Sulfide Stress Cracking
--NACE MR0175-2002, MR0175/ISO 15156
650
Te c h n i c a l
The Details
NACE MR0175, “Sulde Stress Corrosion Cracking Resistant
Metallic Materials for Oil Field Equipment” is widely used
throughout the world. In late 2003, it became NACE MR0175/
ISO 15156, “Petroleum and Natural Gas Industries - Materials for
Use in H2S-Containing Environments in Oil and Gas Production.”
These standards specify the proper materials, heat treat conditions
and strength levels required to provide good service life in sour
gas and oil environments.
NACE International (formerly the National Association of
Corrosion Engineers) is a worldwide technical organization which
studies various aspects of corrosion and the damage that may
result in reneries, chemical plants, water systems and other types
of industrial equipment. MR0175 was rst issued in 1975, but the
origin of the document dates to 1959 when a group of engineers
in Western Canada pooled their experience in successful handling
of sour gas. The group organized as a NACE committee and in
1963 issued specication 1B163, “Recommendations of Materials
for Sour Service.” In 1965, NACE organized a nationwide
committee, which issued 1F166 in 1966 and MR0175 in 1975.
Revisions were issued on an annual basis as new materials and
processes were added. Revisions had to receive unanimous
approval from the responsible NACE committee.
In the mid-1990’s, the European Federation of Corrosion (EFC)
issued 2 reports closely related to MR0175; Publication 16,
“Guidelines on Materials Requirements for Carbon and Low Alloy
Steels for H2S-Containing Environments in Oil and Gas Production”
and Publication 17, “Corrosion Resistant Alloys for Oil and Gas
Production: Guidance on General Requirements and Test Methods
for H2S Service.” EFC is located in London, England.
The International Organization for Standardization (ISO) is a
worldwide federation of national standards bodies from more than
140 countries. One organization from each country acts as the
representative for all organizations in that country. The American
National Standards Institute (ANSI) is the USA representative
in ISO. Technical Committee 67, “Materials, Equipment and
Offshore Structures for Petroleum, Petrochemical and Natural Gas
Industries,” requested that NACE blend the different sour service
documents into a single global standard.
This task was completed in late 2003 and the document was
issued as ISO standard, NACE MR0175/ISO 15156. It is
now maintained by ISO/TC 67, Work Group 7, a 12-member
“Maintenance Panel” and a 40-member Oversight Committee
under combined NACE/ISO control. The three committees are an
international group of users, manufacturers and service providers.
Membership is approved by NACE and ISO based on technical
knowledge and experience. Terms are limited. Previously, some
members on the NACE Task Group had served for over 25 years.
NACE MR0175/ISO 15156 is published in 3 volumes.
Part 1: General Principles for Selection of Cracking-Resistant Materials
Part 2: Cracking-Resistant Carbon and Low Alloy Steels, and the
Use of Cast Irons
Part 3: Cracking-Resistant CRA’s (Corrosion-Resistant Alloys) and
Other Alloys
NACE MR0175/ISO 15156 applies only to petroleum production,
drilling, gathering and ow line equipment and eld processing
facilities to be used in H2S bearing hydrocarbon service. In the
past, MR0175 only addressed sulde stress cracking (SSC). In
NACE MR0175/ISO 15156, however, but both SSC and chloride
stress corrosion cracking (SCC) are considered. While clearly
intended to be used only for oil eld equipment, industry has
applied MR0175 in to many other areas including reneries, LNG
plants, pipelines and natural gas systems. The judicious use of
the document in these applications is constructive and can help
prevent SSC failures wherever H2S is present. Saltwater wells and
saltwater handling facilities are not covered by NACE MR0175/
ISO 15156. These are covered by NACE Standard RP0475,
“Selection of Metallic Materials to Be Used in All Phases of Water
Handling for Injection into Oil-Bearing Formations.”
When new restrictions are placed on materials in NACE MR0175/
ISO 15156 or when materials are deleted from this standard,
materials in use at that time are in compliance. This includes
materials listed in MR0175-2002, but not listed in NACE
MR0175/ISO 15156. However, if this equipment is moved to a
different location and exposed to different conditions, the materials
must be listed in the current revision. Alternatively, successful use
of materials outside the limitations of NACE MR0175/ISO 15156
may be perpetuated by qualication testing per the standard. The
user may replace materials in kind for existing wells or for new
wells within a given eld if the environmental conditions of the
eld have not changed.
Sulfide Stress Cracking
--NACE MR0175-2002, MR0175/ISO 15156
651
Te c h n i c a l
New Sulfide Stress Cracking Standard
for Refineries
Don Bush, Principal Engineer - Materials, at Emerson Process
Management Fisher Valves, is a member and former chair of
a NACE task group that has written a document for renery
applications, NACE MR0103. The title is “Materials Resistant
to Sulde Stress Cracking in Corrosive Petroleum Rening
Environments.” The requirements of this standard are very similar
to the pre-2003 MR0175 for many materials. When applying this
standard, there are changes to certain key materials compared with
NACE MR0175-2002.
Responsibility
It has always been the responsibility of the end user to determine
the operating conditions and to specify when NACE MR0175
applies. This is now emphasized more strongly than ever in NACE
MR0175/ISO 15156. The manufacturer is responsible for meeting
the metallurgical requirements of NACE MR0175/ISO 15156. It
is the end user’s responsibility to ensure that a material will be
satisfactory in the intended environment. Some of the operating
conditions which must be considered include pressure, temperature,
corrosiveness, uid properties, etc. When bolting components
are selected, the pressure rating of anges could be affected. It
is always the responsibility of the equipment user to convey the
environmental conditions to the equipment supplier, particularly if
the equipment will be used in sour service.
The various sections of NACE MR0175/ISO 15156 cover the
commonly available forms of materials and alloy systems. The
requirements for heat treatment, hardness levels, conditions of
mechanical work and post-weld heat treatment are addressed for
each form of material. Fabrication techniques, bolting, platings and
coatings are also addressed.
Applicability of NACE MR0175/ISO 15156
Low concentrations of H2S (<0.05 psi (0,3 kPa) H2S partial
pressure) and low pressures (<65 psia or 450 kPa) are considered
outside the scope of NACE MR0175/ISO 15156. The low stress
levels at low pressures or the inhibitive effects of oil may give
satisfactory performance with standard commercial equipment.
Many users, however, have elected to take a conservative approach
and specify compliance to either NACE MR0175 or NACE
MR0175/ISO 15156 any time a measurable amount of H2S is
present. The decision to follow these specications must be
made by the user based on economic impact, the safety aspects
should a failure occur and past eld experience. Legislation can
impact the decision as well. Such jurisdictions include; the Texas
Railroad Commission and the U.S. Minerals Management Service
(offshore). The Alberta, Canada Energy Conservation Board
recommends use of the specications.
Basics of Sulfide Stress Cracking (SSC) and Stress
Corrosion Cracking (SCC)
SSC and SCC are cracking processes that develop in the presence
of water, corrosion and surface tensile stress. It is a progressive
type of failure that produces cracking at stress levels that are
well below the material’s tensile strength. The break or fracture
appears brittle, with no localized yielding, plastic deformation or
elongation. Rather than a single crack, a network of ne, feathery,
branched cracks will form (see Figure 1). Pitting is frequently
seen, and will serve as a stress concentrator to initiate cracking.
With SSC, hydrogen ions are a product of the corrosion process
(Figure 2). These ions pick up electrons from the base material
producing hydrogen atoms. At that point, two hydrogen atoms
may combine to form a hydrogen molecule. Most molecules
will eventually collect, form hydrogen bubbles and oat away
harmlessly. However, some percentage of the hydrogen atoms will
diffuse into the base metal and embrittle the crystalline structure.
When a certain critical concentration of hydrogen is reached and
combined with a tensile stress exceeding a threshold level, SSC
will occur. H2S does not actively participate in the SSC reaction;
however, suldes act to promote the entry of the hydrogen atoms
into the base material.
As little as 0.05 psi (0,3 kPa) H2S partial pressure in 65 psia
(450 kPa) hydrocarbon gas can cause SSC of carbon and low
alloy steels. Sulde stress cracking is most severe at ambient
temperature, particularly in the range of 20° to 120°F (-6° to
49°C). Below 20°F (-6°C) the diffusion rate of the hydrogen is
Figure 1. Photomicrograph Showing Stress Corrosion Cracking
Sulfide Stress Cracking
--NACE MR0175-2002, MR0175/ISO 15156
652
Te c h n i c a l
so slow that the critical concentration is never reached. Above
120°F (49°C), the diffusion rate is so fast that the hydrogen
atoms pass through the material in such a rapid manner that the
critical concentration is not reached.
Chloride SCC is widely encountered and has been extensively
studied. Much is still unknown, however, about its mechanism.
One theory says that hydrogen, generated by the corrosion
process, diffuses into the base metal in the atomic form and
embrittles the lattice structure. A second, more widely accepted
theory proposes an electrochemical mechanism. Stainless steels
are covered with a protective, chromium oxide lm. The chloride
ions rupture the lm at weak spots, resulting in anodic (bare)
and cathodic (lm covered) sites. The galvanic cell produces
accelerated attack at the anodic sites, which when combined with
tensile stresses produces cracking. A minimum ion concentration
is required to produce SCC. As the concentration increases, the
environment becomes more severe, reducing the time to failure.
Temperature also is a factor in SCC. In general, the likelihood of
SCC increases with increasing temperature. A minimum threshold
temperature exists for most systems, below which SCC is rare.
Across industry, the generally accepted minimum temperature for
chloride SCC of the 300 SST’s is about 160°F (71°C). NACE
MR0175/ISO 15156 has set a very conservative limit of 140°F
(60°C) due to the synergistic effects of the chlorides, H2S and low
pH values. As the temperature increases above these values, the
time to failure will typically decrease.
Resistance to chloride SCC increases with higher alloy materials.
This is reected in the environmental limits set by NACE
MR0175/ISO 15156. Environmental limits progressively increase
from 400 Series SST and ferritic SST to 300 Series, highly alloy
austenitic SST, duplex SST, nickel and cobalt base alloys.
Carbon Steel
Carbon and low-alloy steels have acceptable resistance to SSC and
SCC however; their application is often limited by their low resistance
to general corrosion. The processing of carbon and low alloy steels
must be carefully controlled for good resistance to SSC and SCC. The
hardness must be less than 22 HRC. If welding or signicant cold
working is done, stress relief is required. Although the base metal
hardness of a carbon or alloy steel is less than 22 HRC, areas of the
heat affected zone (HAZ) will be harder. PWHT will eliminate these
excessively hard areas.
ASME SA216 Grades WCB and WCC and SAME SA105 are
the most commonly used body materials. It is Fisher’s policy to
stress relieve all welded carbon steels that are supplied to NACE
MR0175/ISO 15156.
All carbon steel castings sold to NACE MR0175/ISO 15156
requirements are produced using one of the following processes:
1. In particular product lines where a large percentage of
carbon steel assemblies are sold as NACE MR0175/ISO
15156 compliant, castings are ordered from the foundry with
a requirement that the castings be either normalized or stress
relieved following all weld repairs, major or minor. Any
weld repairs performed, either major or minor, are
subsequently stress relieved.
2. In product lines where only a small percentage of carbon steel
products are ordered NACE MR0175/ISO 15156 compliant,
stock castings are stress relieved whether they are weld repaired
by Emerson Process Management or not. This eliminates the
chance of a minor foundry weld repair going undetected and not
being stress relieved.
ASME SA352 grades LCB and LCC have the same composition
as WCB and WCC, respectively. They are heat treated differently
and impact tested at -50°F (-46°C) to ensure good toughness in low
temperature service. LCB and LCC are used in locations where
temperatures commonly drop below the -20°F (-29°C) permitted for
WCB and WCC. LCB and LCC castings are processed in the same
manner as WCB and WCC when required to meet NACE MR0175/
ISO 15156.
For carbon and low-alloy steels NACE MR0175/ISO 15156
imposes some changes in the requirements for the weld procedure
qualication report (PQR). All new PQR’s will meet these
requirements; however, it will take several years for Emerson
Process Management and our suppliers to complete this work. At
this time, we will require user approval to use HRC.
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Figure 2. Schematic Showing the Generation of Hydrogen Producing SSC
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