2009

Copyright 2009 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be in writing to NACE International, Copyright Division, 1440 South creek Drive, Houston, Texas 777084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.

Erosion-Corrosion in Oil and Gas Stainless Steel under de-aerated slurry impingement attack

Inga Bargmann1, Anne Neville, Staffan Hertzman2, Faizal Reza3, Xinming Hu

Institute of Engineering Thermofluids, Surfaces and Interfaces, School of Mechanical

Engineering, University of Leeds, Leeds, United Kingdom 2Outokumpu Stainless Research Foundation, Avesta Research Centre,

SE-774 22 Avesta, Sweden 3Petronas Carigali SDN. BHD., 50088 Kuala Lumpur, Malaysia

1Corresponding e-mail address: menibmba@leeds.ac.uk

ABSTRACT

Stainless steels are known for their excellent corrosion properties when they can rely on their passive film. Passivity is attributed to a self-healing oxide passive layer on the metallic surface with a thickness of around 2-4 nm [1], that establishes as soon as it is exposed to an oxygen-containing environment. The response of passivating materials to the combined attack of erosion and corrosion is still not well understood for materials exposed to oil and gas production environments that are saturated with CO2. In this study a superaustenitic stainless steel was tested under erosion-corrosion attack in de-aerated and carbon dioxide saturated artificial seawater. The focus was on understanding the role of the surface film and the influence of changing velocity and solid loading. The contribution of both erosion and corrosion will be analysed and integrated electrochemical measurements are used to enable the corrosion rate to be assessed as a function of solid content and flow velocity. Keywords: Erosion-corrosion, stainless steel, de-aerated artificial seawater

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Paper No.

09483

INTRODUCTION

Investigations of erosion-corrosion mechanisms receive much attention in the oil and gas processing industry, due to the widespread occurrence and cost intensive failures of equipment [2-4]. A flowing medium like crude oil, water, gas or a mixture of these phases containing aggressive ions, solid particles and/or corrosion products can cause a number of different degradation mechanisms which make it difficult to predict the performance [5]. Erosion-corrosion is the degradation which results from the conjoint effects of erosion and corrosion. Corrosion attacks the material by dissolving the surface causing a higher roughness and destabilisation of surface films on certain materials. Erosion destroys the surface by impact leading to material loss, material deformation, surface hardening and/or surface fatigue. Both mechanisms can also interact with each other, in a synergistic manner and the presence of such synergism can cause higher damage to materials, than the sum of erosion and corrosion [6, 7]. Several laboratory experiments have already been conducted to determine the contribution of each component of erosion-corrosion to the overall weight loss and critical parameters (e.g. velocity, solid loading) were formulated [6, 8-10]. Models have also been derived and are often helpful for handling the large amount of influencing parameters, especially in the case of CO2 corrosion [11-13], but to date no models exist to accurately predict erosion-corrosion where electrochemical and mechanical factors and their interactions are important. Regarding stainless steels, the combination of corrosive and erosive mechanisms can cause multiple types of material degradation. Firstly the passive film is removed, then the underlying, unprotected material is activated [5, 14]. It is therefore necessary to understand the role of such passive films on stainless steel and their performance in a corrosive environment under liquid-solid attack to improve and predict service life of offshore equipment and to reduce maintenance costs [2, 15]. Whilst there is a large volume of literature about carbon steel and CO2 erosion-corrosion [16-21], the passivity of stainless steels in such environments, where breaches of the passive film can result from erosion, has received much less attention. Typical offshore conditions, such as de-aerated high saline CO2 environments, are barely found in combination with performance tests of passive films under erosion-corrosion conditions. To focus on these conditions, a high saline solution with minimized content of dissolved oxygen was used for measurement of the total weight loss. Furthermore in-situ electrochemical measurements were conducted and compared. The contribution of the corrosion and erosion components to the overall total weight loss is evaluated and furthermore the influence of the factors of velocity and particle concentration analysed.

EXPERIMENTAL

Procedure

The material used in this study is a high alloyed super grade of an austenitic stainless steel according to UNS S31254. Its composition and Vickers hardness is given in Table 1.

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Table 1: Composition of UNS S31254

UNS S31254

C Si Mn P S Cr Ni Mo N Cu HV0.5kg

0.01 0.30 0.34 0.02 0.00 20.16 18.12 5.98 0.21 0.60 200

Sample preparation

The stainless steel specimens were cut from plates into (20x20) mm2 pieces. The surface of all materials was ground to a surface finish using P1200 SiC paper. Samples for polarization measurements were first mounted into epoxy resin with a wire attached and then ground. All samples were cleaned, rinsed with water and dried with air. The un-mounted samples were subsequently rinsed with acetone and weighed. The day of the sample preparation was the day of the test run and all test samples were kept in a vacuum-desiccator. After a test was stopped, the samples were immediately taken out of the rig, cleaned, rinsed with water and dried with air. Acetone was taken for cleaning in the case of unmounted samples and the weighing done the same or the next day.

Apparatus

Erosion-corrosion tests have been conducted using a re-circulating flow loop involving impingement of a liquid/solid slurry onto the specimen surface, in a sealed re-circulating jet impingement test rig (Figure 1). The sample holder with the nozzles is immersed into the solution and kept below the water level during the test run. The diameter of the nozzles is 4 mm, pointing the liquid towards the sample at a 90 degree angle. The samples were placed at a distance of 5 mm from the nozzles. A computer controlled potentiostat was used to measure the open circuit potential (OCP) and to conduct linear polarization (LP) measurements.

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CO2 sparging

Electrolyte

Specimens

ReferenceElectrode

CounterElectrode

Potentiostat and scan generator

Reservoir

Pump

Water JetSpecimen

AV

(a) (b)

Lid with Seal

Figure 1: a) Re-circulating jet impingment rig for liquid slurry erosion-corrosion tests;

b) Sample position [10]

Test conditions

The rig was filled with a sodium chloride solution, containing the same chloride concentration (53g/l) as a brine produced in the Forties field. A 4-hour sparging time with CO2 ensured carbon dioxide saturation, resulting in a pH of ~4.00. The temperature was set and controlled at 50 degrees. After the start, the water was circulated by a pump in a closed loop during the 4 hours test run at a constant speed. The specific solids loading (sL) was added to the solution before the test was started. The test was not started before the oxygen level had dropped below 100 ppb. After the test, the oxygen level was measured again and found to be far below that level, at around 50 ppb. The test conditions are stated in Table 2 and are classified according to their severity mild, severe and two intermediate conditions:

Table 2: Test conditions for the impingement tests V1 = 7 m/s V2 = 20 m/s

sL1 = 50 ppm Mild Int II

sL2 = 500 ppm Int I severe

Test Procedures

The total weight loss (TWL) was determined by weighing the unmounted samples before and after the test-run. It is then expressed as erosion-corrosion (E-c) rate in mm/y. The area taken for calculation of the thickness loss is the centre circle (area 1, presented in Figure 2) on the

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sample surface that was formed during the jet impingement. It was found to be the area with the main material loss. The area was measured to be 28.3 mm2 compared to 400 mm2 for the entire surface. To analyse the corrosion response (CE) , a mounted specimen was connected to a three electrode set-up as the working electrode. The measured potential was referred to a saturated Ag/AgCl electrode and a platinum grid was used as a counter electrode. The corrosion rate is expressed as the thickness loss in mm/y.

Test Methods

Integrated electrochemical measurements enable the corrosion rate to be assessed as a function of solid particle content and flow velocity. The open circuit potential (OCP) is defined as an electrode potential measured when anodic and cathodic reactions are in dynamic equilibrium. The OCP was plotted for a full test run of 4 hours. The polarization resistance (Rp) is conducted by shifting the potential of the sample from -20mV to +20mV versus the OCP and measuring the resulting current density (linear polarization measurement). The Rp value is the slope of the linear polarization curve (i.e. the measured voltage over the measured current density). The measurements were taken every 15 minutes during the first hour, every 30 minutes during the second hour and every 60 minutes during the last 2 hours of each test resulting in 9 measurement points for a four hour test-run. To obtain material loss due to pure mechanical erosion (E), cathodic protection (C.P.) was applied to the test sample using the impressed current method followed by weight measurement. The cathodic potential for was 0.8V at 50C without sparging CO2 to minimize corrosion and to avoid hydrogen imbrittlement on the material surface. The hardness of the sample surface was measured for untested and tested conditions. For the tested samples the measurement points were taken in a line across the entire surface, crossing the centre of the inner circle, as demonstrated in Figure 2. The three areas present at the surface are formed due to the jet profile that develops underneath the jet. The diameter of the inner circle was found to be very similar to the nozzle of the jet impingement rig. It is the area with the highest material loss.

5

3

2 1

Figure 2: Direction of hardness measurement of sample surface (arrow)

RESULTS

The components of total weight loss under erosion-corrosion is presented as follows:

SCETWL E ++= ( 0.1) where E is the weight loss due to pure mechanical erosion. CE is the weight loss due to electrochemical corrosion enhanced by erosion. S is the synergism, the interaction of mechanisms, also stated as the effect of corrosion on erosion [22]. So equation (3.1) can be written as:

CE ECETWL ++= ( 0.2) with EC being the corrosion enhanced erosion [23]. In the following section the parts of the overall TWL are determined experimentally and the influence of velocity, solids loading and their interaction is determined.

TWL

The results of the erosion-corrosion rate are presented in Figure 3 and Figure 4. Firstly the rate is presented over the two different levels of particle loading at a) 7 m/s and b) 20 m/s. At 7 m/s a very low increase of material loss is shown. At 20 m/s, the rate increases more significantly, shown by a bigger slope between the two particle loadings. The particle frequency apparently becomes more important with velocity.

6

050

100

150

0 200 400 600

Particle concentration [ppm]

E-c

rate

[mm

/y]

0

50

100

150

0 200 400 600

Particle concentration [ppm]

E-c

rate

[mm

/y]

a) b)

Figure 3: Erosion-corrosion rate as a function of particle loading at velocities of a) 7 m/s, b) 20 m/s

Afterwards the erosion-corrosion rate is presented over the velocities with a) 50 ppm and b) 500 ppm of particle loading. Apparently an increase of velocity has a minimal effect on the material loss when the particle loading is very low. At a higher particle loading the particle velocity soon increases the destructiveness of solid impacts as expected.

0

50

100

150

0 10 20 30velocity [m/s]

E-c

rate

[mm

/y]

0

50

100

150

0 10 20velocity [m/s]

E-c

rate

[mm

/y]

30

a) b)

Figure 4: Erosion-corrosion rate as a function of velocity at particle loadings of a) 50 ppm. b) 500 ppm

In Figure 5 and 6 the free corrosion potential trends for all four test conditions are presented. As shown, two main regions of potential are established during the test run. The conditions at 7 m/s show a noble corrosion potential, with many oscillations during the Int I test. At 20 m/s the potential quickly becomes very active after the test is started. For the Int II conditions an induction time was observed for the potential to reach the same potential that was measured under severe conditions.

7

-500

-400

-300

-200

-100

00 60 120 180 240

time [min]

E [m

V]

mildInt I

Figure 5: Free corrosion potential(Ecorr) under erosion-corrosion of UNS S31254 for mild and Int I conditions

-500

-400

-300

-200

-100

00 60 120 180 240

time [min]

E [m

V]

Int IIsevere

Figure 6: Free corrosion potential (Ecorr) under erosion-corrosion of UNS S31254 for Int II

and severe conditions

The polarization resistance Rp for all four test conditions is presented in Figure 7 with the corresponding calculated corrosion rates as a percentage of the overall erosion-corrosion rate presented in Table 3. An increasing corrosion resistance was observed during the mild test conditions up to a value of over 30 k/cm. Possibly, the passivity is able to increase during a mild flow velocity with a low concentration of solid particles transported in the flowing liquid. The resistance for Int I conditions starts with the same initial value like the mild test conditions, but remains on a steady level throughout the whole test run. The tests conducted under Int II and severe conditions show the lowest values for the resistance from the beginning. Those values remain stable during the whole test run.

8

010

20

30

40

50

0 60 120 180 240

time [min]

Rp

[k

*cm

] mildInt IInt IIsevere

Figure 7: Corrosion resistance (Rp) under erosion-corroion over time for UNS S31254

The measured high resistance for the mild conditions results in a very low corrosion rate as expected. The rates for the conditions Int I and Int II also show low results. The highest rate was calculated for the severe conditions. The corrosion rate is given as a percentage of the erosion-corrosion rate in Table 3.

Table 3: Corrosion rate (CE) as a percentage of the erosion-corrosion rate Mild

[%] Int I [%]

Int II [%]

Severe[%]

UNS S31254 0.4 0.4 0.8 1.0

The results for the hardness measurement are presented in Figure 8 to Figure 12. Firstly the hardness profiles for the conditions at 7 m/s are shown in Figure 8. With 50 ppm solids loading, the values for the surface hardness do not differ from those of the untreated samples over the whole length of the sample. At 500 ppm the center part of the sample surface shows a slight increase in hardness of 33%, over a length of 6 mm.

100150200250300350400450

0 5 10 15 20

sample length [mm]

HV

500

g

mildInt I

Figure 8: Hardness (HV) measurement for 7 m/s at different particle loadings

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At 20 m/s and both solid loadings the centre of the specimens experience a high work hardening effect for both conditions. For 50 ppm and 500 ppm solid loading the difference in central hardness is approximately 5%. However, in the severe conditions (500ppm) the outside area of the wear scar is also work hardened.

100150200250300350400450

0 5 10 15 20

sample length [mm]

HV

500

gInt IIsevere

Figure 9: Hardness (HV) measurement for 20 m/s at different particle loading

In Figure 10 an extra test condition, Int III, is introduced, for which the hardness profile was measured. It was conducted with the same particle loading as the severe and Int I conditions (500ppm) with a velocity of 16.8 m/s. The center of the specimen surface, with an impingement angle of 90, responds almost directly to increasing velocity and shows similar hardness results to the severe conditions, indicating the very small difference between two velocities. The hardness profile at the edge of the sample, where the impingement angle is much smaller, the values of Int III show similarities to Int I condition, which were stated in Figure 8. This is likely to be due to the particles ability to penetrate the squeeze film and impact the surface at different impingement angles.

100150200250300350400450

0 5 10 15 20

sample length [mm]

HV

500

g

severeInt III

Figure 10: Hardness (HV) measurement for 500 ppm at different velocities

The erosion rates for the two test conditions at 7 m/s and the Int II condition, with applied cathodic protection, give an erosion rate that exceeds the overall erosion-corrosion rate. The

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mild condition give an 8.5 times higher erosion rate compared to the erosion-corrosion rate. For the Int I and the Int II conditions the results of the erosion rate are 2.5 and 1.2 times higher than the overall degradation rate. The erosion rates for these three conditions have very similar values and show lower dependency on the changing velocity and level of solids loadings than the erosion-corrosion rate or the pure corrosion rate. The highest erosion rate was measured for the severe conditions. For this condition, the erosion rate was found to be 36% of the erosion-corrosion rate.

01020304050

0 200 400 600

Particle concentration [ppm]

Eros

ion

rate

[mm

/y]

01020304050

0 200 400 600

Particle concentration [ppm]

Eros

ion

rate

[mm

/y]

a) b)

Figure 11: Erosion rate over particle loading at velocities of a) 7 m/s, b) 20 m/s

The hardness of the erosion samples was measured for the two most extreme conditions, mild and severe. The work hardening effect of the material surface is the same under applied protection and without protection. The corrosion has apparently no influence.

100150200250300350400450

0 5 10 15 20

sample length [mm]

HV

500

g C.P. mildC.P. severeTWL mildTWL severe

Figure 12: Hardness (HV) measurement with cathodic protection applied for mild and

severe conditions

DISCUSSION

The percentage contribution of the components of the erosion-rate are presented in Figure 13. As the erosion rate was found to be higher than the erosion-corrosion rate, the calculation of

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the synergy leads to a negative value, which becomes smaller, the more severe the test conditions get. It becomes positive for the test condition severe.

-200

20406080

100120

m

E-C

rate

[mm

/y]

ErosionCorrosionSynergy

Figure 13: Percentage cFor evaluation of the percentathe ANOVA (Analysis of Varianinfluences were calculated for tand erosion. The values can byet. The negative values that investigations. The graphic shows the velocitand the interaction of both factfor the velocity.

E-C

E

C

Figure 14:

As the velocity is of higher inflbe strongly connected to the topresent the erosion-corrosion rsevereild Int I Int II

Test conditions

ontribution of the compontents to the erosion-corrosion rate

nvironmental factors and their interaction ge influence of the e

ce) was used and the percentage contribution calculated. The

he overall erosion-corrosion rate and the components corrosion e found in Figure 14. The synergy component is not presented were found for three test conditions will be subject to future

y being the strongest influential parameter. The solids loading ors are comparable in their influence, with smaller impact than

velocity particle loadinginteraction

46% 27% 27%

37% 31% 32%

38% 32% 30%

Percentage influence of environmental factors

uence than the particle loading the kinetic energy is thought to tal damage at the material surface. Figure 15 and Figure 16 first ate and afterwards the corrosion rate versus the kinetic energy.

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020406080

100120140

0 10 20 30 4

Kinetic energy [mW]

E-c

rate

[mm

/y]

0

Figure 15: Erosion-corrosion rate versus the kinetic energy

0.00.20.40.60.81.01.21.4

0 10 20 30 4

Kinetic energy [mW]

Cor

rosi

on ra

te [m

m/y

]

0

Figure 16: Corrosion rate versus the kinetic energy The correlations of the two rates with the kinetic energy seem comparable. Though the corrosion is of several magnitudes smaller than the erosion-corrosion rate, it seems to be a good indicator for the overall performance of the material under erosion-corrosion tested under these conditions. The erosion-corrosion rate as a function of the corrosion rate is stated in Figure 17.

13

020406080

100120140

0.0 0.5 1.0 1.5

Corrosion rate [mm/y]

Eros

ion

Rat

e [m

m/y

]

Figure 17: Degradation rate of erosion-corrosion over corrosion resistance

More conditions would be needed to find out the type of dependency between the erosion-corrosion rate and the corrosion rate. Apparently, the corrosion mechanism plays a big role for the overall material loss and the interaction with the erosion mechanism, though the actual value of the pure corrosion component is very small. Possibly, the reason for the synergy component found to be the biggest component of all components of the overall total weight loss in this study.

CONCLUSIONS

The weight loss of a super austenitic steel subjected to corrosion and erosion is influenced primarily by the velocity of the solids in the fluid. Cathodic protection of the stainless steel leads to unexpected weight loss values values. With the observed values the synergy would become negative for three of the four test conditions. The most severe condition showed positive values. It was shown that the kinetic energy correlates well with the observed material deterioration. The corrosion rate seems a good indicator for an estimation of the erosion-corrosion rate. The corrosion rate is about 100 times smaller then the overall degradation rate, but the influence of environmental factors seem comparable and the connection of the degradation and the corrosion rate was shown.

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