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Corrosion fatigue of welded stainless steels for deepwater riser applications

P Woollin, S J Maddox and D J Baxter

Proceedings of OMAE 2005: 24th International Conference on Mechanics and Arctic Engineering (OMAE 2005) Halkidiki Greece, 12-16 June 2005.

Abstract

Steel risers for deepwater offshore oil and gas field developments are subject to seawater on the external surfaces, produced fluids on the internal surfaces and to fatigue loading. This paper reviews current knowledge of the corrosion fatigue behaviour of welded stainless steel for risers and presents results of testing of supermartensitic, duplex and superduplex grades in relevant environments.

Introduction

Catenary risers and top tension risers are subjected to fatigue loading from wave and tidal motion and vortex induced vibration, whilst simultaneously being exposed to seawater, typically with cathodic protection (CP), on the outside and corrosive produced fluids on the inside. The produced fluids typically contain water acidified by the presence of hydrogen sulphide (H2S) and carbon dioxide (CO2) and recent attention has been paid in particular to the detrimental effects of fluids containing H2S (sour fluids) on the fatigue performance of riser girth welds in C-Mn steel pipe. [1-3] Results published to date indicate that a severe reduction in fatigue life may occur in sour fluids by a factor of up to 20 particularly at higher stress ranges. Consequently, there is a need to explore the use of corrosion resistant alloys (CRAs), either stainless steels, nickel alloys or titanium alloys as alternative sour riser materials, either as a solid or clad product. The main candidate stainless steels are high strength grades such as 12%Crsupermartensitic, 22%Cr duplex and 25%Cr superduplex grades, or austenitic grades such as 316 stainless steel or Ni-Fe alloy 825 but these have fairly low strength and would have to be used as a clad product.

This paper considers existing corrosion fatigue data of relevance to welded stainless steel risers and presents results of crack growth rate testing of welded supermartensitic and superduplex stainless steel in seawater with cathodic protection, welded superduplex stainless steel in a sour environment and a simulated HAZ microstructure in a duplex stainless steel in 3%NaCl solution.

Review of existing corrosion fatigue data for CRAs

Supermartensitic stainless steel

Few fatigue data exist for 12%Cr supermartensitic grades, although there are more data for the conventional, higher carbon 13%Cr martensitic stainless steel grades. [4-7] The limited data suggest that fatigue behaviour of welded joints in air is very similar to that of C-Mn steels. Testing of conventional martensitic stainless grades in seawater has been performed and gave reduced life compared to tests in air but this was apparently dominated by the highly corrosive nature of seawater with respect to 13%Cr steel, which gave significant surface corrosion. [4,5] Reducing the frequency gave increased crack propagation rates. Pit formation was shown to be a critical factor controlling fatigue crack development in martensitic stainless steels, which is time dependent. Hence, any meaningful testing to establish fatigue lives must allow time for any such time-dependent corrosion processes. No published data are available for the behaviour of supermartensitic or conventional martensitic stainless steels in seawater with cathodic protection or in produced fluids.

Duplex and superduplex stainless steels

The fatigue performance of welded joints in duplex and superduplex stainless steels in air is broadly similar to that of C-Mn structural steels. [6,7] When exposed to seawater, both with and without cathodic protection, higher fatigue crack propagation rates were observed when K max was above around 700-1000N/mm 3/2 . [6,8-13] Below this level, especially in the near threshold regime, crack propagation behaviour was similar to that in air, as was the fatigue threshold. The increase in propagation rate was greater under cathodic protection. This behaviour was associated with the appearance of a cleavage-like fracture morphology, suggesting an effect of hydrogen embrittlement in both cases. [8,9,12,14] Without cathodic protection, the active hydrogen presumably arises from corrosion at the crack-tip. Crack propagation rates increased with decreasing frequency. It was found that corrosion fatigue in seawater was sensitive to microstructure, with a grain coarsened heat affected zone with high ferrite content showing higher propagation rates than parent steel. [13] Overall, the data are consistent with a corrosion fatigue mechanism including a static mode of crack extension, where the static mode is hydrogen embrittlement. It is likely, however, that dynamic straining of the crack-tip encourages local corrosion and hydrogen generation, so that the hydrogen embrittlement component may become active under less severe environmental conditions than might be expected from static load data. No published data were found for testing in sour or sweet media.

Austenitic stainless steels

Austenitic stainless steel welds have generally similar performance to C-Mn steel welds in air and it has been proposed that the same design S-N curves can be used for both providing there is no influence of the environment. [6] Testing of austenitic stainless steel in seawater gave a reduction of fatigue life by a factor of approximately two when compared to air. [6,7]

Overall, few fatigue data are available for austenitic stainless steel welds in corrosive environments [7] and no data from tests in produced fluids or seawater with CP have been reported. However, materials with austenitic microstructure, including austenitic stainless steels and nickel alloys, are expected to be less susceptible to hydrogen embrittlement in either parent steel or a weld HAZ than the competing supermartensitic and duplex/superduplex grades. Such grades are particularly suitable when clad onto a C-Mn steel substrate, as they have inherently low strength in the solution annealed condition and are unlikely to be used in monolithic form.

Experimental programme

Materials

Three high strength stainless steels, which could be used to construct a riser without the need for clad product, were chosen: (i) UNS S41426 12%Cr supermartensitic stainless steel welded by the pulsed MIG process using aproprietary superduplex wire (Zeron 100X), (ii) UNS S31803 22%Cr duplex stainless steel plate, heat treated to 1350°C and immediately water quenched to simulate a high temperature HAZ microstructure with 62% ferrite, and (iii) UNSS39274 25%Cr superduplex stainless steel welded by the TIG process with a proprietary superduplex wire (Zeron 100X). A summary of the materials is given in Table 1.


Table 1 Materials (nominal compositions)

Grade UNS No. Element, wt%
C Mn Si Cr Mo Ni W Cu N
12Cr supermartensitic S41426 0.01 0.4 0.2 12.2 2.5 6.4 0 0 ≤0.01
22Cr duplex S31803 ≤0.03 0.5 1.0 22.0 3.2 5.3 0 0 0.17
25Cr superduplex S39274 ≤0.03 1.0 0.8 25.0 3.5 7.0 2.0 0.5 0.28
Zeron 100X wire None ≤0.03 0.7 0.4 25.0 3.0 9.3 0.6 0.7 0.23


Table 2 Corrosion fatigue test matrix

Parent steel Weld metal Notch location Test environment Potential (mV) Frequency (Hz) R-ratio Test temperature (°C)
12Cr
12Cr
12Cr
12Cr
25Cr
25Cr
25Cr
25Cr
Parent
Parent
Cap HAZ
Cap HAZ
Seawater
Air
Seawater
Air
-1100
NA
-1100
NA
0.5
15
0.5
15
0.5
0.5
0.5
0.5
20
20
20
20
22Cr
22Cr
NA
NA
HAZ sim
HAZ sim
Air
3%NaCl
NA
Ecorr
10-20
1
0.5
0.5
15
15
25Cr
25Cr
25Cr
25Cr
25Cr
25Cr
25Cr
25Cr
25Cr
25Cr
25Cr
25Cr
25Cr
25Cr
Cap HAZ
Cap HAZ
Cap HAZ
Cap HAZ
WMCL
WMCL
WMCL
Seawater
Air
Seawater
Air
Sour
Air
Sour
-1000
NA
-1000
NA
Ecorr
NA
Ecorr
0.5
20
0.5
20
0.5
20
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
20
20
80
80
20
20
80

Ecorr = natural corrosion potential, NA = not applicable, WMCL = weld metal centre line, 12Cr = supermartensitic stainless steel, 22Cr = duplex stainless steel, 25Cr = superduplex stainless steel


Fatigue crack growth rate testing

General

The fatigue crack growth rate tests were conducted on single edged notched bend specimens in 3 point bending, in servo-hydraulic machines. The tests were conducted according to ASTM E647 using a constant load range (i.e. under increasing ΔK conditions) or using load shedding to approach the fatigue threshold (i.e. under decreasing ΔK conditions), mostly at a stress ratio of R=0.5 which is relatively high, to simulate the in-service loading conditions for a riser and the presence of tensile residual stress, which is typically associated with welds. All tests used sinusoidal loading. Crack growth was monitored by the direct current potential drop (dcpd) method. The results were processed to obtain crack growthrates (da/dN) and corresponding values of stress intensity factor range (ΔK), following methods presented in BS 6835.

In air

Testing of each material was performed in air at ambient temperature, with a frequency of 10-20Hz and a stress ratio of 0.5. This included specimens sampling the HAZ (surface notched from the weld cap) for the 12%Cr supermartensitic stainless steel and the 25%Cr superduplex stainless steel and sampling 25%Cr superduplex weld metal notched from the root. Samples of the bulk simulated HAZ microstructure in 22%Cr duplex stainless steel were tested also. For comparison, one specimen of 25%Cr superduplex stainless steel cap HAZ and one of root weld metal were tested at 80°C.

In seawater without cathodic protection

Testing was undertaken on specimens of the simulated HAZ microstructure in 22%Cr duplex stainless steel material in 3%NaCl at 15 ± 1°C at 1Hz and at a stress ratio of 0.5. Each specimen was heat treated individually.

In seawater with cathodic protection

Testing of welded 12%Cr supermartensitic and 25%Cr superduplex stainless steel specimens sampling the cap HAZ, was undertaken in seawater with an applied potential of -1100mV SCE or -1000 mV SCE at 20°C with a frequency of 0.5Hz and a stress ratio of 0.5. The specimens were surface notched from the toe of the weld cap, to simulate the service situation. One specimen of 25%Cr superduplex stainless steel was tested at 80°C for comparison.

In a sour brine

Testing of 25%Cr superduplex stainless steel specimens was undertaken in a sour brine, with samples surface notched on the weld metal centreline, from the weld root, to simulate the service situation. The environment consisted of21.6g/l NaCl, 1.3g/l Mg Cl2 , 1.3g/l NaCH3 COO, 0.5g/l CaCl2 , 0.7g/l Na HCO3 and 0.4g/l KCl in deionised water deoxygenated to <10ppb oxygen and acidified with 0.975 bar CO2 and 0.025 bar H2S. Tests were undertaken at 20°C and 80°C with a frequency of 0.5 Hz and a stress ratio of 0.5.

Post test evaluation of test specimens

After test, the crack growth rate specimens were broken open and the fracture faces examined in a scanning electron microscope, to characterise the crack face morphology. Sectioning perpendicular to the fracture face was undertaken to determine the relationship between the crack path and microstructure in the superduplex specimens.

Results

Fatigue of 12%Cr supermartensitic stainless steel in seawater with cathodic protection

Figure 1 shows a comparison of fatigue crack growth rates in air and in seawater with CP at -1100mV SCE for the HAZ notched 12%Cr supermartensitic stainless steel specimens. It can be seen that crack growth rates in the HAZ are up to about a factor of 5-7 higher than in air for ΔK >600 N/mm3/2 . At low ΔK the data under CP fall below the air data suggesting a higher threshold in the former case.


Fig.1. Fatigue crack propagation data for supermartensitic stainless steel HAZ notched specimens tested in air and seawater with cathodic polarisation to -1100mV
Fig.1. Fatigue crack propagation data for supermartensitic stainless steel HAZ notched specimens tested in air and seawater with cathodic polarisation to -1100mV

 

Fatigue of 22%Cr duplex stainless steel simulated haz in 3%NaCl

Figure 2 shows the data for the 22%Cr duplex stainless steel simulated HAZ specimens tested in air and 3%NaCl. The results at R=0.5 in air were consistent with published data for parent plate in air but there was a large amount of scatter in the near threshold region. The data in air are very similar to the crack growth rate data obtained for the 12%Cr supermartensitic stainless steel HAZ under similar conditions.

The rate of fatigue crack growth was increased in the 3%NaCl solution by a factor of 5-7 times at high ΔK (above about 600N/mm 3/2 ) but was very similar to or below the air data at lower ΔK. This was investigated further by examining the microstructure and fracture faces in detail. This revealed a crystallographic fracture mode and some secondary cracks after testing in 3%NaCl.

Fatigue of 25%Cr superduplex stainless steel in seawater with cathodic protection

Figure 3 shows the results of testing of the superduplex stainless steel HAZ in seawater with CP. In seawater, a step change in behaviour was observed at about ΔK = 600N/mm 3/2 . Below this level, there was most scatter in the data but there was a difference between the two temperatures in this regime indicating slightly lower fatigue crack growth rates at 80°C than at room temperature. At high ΔK, the data at 80°C lie above the room temperature data but again the effect is fairly small compared with the level of data scatter.

The crack growth rates in seawater with CP are similar to those in air at low ΔK, below about 400N/mm 3/2 . At higher ΔK, the crack growth rates in seawater are about three times higher than the corresponding air data. The seawater data exhibit higher slope than for air in the near-threshold region and there is a suggestion that the threshold may be higher in seawater with CP than in air. This may be due to closure arising from the formation of calcareous deposits under CP. However, because of the degree of scatter in the analysis, it must be assumed that the crack growth rates were not significantly different.


Fig.2. Fatigue crack propagation data for 22%Cr duplex stainless steel simulated HAZ specimens tested in air and 3% NaCl solution without cathodic polarization.
Fig.2. Fatigue crack propagation data for 22%Cr duplex stainless steel simulated HAZ specimens tested in air and 3% NaCl solution without cathodic polarization.

 

Fatigue of 25%Cr superduplex stainless steel in a sour environment

Figure 4 shows the results for the sour brine. At both test temperatures, crack growth rates were substantially higher in the sour environment than in air. Crack growth rates were up to ten times higher than in air at higher ΔK values, particularly in the mid- ΔK range. At high and low ΔK values, the effect of the environment is apparently reduced, especially at 20°C in the high ΔK range, although it is noted that only a fairly small amount of data is available in the low ΔK regime.


Fig.3. Fatigue crack propagation data for superduplex stainless steel HAZ notched specimens tested in air and seawater with cathodic polarisation to -1000mV.
Fig.3. Fatigue crack propagation data for superduplex stainless steel HAZ notched specimens tested in air and seawater with cathodic polarisation to -1000mV.
Fig.4. Fatigue crack propagation data for superduplex stainless steel weld metal centreline notched specimens tested in air and a sour brine with 25mbar H 2 S.
Fig.4. Fatigue crack propagation data for superduplex stainless steel weld metal centreline notched specimens tested in air and a sour brine with 25mbar H 2 S.


Examination of the fracture faces of the samples tested in the sour brine showed crack propagation predominantly by transgranular cleavage suggesting an effect of hydrogen embrittlement. This was through the ferrite phase but also, to a lesser extent, apparently through austenite, giving rise to large facets on the fracture faces. There was very little evidence of ductile tearing. The samples tested in air showed crack surfaces more typical of fatigue with no environmental action, i.e. irregular transgranular features with little evidence to indicate whether propagation was through ferrite or austenite phases. Fatigue striations were also observed.

Sectioning confirmed the findings of fractography. The samples tested in the sour brine demonstrated a transgranular crack path showing large facets with little deviation from the macroscopic crack path. There was no evidence of preferential crack propagation through either the ferrite or the austenite phase. In contrast, the samples tested in air, showed an irregular crack morphology. Very little crack branching was observed in either sample.

Discussion

Supermartensitic stainless steel

The supermartensitic stainless steel HAZ notched specimens showed a factor of 5-7 increase in crack growth rate when tested in seawater with cathodic protection. Cathodic polarisation will give hydrogen uptake ahead of the crack-tip due to the forced generation of atomic hydrogen on the steel surface. The cap heat affected zone typically consists of virgin martensite due to the high hardenability of the steel and the absence of reheating by subsequent passes but the remainder of the HAZ will have some reheating and will consist of partially tempered martensite. Embrittlement ahead of the crack-tip by hydrogen generated under cathodic protection would be expected to increase crack propagation rates in supermartensitic steel. The magnitude of this effect would be expected to be increased in the untempered and partially tempered martensite of the HAZ compared to the fully tempered martensite of the parent steel. Hydrogen embrittlement is a static mode of fracture and the crack growth rates would be expected to be sensitive to the maximum applied K value under such conditions, with higher cyclic growth rates as the maximum K increases. It is noted that the increased crack growth rates are only observed above about 600N/mm3/2 , with near threshold crack growth rates being similar to or lower than the air rates. This is probably due to closure resulting from formation of calcareous deposits within the fatigue crack and may be particularly important for risers, which would be expected to spend most of their life in the near threshold regime. More data are required in the threshold region to confirm this effect.

Duplex stainless steel

The faceted appearance of the duplex stainless steel fracture faces and presence of secondary cracks indicated that hydrogen embrittlement had again contributed to crack propagation in the 3%NaCl solution without CP, presumably relating to hydrogen generated by corrosion at the crack-tip, where the passive film is repeatedly ruptured. This was apparently sufficient to give increased crack growth rates compared to air. Coarse, high ferrite microstructures are typically fairly sensitive to hydrogen embrittlement and the simulated HAZ had 62% ferrite compared to 45% in the parent steel. Grain coarsening and increased ferrite levels, to around the level employed here, are inevitable in the HAZs of fusion welds in duplex stainless steels. The extent of the effect will depend on material composition, and will be most marked in steels with fairly low nitrogen level, say below about 0.12-0.13% (the steel used here had 0.17%).

Sensitivity to hydrogen embrittlement effects increases at high ferrite contents, and in principle high nitrogen contents or perhaps high arc energy welding conditions are to be preferred to promote austenite formation during the weld thermal cycle, although upper limits of heat input are specified to avoid formation of intermetallic phases.

Superduplex stainless steel

It should be noted that fatigue crack growth rates obtained for the sour brine and seawater environments show similarity to previously published data [16-18] for UNS S32760 superduplex steel tested in gaseous hydrogen and water with a range of oxygen contents. Increased crack growth rates were reported in both hydrogen and water compared to air. In addition, formation of transgranular cleavage facets was observed in the ferrite phase in both water and hydrogen, and in the current work in seawater with CP and sour brine. Consequently, it seems that increased crack propagation rates in superduplex stainless steel may be obtained in water or brine with or without cathodic polarisation, gaseous hydrogen and brine with H2S, i.e. a range of environments, which may introduce atomic hydrogen at the fatigue crack-tip. This is consistent with the 22%Cr duplex stainless steel data derived from testing in 3%NaCl without CP.

This again indicates the sensitivity of the ferrite phase to the presence of absorbed hydrogen [16-18] , although it seems from the results that the austenite might also show some sensitivity. Whilst the H2S level employed in the sour tests was below the maximum level allowed for UNS S39274 superduplex stainless steel in ISO 15156, [19] fatigue crack growth tests repeatedly expose virgin material at the crack-tip and some degree of hydrogen generation and pick-up may be anticipated for any aqueous environment. The addition of H2S tends to enhance hydrogen uptake as it is a cathodic 'poison', whilst the presence of CO2 reduces the solution pH and encourages corrosion. In the present tests, the uptake of hydrogen at the crack-tip has again reduced material ductility ahead of the crack-tip, leading to increased growth rates and the observation of a cleavage-like hydrogen embrittlement mode of failure through both the ferrite and austenite phases. Similar fracture faces have previously been observed in hydrogen-charged material [16-18] .

General comments

The study has examined three stainless steels with fairly high strength, which may be considered as candidate materials for steel catenary risers and top tension risers. The various stainless steel air data were all similar and lie within the scatterband for C-Mn steels. [15] Crack growth rate testing indicates that in oxygenated brines, in seawater with cathodic protection and in deoxygenated sour produced brines, fatigue performance may be reduced compared to air by a factor of up to 10. These detrimental effects are due to hydrogen pick-up, either as a result of corrosion at the crack-tip due to passive film rupture, or from cathodic protection. The cracking morphology typically becomes more facetted under such testing conditions. However, it is shown that the magnitude of any degradation is dependent upon the applied ΔK level and indeed the effects are typically least in the low ΔK region, implying that the effect of these environments on fatigue life of girth welded pipes, when much of the time is spent in the near threshold region, may be less than crack growth rate testing suggests. This may be due at least in part to closure either resulting from corrosion products in the crack or the change of crack morphology.

The observed effects of the various environments on fatigue crack growth rates may provide the basis of preliminary design recommendations. Thus, in the presence of brine with or without CP or deoxygenated sour brine, for a structure which experiences low frequency fatigue loading, a fatigue life reduction factor of at least 5-10 may be required for supermartensitic, duplex and superduplex stainless steel welds. However, since this appears to be particularly relevant at relatively high crack growth rates it might not be necessary to apply such factors, except for low-cycle fatigue applications. There is a need therefore for endurance testing of specimens with geometry representative of real components to determine accurately the fatigue life in such corrosive environments. It is not clear whether fracture mechanics calculations based on data from crack growth rate testing can provide an accurate estimate of life on the basis of the current data.

The sensitivity to hydrogen of these high strength stainless steels and the consequent reduction of fatigue behaviour under conditions that introduce hydrogen at the crack-tip suggests that a more suitable choice for a sour-service corrosion resistant riser may be a material with good intrinsic resistance to hydrogen embrittlement, such as an austenitic stainless steel or a nickel alloy. Due to their low strength, it is most appropriate to use these in the form of internal cladding on a higher strength C-Mn steel pipe. A second alternative would be a material with very highly stable passive film, such as a titanium alloy, such that no appreciable hydrogen pick-up occurs at the crack-tip. Further data are required to explore the most suitable alternative.

Conclusions

  1. Fatigue crack growth rate testing of duplex, superduplex and supermartensitic stainless steels indicates that environments including 3%NaCl, seawater with cathodic protection and deoxygenated sour brine may all give increased weld area fatigue crack propagation rates relative to air.

  2. Crack propagation in a 22%Cr duplex stainless steel simulated HAZ microstructure in 3%NaCl was up to a factor of seven times higher than in air.

  3. Crack propagation in a 25%Cr superduplex stainless steel HAZ in seawater with cathodic protection at -1000mV was up to a factor of three times higher than in air at 20°C and 80°C. A sour brine with 25mbar H2S gave an increase by a factor of about ten.

  4. Crack propagation in a 12%Cr supermartensitic stainless steel HAZ in seawater with cathodic protection at -1100mV was up to a factor of seven times faster than in air.

  5. In all of the tests, crack growth rates in the various environments were closer to the air data in the near threshold region than at higher stress intensity ranges.

  6. Existing data suggest that austenitic stainless steels and nickel alloys may be preferred materials for corrosion resistant steel catenary risers operating in sour service due to their low susceptibility to hydrogen embrittlement by the internal environment.

  7. Endurance testing is required to give a more complete understanding of the corrosion fatigue behaviour of girth welds in candidate stainless steels for riser applications.

 

Acknowledgments

Statoil are thanked for providing permission to publish the results of testing of the superduplex stainless steel.

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