EFFECT OF NACE TM0284 TEST MODIFICATIONS ON THE HIC PERFORMANCE OF LARGE-DIAMETER PIPES

C. Bosch and T. Haase Salzgitter Mannesmann Forschung GmbH

P.O. Box 25 11 16 47251 Duisburg

GERMANY

A. Liessem Europipe GmbH

P.O. Box 10 05 04 45405 Muelheim a. d. Ruhr

GERMANY

J.-P. Jansen Europipe France SA

BP 5527 5983 Dunkerque, Cedex 1

FRANCE

ABSTRACT

The test method described in NACE TM0284-2003 'Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking (HIC)' is intended to provide reproducible test environments capable of distinguishing the susceptibility of different steel samples to HIC in a relatively short time. Recently, several significant modifications have been made or proposed to the test procedure of this standard. Amongst others, these modifications affect the test solution chemistry, pH limitations and solution volume to specimen surface area ratio. Frequently, the acidified acetate buffer test solutions, as proposed in the EFC 16 recommendations and initially intended for use in sulfide stress cracking (SSC) tests, have been adopted for HIC testing. A frequent difficulty associated with the EFC 16 test solution A (pH 3.5) is the requirement of pH drift limitation within 0.1 pH units. The paper describes how to overcome the difficulty of frequent manual pH adjustment by automation of pH control and adjustment. Other HIC test modifications, e.g. with regard to changes in solution volume to specimen surface area ratios are also addressed. Based on the test results, the need for sophisticated modifications of the NACE TM0284 standard HIC test is discussed. Keywords: Sour service, HIC, Large-Diameter Pipe, NACE TM0284, EFC 16, NACE MR0175 / ISO 15156,

INTRODUCTION

Pipelines are the preferred means of transportation of a liquid or gas over a long distance. Wet sour gas and H2S (hydrogen sulfide) containing media can be extremely damaging to the pipeline steel. In the presence of sour conditions (wet hydrogen sulfide) the anodic dissolution of the steel is accelerated. Hydrogen atoms originating from this corrosion reaction can diffuse into the steel resulting in cracking phenomena.1 Different forms of cracking may occur including hydrogen-induced-cracking (HIC), sulfide-stress-cracking (SSC) and stress-oriented-hydrogen-induced cracking (SOHIC).2 Due to the sudden occurrence of these cracking phenomena severe damage of the pipeline can result. This form of corrosion is much more dangerous than weight-loss corrosion that only reduces the wall thickness of a pipe. To avoid cracking in sour service it is very important that pipeline steel is tested for cracking resistance before pipelines are put into operation.

For HIC testing NACE TM0284-2003, which was revised in 2003, is the current test standard.3 This test method consists of exposing unstressed test specimens to a standard test solution saturated with gaseous H2S at ambient temperature and pressure. After 96 h testing time the test specimen is removed and evaluated. Two options are given for the test solution: solution A, which consists of 5 weight % sodium chloride and 0.5 weight % acetic acid in distilled water and test solution B, which is synthetic seawater. The following discussion is based only on test solution A, which has an initial pH of 2.6 to 2.8. The test is performed on rectangular, flat test specimens with a size of 100 mm in length and 20 mm in width with a thickness close to the wall thickness of the tested plate or pipe. The location, where test specimens have to be cut from the plate or pipe is clearly specified in NACE TM0284-2003. For testing the test specimens are placed in a test vessel and exposed to the test solution which is saturated with H2S gas. The ratio of test solution volume to total surface area needs to be a minimum of 3 ml per cm2 specimen surface. The pH is measured directly after H2S saturation and shall not exceed 3.3, and at test end, where it shall not exceed 4.0 for a valid test. After removal from the vessel the test specimens are sectioned for further metallographic evaluation. Sectioning is in general undertaken at three equidistant locations transverse to the rolling direction of the steel. For a base material specimen the sectioning positions are 25 mm, 50 mm and 75 mm of the specimen length. For each section the CLR (crack length ratio), CTR (crack thickness ratio) and CSR (crack sensitivity ratio) are calculated based on visible cracks identified under the microscope at magnifications up to 100x. The values for each section and the average values of CLR, CTR and CSR are reported at the end of the test. NACE TM0284-2003 does not include acceptance or rejection criteria for HIC testing. Acceptance criteria can be found e.g. in the NACE MR0175 / ISO 15156-2 standard with CLR 15 %, CTR 5 % and CSR 2 4 for tests in TM0284 test solution A.

NACE TM0284-2003 is the current test standard for HIC-testing of pipeline steel. In the recent past modifications to the test procedure and test evaluation have been suggested.

NACE TM0284 TEST MODIFICATIONS Modifications to the ratio of the volume of test solution to the total surface area

The ratio of the volume of test solution to the total surface area of the test specimen shall be a minimum of 3 ml/cm2. 3

It has been experienced that this test solution volume to specimen surface area ratio is sufficient to ensure that the pH does not exceed 4.0 at the end of the test after 96 hours. However, for the same test solution chemistry (test solution A) in NACE TM0177-2005 5 method A, 30 10 ml/cm2 are required for SSC tests, but for an extended test duration of 720 hours.

In addition to the larger quantity of test vessels needed to perform the same number of such HIC tests with 30 ml/cm2 test solution volume, the pH at the end of the NACE TM0284 test is lower compared to standard TM0284-2003 conditions. The end pH when using a ratio of 3 ml test solution per cm2 specimen surface is normally between 3.8 and 3.9. When a ten times higher ratio is used an end pH of only 3.5 to 3.6 is reached. This is easy to understand. Due to surface corrosion reactions (iron sulfide precipitation) during the HIC test the pH is rising from the initial value of 3.3. The buffering effect of the solution is low, but it helps not to exceed an end pH of 3.8 to 3.9 under the NACE TM0284-2003 conditions. If the ratio of test solution to specimen surface area is increased the pH drift is decreased due to a larger quantity of acetic acid, resulting in an end pH of 3.5 to 3.6. As the pH limit for test validity required by TM0284, is still 4.0 (regardless of the volume to surface area ratio), conducting a TM0284 test at conditions of 30 ml/cm2 generates a more severe environment than required by the standard TM0284 test.

First experiments performed on steel of known HIC sensitivity under TM0284 standard conditions (3 ml/cm2 solution volume) revealed that the extent of cracking is up to 4 times higher when a solution volume of 30 ml/cm2 is used.

Increasing the minimum solution volume from NACE TM0284 also has a huge influence on test capacity. In Table 1 the two test conditions are compared. In the authors' lab a 30 l test vessel (height 40 cm) is typically used to perform HIC-test on a larger quantity of specimens. For a specimen thickness of 20 mm the test vessel can contain up to 90 test specimens at a test solution volume to specimen surface area ratio of 3 ml/cm2. The total volume of test solution would be 24 l to cover the total specimen surface of 7920 cm2. The fill level of the vessel would be 39 cm resulting in a completely filled test vessel, which is in accordance with NACE TM0284-2003. As long as the specified ratio of volume of test solution to test specimen surface area is maintained, as many test specimen as will fit in the test vessel fully submerged and without touching may be exposed at one time.3

When the ratio of solution volume to specimen surface area is increased to 30 ml/cm2, in the same test vessel only 10 specimen of the same size can be tested at one time. They result in a total specimen surface area of 880 cm2 with a specimen volume of 400 ml. Using 26.5 l test solution makes the required ratio possible with a vessel fill level of 38 cm. In Figure 1 an image of a test vessel containing only one layer of test specimen is shown. It can be easily noted that most of the test vessel volume is used for storage of test solution. With a ratio of 30 ml test solution per cm2 specimen surface the effort to perform a HIC test is much higher compared to standard NACE TM0284-2003 conditions. More test vessels to test the same specimen quantity are needed; in the noted example instead of one vessel, nine test vessel would be required to perform a HIC test on 90 specimens. As each test vessel needs connections to the H2S dosage system and must be placed in a hood this could easily affect the capacity of small test labs. Also a larger amount of test solution and test gas is needed, that has to be disposed after the test in an environmentally friendly way.

Modifications to the test solution with a constant pH

The pH at the start of the test shall not exceed 3.3 and at the end of the test the pH shall not exceed 4.0 for the test to be valid. 3

In the recent past HIC testing according to NACE MR0175 / ISO 15156-2 in a solution different from NACE TM0284-2003 solution A at a constant pH 0.1 pH units has gained increasing recognition for HIC tests for specific or less severe duty. The test solution has been adopted from EFC 16 where it was initially intended for use in SSC testing only.4,7 The EFC 16 solution contains 5 weight % sodium chloride and 0.4 weight % sodium acetate. This solution is adjusted to an initial pH of e.g. 3.5, which is representative for various applications. Due to the presence of sodium acetate, which is the sodium salt of acetic acid with a pKa of 3.78, a pH buffering effect can be achieved. However, the buffer capacity of this solution is not sufficient to keep the pH in the range of 3.5 0.1. From the authors' experience, the pH at test end approaches 4.4 to 4.5 when the EFC 16 solution is used in a HIC test with a test solution to specimen surface area ratio of 3.0 ml/cm2. Already after 20 h of test duration a pH of 3.8 is reached, which is out of the intended range of 0.1 pH units. One option to keep the pH constant during the test is readjustment of the pH by adding small quantities of acid as suggested by the EFC 16 protocol. This can be achieved by manual adjustment before the pH reaches its upper limit of 3.6 (3.5 0.1). However, for this procedure the test vessel has to be reopened with the risk of disposal of toxic H2S to the environment. Also contamination of the test solution by ingress of oxygen is possible. By using a well designed test vessel system these problems should be solvable.

The main disadvantage of a manual pH adjustment circle can be seen in Figure 2. By repeated manual pH adjustment of a HIC test solution the pH can be easily maintained within pH = 3.5 0.1 during daytime, when lab staff is available. During nighttime and on weekends, when no lab staff is in operation, the pH drifts up to 3.8 (at 20 h test time) exceeding the pH upper limit and invalidating the test. Thus a manual pH adjustment is not practicable under standard lab conditions. The idea to increase the solution volume to test specimen surface area to about 10 ml/cm2 reduces the pH drift, but the necessity of frequent pH adjustment remains (Figure 3).

To improve the adjustment circle an automatic pH adjustment system has been developed in the authors' lab. The general principle of this system is continuous measurement of the pH of the test solution along with automatic addition of acid for readjustment when the pH reaches an upper limit. The schematic sketch for this adjustment system can be found in Figure 4. The test vessel is equipped with an additional pH measuring electrode. This pH electrode is connected to a transducer, where the upper and lower pH limit can be set. When the upper pH limit is reached a signal is sent to a magnetic dosage pump to start the periodical dosage of hydrochloric acid from a reservoir through a transport line directly into the test vessel. When the pH in the test vessel reaches a lower limit the pump again receives a signal to stop the adjustment process until the upper limit is reached again.

For operation of this system special care has to be taken. The over-dosage of acid may bring the pH below the lower limit of 3.4. As a rule of thumb an amount of 0.4 ml concentrated hydrochloric acid is needed to reduce the pH by 0.1 units at an interval of 2 hours. This system can be used for HIC testing under a constant pH in one test vessel. However, usage of this system on a larger quantity of test specimen would require installation of an automatic

adjustment system on every test vessel, making an impact on test complexity and costs. An additional aspect that has to be taken into account is the performance of the pH electrode. The requirements for the pH electrode in the system are very high. The electrode is exposed to H2S containing solution for at least 96 h. H2S is known to poison pH electrodes; it reacts with the electrolyte of the electrode which can result in incorrect pH values during the readjustment cycle. Electrodes that are resistant in such media for a long time remain stable by leakage of electrolyte in the solution to resist poisoning. As this might alter the composition of the test solution to some extent these electrodes should not be used. For a standard electrode after a 96 h HIC test the measured pH values are only slightly different ( 0.02 pH units) compared to calibration standard solutions (pH = 2.00, 4.00 and 7.00). This difference can be taken into account by setting the adjustment circle parameters. However, a 720 h SSC test at a constant pH, which is also mentioned in NACE MR0175 / ISO 15156-2 4, would not be possible with such an electrode.

HIC test have been performed under NACE TM0284-2003 conditions in EFC 16 solution A with pH adjustment to 3.5 on grade X60 steel. The chemical composition of this steel can be found in Table 2. In each solution three test specimens from the 300, 600 and 1200 position of a large-diameter pipe have been tested. Details of the test conditions are given in Table 3. For each test specimen the individual CAR has been determined by ultrasonic testing using an in-house technique. The average CAR for each test position has been calculated. The results are shown in Figure 5. The highest individual CAR value and average CAR can be found for specimens tested in NACE TM0284-2003 solution without pH adjustment. The distribution of the CAR values shows that the CAR of the specimens tested in EFC 16 solution with pH adjustment to pH 3.5 are in general slightly below the CAR of the specimens tested in NACE TM0284-2003. The results of testing at a constant pH of 3.5 0.1 with pH adjustment indicate that the efforts of automatic pH adjustment in HIC test need to be carefully balanced against the expected benefits, as the results at pH 3.5 0.1 are comparable to the NACE TM0284-2003 solution A standard conditions.

Metallographic evaluation of NACE TM0284-2003 specimens

After testing each pipeline test specimen shall be sectioned and the indicated surfaces examined. 3

In the NACE TM0284-2003 test for HIC only a small section of pipe material is tested. Due to the destructive testing method every additional test location will reduce the length of the remaining pipe. This test method is acceptable, because possible triggers for HIC initiation (e.g. segregation areas, nonmetallic inclusions) are distributed in the pipe on a statistical (random) basis. After exposure to the sour environment each specimen is cut at three defined locations, e.g. for a base material specimen in a distance of 25 mm, 50 mm and 75 mm apart from the specimen edge. Based on the individual values of the CLR, CTR and CSR of these three cuts, an average CLR, CTR and CSR is calculated for each specimen. Normally three specimens are taken around the pipe circumference, and when only one specimen fails the test, which means that either the CLR, or the CTR or the CSR is outside the range of agreed acceptance criteria, this pipe, and usually the related heat of steel is rejected. Typical acceptance criteria for the HIC test are given in common standards 4,6 and in the EFC 16 Guidelines 7.

Recently, ultrasonic testing (UT) of the specimens has been adopted in order to rank different steels with respect to their HIC resistance. 8 As a result from ultrasonic testing, the specimen area, which may be affected by HIC cracking, can be obtained. The typical value obtained from UT testing is the CAR (crack area ratio). The CAR depends on the applied ultrasonic technique and the calibration and therefore, in contrast to CLR, CTR and CSR, CAR is not an absolute measure.

In the plots of specimens of a large-diameter pipe with limited resistance to HIC (I and II in Figure 6), which resulted from UT testing applying an in-house technique, the distribution of HIC cracks and blisters in the specimen is shown. Sectioning of the two specimens (no. I at 3h and no. II at 6h position of the pipe), which have been HIC tested according to NACE TM0284-2003 at a ratio of 30 ml test solution per cm2 specimen surface area is illustrated as an example. The sections marked with A, B and C represent equidistant cutting according to NACE TM0284-2003. Each specimen has been sectioned at the indicated positions and the sections have been polished metallographically and etched. For comparison, an additional section D as indicated in Figure 6 was made in specimen II through the same UT indication, but at a different position, revealing higher CLR, CTR and CSR values.

The obtained values for CLR, CTR and CSR for each section and the average values for each specimen are given in Table 4. Some examples for HIC cracks are given in Figure 7. As the results show, it is likely that cracking will be found in the metallographic evaluation of equidistant sections for the case that smaller cracks and/or surface blisters are randomly distributed in the volume of the specimen (Figure 6, specimen I). The fact that larger cracks are not necessarily found or fully represented by this evaluation method (Figure 6, specimen II) underlines the statistical character of the test.

CONCLUSIONS

Modifications of the test standard NACE TM0284-2003 for HIC testing have been investigated. In detail these are the influence of the test solution volume to specimen surface area ratio and the influence of a constant pH during the test. The NACE TM0284 metallographic evaluation procedure has also been taken into account.

Basically, neither a multiplication of the test solution volume nor testing at a constant pH of 3.5 0.1 reveals a remarkable improvement in the HIC test method. A tenfold increase of the solution volume during the test enhances the test expenses nearly ten times and reduces the test capacity dramatically to 1/10 of the initial capacity. Also the test severity appears to be increased due to a lower pH drift during the test, which leads to a lower final pH of about 3.5, which is far below the pH limit of 4.0 of the well established NACE TM0284-2003 standard.

Testing at constant, but low, pH of 3.5 during the HIC test requires frequent readjustment of the test solution pH during the test, even in the case that the EFC 16 acetate buffer solution is used. Manual readjustment is accompanied by practical challenges, as a test lab is normally not staffed 24/7. It has been shown that it is possible to use an automatic pH adjustment system, but apart from the investments in such a system the long-term stability of pH electrodes in sulfide containing media needs to be addressed. A comparison of specimens from the same steel tested without and with pH adjustment revealed that pH adjustment to pH 3.5 results in HIC performance comparable to the standard conditions from NACE TM0284-2003.

Equidistant sectioning of test specimens following NACE TM0284-2003 is in agreement

with the statistical appearance of possible HIC initiation sites in pipeline steels. Related acceptance criteria for CLR, CTR and CSR published in common standards are based on this approach of evaluation.

REFERENCES 1. Dahl, W.; Stoffels, H.; Hengstenberg, H.; Dueren, C: Untersuchung ber die Schdigung

von Sthlen unter Einfluss von feuchtem Schwefelwasserstoff (in German). Stahl und Eisen, 87 (3) (1967) 125-136

2. Poepperling, R.; Benett, C.; Brown, A.; Pontremoli; N.; Provou, Y.: Results of Full Scale

Testing and laboratory Tests of Line Pipe Steels. CORROSION 91, paper 16, NACE International Houston/Tx, USA, 1991

3. NACE TM0284-2003: Evaluation of Pipeline and Pressure Vessel Steels for Resistance

to Hydrogen-Induced-Cracking. NACE International, Houston/Tx, USA, 2003 4. NACE MR0175 / ISO 15156-2: Petroleum and natural gas industries Materials for use

in H2S-containing environments in oil and gas production Part 2: Cracking-resistant carbon and low alloy steels and the use of cast irons. ISO, 2003

5. NACE TM0177-2005: Laboratory Testing of Metals for Resistance to Sulfide Stress

Cracking and Stress Corrosion Cracking in H2S Environments. NACE International, Houston/Tx, USA, 2005

6. ISO 3183, "Petroleum and natural gas industries Steel pipe for pipeline transportation

systems", ISO, 2007 7. Guidelines on Materials Requirements for Carbon and Low Alloy Steels for H2S

Containing Environments in Oil and Gas Production. EFC Publication No. 16, European Federation of Corrosion, 1995

8. Bosch, C.; Schroeder, J.; Kulgemeyer, A.: Performance of X65 to X100 Large-Diameter

Pipes under various Hydrogen Charging Conditions. CORROSION 2005, paper 05118, NACE International, Houston/Tx, USA, 2005

TABLE 1 Comparison of test conditions for HIC testing with different ratios of test solution to specimen

surface area

volume of test solution to specimen surface area ratio [ml / cm2]

3 30

total specimen surface area [cm2] 7920 880total specimen volume [ml] 3600 400test solution volume [ml] 24000 26500number of specimen 90 10vessel filling height [cm] 39 38pH at test end 3.8-3.9 3.5

TABLE 2 Chemical composition of the investigated grade X60 steel in mass-%

C Si Mn P S Al Cu + Ni + Cr Others

0.036 0.260 1.300 0.009 0.0004 0.020 0.090 Nb, Ti

TABLE 3 Test conditions without and with pH adjustment

test solution NACE TM0284-2003 solution AEFC 16

solution Atest duration 96 h 96 hH2S pressure 1 bar 1 barpH before H2S saturation 2.7 3.4pH after H2S saturation 3.2 3.5pH at test end 3.8 3.5pH adjustment no yesratio specimen surface to test solution 3.1 ml / cm

2 3.1 ml / cm2

TABLE 4 CLR, CTR and CSR values for the specimen I and II. The sections A, B and C are based on equidistant sectioning according to NACE TM0284-2003. Section D in specimen II was made

in the same UT indication, but at a different position.

I: section CLR [%] CTR [%] CSR [%]A 19.00 14.84 1.42B 0.00 0.00 0.00C 3.35 0.38 0.01

average 7.45 5.07 0.48

II: section CLR [%] CTR [%] CSR [%]A 8.45 2.04 0.11B 16.80 6.13 0.59C 0.00 0.00 0.00

average 8.42 2.72 0.24D 47.40 19.82 2.81

FIGURE 1 - Image of a test vessel containing only one layer of specimens intended for HIC

testing with a ratio of test solution volume to specimen surface area of approximately 30 ml/cm2

FIGURE 2 Changes of pH during a HIC test in EFC solution A with manual adjustment to pH 3.5 at a solution volume to specimen surface area ratio of 3.1 ml / cm2

FIGURE 3 Changes of pf pH during HIC test in EFC solution A with manual adjustment to

pH 3.5 at a solution volume to specimen surface area ratio of 9.4 ml / cm2

3,3

3,4

3,5

3,6

3,7

3,8

3,9

0 10 20 30 40 50 60 70 80 90 100

Test duration [h]

pH

3,3

3,4

3,5

3,6

3,7

3,8

3,9

0 10 20 30 40 50 60 70 80 90 100

Test duration [h]

pH

FIGURE 4 Test set-up for an automatic pH adjustment system

FIGURE 5 Individual and average CAR values for HIC testing without and with pH adjustment

0

2

4

6

8

10

individual CAR average CAR

CA

R [%

]

300 600 1200 300 600 1200 position NACE TM0284 EFC 16 (pH 3.3 - 4.0) pH 3.5 (adjusted)

N2, H2S, CO2

pH 3,5

HCl

HIC test vesseltransducer magnetic

dosage pump acid reservoir

N2, H2S, CO2

pH 3,5

HCl

HIC test vesseltransducer magnetic

dosage pump acid reservoir

pH 3,5pH 3,5

HCl

HIC test vesseltransducer magnetic

dosage pump acid reservoir

FIGURE 6 - Results from ultrasonic testing of specimens I and II. Sectioning according to

NACE TM0284-2003 (A, B and C). Additional section D for comparison

A

B

C

A

B

C

D

specimen no. I specimen no. II

Top view Side view Side view Top view

FIGURE 7 Image of cracks from specimen II in a) section B and b) section D

a) b)

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