246 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION
out procedure at temperatures from 175 ° C to 205 ° C
(from 350 ° F to 400 ° F) for a period of time depending
on the size of the part in question.
90 – 96
Unfortunately,
there is no guarantee that all of the hydrogen will be
removed from the metal.
Environmental exposure can lead to hydrogen
embrittlement of galvanized or electroplated high -
strength fasteners at coating holidays, which are inevi-
table. Hydrogen from atmospheric condensation
(typically around pH 5) is enough of a concern that
high - strength fasteners are not galvanized because of a
concern with hydrogen embrittlement.
97,98
NACE RP 0176/ ISO 15156 - 2 Guidelines This stan-
dard applies to environmental cracking in the presence
of H
2
S. Bolting that will be directly exposed to sour
(H
2
S) environments or that will be buried, insulated,
equipped with fl ange protectors, or otherwise denied
atmospheric exposure must meet the requirements of
this standard, which restricts bolts to the materials
shown in Table 8.12 and to a maximum hardness of
HRC 22.
The two materials listed are the same, but the ASTM
Grade L7M is for use at low temperatures and has frac-
ture toughness testing requirements missing from the
more commonly used ASTM A 193 Grade B7M stan-
dard.
99,100
The M in the grade designation indicates that
the hardness levels for these bolts are held to a maximum
of HRC 22, in accordance with the limitations of the
NACE/ISO standard. This reduced hardness means that
fl anges may be derated to lower pressures than would
be allowed if standard bolting were used.
Subsea Embrittlement by Cathodic Protection At
one time, it was thought that limiting subsea bolting to
materials that met the requirements of NACE MR0175
would prevent hydrogen embrittlement of fasteners on
cathodically protected subsea assemblies. Unfortunately,
alloys in the following groups have been found to have
hydrogen embrittlement problems when used as fasten-
ers on cathodically protected equipment:
75,101 – 104
•
Martensitic stainless steels
•
Ferritic stainless steels
•
Duplex stainless steels
•
Nickel - based alloys
It is possible that the reported problems were associated
with using metals that were too hard, and studies indi-
cate that hydrogen embrittlement should not be a
problem for fasteners if the hardness level is kept at
HRC 34 or lower.
75,77,88,89
These hardness levels are much
higher than the HRC 22 restrictions for H
2
S service. The
discrepancies between reports that hardness levels in
•
UNS S66286, a precipitation - hardened stainless
used for low pressure units
•
UNS R30035, a nonmagnetic nickel - cobalt -
chromium - molybdenum alloy (MP35N) used at
higher pressures
•
UNS N09925, aprecipitation - hardening nickel -
iron - chromium alloy with molybdenum and copper
additions
•
UNS N07718, a precipitation - hardening nickel
chromium iron alloy
•
UNS N07725, a precipitation - hardening nickel
chromium iron alloy with higher alloying content
than UNS N07718. This alloy has better pitting
resistance than the others on this list and is prob-
ably most suitable for subsea applications.
75
Several reports indicate that the two precipitation -
hardened nickel alloys, UNS N77018 and N07725, are
probably the best CRAs for subsea service.
75,89
Copper - based systems often use aluminum bronze
(UNS C63000, ASTM B150) bolts, and most titanium
bolts are either UNS R50400, commercially pure tita-
nium, or UNS R56400, titanium with 6% aluminum plus
4% vanadium additions. While CRA alloy bolts are
commercially available, they are seldom used because
low - alloy heat - treatable steel bolts are much stronger
in most cases. Electrical isolation practices like those
shown in Figures 5.9 – 5.11 are available, although some
authorities are of the opinion that they are likely to be
overcome through inadvertent grounding in many, if not
most, instances.
Embrittlement Concerns The high - strength nature of
most industrial bolts means that they are often made
from materials subject to hydrogen embrittlement or
environmental cracking. Bolt embrittlement can occur
during the manufacturing process, during transportation
and storage, or in use. During manufacturing, the alloy
steels are quenched and tempered to produce the
appropriate strength levels. If this is done improperly, it
can lead to brittle bolts.
A more common concern is the effect of corrosion
resistant coatings, usually zinc or cadmium, which are
applied for atmospheric corrosion control. The pickling
process (acid cleaning) prior to coating can result in
hydrogen entry into the steel. Electroplating processes
also introduce hydrogen. This hydrogen entry is acceler-
ated in most electroplating baths because they usually
contain cyanides, which help produce quality electro-
plates but also act as hydrogen - entry poisons in much
the same manner as environmental H
2
S. The standard
means of controlling hydrogen embrittlement in elec-
troplated metal is by using a dissolved hydrogen bake -
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