Effect of NACE TM0284 test modifications on the HIC performance of Large-Diameter Pipes

13 Pages·2.722 MB·English

by  Bosch C. et al.

Paper No.
08109
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 J.-P. Jansen
Europipe GmbH Europipe France SA
P.O. Box 10 05 04 BP 5527
45405 Muelheim a. d. Ruhr 5983 Dunkerque, Cedex 1
GERMANY 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,
Copyright
©2008 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.
1
INTRODUCTION

Pipelines are the preferred means of transportation of a liquid or gas over a long
distance. Wet sour gas and H S (hydrogen sulfide) containing media can be extremely
2
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 H S at ambient temperature and pressure. After 96 h testing
2
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 H S gas. The ratio of test solution volume to total surface area needs to
2
be a minimum of 3 ml per cm2 specimen surface. The pH is measured directly after H S
2
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
2
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 H S dosage system and must be placed in a hood this could easily affect
2
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.

3
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 pK of 3.78, a pH
a
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 H S to the environment. Also contamination of the
2
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
4
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 H S
2
containing solution for at least 96 h. H S is known to poison pH electrodes; it reacts with the
2
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.

5
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.
6
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 über die Schädigung
von Stählen 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 H S-containing environments in oil and gas production – Part 2: Cracking-resistant
2
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 H S Environments. NACE International,
2
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 H S
2
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

7
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
3 30
2
surface area ratio [ml / cm ]
2
total specimen surface area [cm ] 7920 880
total specimen volume [ml] 3600 400
test solution volume [ml] 24000 26500
number of specimen 90 10
vessel filling height [cm] 39 38
pH 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

NACE TM0284- EFC 16
test solution
2003 solution A solution A
test duration 96 h 96 h
H2S pressure 1 bar 1 bar
pH before H S saturation 2.7 3.4
2
pH after H S saturation 3.2 3.5
2
pH at test end 3.8 3.5
pH adjustment no yes
ratio specimen surface
2 2
3.1 ml / cm 3.1 ml / cm
to test solution

8
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 1.42
B 0.00 0.00 0.00
C 3.35 0.38 0.01
average 7.45 5.07 0.48
II: section CLR [%] CTR [%] CSR [%]
A 8.45 2.04 0.11
B 16.80 6.13 0.59
C 0.00 0.00 0.00
average 8.42 2.72 0.24
D 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
9
3,9
3,8
3,7
H 3,6
p
3,5
3,4
3,3
0 10 20 30 40 50 60 70 80 90 100
Test duration [h]

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

3,9
3,8
3,7
H 3,6
p
3,5
3,4
3,3
0 10 20 30 40 50 60 70 80 90 100
Test duration [h]

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

10