1

Hydrogen Induced Stress Cracking of Stainless Steel in Seawater – what do we know and what is still unknown?

Roy Johnsen

Norwegian University of Science and Technology, Norway, roy.johnsen@ntnu.no

Abstract During the last 20 years, Hydrogen Induced Stress Cracking (HISC) has been a potential threat in the oil & gas industry to Corrosion Resistant Alloys under Cathodic Polarization in seawater. In 1999 the first reported serious failure due to HISC occurred on a subsea component made from 22% Cr duplex stainless steel at the Foinaven field on the UK sector. Failure investigations concluded that the failure was caused by a combination of hydrogen from the cathodic protection system, a coarse microstructure and high local stresses. Shortly after this incident, several similar incidents were reported both on duplex stainless steel alloys and on super martensitic stainless steels. Failure investigations and research projects initiated by different research groups were executed to establish necessary knowledge about the phenomena and to establish design rules/recommended practices to reduce the probability of initiating HISC in future field developments. The content of these projects varies from small/full scale testing of components to establish critical stress values to modelling/simulation of hydrogen uptake/diffusion and corresponding crack mode from atomistic via nano to macro scale. Hydrogen embrittlement of metals has grown to be a major research area around the world today. This presentation focuses on duplex stainless steel and includes:

• an overview over reported field failures, • the main outcome from executed R&D projects, • summary of “best industry practice” for reducing the HISC probability for subsea

components connected to the cathodic protection system, • an overview of topics that still need further evaluation.

Keywords; Hydrogen induced stress cracking, duplex stainless steel, cathodic protection, seawater, field failures, lessons learned

2

Introduction [1]

Duplex stainless steels (DSSs) form a group of stainless steels that was developed in the 1930’s [1]. They were first used in the sulphite paper industry due to their excellent resistance to stress corrosion cracking. Later, the application of the steels has spread considerably, and they now include 22% Cr duplex stainless steel (DSS) and 25% Cr super duplex stainless steel grades (SDSS) that were developed in the 1980’s. During the last 20 years, there has been an increasing use of stainless steel alloys as replacement for carbon steel for subsea flowlines and production systems within the oil industry. Even if the use of stainless steels is normally a more robust solution when considering internal corrosion problems, their use has also lead to leakages, production shutdowns and expensive repair work. Several reported failures have been attributed to hydrogen entrapment resulting from external cathodic protection, combined with a certain stress/strain level of the failed component. Atomic hydrogen entering an alloy can weaken the alloy’s mechanical strength, cause cracks and hence destroy the integrity of equipment or a system. Such hydrogen induced stress cracking (HISC1) failures are clearly not acceptable with regard to safety, environmental hazard and cost. The first reported major HISC failure on SDSS was a failure on a forged subsea hub with welded pipe connectors on the Foinaven Field in 1996 [4-7]. After this incident several other similar incidents have happened with DSS, SDSS and 13%Cr supermartensittic stainless steels (SMSS) involved [8-13]. Since the Foinaven incident, many research activities related to HISC of DSS/SDSS and SMSS have been executed in leading research groups around the world. These research activities have been focusing on establishing a better understanding of the phenomena through experimental work and modeling and to establish “best practices” in how to reduce the probability of HISC initiation during operation [14-27]. What is Hydrogen Induced Stress Cracking (HISC)? The three key factors shown in Figure 1; 1) local stress/strain, 2) atomic hydrogen, and 3) susceptible microstructure of the alloy, need to be present simultaneously to initiate HISC. For components exposed to seawater, the main hydrogen source is from the Cathodic Protection (CP) system [6, 8, 28]. Hydrogen can also, however, be available from other sources; e.g. from welding, surface treatment, coating process, and corrosion. When CP is applied, reduction of oxygen according to Equation (1) and hydrogen reduction leading to the formation of hydrogen at the metal surface according to Equations (2) and (3) are the three possible cathode reactions.

O2 + 2H2O + 4e- ↔ 4OH- (1) H2O + e- + � ↔ Hads + OH- (2) H+

aq + e- + � ↔ Hads (3) Here � is a surface site available for hydrogen adsorption. Adsorbed hydrogen Hads, can either recombine to H2 and escape as gas or absorb (Habs) and diffuse in the metal [28-31]. The absorbed hydrogen atoms will be the source for possible hydrogen embrittlement of a component.

1 Called Hydrogen Stress Cracking (HSC) in ISO 21457 and ISO 15156 [2,3]

3

Figure 1. Elements needed to initiate HISC. Figure 2. Failed 6” hub from Foinaven [4]. Reported field failures The discussion about the use of duplex stainless steel in subsea equipment started in the mid 1980-ties. Parallel to this discussion research work was initiated at universities and in research organizations. At UMIST two PhD research projects related to HISC of DSS under CP were executed during the period 1987-1994 [28-29]. In 1996 a PhD thesis titled “Hydrogen transport in duplex stainless steels” were defended at NTNU [30]. Based on the learning’s from the research work, HISC was identified as a potential problem. Even despite the fact that HISC failures had been observed on subsea fasteners made from DSS [32-33], it was, however, assumed that the probability of failure for pipelines and subsea equipment was low and the oil industry started to use both DSS/SDD and SMSS for subsea pipelines and in subsea production systems. In the following are given some examples of reported failures with components made from DSS and SDSS. Similar examples have also been published for SMSS [9,12-14]. These failures will not be presented in this paper. 1996 BP Amoco Foinaven [4-7, 47] A total of 181 hubs made from SDSS (Zeron 100) were installed subsea on the Foinaven Field on the UK sector by BP Amoco in 1996. During pressure testing as part of the final commissioning six months after installation on the seabed, leaks were discovered in two forged connectors with cracking in the transverse direction in the most highly stressed area, see Figure 2. The cause of cracking was identified to be due to HISC. The cathodic protection system generated hydrogen on the non-painted hub surface exposed to seawater. The metallurgical examination also revealed that there was a coarse grain structure (grain size up to 180 µm) in the hub material in the vicinity of the cracks and that the ferrite grain structure was orientated in a through thickness direction. The ferrite content was about 50%, but the microstructure had a relatively high content of carbon-nitrites. 1997 Amerada Hess Scott [34-35] A fracture in a 6” tie-in spool made from DSS was observed after 4 years in service. The crack had initiated in a weld-toe of an anode doubler plate. The operating temperature was 1000C and thermal stresses had caused a steady displacement of the pipeline along the seabed.

4

Examinations showed that the microstructure of the base metal and weld were normal. This means that the initiation at the weld toe was apparently related to the stress concentration rather than the ferrite content. Post-calculations estimated that the crack had initiated at a strain in the order of 2-3%. After removal from the seabed it was observed that the pipeline coating (phenolic-epoxy coating) was severely blistered due to the high operating temperature. A hydrogen analysis showed 29 ppmW in a sample close to the surface. 1998 Conoco Britannia [34] Several 5” tie-in spools made from DSS were installed subsea in 1996. In 1998 leaks were detected in three flowlines. These flowlines were exposed to seabed subsidence and excessive rock dumping which caused gross plastic straining – subsequently estimated to be > 3.5%. The thermal insulation on the flowlines was damaged and partly removed due to distortion. In addition the following information is valid; i) Temperature up to 1090C, 2) High ferrite content, iii) Disbonded fusion epoxy on the spools. 2003 Shell Garn West [8, 34] A 12” inboard machined production hub made from SDSS was installed subsea in 2001. In 2003 a full circumferential-through wall crack was observed, see Figure 3, and the hub was retrieved for replacement. The failure investigation concluded that the flowline loads exceeded the interface design load limitations. Rock dumping and absence of an expansion loop mainly caused the tensile loading. The failure mode was HISC cause by a combination of high stress/strain, access to hydrogen from the CP system and a susceptible microstructure. A hydrogen content of 300 ppmW was measured in an element close to the exposed surface. In addition the following information is valid; i) Temperature up to 500C, ii) Austenite spacing 50-100 µm, iii) No coating on the surface.

Figure 3. Fractured unit from the Garn West field [34].

Figure 4. Photo of cracked forging [11].

2003/2004 ChevronTexaco Kuito [35] Tee connections made from SDSS – OD 120 mm with w.t. 36 mm and OD 76 mm, w.t. 19 mm – associated with connectors of a subsea gas lift system, suffered from fractures in the fillet welds. The components were installed in 1999, but the failures were observed in 2003/2004. Failure investigations showed that the ferrite content in the crack initiation area was > 80% and that cracking was found to follow ferrite/austenite grain boundaries along the large primary ferrite boundaries oriented transverse to the external surface of the weld. In addition the

5

following information is valid; i) Operating temperature like ambient temperature, ii) Hydrogen content in the weld bead 50 ppmW. 2013 Published by Statoil [11] – Case 1 During an inspection of a subsea module after 3 years in service a large crack was observed along a weld between two forged tee components. The failed 6” component made from SDSS (UNS S32760) had a wall thickness of 35 mm and was made from a bored forging. The subsequent failure examination of the component showed that the crack had initiated at the weld toe, propagated through-thickness in the heat affected zone and base material, continuing almost 1800

circumferential before finally being arrested, see Figure 4. Fractographic examination of the crack revealed brittle cleavage type fracture characteristics, with crack propagation preferentially having taken place through the ferrite phase. Hydrogen measurements indicated total hydrogen content of 10 ppmW close to the crack initiation region. Based on the examination it was concluded that HISC was the main contributor to the failure. During the failure investigation chromium nitride precipitates were found in the microstructure when using a special etching procedure; 10% oxalic acid electrolytic etch (5-6V, 5-10s) followed by a 20% NaOH electrolytic etch (2V, 5-10s). A test program comparing HISC susceptibility of samples directly from the failed component (with nitrides) and with heat treated samples to remove nitrides was executed. The outcome of this test program was an increase in the HISC resistance for heat treated test samples (minimized nitride content) compared to samples with nitrides (reference also is given to Figure 8). 2013 Published by Statoil [11] – Case 2 Another case involves the failures of multiple DSS (UNS S31803) and SDSS (UNS S32760) flanges used on vertical column pipes for seawater service. All flanges were of near identical design, but varying in size with normal diameter in the range 18-20” (455 – 505 mm) and wall thickness 4.8 – 5.6 mm. All the cases involved near identical failure modes with cracking – lengths in the order of 100 mm - taking place along the girth weld, see Figure 5. The following failures were reported:

o two forged SDSS flanges – after 6 years in service; unfavourable grain flow direction, o three forged SDSS flanges – after 10 years in service; intermetallic precipitations in

base metal, o two cast DSS flanges – after 16 years in service; coarse microstructure along with

intergranular nitride precipitation. Failure investigation concluded that all the failures were caused by HISC, exhibiting brittle cleavage type characteristics and associated with secondary micro-cracking with propagation preferentially occurring in the ferrite phase. A hydrogen content of 100 ppmW was generally measured near crack initiation sites. One important observation was done during the failure investigation: Crack initiation sites were in all cases associated with start/stop regions or in areas where local weld repairs had been performed on the circumferential weld. This strongly suggested that a residual stress from repair weld was an important contributor to the HISC cracks. High residual stress was also documented by testing in selected cracked units.

6

Figure 5. Crack in flange initiated at girth weld start/stop area in two different units [11]. 2013 Published by Statoil [11] – Case 3 Cold formed couplings for 10 mm umbilical hoses – manufactured in DSS (UNS S31803) and SDSS (UNS S32760) – were used subsea on hydraulic distribution units in sets of 15, see Figure 6. The following observations were done:

o SDSS: After 3 years in service 7 out of 15 contained large cracks or fractures o DSS: After 5 years in service 5 out of 15 contained longitudinal cracks

All the cracked couplings exhibited fracture surface characteristics that were consistent with HISC as the failure mechanism; brittle crack propagation predominately following the ferrite phase as well as some crack branching and bridging. Hydrogen analysis indicated 40 ppmW in SDSS after 3 years in service and 50-60 ppmW in DSS after 5 years. The couplings exhibited fine grained microstructure with austenite spacing less than 20 µm and without any type of detrimental precipitates. The method of attaching the couplings to the hoses involved a swaging process that introduced considerable cold deformation into the material, as well as significant residual stresses in the hoop direction. This process also introduced a through thickness strain hardening with near surface hardness values approaching 500 HV0.0025. According to Statoil the robustness of swaged couplings made from DSS and SDSS used subsea is unacceptable low, and austenitic stainless steels or nickel base alloys are more robust solutions regarding HISC for these couplings.

Figure 6. Cracked DSS 10 mm Hose Coupling (left) and Microstructure with HISC cracking in transverse direction (right) [11].

7

What do we know about HISC challenges today? An overview of duplex pipes used subsea during the period 1975 to 2004 was presented by Byrne during the Stainless Steel World Conference in 2015, see Figure 7. [34] According to this overview and the reported failures the majority of the field failures occurred during the period 1996 to 2004. Due to these serious failures several failure investigations and R&D projects were initiated. SINTEF and Det Norske Veritas (DNV) launched one important Joint Industry Project (JIP) in 2004. This project was called “Hydrogen Induced Stress Cracking of Stainless Steels” and was supported BP, ConocoPhillips, Chevron, Hydro, Petrobras, Shell, Statoil and Total in addition to material suppliers.

Figure 7. Overview of duplex stainless steel in subsea pipes from 1975 to 2004. [34] One important output from this project was the Recommended Practice DNV-RP-F112 “Design of Duplex Stainless Steel Subsea Equipment Exposed to Seawater” [36]. The main focus in this document is critical stress/strain values for typical geometries for subsea components exposed to cathodic protection subsea. Another important output was the development of a test method for testing HISC susceptibility for stainless steels and nickel based alloys [27]. Based on the Learning from the failure investigations and the R&D projects the following “Best industry practice” is established for HISC today: Basic requirements To initiate HISC the three elements i) access to atomic hydrogen, ii) susceptible alloy, iii) stress/strain above critical values, needs to be present simultaneously. Removing one of these elements eliminate the probability of HISC initiation. Access to atomic hydrogen Components exposed subsea are always connected to a cathodic protection system. This is the main source for atomic hydrogen on a metal surface. According to DNV RP-B401 [37] and DNV RP-F103 [38] the protection potential for steel is -800 mV vs. Ag/AgCl and for DSS/SDSS -500 mV vs. Ag/AgCl. Since the equilibrium potential for hydrogen development is in the order of -750 mV vs. Ag/AgCl in seawater, DSS/SDSS can be protected in the potential range -500 to -750 mV vs. Ag/AgCl without suffering from HISC. The problem is that all components normally are connected to steel, which requires -800 mV vs. Ag/AgCl to be protected. There are some options to reduce the probability/speed of hydrogen entry to the DSS/SDSS surface:

8

1. Apply a coating that is 100% dense throughout the lifetime – e.g. rubber lining. 2. Overlay weld critical areas (e.g. welds) with a nickel base alloy with lower hydrogen

diffusion constant (be aware: hydrogen will diffuse through the nickel base coating). 3. Limit the protection potential by use of diode controlled CP [39].

Susceptible alloy Duplex stainless steels (DSS and SDSS) and SMSS are susceptible to HISC. Austenittic stainless steels like e.g. UNS S31254 is less susceptible. Microstructure and austenite spacing Duplex stainless steels (DSS/SDSS) have a microstructure consisting of austenite and ferrite structure preferably with a 50/50 mixture [1]. Austenite has high solubility of hydrogen, low hydrogen diffusion rate but high HISC resistance. Ferrite has low solubility of hydrogen, high hydrogen diffusion rate but low HISC resistance. This is the reason why HISC cracks most often follows the ferrite path in an alloy. Grain size or austenite spacing is another important factor when it comes to HISC resistance [36]. For some of the reported field HISC failures, coarse grain sizes (> 100 µm) have been reported as one of the main reasons for the failure. In the literature an austenite spacing of 30 µm or lower is stated to be favorable to reduce HISC susceptibility [5, 8, 36, 40]. Precipitates of secondary phases like σ-phase, χ-phase and chromium nitrides (Cr2N) have a negative impact on HISC resistance. This has been documented through field failures and laboratory testing [9, 11, 41]. Figure 8 shows test results published by Statoil for the effect of nitrides on HISC susceptibility [11]. The hypothesis is that Cr2N increase the hardness of the ferrite grains which reduce the toughness of the alloy [48].

Figure 8. Cracking in SDSS with nitrides – Microstructure of the material with Cr2N (left) and test results normalized to actual Yield Strength for four (4) parallel forged test samples (right) [11]. Production method Production method influences the microstructure of a component and thereby the HISC resistance. The following ranking from less susceptible to more susceptible has been documented; Hot Isostatic Pressing (HIP) < rolled plates < forged [4, 40]. Cold working [11, 42-44] Statoil has published field data showing that cold forming of DSS and SDSS can cause HISC

9

failures. This phenomenon has also been investigated in other several papers with the same conclusion; cold work significantly reduces the HISC resistance of the alloys. Elhoud et.al. [44] did a detailed investigation and concluded: “The findings were attributed to the damage caused by plastic deformation to the passive film and deformation structures that formed in the ferrite and austenite phases. Examination of secondary cracks confirmed that the ferrite phase acted as a preferential path for crack propagation”. Residual stresses The effect of residual stresses has been a topic for discussion for long. During welding residual stresses will always be introduced in a component. If no-post heat treatment is done, tensile residual stresses combined with load or temperature induced stresses can give high total tensile stress locally. The crack presented in Figure 5 can be attributed to high residual stress in the area close to a start/stop weld. The reason for this hypothesis is that the tensile stress due to load and operation is calculated to be below 50% of Actual Yield Strength for the material. Compressive stress applied by peening is used to improve the fatigue resistance of welded components [47, 49]. Peening has also been used to improve corrosion resistance including stress corrosion cracking (SCC). Lu et.al. documented the effect of laser peening on AISI 304. The peening generated high level of compressive residual stresses and refined original grain in the surface layer. The improved SCC resistance was caused by the combined effect of compressive stresses and grain refinement [50]. Byrne et.al. presented results where forged material and welded samples from seamless pipes – both according to UNS S32760 – where HISC tested. As-produced samples and samples exposed to shot peening were tested [47]. The test results showed that controlled shot peening significantly improved threshold stresses for initiation of HISC with up to i) 10-15% for fine austenite spacing, ii) 14% for weldments, and iii) 5% for forged products with coarse austenite spacing. Hardness ISO 21457 and ISO 13628-1 specify a maximum hardness of 35 HRC or 328 HB for any type of steel under CP, while ISO 15156-3 provides a maximum hardness limit of 36 HRC for DSS/SDSS [2, 3, 45]. Important question; Do we really know the effect of material hardness on HISC susceptibility? Is the background documentation good enough? One indication of effect of hardness is related to the fact that precipitation of Cr2N increase the hardness of ferrite grains and reduce the HISC resistance [47, 48]. Critical stress/strain values DNV recommended practice DNV-RP-F112 [36] was published in October 2008 and is up for revision in 2017. This document provides guidelines for design of subsea components in order to avoid HISC. The main objective of the recommended practice is to be an industry RP defining the best practice for design of duplex stainless steel components for subsea installations. This is achieved by providing detailed recommendations on loads and conditions to be considered, as well as defining other parameters affecting the resistance to HISC. DNV-RP-F112 sets forth the considerations and requirements for carrying out an HISC evaluation of a component. Starting from the loads and material characteristics the utilization of the material can be compared to limits established through testing. Research activities today As seen from the failure histories, the number of reported HISC failures have dramatically been reduced during the last 10 years. This is due to increased knowledge about and focuses on the

10

phenomenon. However, despite the increased knowledge there are still unanswered questions about HISC – both for duplex stainless steel, but also for other Corrosion Resistant Alloys. HISC or the wider concept hydrogen embrittlement (HE) is the topic for a lot of R&D projects in academia and research organizations around the world today. R&D programs covering “Multi-scale – from atomistic level through nano, micro to macro level”. A “Multi-scale” approach links atomistic modeling and experiments of fundamental mechanisms at the nano-scale with finite element methods and materials testing at the macro-scale to provide a basis for preventing and predicting hydrogen induced degradation in metals. This is achieved through models combined with experimental work, which describes and couples environment-assisted hydrogen degradation mechanisms at different length and time scales. Figure 9 shows a schematic overview of the content in the HIPP program: Hydrogen-induced degradation of offshore steels in ageing Infrastructure - models for Prevention and Prediction. This program is executed by NTNU and SINTEF and financed by the Research Council of Norway (RCN) [46].

Figure 9. Schematic view of the content in the HIPP research project [46]. These types of research programs have resulted in close cooperation and partnership between scientists, professors and PhD candidates from different organizations around the world. Parallel to the more fundamental research programs described above, there are still some HISC engineering questions for DSS/SDSS that are not fully understood or clarified: Question 1: What is the connection between microstructure and hydrogen diffusion and

trapping as a function of austenite spacing, austenite/ferrite distribution, texture? Question 2: A wall exposed to CP on one side and production fluid on the other side – can

the hydrogen profile through the wall be established by modeling? Which diffusion constant Deff and sub-surface Co values to be used? How to include the effect of temperature?

Question 3: What is the effect of plastic deformation and cold creep on HISC resistance? Question 4: What is the effect of residual stresses on HISC? How to combine residual stress

and actual (load) stress to define input stress in DNV-RP-F112? Question 5: Can HISC be initiated from embedded defects? Question 6: What is critical hydrogen content for initiation of HISC?

11

Question 7: In practice; is there any difference between the HISC susceptibility for DSS and SDSS? Is there any difference between SDSS alloys with different UNS number?

Conclusions Duplex stainless steels are susceptible to HISC when atomic hydrogen is available. Field experiences and R&D projects have shown that i) implementing DNV RP-F112 and ii) keeping the grain size/austenite spacing smaller than 30 µm will reduce the risk for initiation. This is documented through the reduced number of reported field failures during the last 10 years. There are still questions related to HISC for DSS/SDSS that are not fully understand and further work is needed to increase the knowledge, improve standards and Recommended Practices and finally reduce the risk level for HISC. References 1. Olsson J., Liljas M., “60 years of DSS applications”, in NACE Corrosion 94 Conf., paper

No. 395, Baltimore, MD, 1994. 2. ISO 21457:2010 (latest revision), “Petroleum, petrochemical and natural gas industries –

Materials selection and corrosion control for oil and gas production systems” (Geneva, Switzerland: ISO)

3. ISO 15156-3:2009 (latest revision), “Petroleum and natural gas industries – Materials for use in H2S-containing environments in oil and gas production – Part 3: Cracking-resistant CRAs (corrosion resistant alloys) and other alloys” (Geneva, Switzerland: ISO)

4. T.S.Taylor, T.Pendington, R.Bird: Foinaven Super Duplex Materials Crack Investigation. Offshore Technology Conference (OTC) 1999, paper No. 10965.

5. P. Woolin and A. Gregori: Avoiding hydrogen embrittlement stress cracking of ferritic austenitic stainless steels under cathodic protection, in conference proceedings of OMAE04, June 20-25, 2004, Vancouver, Canada.

6. T.S. Taylor, T. Pendington and R. Bird: Foinaven Super Duplex Materials Cracking Investigation, in proceedings of Offshore Technology Conference-OTC, paper No. 10965, Houston, Texas, 3-6 May 1999.

7. P.Wollin and W.Murphy: Hydrogen embrittlement stress corrosion cracking of superduplex stainless steel, Corrosion 2001, paper no. 01018.

8. S.Huizinga, B.McLoughlin, I.M. Hannah, S.J. Paterson and B.N.W. Snedden: Failure of a subsea super duplex manifold hub by HISC and implications for design, Corrosion 2006, paper no. 06145.

9. T. Cassagne, F. Busschaert: A review on hydrogen embrittlement of duplex stainless steels, Corrosion 2005, paper No. 05098.

10. S. Olsen, S.M. Hesjevik: Hydrogen embrittlement from CP on supermartensitic stainless steels- recommendations for new qualification methods, Corrosion 2004, paper No.04546.

11. M.Aursand, G.Rørvik, L.A.Marken, I.M.Kulbotten; Experiences with Hydrogen Induced Stress Cracking of Dulex Stainless Stel Components in Subsea Services with Cathodic Protection. NACE Corrosion 2013, Paper no. 2490.

12. R.Mollan; Experiences with 13Cr supermartensittic stainless steel in the Tune Submarine flowlines. NACE Corrosion 2005, Paper no. 05092

13. S.M.Hesjevik, S.Olsen; Hydrogen embrittlement from cathodic protection on supermartensittic stainless steels – Case History. NACE Corrosion 2004, Paper No. 04545.

14. S. Hashizume, Y. Inohara and K. Masamura: Effect of stress corrosion cracking resistance of martensitic stainless steels, NACE Corrosion 2001, paper No. 85

15. A.Mikkelsen, S.Wästberg, R.Johnsen, B.Nyhus, T.Rogne: Influence of ambient pressure on

12

Hydrogen Induced Stress Cracking (HISC) of Duplex Stainless Steels under Cathodic Protection, Corrosion 2006, paper No. 06499.

16. G.Byrne, G.Warburton, Z.Schulz, R.Francis; Advanced Forging Process (AFPTM), Super Duplex Stainless Steel for Increased Low Temperature Impact Toughness and Resistance to Hydrogen Induced Stress Corrosion Cracking (HISCC) due to Cathodic Protection of API Forgings for Subsea Applications.

17. S.Ronneteg, A.Juhlin, U.Kivisäkk; Hydrogen embrittlement of duplex stainless steels testing of different product forms at low temperature. NACE Corrosion 2007, Paper no. 07498.

18. X.An, A.Dobson; An influence of cathodic protection potential on the mechanical properties on Super Duplex Stainless Steel tube. NACE Corrosion 2009, Paper no. 09297.

19. W.Cheng et.al.; Effect of Cahodic Polarisation Potential on Hydrogen Induced Stress Cracking of Duplex Stainless Steel. NACE Corrosion 2013, Paper no. 2384.

20. F.Salvio, R.S.Silva, D.S.dos Santos; On the role of HISC on Super and Hyper Duplex Stainless Steel. OTC 24289, OTC Conference Rio de Janeiro, Brazil, 29-31 October 2013.

21. V.Olden, C.Thaulow, E.Østby, T.Berstad, R.Johnsen: Influence of hydrogen from CP on the fracture susceptibility of 25% Cr duplex stainless steel – constand load SENT testing and FE modeling using hydrogen influenced cohesive zone elements. Accepted for publication in Engineering Fracture Mechanics, November 2008.

22. V. Olden, C. Thaulow, R. Johnsen, E. Østby: “Cohesive zone modeling of hydrogen-induced stress cracking in 25% Cr duplex stainless steel”, Scripta Materialia 57, 2007, p. 615-618.

23. V. Olden, C. Thaulow, R. Johnsen, E. Østby, T. Berstad: "Application of hydrogen influenced cohesive laws in the prediction of hydrogen induced stress cracking in 25%Cr duplex stainless steel", Engineering Fracture Mechanics, 75(8), 2008, p.2333-2351.

24. V.Olden, C.Thaulow, R.Johnsen: Modeling of hydrogen diffusion and hydrogen induced cracking in supermartensittic and duplex stainless steels. Materials & Design 29 (2008), p. 1934-1948.

25. V.Olden, L.Jemblie, R.Johnsen; FE simulation of hydrogen diffusion in a duplex stainless steel -influence of phase shape and size and embedded defects. ISOPE-2013, Anchorage, Alaska, June 30 – July 5, 2013

26. V.Olden; FE-modellling of hydrogen induced stress cracking in 25% Cr duplex stainless steel. Doctoral Thesis at NTNU, 2008:129, ISBN 978-82-471-8625-1.

27. R.Johnsen, B.Nyhus, S.Wästberg: Hydrogen induced stress cracking (HISC) of stainless steels under cathodic protection in seawater – Presentation of a new test method. OMAE 2009, Paper no. OMAE2009-79325.

28. Valdes-Vallejo FR. PhD Thesis “The Cathodic Hydrogen Embrittlement of Duplex Stainless Steel” UMIST, June. 1990.

29. Hutchens D. PhD Thesis “The Hydrogen Embrittlement of Duplex Stainless Steel” UMIST, March. 1994.

30. E.Lembach-Beylegaard; Hydrogen transport in duplex stainless steels, PhD thesis at NTNU, Trondheim 1996.

31. S.P.Lynch; Progress towards understanding mechanisms of hydrogen embrittlement and stress corrosion cracking. NACE Corrosion 2007, Paper no. 07493.

32. Francis R. and Campbell H. BNF Technology Centre Report R 538, June 1986. 33. Wolfe LH. Joosten MW. SPE Prod. Eng. Vol. 3, No. 3, p 382. 1988. 34. G.Byrne; A historical overview of HISC failures in duplex, super duplex and low carbon

martensitic stainless steels due to cathodic protection (CP). Stainless Steel World Conference & Expo 2015, Mastricht, Nederlands

35. DNV “Duplex stainless steels – Field failures and acceptance criteria data”. Technical

13

report No. 2004-3471 in the JIP HISC project 2004. Not published report. 36. DNV-RP-F112 “Design of Duplex Stainless Steel Subsea Equipment Exposed to seawater”,

October 2008. 37. DNV-RP-B401 Cathodic Protection System 38. DNV RP F103 “Cathodic Protection of Submarine Pipelines by Galvanic Anodes”, October

2010. 39. A. Sjaastad, H. Osvoll, O.J. Hauge, R.E. Lye and G. Bärs: CP design of a super 13% Cr

flowline, Corrosion 2004, paper No. 04092. 40. G.Ø.Lauvstad, R.Johnsen, I.Asbjørnsen, M.Bjurstrøm, C.G.Hjort: The resistance towards

hydrogen induced stress cracking (HISC) of hot isostatically pressed (HIP) duplex stainless steels under cathodic protection. Paper presented at Eurocorr 2007, Freiburg, Germany

41. Aursand M., Rørvik G., and Sjølstad K., “Chromium nitride precipitates in large super duplex stainless steel forged components”, Stainless Steel World 2011, Maastricht, Nederland.

42. U.H.Kivisäkk, M.Holmquist; Influence of cathodic protection on hydrogen embrittlement on annealed and cold worked duplex stainless steels. NACE Corrosion2001, Paper no.: 01019, 2001

43. P. Sentence: Hydrogen embrittlement of cold worked duplex stainless steel oilfield tubulars. In: Duplex stainless steel 91, 28 - 30 October, Beaune Bourgogne, France; 1991.

44. A.M. Elhoud, N.C. Renton, W.F. Deans: Hydrogen embrittlement of super duplex stainless steel in acid solution, Int. National Journal of Hydrogen Energy 35 (2010), p. 6455-6464.

45. ISO 13628-1:2005/Amd.1:2010 (latest revision), “Petroleum and natural gas industries – Design and operation of subsea production systems – Part 1: General requirements and recommendations” (Geneva, Switzerland: ISO)

46. HIPP Project at Norwegian University of Science and Technology, Trondheim - Norway. www.ntnu.edu/kt/research/hipp

47. G.Byrne, R.Francis, G.Warburton; Hydrogen Induced Stress Cracking (HISC) Resistance and Improved Methods for Super Duplex Stainless Steels. NACE Corrosion 2016, Paper no. 6981.

48. T.H.Iversen, PhD Thesis “Intergranular Chrominum Nitride Precipitates in Duplex and Super Duplex Steels”. Norwegian University of Science and Tecgnology, NTNU-Trondheim, Norway, June 2012.

49. J.L.Curtat, J.Lanteigne, H.Champliaud, Z.Liu, J-B.Levesque; Influence of hammer peening on fatigue life of E309L steel used for 13Cr-4Ni blade runner repair. Int. Journal of Fatigue 100 (2017) 68-77.

50. J.Z.Lu et.al.; Effects of laser peening on stress corrosion cracking (SCC) of ANSI 304 austenitic stainless steel. Corrosion Science 60 (2012) 145 – 152.