INFLUENCE OF CATHODIC PROTECTION ON HYDROGEN ENBRITTLEMENT ON ANNEALED AND COLD WORKED DUPLEX STAINLESS STEELS

U.H. Kivis~d and M. Holmquist AB Sandvik Steel

SE-811 89 SANDVIKEN SWEDEN

ABSTRACT

The resistance to hydrogen embrittlement has been investigated for two of the most common duplex stainless steels, UNS $31803 and UNS $32750. The test environment was set up to simulate the influence of cathodic protection in seawater with the test solution as synthetic seawater at 80C. As a substitute for cathodic protection, zinc was coupled to samples. In addition, the samples were electrochemically pre-charged with hydrogen. Both annealed and 20% cold worked material were tested using a constant load test at 90% of tensile strength. Cold worked material was also tested with the slow strain rate technique.

This investigation indicates that there is a very low risk of hydrogen embrittlement for these duplex steels in seawater when cathodically protected with a potential corresponding to Zn. However, for cold worked UNS $32750, plastic straining during service may cause hydrogen embrittlement. The test indicates that the risk is lower for UNS $31803.

Keyword: hydrogen embrittlement, duplex stainless steels, cathodic protection, cold work

INTRODUCTION

Over the years, some duplex stainless steel failures have occurred which were blamed on hydrogen embrittlement caused by cathodic protection. In 1997, Olsson and Delblanc-Bauer(1) reached the conclusion that UNS $32750 is not susceptible for hydrogen embrittlement. This work was continued and in this paper additional results of cold worked samples are presented to complete that study. Recent

year's practical experiences have indicated that there is no problem with duplex stainless steel in combination with cathodic protection if the material is not substandard, over stressed or polarized to potentials that are too negative. It should be noted that no failures have been reported for wrought tube material.

In order to protect less noble materials in seawater, cathodic protection is frequently used. Normally, duplex stainless steels have no need for such protection. However, duplex stainless steels can be influenced by a cathodic protection intended to protect less resistant materials, for instance a carbon steel structure. Cathodic protection means that the steels potential is, by an either impressed current or by sacrificial anodes, shifted down to a more negative potential where the steel is passive. Normally this shift is to a potential of about -1000 mV vs. SCE. Such potentials also lead to the evolution of hydrogen on a steel surface.

Hydrogen evaluation on the steel surface could cause cracking due to hydrogen embrittlement or hydrogen induced stress cracking. Hydrogen embrittlement occurs due to decreased ductility caused by high concentrations of hydrogen that have penetrated into the bulk material from the surface. If the material is overstressed, by external or internal stress, cracking can occur. Under the influence of internal or external stresses, the cracking starts at the surface and propagates when hydrogen is transported into the crack. In this case, the hydrogen is charged externally, for instance electrochemically, and no diffusion of hydrogen into the bulk metal is needed.

EXPERIMENTAL

The materials in the investigation are the normal duplex stainless steel UNS $31803, that in this case is equivalent to UNS $32205, and the so-called superduplex UNS $32750, with a composition shown in Table 1 below. After annealing at 1050 - 1100C, the material was quenched rapidly in water. Samples were machined and polished to 600 mesh. Cold working was performed before testing using a standard tensile test machine that elongated the samples 20% in the longitudinal direction.

In order to simulate cathodic protection, the samples were connected to a zinc bleak. The zinc bleaks were strapped onto both the upper and lower parts on the sample. Galvanostatic polarization at - 500 mA/cm 2 was used to pre-charge hydrogen in the bulk metal in a solution of 10 % sulfuric acid with an addition of As203 at a concentration of 30 rag/1. The tests were performed directly after pre-charging in order to avoid the loss of hydrogen from the bulk. Previously it has been shown that this kind ofpre- charging results in about 80 ppm of hydrogen charged into the steel. (1)

In this investigation, constant load and slow strain rate testing, SSRT, were used. The same environment was used for both techniques, synthetic seawater (ASTM D1141-90) at 80C. The solution was pre-saturated with air, A load corresponding to 90% of the actual tensile strength at 80C was used in the constant load technique. Testing time was 500 hours. The actual tensile stress was determined by tensile tests at the actual temperature, 80C. In the SSRT test, a strain rate of 1 x 10 -6 s -~ was used. Reference tests were performed in synthetic seawater and the results are shown as ratios of elongation when coupled to zinc versus uncoupled in seawater.

~S~TS

In the constant load tests, no failures were observed for any of the tested samples after the testing time of 500 hours. The results are shown in Table 2. Therefore, no influence of the cathodic protection by the zinc anode or the pre-charging with hydrogen could be observed.

In Table 3, the results from the SSRT-tests are shown. The results are shown as ratios between the elongation of exposed sample and a reference sample. Only cold worked samples were tested and, for UNS $31803, the ratios were between 0,91 and 0,95 for both Zn-coupled as well as for those also precharged with hydrogen. For UNS $32750, the ratios were somewhat lower, between 0,81 and 0,85 and the precharged samples were slightly more ductile than the samples without pre-charging. No secondary cracks could be found after testing.

DISCUSSION

Interpreting constant load tests is easy, either the sample has failed or not. For SSRT, it is more difficult to interpret the results. A decrease in ductility compared to the reference tests may indicate increased susceptibility for cracking due to hydrogen embrittlement. It is easy to understand that when the ratio is 1, no influence of hydrogen has occurred. But there is scatter in the results from SSRT-tests. Since there is almost always a decrease in ductility when the sample is exposed to the corrosive environment, SSRT is a very good ranking test compared to constant load tests. But the SSRT-results do not give a clear pass or fail as the constant load tests do. It should also be remembered that SSRT is a very severe test where the sample is subjected to plastic deformation during the test. The acceptance criterion used for passing the SSRT ws an elongation ratio of 0,85 or above. Weather this limit is relevant or not, and the background to this limit is unclear. However, it is commonly accepted that even if the ratio is below 0,85 and no secondary cracks can be found, the sample has passed the test. In EFC 17 (2), a guideline for Sour Service Testing of CRA the following is stated:

"Constant load, sustained load and constant total strain tests establish resistance to SCC/SCC under normal static conditions.

SSRTs may:

- demonstrate a material to be immune to SSC/SCC when subjected to plastic straining; - identify materials that are susceptible to SSC/SCC when subjected to plastic straining; - provide a comparative measure of cracking susceptibility based on conventional measures such as ductility or maximum load ratios."

Both AB Sandvik Steel and the Swedish Institute for Metals Research, SIMR, have earlier published studies on embrittlement of duplex stainless steel at low potentials. In Sandvik's study, UNS $32750 was immune to hydrogen embrittlement. SIMR found a ratio of about 90% observed for UNS $32750 (3), which shows a very low degree of brittle behavior.

Therefore, this investigation has been focused on cold worked UNS $32750 and the SSRT-tests. Cold working of a steel normally makes the steel more susceptible to hydrogen cracking. It has been shown by Sentence (3) that cold work may be detrimental for duplex stainless steels resistance against hydrogen embrittlement. In this investigation, only a few tests were performed. The deformation, which will include dislocations in the material, will help the diffusion on hydrogen and can increase the risk for hydrogen embrittlement. Another important factor can also be the tendency for the austenite to

form deformation martensite as an effect of cold work. The deformed martensite phase will not have the same tendencies for blocking of cracks as the austenite. (1) SSRT-testing, in the same conditions and no cold work, gave no decrease in ductility for UNS $32750. The ductility ratios were found to be about 0,80-0,85. No secondary cracks could be observed and there were no failures in the constant load tests. This indicates that the material is not immune, as none cold worked UNS $32750 is, to hydrogen cracking when subject to plastic straining during service. However, for material that is not plastic strained during service, there is no risk for hydrogen embrittlement.

Hydrogen pre-charging may make the test more severe. In this case, the hydrogen is in the bulk of the steel before the test. SSRT results indicate lower ductility, but not necessarily any extensive hydrogen embrittlement or hydrogen induced cracking since no secondary cracks were observed. Also, the results were very similar to the samples without pre-charging that which were only coupled to zinc. A small decrease in ductility could be observed, but the ratios were 0,80 and 0,95 and no secondary cracking could be observed. The constant load tests all indicated that hydrogen embrittlement is not a problem when the steel is both coupled to zinc and precharged in hydrogen.

The SSRT and constant load results indicate that UNS $31803, cold worked as well as not, is not susceptible to hydrogen embrittlement. A small decrease in ductility for the cold worked material could be seen in SSRT but it is hardly significant. Normally it is said that a higher alloyed duplex stainless steel has a better resistance towards hydrogen embrittlement than a lower allowed duplex stainless steel. Therefore the results in this investigation are surprising.

In general, this investigation indicates that there is a very low risk for hydrogen embrittlement when using UNS $31803 and UNS $32750 in seawater when cathodically protected with a potential corresponding to Zn. For cold worked UNS $32750, plastic straining during service should be avoided because it may cause hydrogen embrittlement. The risk is less for UNS $31803 when plastically strained during service.

CONCLUSIONS

UNS $32750 and UNS $31803, cold worked to 20% or not, did not show any cracking in constant load tests for 500 hours when coupled to zinc. The same result was obtained when the samples were precharged with hydrogen.

In the SSRT-results, no significant influence ofprecharging with hydrogen could be found when the samples were coupled to zinc.

UNS $31803, cold worked or not, showed a low risk for hydrogen embrittlement in the SSRT results.

Cold worked UNS $32750 showed a small decrease in ductility in the SSRT results, which indicates that plastic straining during service should be avoided.

In general, this investigation indicates that there is a very low risk for hydrogen embrittlement when using UNS $31803 and UNS $32750 in seawater when cathodically protected with a potential corresponding to Zn.

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REFERENCES

Olsson P. & Delblanc-Bauer A, "Hydrogen embrittlement of duplex grades UNS $32750 and UNS $31803 in connection with cathodic protection in chloride solutions", Sandvik lecture S-33-48-ENG, Sandviken, 1997

Corrosion resistant alloys for oil and gas production: guidance on general requirements and test methods for H2S-service, EFC publication NR 17", The institute of materials, 1996

Wessman S.M. & Jargelius Pettersson R.F.A, Hydrogen cracking of duplex stainless steels and weld metals cathodically polarized in 3,5 % sodium chloride solution, Swedish institute for metals research, IM-report 99/044, 1999

Sentence P, "Hydrogen embrittlement of cold worked duplex stainless steel oilfield tubulars," presented at Duplex Stainless Steel'91, 1991

TABLE 1 THE NOMINAL CHEMICAL COMPOSIOTION OF USED MATERIAL, IN WEIGTH PERCENT

C Si Mn P S Cr Ni Mo N max max max max max

UNS $31803 0,030 1,0 2,0 0,030 0,015 22 5 3,2 0,18 UNS $32750 0,030 0,8 1,2 0,035 0,015 25 7 4 0,3

TABLE 2 THE RESULTS FROM THE CONSTANT LOAD TESTS, USED LOAD WAS 90% OF ACTUAL

TENSILE TEST TEMPERATURE

Material Cold work Zn- coupled Precharged Failed samples Time to failure UNS $31803 0% Yes No 0/2 - UNS $31803 0% Yes Yes 0/2 UNS $31803 20% Yes No 0/2 - UNS $31803 20% Yes Yes 0/2 UNS $32750 0% Yes No 0/2 UNS $32750 0% Yes Yes 0/2 UNS $32750 20% Yes No 0/2 - UNS $32750 20,/0 Yes Yes 0/2

TABLE 3

RESULTS FROM SSRT, PRESENTED AS THE RATION BETWEEN ELONGATION IN CORROSIVE MEDIA AND REFERENCE

Material Cold work Zn- coupled Precharged Ratio UNS $31803 20% Yes No 0,91 / 0,95 UNS $31803 20% Yes Yes 0,92 / 0,94 UNS $32750 20% Yes No 0,81 / 0,80 UNS $32750 20% Yes Yes 0,85 / 0,82

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