Introduction NACE
Carbon steels in service in the offshore and oil refinery industries are susceptible to a
cracking mechanism known as sulfide stress cracking (SSC) or hydrogen stress cracking
(HSC) when in sour service, ie when hydrogen sulfide (H2S) is present in the process
fluid. Although the cracking is described as stress cracking, the main problem is the
hardness of the parent metal, weld metal and heat-affected zone (HAZ).
NACE (formerly the National Association of Corrosion Engineers) has published two
specifications that provide guidance on reducing the risk of in-service cracking, these
also being ISO standards. The major difference between these two primary
specifications is the environmental and service condition.
The first standard, NACE MR0175/ISO 15156, ‘Petroleum, petrochemical and natural gas
industries – Materials for use in H2S-containing environments in oil and gas production’,
is intended for offshore applications. The second standard, NACE MR0103/ISO 17945,
‘Petroleum, petrochemical and natural gas industries – Metallic materials resistant to
sulfide stress cracking in corrosive petroleum refining environments’, is intended for
onshore process plant.
Both MR0175 and MR0103 have virtually identical requirements for specifying parent
metal properties of carbon steels for sour service; the major concern of the standards is
the requirement of a maximum hardness. All steels that have been cold-worked must be
stress-relief heat-treated to ensure the hardness is less than 22HRC (Rockwell hardness,
equivalent to 248Hv or 237HB). Carbon steels other than P1 can be used provided that
their hardness is also less than 22HRC (237HBW). Consideration must also be given to
the resultant hardness of welds.
Standard practice guidance
MR0103 refers to a standard practice document for controlling welding activities, NACE
SP0472, ‘Methods and controls to prevent in-service environmental cracking of carbon
steel weldments in corrosive petroleum refining environments’. SP0472 and this article
are concerned with the methods used to control the weldment hardness to prevent SSC
and HSC. SP0472 also gives some consideration to prevention of alkaline stress
corrosion cracking (ASCC) via post-weld heat treatment (PWHT) but this will not be
covered in this article.
Whilst both MR0175 and MR0103 cover a wide range of materials (carbon steels,
stainless and duplex steels, nickel alloys and aluminium alloys), SP0472 is only
concerned with carbon steels, classified as P1, Group 1 or 2 in ASME IX. These are hot
finished carbon steels with a specified ultimate tensile strength less than 485MPa
(70,000p.s.i.). Note that the BS EN 10028 steels are now assigned P numbers in ASME IX.
It should be remembered that parent metals may be weld-repaired as part of the plate
production process. These base metal repairs must also comply with the NACE
requirements with respect to weld metal and HAZ hardness. In addition, although
SP0472 is concerned with the results of welding, any thermal cutting process will
produce a heat-affected zone, which if not removed or welded over may result in HSC.
In these cases, it is generally considered necessary to remove approximately 3mm of
material to ensure that there are no areas of unacceptably high hardness.
Welding processes covered by SP0472 are the more common processes; manual metal
arc (MMA), metal active gas (MAG), flux-cored arc welding (FCAW), tungsten inert gas
(TIG) and submerged arc welding.
SP0472 provides a ‘Road Map’ of guidelines, with the aim of preventing the two
cracking mechanisms by controlling weldment hardness. There are three major
components to this. The first is prevention of the HSC or SSC cracking mechanism by
control of the weld deposit. The second is prevention of the HSC or SSC cracking
mechanism by control of the HAZ. The third is prevention of the ASCC cracking
mechanism by PWHT of the entire weldment.
What is NACE
NACE was established since 1943, full name is The National Association of Corrosion
Engineers. The founders were 11 engineers of anti-corrosion of pipeline industry.
Till now, NACE has been the largest organization to broadcast knowledge of anti-corrosion and
improve skills of corrosion prevention.
In NACE standards, MR0175 is the standard of material requirements for H2S containing oil and
gas production and equipment.
As a result, Nace pipe is used in H2S containing gas and oil transportation.
You could find NACE MR0175 standard specification from here.
NACE MR0175 or ISO 15156 Scope
NACE MR0175/ISO 15156 provides common rules, gives suggestions and requirements to
select qualified steel materials, that served in equipment used in oil and gas production, and in
natural gas sweentening plants in H2S-containing environments. Where if such equipment failed
could make a risk to human health and safety, and damage to the environment.
It is also a federal standard in US as recognized as ISO 15156, for Petroleum and natural gas
industries that materials applied in H2S-containing environments in oil and gas production.
Cracking Types
This standard NACE MR0175/ISO 15156 addresses all mechanisms of cracking that can be
caused by H2S, including below cracking conditions.
Sulfide stress cracking (SSC)
Stress corrosion cracking
Stepwise cracking
Soft zone cracking
Stress-oriented hydrogen-induced cracking
Hydrogen-induced craking
Galvanically induced hydrogen stress cracking
How to select material
Steel materials that selected or qualified using NACE MR0175/ISO 15156 are resistant to
cracking in defined H2S-Containing environments in oil and gas production, but are not
nesessarily immune under all service conditions.
Consequently,
It has been taken as the instructions for selecting the qualified material of carbon and low-alloy
steels, corrosion resistant alloy steels and other alloy materials that against in the H2S contained
environments. Meanwhile, the standard also suitable for API, ASTM, ASME and ANSI etc.,
(In some cases, NACE pipe is also equivalent to sour service pipe, that applied in H2S
environments.)
Equipment that applied with NACE MR0175
We know that there are a lot of equipment applied with NACE MR0175/ISO 15156 standard,
mostly in oil and gas drilling, construction, lifting activities.
For example drilling as drill bits; Subface equipment like sucker rods pumps and sucker rods,
electric submersible pumps, and other artificial lift equipment;
Sour water handling equipment, natural gas treatment plants.
And most important, steel pipe and fittings that for transportation pipelines of oil and gas,
liquids, and multiphase fluids.
This is something we are going to talk about.
NACE Carbon Steel Pipes Related
Carbon and alloy steel containing up to carbon and up to manganese and residual quantities of
other elements, except those intentionally added in specific quantities for deoxidation (usually
silicon and/or aluminum)
Please note:
In standard NACE MR0175 carbon steel pipes and fittings material used in petroleum industry
usually contain carbon less than 0.8%.
Carbon and low alloy steels includes plate, pipe, pipe fitting that complied with NACE MR0175,
with stated exceptions, further SSC testing is not required. Nevertheless, any SSC testing that
forms part of a materials manufacturing specification shall be carried out successfully and the
results reported.
Please note, the carbon and alloy steel pipes and fittings described/listed in previously NACE
MR0175 (all versions) were identified by extensive correlations of filed failures/successes and
laboratory data. The hardness limit of HRC 22 applied to most carbon and low alloy steels was
based upon correlations of heat treatment, chemical composition, hardness and failure
experience. The higher hardness limits for the chromium-molybdenum steels were based upon
similar considerations.
Carbon Steel Pipe Standards Refer NACE
NACE MR0175 standard includes below standard steel pipes.
API 5L Seamless Grade B to X42, to X65
ASTM A53 Pipe
ASTM A106 Grade B, A, and C
ATSM A333 Grade 1 and 6
ASTM A524 Grade 1 and 2
ASTM A 381 Class 1 Y35 and to Y65
ISO 3183-3 Grade L245 to L450
Casing and Tubing from API 5CT J55/K55 to T95
NACE MR0175 Pipe and Fittings
Steel pipe and related fittings which are made from the NACE material (complied with NACE
MR 0175 or ISO 15156 standard). We call them NACE pipe, or NACE pipe fittings.
Therefore, these products are specially used in the oil and gas environments that contain the H2S
etc corrosive chemicals.
Chemical Composition
For carbon and alloy steel pipes, NACE MR0175 has limits for content Sulfur (S ≤ 0.002%),
Phosphorus (P ≤ 0.020%) and Carbon (C ≤ 0.10%).
Mechanical Strength
Mechanical properties of tensile strength, and yield strength is same with general pipes referred
to corresponding standards.
NACE Test Methods
For sour service steel pipe, plate, fittings, commonly there are two test methods referred NACE.
HIC and SSC.
HIC is Hydrogen Induced Cracking. HIC test is a mandatory test for NACE pipe and fittings. But
if there is certificate to proof the material (Raw material for making pipes or fittings) comply to
NACE MR0175, then there is no need to do this test again.
SSC is Sulfide Stress Cracking. SSC test is a long time lasting experiment. The test is specially
to alloy steel, to put test material in corrosion environment plus with constant pulling force
(Under situation of working force and metal inside force). In these two factors (Corrosion +
Force) affects, material havs the brittle rupture. So this is SSC cracking.
(SSC cracking is one of the most harmful corrosion types, during corrosion, if there is any small
cracks, the broken speed is far more soon than other type of corrosion. So SSC corrosion is an
“Corrosion Disaster”. This happened in Bridge damage, Airplane accidents, boiler tank
explosion. These are bring big loss for people’s life and property. Other than this, power station,
Ship, boiler tank, petroleum industries all happened SSC accidents.)
NACE MR0175 pipe and fittings Applications
The material shall be used in water-containing liquid and media which include all kinds of gas,
or H2S containing crude oil, the H2S absolute partial pressure ≥0.0003 Mpa (0.05 psia).
On the other hand, under these conditions there is no need apply to MR0175. (It’s up to users.)
a. Low pressure gas: the total pressure is lower than 0.4 MPa (65 psia).
b. Low pressure gas and oil multi phases media: the total pressure is lower than 1.83 MPa.
MR0175 standards of general material requirements for selecting of cracking-resistant material,
cracking-resistant carbon and low-alloys steels, cast irons, cracking-resistant alloys and other
alloys. And details requirements for H2S containing equipment.
SSC Cracking factors
Sulfide Stress cracking affected by following factors:
1) Subjected by chemical ingredients, hot treatment and micro structural etc. Influences
metallurgy conditions and strength.
2) Hydrogen iron concentration in aqueous phase (Ph index)
3) H2S partial pressure, that is H2S concentration and the overall absolute pressure ratio.
4) Total tensile stress
5) Temperature
6) Exposure time to acid environment
7) Power storage effect
8) Chloride or other halide ions concentration
9) Oxidizing agent
10) Non-secondary fluid (including fluid used for acid excitation and packers)
As a matter of fact,
Pipe corrosion speed also influenced by cold working of metal.
Although steel pipes, pipe fittings, production and equipment meets the standard NACE MR
0175, have the function of anti-H2S corrosion. But in case unsuitable design, manufacturing,
installation, material’s choosing and processing etc, all these factor could leading corrosion-
resistant materials to sensitivity.
Material Cost of NACE MR0175 than General pipe
The price for NACE pipe itself is not too much higher than general steel pipe, (Especially for
API 5L pipe or ASTM A106 pipe, the price is higher but not more than 100 dollars).
On the other hand, the hard and expensive part is HIC and SSC test fees. If the quantity is very
less, and there is no stock available, it is almost impossible to get this material.
Working conditions for the NACE MR 0175 steel pipe
The standard should suitable for destruction by SSC or SCC in H2S containing productions and
equipment in the following conditions:
1) Resulting in equipment cannot work as usual under pressure.
2) Could not make sure to bear the pressure of the system.
3) Lead to equipment basic function cannot sustain. But under usual or low pressure.
It shall be noted that,
To choose the NACE material in these working conditions: such as water treatment equipment,
sucker rods and liquid pump etc. All these products have detailed specifications, but not belong
NACE MR0175 standards scope.
Things you should know before purchase NACE MR0175
steel pipe and fittings
The responsibility of users is to make sure whether the standards is suitable for the work
conditions.
The materials of NACE steel pipe and fittings that included by this standards could be used for
any given parts.
From manufacturers side, their responsibility is to make sure the specified material satisfied
requirements of metallurgy, while the users’ responsibility is to make sure selecting the right
material for the environments.
Users could select by working condition (include pressure, temperature, causticity and
characteristics of liquid etc.,)to choose specific material.
