SUMMARY. 1 Issued as Technical Report No.: TR3804/APP95005(Part-II) in March 2002.

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1 SLOW STRAIN RATE TESTING TO DETERMINE HYDROGEN EMBRRITTLEMENT OF MARTENSITIC STAINLESS STEELS USED IN LINE-C DURING CATHODIC PROTECTION 1 TASK 1, PART-II APP Ismail Andijani and Anees U. Malik Research & Development Center Saline Water Conversion Corporation P.O.Box 8328, Al-Jubail , Saudi Arabia Tel: , Fax: rdc@swcc.gov.sa SUMMARY The Task No. 1, (Part-II) of the recently concluded project The effect of changing from MSF to RO water on the internal corrosion of Line-C (Line 3) was concerned with the Slow Strain Rate Testing (SSRT). The objective of this task was to determine the effect of cathodic protection (CP) applied to pumps and/or valves made of ferritic or martensitic stainless steels. As a result of C.P., there is a possibility of hydrogen embrittlement of the material due to hydrogen ingress. Owing to nonavailability of SSRT during the execution of Line-C project, this task could not be carried out and was initiated only when the SSRT machine (manufactured by M/s Cortest, U.K.) was acquired by R&D Center. In SSRT, a tensile specimen is subjected to a steadily increasing stress in a given environment. This procedure results in rupture of surface films and thus tends to eliminate initiation time required for surface crack to form. The test is continued till the specimen fails, the ductility and strength parameters coupled with surface morphology of the fracture provide information about the mode of failure. Martensitic stainless steels, 410, 420 and 431 in environments of air or salt solution under controlled condition or charged conditions were subjected to 1 Issued as Technical Report No.: TR3804/APP95005(Part-II) in March ١

2 SSRT at room temperature. It has been found that SSRT in air and charged salt solution resulted in significant drop in % elongation of all stainless steels. The loss in % elongation was higher in charged salt solution than in air. SSRT studies on control specimens in air indicate that all the martensitic steels undergo ductile fracture by elastic-plastic deformation. Precharged (-1.1V for 2 weeks) steel specimens subjected to SSRT indicate failure by brittle fracture without necking and cup and cone. Under charged conditions, the fracture surface show features resembling those of hydrogen embrittlement indicating thereby the possibility of hydrogen damage during C.P. 1. INTRODUCTION Recently concluded project entitled The effect of changing from MSF to RO water on the internal corrosion of Line-C (Line 3) had contained a task (Task No. 1, Part-II, Project No. APP 95005), namely Slow Strain Rate Testing (SSRT). The SSRT is carried out to determine the effect of hydrogen embrittlement susceptibility of the materials of valves and pumps which are cathodically protected. The hydrogen ingress into the material is one of the risk, associated with the cathodic protection particularly when they are constructed out of ferritic and martensitic stainless steels. Owing to non-availability of SSRT machine at the Corrosion Department, the task pertaining to SSRT could not be carried out during the execution of Line-C project program. The SSRT machine manufactured by M/s Cortest, U K had been acquired later on and the aforementioned task was carried out. In SSRT, a tensile specimen is subjected to a steadily increasing stress in a given environment. This procedure results in rupture of surface films and thus tends to eliminate initiation time required for surface crack to form. In this test the extension rate which is normally in the range of 10-6 to 10-8 inch sec -1, is held constant and is low enough to allow corrosion and adsorption processes to interact with material freshly exposed to the environment through plastic deformation. The test is continued till the specimen fails, the fracture and ductility obtained in the test are compared with the specimen fractured in an inert environment to assess the mode of fracture. 1.1 Literature Review ٢

3 The SSRT technique was developed initially as a laboratory tool for rapid sorting of material-environment combination displaying sensitivity to stress corrosion cracking (SCC) [1]. Its most attractive benefits over static loading, SCC tests are the relatively short period of time within which results can be obtained and the definite end when specimens have failed by hydrogen embrittlement, environmentally assisted cracking or by ductile fracture. SSRT is used widely and is an established technique in the nuclear power industry, in oil and gas production and in chemical process industry as a useful method of material evaluation and quality assurance [2]. The research activities relating to standardization of methodologies have been reported recently [3]. Potentiostatic SSRT were performed to investigate the susceptibility of intergranular stress corrosion cracking (IGSCC) of type 304LSS in 288 o C water, focusing on corrosion potential and the degree of sensitization [4]. The results of the tests indicate that SSRT fracture mode changed from ductile-transgranular stress corrosion cracking (TGSCC) to intergranular stress corrosion cracking (IGSCC), with increasing corrosion potential, water conductivity and the electrochemical potentiokinetic reactivation (EPR) value. The IGSCC onset occurred at 375 mv vs standard hydrogen electrode (SHE) at corrosion potential for type 304L SS with the EPR value >3%, but it did not occur when the EPR value is < 1%. Even though, the microstructure exhibited a step structure, slight Cr depletion occurred and the precipitation of C r23 C 6 was observed at grain boundaries after sensitization heat treatment. After steels, aluminum alloys have been tested most widely by this technique [5-9], it appears to be a promising SCC testing method for these alloys. The SSRT technique was used to investigate the SCC behavior of aluminum alloys Al 7050 (UVS A97050) in different tempers in various electrolytes at the free corrosion potential [5]. Smooth tensile specimens were strained dynamically in the short transverse direction under permanent immersion conditions. Strain rates were from 5 x 10-8 inch/s. Using substitute ocean water, Al 7050 was found highly sensitive to environmentally assisted cracking in the peak aged temper T651 and the overaged temper T7351, respectively. SSRT performed in the mixed salt aqueous solution containing Na 2 SO 4, NaCl, Na NO 3 and Na HCO 3 at ph 3.5 indicated SC susceptibility for Al 7050-T651. Scatter was observed in the fracture energy data of Al 7050-T7351 specimens dynamically strained ٣

4 in the mixed salt solution. Deterioration was attributed to pitting attack, as supported by fractography. In the present studies related to Task No.1, Part-II, laboratory SSRT were carried out in synthetic environment relevant to service conditions. The hydrogen embrittlement effect was identified by the reduction in strength or ductility of the test material as determined by plots of load versus strain for control and charge samples. The embrittle effect was also visualized by the examination of fracture topography in the scanning electron microscope. The results of the studies provide information whether cathodic protection (CP) could be applied to AISI 410, 420 and 431 stainless steels specimens without embrittlement. 2. OBJECTIVE To determine the effect of hydrogen ingress on the ductility and strength of control and charged samples representing components in water transmission system, in particular valves, which are made of martensitic stainless steels. 3. EXPERIMENTAL METHODOLOGY 3.1 Test Solution RO Water, ULW (upper limit water) of the following composition (in ppm) was used during SSR testings. Sodium Chloride Sulphate ph Materials Martensitic chromium steels AISI 410, 420 and 431 were used during the experiments. The chemical composition of the alloy is given in Table 1. ٤

5 3.3 Technique SSRT SSRT were carried out with the help of Cortest Constant Extension Rate Tester Floor Model # Standard tensile specimen with 6.4 mm diameter, 25.4 mm gauge length machined out from 15 mm dia round samples were employed. The samples were encapsulated in an environmental chamber with a ratio of test solution volume to specimen surface of 32 ml/cm 2. Figure 1 shows a photograph of the SSRT machine used during the investigations Precharging Precharging of samples in duplicate was carried out in test solution with specimen potential 750 mv vs SCE and mv vs SCE. Duration of precharging was 1, 2 and 4 weeks Strain Rates A strain rate of 2 x 10-6 inch/second was used on 1 inch gauge length for all samples. The control samples (precharged to respective periods) were tested in air at normal strain rate of 3.3 x 10-3 inch/second. Tests were carried out at ambient temperature. 3.4 Scanning Electron Microscopic (SEM) Studies The visual examination of the fractured surface and subsequently the SEM studies will provide information regarding the mode of fracture. 4. RESULTS AND DISCUSSION 4.1 SSRT Studies Figure 2 represents load vs strain plots of control samples: SS 431, SS 420 and SS 410. The results show a comparison between different martensitic steels. Load-Strain behavior of SS 420 and SS 410 are similar where the elongations were 37% and 38% and the maximum load were 1640 Kg and 1630 Kg, respectively. SS 431 has lower elongation (20.5%) and higher maximum load (3000 Kg). Results indicate that SS 431 has higher strength and more resistant to fracture than SS 420 and SS 410 in air. Figure 3 represents load vs strain plots of SS 420 under control and polarized (charged) ٥

6 conditions tested in air. The results show the effect of charging on the ductility of SS 420 under high precharged, low precharged and control conditions. Precharging of the samples was carried out in ULW solution at -750 mv and mv (Vs. saturated calomel electrode, SCE) for a duration of 2 weeks. Percent elongations of SS 420 samples under control condition, precharged to -750 mv and precharged to mv were 35, 32.9 and 17, respectively. Maximum loads reached were 1640 Kg, 1740 Kg and 1695 Kg under control condition, precharged to -750 mv and mv, respectively. The results indicate significant reduction in percent elongation on charging the steel in ULW medium. Charging to -750 mv reduced the ductility by 6% and charging to mv reduced it by 52%. No significant effect on maximum load was observed on charging. It appears that the process of charging the steel in ULW hastens the fracture of steel. Figure 4 shows load vs. strain plots of control and polarized SS 431 samples tested in air. The results show the effect of charging on the ductility of SS 431 alloy. Precharging of sample was carried out in ULW solution at a potential of mv for a duration of 2 weeks. Elongation of control sample was 20.5% and that of precharged was 11% and maximum load achieved was 3000 Kg. The results indicate that prior charging in ULW reduced the ductility of SS 431 to 47%. Thus a sharp drop in elongation is noticed as a result of precharging of SS 431 sample. A maximum load of 3000 Kg was reached in both cases indicating that there is no change in ultimate tensile strength in both the cases. The load vs strain plots for SS 431 tested in ULW solution and under air exposure condition are shown in Fig. 5. Percent elongation for control sample tested in air was found to be 20.5 whereas for control sample tested in ULW solution it was 22. Maximum load reached was 3000 Kg for both the conditions. Results indicate that there is no significant effect on stress-strain behavior by changing the medium. Figure 6 shows load vs. strain plots for SS 420 tested in ULW solution and in air. The results show the effect of test medium on the load-strain behavior of SS 420 alloy. Percent elongation for control sample tested in air was 35% while that for control sample tested in ULW solution was 36.6%. The results indicate that the effect of medium on the load-strain behavior is insignificant. Maximum loads reached for samples tested in air and ULW solution were 1640 Kg and 1800 Kg, respectively. ٦

7 Table 2 summarizes the results of SSRT studies carried on alloys SS 410, SS 420 and SS 431 in ULW and air under control and charged conditions. From SSRT studies, the following points emerge: (1) Load vs strain plots for SS 410 and 420 in air are similar showing same elongation (37% and 38%) and same maximum load (1630 Kg). (2) SS 431 has higher strength than SS 410 and SS 420 having higher ultimate tensile strength (max. load : 3000 Kg) and much lower elongation (20.5%). (3) Upon charging, all the alloys show significant drop in % elongation although there is not any remarkable influence on maximum load. (4) Without charging, the effect of medium (air or ULW) is minimal in % elongation. (5) Upon charging in ULW medium, a significant drop in %elongation is noted without change in maximum load in all the alloys. 4.2 Fractography Figure 7 shows fracture surface of the SS 410/8 control sample slow strain rate tested in air. Cup and cone fracture occurred as a result of plastic deformation. This is a characteristic of ductile fracture. SEM picture (Fig. 8) of the fractured sample at the bottom of cup shows features typical of ductile fracture. The micrograph also shows the presence of some inclusions. Figure 9 shows surface photograph of 420/8 control sample tested in air. The fracture surface shows cup and cone pattern as a result of ductile fracture. The scanning electron micrograph (Fig. 10) of the fracture surface shows features typical of ductile fracture through elastic-plastic deformation through necking. The photograph of fractured surface of SS 431/3 control sample tested in air shows cup and cone fracture typical of ductile fracture (Fig.11). The SEM picture at the tip of the ٧

8 fracture indicates ductile fracture at the bottom of cup and coalescence of the grains (Fig. 12). SS 420 alloy sample slow strain rate tested in ULW, shows fracture by cup and cone pattern through plastic deformation (Fig.13). The scanning micrograph (Fig.14) of the fracture shows a mixed ductile-brittle fracture pattern. The fractured surface of alloy SS 431/2 sample precharged at -1.1V for 2 weeks and tested in air shows a brittle fracture as a result of transgranular or intergranular cracking (Fig.15). The scanning photomicrograph shows a 2-phase structure indicating a brittle mode of fracture (Fig. 16). The SS 420/2 sample precharged at -1.1 V for 2 weeks and tested in air shows features typical of brittle fracture, there is no necking and no cup and cone formation (Fig.17). The SEM picture of the fracture surface indicates a macrostructure typical of brittle fracture showing intergranular cracking pattern (Fig.18). The results of SEM studies carried out on steel samples fractured during SSRT testing are summarized in Table 3. The fractographic analysis of the slow strain rate tested steel specimens in air indicate that all the martensitic alloys undergo ductile fracture by elastic-plastic deformation with a cup-cone fracture pattern. In ductile fracture, during elastic-plastic deformation voids are developed within the necked region of the tensile specimens and coalescence of voids occurred to produce an internal crack by normal rupture. Final separation took place by shear rupture. Development of voids in ductile fracture surface can be seen in the form of dimples in the microstructure. Macroscopic examination of the precharged (-1.1 V) steel specimens subject to SSRT indicate failure by brittle fracture with no necking and absence of cup and cone. The microstructures show a ductile-brittle fracture mode indicating the failure of specimens under applied voltage condition. Brittle fractures are characterized by rapid crack propagation with less energy consumption than ductile fracture. Brittle fracture has a bright granular appearance and exhibit little or no necking, features which are exhibited ٨

9 by the precharged specimens. The fracture surface of steel specimens also show the presence of shear lip along the periphery where crack emerges from the interior of surface. Martensitic stainless steels have high strength and are subjected to hydrogen embrittlement under condition of impressed voltage greater than required for protection. SSRT studies provide useful information regarding the behavior of martensitic stainless steel under charged and uncharged conditions. Under charged conditions, a significant drop in % elongation is noted pointing to the hydrogen embrittlement and mode of fracture appears to be brittle on rupture. The fractography of the charged specimens support the view point that failure occurs by brittle fracture. 5. CONCLUSIONS 1. Load vs. strain plots for martensitic steels obtained from SSRT studies indicate significant loss in % elongation of all stainless steels (SS 410, SS 420 and SS 431) on high charging (-1.1V) although there is no significant effect on maximum load (ultimate tensile strength). 2. Without charging, the effect of medium is minimal on % elongation of the steels. 3. SSRT studies on control steel specimens in air indicate that all the martensitic alloys undergo ductile fracture by elastic-plastic deformation. 4. Precharged (-1.1V) steel specimens subject to SSRT indicate failure by brittle fracture with no necking and absence of cup and cone. 5. Under highly charged conditions, the fracture surface show features resembling those of hydrogen embrittlement. This indicates the possibility of hydrogen damage of the alloys during cathodic protection (C.P.). ٩

10 6. RECOMMENDATIONS There appears to be a tendency of hydrogen embrittlement in martensitic stainless steels (SS 410, SS 420 and SS 431) during cathodic protection (CP). We, therefore, advise that detailed and systematic studies are to be carried out in order to establish firmly the possibility of steel components being damaged during C.P. in water transmission lines. ١٠

11 Table 1. Chemical Composition of the Alloys Used in SSRT S. Alloy AISI UNS % C %Cr % Ni Other No. Type No. Elements 1 Martensitic chromium stainless steel 410 SS S Max Martensitic 420 SS S Max - chromium stainless steel Max 3 Martensitic chromium stainless steel 431 SS S Max Table 2. Results of SSRT Tests S. Sample Charge Duration Medium S.R. Eln. R.A. Time Max. Remarks No. (V) (weeks) (inch/sec) (%) (%) Taken to Load Break Hrs Kg /1 - - Air 1 x Trial test 2 316/2 - - Air 7.3 x Trial test 3 420/1 - - Air 7.3 x Control sample 4 420/ Air 7.3 x Ref.Fig /1 - - Air 7.3 x Control sample 6 410/ Air 7.3 x Charged sample 7 410/ Air 1.7 x / Air 1.5 x / Air 2.4 x / Air 2.8 x /1 - - ULW 7.3 x Control sample in solution /2 - - Air 7.3 x Control sample in air /3 - - Air 7.3 x Control sample in air /4 - - Air 7.3 x Ref. Fig. 13A /5 - - ULW 7.3 x Ref. Fig ULW 7.3 x Ref. Fig / Air 7.3 x Charged sample / Air 7.3 x / Air 7.3 x Ref. Fig / Air 7.3 x Charged sample /8 - - Air 7.3 x Ref. Fig.2 & /8 - - Air 7.3 x Ref. Fig /9 - - Air 7.3 x Ref. Fig Air 7.3 x Charged sample ULW 7.3 x Ref. Fig. 3 ١١

12 Table 3. Results of Fractographic Analysis S. Specimen Condition Visual SEM Studies Remarks No. Examination 1 SS 410/8 Control Cup and cone Microstructure Structure sample in fracture with showing net type indicates ductile air necking (Fig. 7) structure with fracture dimples. Inclusions can be seen (Fig. 8) 2 SS 420/8 Control sample air 3 431/3 Control sample air Cup and cone Microstructure in fracture with showing net type necking (Fig. 9) structure with dimples. Inclusions can be seen (Fig. 10) in 4 SS 420 Tested in ULW Cup and cone fracture with necking (Fig. 11) Microstructure showing highly dimpled structure, (Fig.12) Cup and cone Microstructure Ductile fracture with showing net type necking (Fig. structure with 13) inclusions (Fig.14) Structure indicates ductile fracture Ductile fracture is most probable brittle fracture mode is most probable 5 SS 431 Charged at Fracture 2-phase structure Structure 1.1 V for 2 without cup and characteristic of brittle weeks and cone showing brittle fracture (Fig. mode tested in air granular 16) structure (Fig. 15) 6 SS 420/2 Charged at Fracture The microstructure 1.1 V for 2 without cup and shows integranular weeks and cone and cracking (Fig. 18) tested in air necking showing granular appearance (Fig. 17) shows fracture Fracture mode is predominantly brittle ١٢

13 Figure 1. Photograph of the Slow Strain Rate Testing (SSRT) machine using in C.P. investigation Figure 2. Load strain plot of different stainless steels tested in air condition ١٣

14 Figure 3. Load strain plot of control and polarized SS420 tested in air condition Figure 4. Load strain plot of control and polarized SS431 tested in air condition ١٤

15 Figure 5. Load strain plot of SAS431 tested in ULW solution and at air condition Figure 6. Load strain plot of SS420 tested in ULW solution and at air condition ١٥

16 Figure 7. Photograph of the fractured SS410/8 control sample SSR tested in air Figure 8. Scanning electron micrograph of SS 410/8 control sample SSR tested in air showing ductile fracture mode ١٦

17 Figure 9. Photograph of the fractured SS420/8 control sample SSR tested in air Figure 10. Scanning electron micrograph of SS 420/8 control sample SSR tested in air showing ductile fracture mode. ١٧

18 Figure 11. Photograph of the fractured SS 431/3 control sample SSR tested in air Figure 12. SEM picture of SS 431/3 control sample SSR tested in air showing highly dimpled microstructure ١٨

19 Figure 13. Photograph of the fractured SS 420 control sample SSR tested in ULW solution Figure 14. SEM photograph of 420 control sample SSR tested in ULW solution showing a mixed ductile-brittle fracture mode ١٩

20 Figure 15. Photograph of the fractured SS 431/2 charged at 1.1V for 2 weeks then SSR tested in air Figure 16. SEM photograph of SS 431 charged at 1.1V for 2 weeks then tested in air showing brittle fracture mode ٢٠

21 Figure 17. Photograph of the fractured SS 420/2 charged at -1.1V for 2 weeks and tested in air Figure 18. Scanning electron micrograph of SS 420/2 charged at -1.1V for 2 weeks then SSR tested in air showing a predominately brittle fracture mode ٢١

22 REFERENCES 1. Parkins, R.N., (1993), Slow Strain Rate Testing-25 Years Experience in Slow Strain Rate Testing for the Evaluation of Environmentally Induced Cracking: Research and Engineering Applications, ASTM STP 1210, Ed. R.D. Kane (Philadelphia, P.A: ASTM). 2. Kane, R.D., (1993), Slow Strain Rate Testing for the Evaluation of Environmentally Induced Cracking: Research and Engineering Applications, ASTM STP 1210, Ed. R.D. Kane (Philadelphia, P.A: ASTM). 3. Kane, R.D., Willhelm, S.M., (1993), Status of Standardization Activities on Slow Strain Rate Testing Techniques in Slow Strain Rate Testing for the Evaluation of Environmentally Induced Cracking: Research and Engineering Applications, ASTM STP 1210, (Philadelphia). 4. Saito, N., Tsuchiya, Y., Kano, F. and Tanaka, N, (2000), Variation of Slow Strain Rate Test Fracture Mode of Type 304L Stainless Steel in 288 o C Water, Corrosion, 56, Braun, R. (1997), Slow Strain Rate Testing of Aluminum Alloy 7050 in Different Tempers Using Various Synthetic Environments : Corrosion, 53, Holoroyd, N.J.H., Scamans, G.M., (1984), Slow Strain Rate Stress Corrosion Testing of Aluminum Alloys in Environment-Sensitive Fracture : Evaluation and Comparison of Test Methods, ASTM STP 821, Eds. Dean, S.W., Pugh, E.N, Ugiansky, G.M. (Philadelphia, P.A.) Lyon, S.B., Thompson, G.E., Johnson, J.B., (1992), Materials Evaluation Using Wet-Dry Mixed Salt Spray Tests in New Methods for Corrosion Testing of Aluminum Alloys, ASTM STP 1134 Eds Agarwala, V.S., Ugiansky, G.M. (Philadelphia, P.A.:ASTM). 8. Braun, R. (1994), Evaluation of the SCC Behavior of Alloy 7475 Using Slow Strain Rate Technique, Werkst, Korros, 45, Braun, R. (1995), Stress Corrosion Cracking Behavior of 8090-T8171 Aluminum Alloys, Mat. Sci. Eng., A190, 143. ٢٢