- Corrosion Cracking of Stainless Alloys with Different Nickel Content in Boiling Acidified Sodium Chloride Solution. 9] ASTM G3 Standard Practice for Making and Using U-Bend Stress-Corrosion Test Specimens. Jan Björk, Hans-Erik Peth, Marie Almén and Mikael Schönning are greatly acknowledged for carrying out the corrosion experiments. Reproduced with permission from NACE International, Houston, TX. All rights reserved. Paper C presented at CORROSION/215, Dallas, TX. NACE International 215.

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