Corrosion control and materials for Oil & Gas CAP.4

Part 4

Corrosion Resistant

Alloys

34 Localised Corrosion of some Selected Corrosion Resistant Alloys in the Presence of Very High

Salinity

T. CHELDI and L. SCOPPIO Eni Agip Exploration and Production, Italy

Centro Sviluppo Materiali S.p.A., Italy

ABSTRACT Oil well reservoir water can have very high salinity values of more than 200 gL -1 as NaC1, and have weakly 'sweet' environments (pCO 2 = 0.2 MPa) and medium temperatures (100-110C). Experimental work was carried out to investigate the corrosion resistance of some CRAs (Corrosion Resistant Alloys) in these conditions and to verify their suitability for tubing strings in wells. Autoclave exposures and potentiodynamic tests were conducted in simulated environments to verify the resistance to uniform and localised (pitting, crevice) corrosion. Results showed that duplex and superduplex stainless steels present good resistance to pitting and crevice corrosion. Supermartensitic steels are affected by rare pitting; potentiodynamic test results revealed significant differences between the vaious tested supermartensitic steels showing differing pitting resistances.

1. In t roduct ion

In Africa, several oil fields have similar environmental aggressiveness which is mainly characterised by the very high salinity of the reservoir water. Well condit ions of some selected oil fields are summar ised in Table 1.

Based on literature data [1-3], the available material selection expert systems [4,5]

Table 1. Some selected field conditions characterised by very high salinity reservoir water

Geographical B.H.T. B .EP. B.H.P. Area (C) (MPa) (MPa)

Algeria (on-shore)

Congo (off-shore) . . . .

Congo (off-shore) . . . . .

Libya (on-shore)

90 14 32

100-120 26-29 26-32

80-90 13-20 22

120 21 21

CO 2 in gas phase at

BPP (%mol.)

Formation water

salinity (gL -1)

in situ pH

1 250-330 4.5

0.8-1 190 4.6

3-3.5 250-300 3.7-3.9 (1}

3.8 250 //(2)

B.H.T. = bottom hole temperature; B.P.P. = bubble point pressure; B.H.P. = bottom hole pressure. (1) In situ pH values have been calculated on the basis of dissolved CO 2, ignoring any buffering effects from bicarbonate ions as no data were available on water chemical composition. (2) No data were available on water chemical composition.

344 Advances in Corrosion Control and Materials in Oil and Gas Production

and manufacturers engineering field diagrams and guidelines, it was concluded that materials as noble as duplex stainless steel 22 or 25%Cr or higher, had to be selected to avoid corrosion in such environments. Carbon steel use is considered a high risk because of the potentially high corrosivity of the produced water; the consequent high corrosion rates mainly depend on the water cut, water chemical composition and on the oil filming efficiency. Other solutions to corrosion mitigation, such as internally coated carbon steels have been considered to be unsafe for production environments where damage to the coatings during wire line operations cannot be excluded in advance [6].

Martensitic stainless steels are normally considered safe up to a medium value of 80-150 gL -1NaC1 depending on the temperature [4]. The following empirical formulae are also used to provide limits for the applicability of martensitic and supermartensitic stainless steels, in the absence of H2S, depending on salinity and temperature:

$ if No No NaCI (gL -~) < 130-2/3 T(C) ~ NaCI (gL -1) < 200-8/15 T(C) ~ higher resistant steels

$ $ yes yes

martensitic SS13Cr supermartensitic SS S13Cr or 15Cr

Nickel Development Institute (NIDI) guidelines [7] limits the use of martensitic 13%Cr to a maximum temperature of about 80C with 250 gL -1 NaC1 depending on CO 2 content; the temperature limit decreases if the NaC1 content is increased.

The behaviour of the modified martensitic and duplex stainless steels at high chloride contents (> 20 % NaC1) has not yet been studied in detail [8]. In 200 gL -1 NaC1, low H2S (0.005 MPa) conditions, pitting occurs in the range 70-130C on martensitic steel and slight general corrosion and severe general corrosion are observed in the range 130-180C and above 180C respectively [6].

Experimental work has been carried out to investigate the corrosion behaviour of the most used CRAs (Corrosion Resistant Alloys) for well completions in slightly sweet environments, i.e. as characterised by low CO 2 contents, medium temperatures of up to 120C, very high salinity values > 200 gL -1, with the objective of selecting the most cost'effective CRAs.

2. Experimental 2.1. Materials

All samples used in this investigation were taken from production tubing of oil country tubular goods. For this study martensitic and some supermartensitic stainless steels provided by different steel makers (indicated as 'A', 'B' and 'C' in Table 2), were chosen. Supermartensitic stainless steels (SM) were characterised, from the corrosion resistance point of view, by a slight difference in Mo and Ni contents.

Duplex 22%Cr (D22), 25%Cr (D25) and a superduplex stainless steel (SD) alloyed with 0.7% Cu and 0.7% W were also tested. Carbon steel (CS) was selected as a reference material. The chemical compositions of the tested materials are shown in Table 2.

Localised Corrosion of some Selected CRAs in the Presence of Very High Salinity

Table 2. Chemical composition of the tested steels (wt%) (m nil or not tested)

Steel UNS CS

13Cr $42000

15Cr 'B' $42500

15Cr 'C' $42500

SM 'A' $41424

SM 'B' $41425

D22 $31803

D25 $31260

SD $32760

C Mn Cu Ni Cr Mo W N

0.290 0.67 0.17 0.10 1.07 0.53 0.01

0.200 0.81


Critical current density (i c ), passivity potential (E p), passive current density (ip), P P

pitting potential (Ep), transpassivity (Et) and corrosion potential (Ec) were determined for each test.

Specimens were polarised from a cathodic potential E c = -1.0 V in the anodic direction until pitting occurred. Pitting potential values were taken as the last value at which the current was as low as that of a completely passive specimen.

The environment for the pitting corrosion experiments was a deaerated NaC1 solution (330 gL-1). Measurements were made at 90C with a conventional glass cell.

A scanning rate of 1 mVs -1 was chosen for all the tests. Tests were considered completed when either the transpassivity E t or the pitting potential Ep value was reached.

2.4.2. Crevice The potentiodynamic method developed by Crolet et al. [9] was used in order to assess the crevice corrosion resistance of the selected materials. This is based on determining a depassivation pH, that is, the pH at which the passive film no longer offers protection against corrosion. Polarisation curves were a deaerated 330 gL -1 NaC1 solution at 90C at various pH values. The height of the active anodic peak was plotted as a function of pH. The pH values at which the active peak reached a nominal value of 0.1 Am -2 was taken as the depassivation pH [10].

3. Results and Discussion 3.1 Autoclave Exposures

3.1.1. Mass loss The average corrosion rates of the flat coupons exposed to the environmental (see Table 3) conditions in the autoclave are reported in Table 4. Higher mass loss values were obtained in the simulated oilfield solution a where the chloride content was 330 gL -1.

Carbon steel coupons were affected by general corrosion and high values of the mass loss were obtained: 450-459 mm/year and 172-210 mm/year for solution a and b respectively.

As expected [11,12], the corrosion rate of supermartensitic and duplex stainless steels was practically nil, i.e. lower than 4 mm/year for all the coupons exposed in both environments (Table 4).

3.1.2. Crevice corrosion Carbon steel was not considered since it was affected by general corrosion.

Specimens of duplex and superduplex stainless steel exposed in the simulated environment were only slightly affected by crevice corrosion. (Fig. 1).

Specimens of 13%Cr, 15%Cr and supermartensitic steels were prone to crevice corrosion (4-6 out of 80 corroded sectors).

However, it is worthy of note, that in all the examined cases, the affected sectors were only partially corroded.

3.1.3. Pitting corrosion Table 5 summarises the visual inspection of the pitting corrosion specimens after the test completion.


Table 4. Results of the autoclave exposure tests: mass loss determination

347

Environment a Environment b

Steel Multicrevice corrosion rate

(Bm/y)

Flat coupons corrosion rate

(Bin/y)

Multicrevice corrosion rate

(~tm/y)

Flat coupons corrosion rate

(~tm/y)

CS 450 459 210 172

13Cr Not tested Not tested 59 94

15Cr 'B' Not tested Not tested 24.7 17.7

15Cr 'C' Not tested Not tested 17.7 16.5

SM 'A' 1.3 1.4 0 0

SM 'B' 0.5 0.7 0 0.1

D22 Not tested Not tested 4.2 3.4

D25 0.2 0 0 0

SD 0.1 0 0 0

.~9 5 82

0 0 o < rn o o o Steel

, - ~- 03 09 04 04

Crevice susceptibility

Fig. 1 Crevice corrosion susceptibility as a function of the environment salinity. Number of corroded sectors out of 80 (series I = 330 gL -1 NaCI (a), series 2 = 315 gL -1 NaCI (b)).

All the specimen surfaces were very shiny except for carbon steel and martensitic stainless steel specimens.

Diffuse pitting was seen on 13%Cr steel. Supermartensitic stainless steel specimens exposed to environment a were slightly

prone to pitting: both SM 'A' and SM 'B' showed very few pits on at least one of the specimen surfaces.

Pit depth was evaluated by means of the stereomicroscope in order to determine the seriousness of the localised corrosion attack. Pits were mainly 5-10 ~tm deep and as a consequence the pitting corrosion rate was estimated to be 60-120 t, tm/y.

Samples of duplex (D25) and superduplex stainless steels (SD) showed good resistance to pitting corrosion.


Table 5. Pitting corrosion susceptibility of the tested steels exposed in the autoclave

Environment a Environment b Steel pitting (side 1) pitting (side 2) pitting (side 1) pitting (side 2)

CS General corrosion

13Cr Not tested 15Cr 'B' Not tested 15Cr 'C' Not tested SM 'A' No SM 'B' Very rare D22 Not tested D25 No SD No

General corrosion General corrosion General corrosion ,

Not tested Diffuse Diffuse

Not tested Rare Rare Not tested Rare Very rare Very rare No Very rare

Rare No Very rare

Not tested No No No No No No No No

, .

3.2. Potent iodynamic Tests

Potentiodynamic tests were performed on some duplicate material (see Table 6). Duplex stainless steel (D25) had the highest observed E value, namely 991 mV,

confirming an excellent resistance of the passive film to [ocalised breakdown by pitting.

Superduplex stainless steel (SD), although characterised by a higher PREN, had an E value (804 mV) lower than D25, possibly due to the presence of some inclusions in thPe microstructure.

Both supermartensitic stainless steel SM 'A' and SM 'B' pitting potential values lower than those of duplex stainless steels, the mean values being 30 and --47 mV, respectively. A significant difference of around 100 mV was observed between the two steels; SM 'A' showing the better pitting resistance. The anodic polarisation curves of the tested steels are shown in Fig. 2.

Figure 3 shows the ranking of the tested steels based on the depassivation pH value. The higher the pH at which the passive film breaks down, the less resistant is the material to crevice corrosion.

Decidedly higher crevice corrosion resistance was shown by the stainless duplex 25%Cr steel (D25). Some difference was observed between the two supermartensitic stainless steels in that SM 'A' was less susceptible to crevice than SM 'B' (Fig. 3).

The depassivation pH of 15%Cr steels was approximately one pH unit greater than the values for supermartensitic steels.

Table 6. Potentiodynamic test potential values (Ecorr , E ) and current density (Ipass) p' Steel E (mV) E (mV) I (mAm -2)

corr p pass

SM 'A' -650 -680 10 50 19 17 SM 'B' -563 -720 50 -44 17 19 D25 -650 -700 991 991 15 15 SD -617 -630 804 804 13 13

> E

v

tu

Localised Corrosion of some Selected CRAs in the Presence of Very High Salinity 349

1000-

600

200

-200

-600

-1000

. . . . . . . . . . . . . . . . . . .

D25 .~...~ . . . . . . SD -'"" ~o SMA i! SMB i /

i.." ~11 ,

i i i 1 u111~ . . . . . . . . . . . . .~. . . . ,~. . . ,~ . . . . .

" - - . o .......... '~" :Z '~ ,, ,, ...,,..... ...

I I I I I I

-9 -8 -7 -6 -5 -4

current density (Acm -2) -3

Fig. 2 Anodic polarisation curves of supermartensitic steels (SM 'A'and SM 'B'), duplex (D25) and superduplex steels (SD).

Fig. 3 Critical depassivation pH of the tested steels in 330 gL -1 NaCI, 90C.

Resistance to crevice corrosion

D 25

SMA SM B

15CrB 15CrC

Depassivation pH


3.3. Discussion

All the stainless steel specimens examined after autoclave exposure in simulated oil field environments showed no general corrosion.

Martensitic stainless steels showed a high susceptibility to localised corrosion in these environments. The 15Cr steels showed some tendency to localised corrosion with pitting and crevice corrosion being detected. Critical depassivation pH values are in fact higher than those of supermartensitic stainless steels.

Supermartensitic stainless steels present a slight susceptibility to localised attack and proneness to pitting and crevice corrosion should be taken into account. Resistance to pitting attack of steel 'A' was slightly superior to that of supermartensitic stainless steel 'B'.

In particular, the positive effect of a higher molybdenum content of the supermartensitic stainless steel 'A' is shown by potentiodynamic tests, highlighting the different film self-repairing capacity of the steels.

Duplex 25%Cr and superduplex stainless steels tested at 115C and in high chloride solutions (concentration up to 330 gL -1NaC1) gave good localised corrosion resistance. No susceptibility to pitting corrosion was observed and passive film resistance to pitting attack was satisfactory. As regards crevice corrosion resistance although a few examples of attack were observed in autoclave tests an excellent critical depassivation pH was found.

4. Conc lus ions

Experimental work has been carried out to investigate the corrosion behaviour of the most used CRAs for well completions in environments characterised by low CO 2 contents, medium temperatures up to 120C, very high salinity values of > 200 gL -1, with the purpose of selecting the most cost-effective material. Based on the results the following conclusions have been drawn:

13%Cr and 15%Cr steels are not to be considered safe and applicable in the tested environments because of their proneness to localised corrosion.

supermartensitic stainless steels can be used up to a salinity value of about 300 gL -1 in a temperature range of 100-120C with a low content of CO 2. However, in these environmental conditions supermartensitic stainless steels are not completely immune to localised corrosion.

duplex and superduplex stainless steels have higher corrosion resistance.

It is worthy of note that these environmental conditions are the 'border line' and consequently the chemical composition which characterises the various martensitic steels will affect the corrosion resistance mainly towards localised corrosion.


5. Acknowledgements

351

The authors would like to thank ENI-Agip Division for the kind permission to publish the results presented in this paper and steel makers for the supply of tubular materials for testing.

References

1. M. Barteri et al.,"Conservative Criteria for Application of Stainless Alloys in Gas & Oil Sour Wells" Innovation Stainless Steels Conference, Florence, Italy, 1993. 2. M. Schofield et al., "Comparison of Duplex and Modified 13Cr Martensitic Stainless Steels in High Temperature, Slightly Sour Conditions" Proc. Int. Conf. on Duplex Stainless Steels'97, The Hague, The Netherlands, 97. 3. M. Verneau et al., "Evaluation of the Corrosion Resistance of Duplex Stainless Steel in H2S and Chloride Containing Media" Proc. Int. Conf. EUROCORR "96, Paper 6, Nice, France. Cefracor/Soc. Chim. Ind., Paris 1996. 4. A. Kopliku et al., "An Expert System to Assist Corrosion Engineers in Material Selection for Well Completion", Corrosion "97, Paper No. 327, NACE International, Houston, Tx, 1997. 5. S. Srinivasan et al., "Selection of Martensitic and Duplex Stainless Steels for Oil and Gas Production Service", Corrosion "92, Paper No. 56, NACE International, Houston, Tx, 1992. 6. D. Condanni et al., "Carbon Steel Tubing Internally Coated with Resins: Mechanical and Corrosion Testing for Application in Corrosive Environments, Corrosion "97, Paper No. 65, NACE International, Houston, Tx, 1997. 7. B. D. Craig, "Selection Guidelines for Corrosion Resistant Alloys in the Oil and Gas Industry, NiDi Technical Series No. 10 073. 8. C. D. S. Tuck et al., "The Influence of Copper on Pitting Corrosion of Duplex Stainless Steels in Saline and Acid Media" Proc. Int. Conf. on Duplex Stainless Steels "97, The Netherlands, 1997. 9. J .L. Crolet, J. M. Defranoux, L. Seraphin and R. Tricot, Metaux-Corros. Ind., 1975, 599, 262. 10. J. Oldfield, "Test Techniques for Pitting and Crevice Corrosion Resistance of Stainless Steel and Nickel base Alloys in Chloride-containing Environments", NiDi Technical series No. 10016. 11. S. Huizinga et al., "Limitation for the Application of 13Cr Steel in Oil and Gas Production Environments", Corrosion '97, Paper No. 39, NACE International, Houston, Tx, 1997. 12. P. J. Cooling et al., "The Application Limits of the Alloyed 13%Cr Tubular Steels for Downhole Duties", Corrosion "98, Paper No. 94, NACE International, Houston, Tx, 1998.

Front MatterTable of ContentsPart I. Keynote PapersPart II. Carbon and Low Alloy SteelsPart III. Martensitic Stainless SteelsPart IV. Corrosion Resistant Alloys34. Localised Corrosion of some Selected Corrosion Resistant Alloys in the Presence of Very High Salinity34.1 Introduction34.2 Experimental34.3 Results and Discussion34.4 Conclusions34.5 AcknowledgementsReferences

35. High-Strength Corrosion Resistant Nickel-Base Alloys for Oilfield Applications35.1 Introduction35.2 Data Review35.3 SummaryReferences

36. Effect of Alloy Nickel Content vs Pitting Resistance Equivalent Number (PREN) on the Selection of Austenitic Oil Country Tubular Goods for Sour Gas Service36.1 Introduction36.2 Experimental36.3 Results and Discussion36.4 ConclusionsReferences

37. Effect of Grain Size on Stress Corrosion Cracking Resistance of Alloy G-3 (UNS N06985) OCTG in Sour Gas Environments37.1 Introduction37.2 Experimental37.3 Results and Discussion37.4 ConclusionsReferences

Part V. Galvanic CorrosionPart VI. Corrosion InhibitorsPart VII. Non-MetallicsIndexACDEFGHIKLMNOPRSTUVW

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Corrosion control and materials for Oil & Gas CAP.4