Uoderwater Tecbnology
The Effective U se of Titanium in Subsea. Applications
by D. K. PEACOCK*
Experience from this and from similar applications,
dating back more than thirty years, is available to
guide both existing and new users to the best alloys
and manufacturing processes for each application.
Abstract
The effective and successful use of titanium in subsea
applications requires the recognition, understanding
and correct application of the combination of usefuJ,
and unique physical, mechanical and corrösion
resistant properties of titanium and its alloys. These
properties are reviewed and guidance is given on
design, and fabrication using available products and
cost-effective manufacturing methods.
Mechanical properties of titanium and its alloys
The range of grades and alloys of titanium available
to the designer makes possiblethe
selection of a
combination of properties appropriate both for fabrication and end use. Three types of alloy are described
generically by their metallurgical structure, alpha,
alpha-beta or beta (Table 2).
Introduction
The principal factors to be considered in successful
and effective use of titanium subsea are its basic
physical and mechanical properties, corrosion resistance, and practical aspects of fabrication and
installation.
Titanium is light, being little more than half the
density of steel, and its alloys are available to match
or exceed the strength of steels commonly used subsea.
Table 1 shows the favourable strengthjweight ratio of
titanium in comparison with other engineering alIoys.
The modulus of titanium is also about half that of
steel. This may require some change of design, but is
a positive factor both in the high damage tolerance
and shock resistance of titanium alloys, and their
suitability for flexible risers and flowlines.
Titanium is highly resistant to corrosion in seawater,
brines and brackish waters, and to almost all conditions
encountered subsea and in oil and gas extraction. In
many subsea environments titanium will be immune
to corrosion.
Fabrication techniques for machining, forming and
welding titanium are weil understood, and whilst
different in some respects from those used for other
metals, they are not significantly more costly, nor more
difficuIt to apply successfully.
Titanium and its alloys are being specified and
used offshore and subsea to an unprecedented extent.
ASTM specijications
A convenient and widely-used system for specific
identification of the various grades of commercially
pure titanium and titanium alloys used for engineering
and corrosion resisting applications is provided by
ASTM.
Grades 1,2,3 and 4 are commercially pure (alpha)
titanium, used primarily for corrosion resistance.
Strength and hardness increase, and ductility reduces
with grade number. Grade 2 is the most widely used
specification in all product forms. Grade 1 is specified
when superior formability is required. Grades 3 and
4 are used where higher levels of strength are necessary.
Grades 7, 11, 16 and 27 contain palladium and
provide superior corrosion resistance, in particular to
reducing acid chlorides. The mechanical properties of
grades 7 and 16 are identical to those of grade 2. The
mechanical properties of grades 11 and 17 are similarly
identical to those of grade 1. Grade 12 also offers
corrosion resistance superior to commercially pure
titanium, but is stronger and retains useful levels of
strength up to 300 oe.
Grade 5 is the 'workhorse' alpha-beta alloy of the
titanium range. It is als~fied
with reduced oxygen
content (EU) for enhanced toughness (grade 23), and
with addition of 0.05% palladium for added corrosion
resistance (grades 24 (EU) and 25). Restrictions on
fabricability may limit availability in certain products.
* Titanium Meta/s Corporation, 17 Woodford Trading Estate,
Southend Road, Woodford Green, Essex 1GB BHG, UK.
(Chairman, Titanium Information Group)
Table 1 Strength/weight
ratio of titanium compared with other engineering alloys
Material
Vield strength
20·C MPa
Titanium Grade 2
Titanium Grade 5
316 Stainless
254 SMO
2205 Duplex
Monel400
Inconel625
Hastelloy C-276
70/30 Cu-Ni
275
830
230
300
450
175
415
355
120
at
Density g/cm3
Vield strength
density ratio
% Ratio relative to
Ti-grade 2
%
4.51
4.42
7.94
8.00
7.80
8.83
8.44
8.89
8.90
61
188
29
38
58
20
49
40
13
100
308
48
62
95
32
80
66
21
32
100
15
20
31
11
26
21
7
23
Ratio relative to
Ti-grade 5
Volume 21 Number 4
Underwater Technology
Table 2 Summary
of key properties
Description
Alloy
type
Mechanical Properties
0.2% Proof stress
Typical range
UTS
Typical range
Elongation
Hardness
Tensile modulus
Torsion modulus
Physical properties
Density
Thermal expansion
(0-200°C)
(32-400°F)
Thermal conductivity
At room tempo
Specific heat
At room tempo
of titanium
alloys
Commercially
pure
Titanium
(CP)
Medium strength non-heat
treatable Ti-6AI-4V
Highest
alloys
Units
Alpha
Alpha-beta
Beta
MPa
ksi*
MPa
ksi
%
HV
GPa
ksi x 1000
GPa
ksi x 1000
345-480
50-70
480-620
70-90
20-25
160-220
103
14.9
44.8
6.5
830-1000
120-145
970-1100
140-160
10-15
300-400
114
16.5
42
6.1
1100-1400
160-203
1200-1500
174-217
6-12
360-450
69-110
10-16
38-45
5.5-6.5
4.81-4.93
0.174-0.178
strength
g/cm3
Ib/in3
4.51
0.163
4.48
0.161
10-6rC
1O-6rF
8.9
4.9
8.3
4.6
7.2-9.0
4.0-5.0
6.7
3.87
46.5
6.3-7.6
3.65-4.40
44-53
565
0.135
490-524
0.117-0.125
W/mk
Btu/ft.h.oF
Btu.in/ft2.h.oF
22
12.5
150
jJk9rC
Btu/lbrF
515
0.123
* ksi = Ib/in2 x 1000
Grade 9 (alpha-beta), has good fabricability and
medium levels of strength. Grade 18 (grade 9 +
0.05% palladium) ofTersenhanced corrosion resistance.
Beta-C and TIMETAL™ 21S* are very high
strength, highly corrosion-resistant beta alloys, now
included in the ASTM range. They are respectively
grade 19, and grade 21. (Their counterparts with
0.05% palladium are grades 20 and 22.)
The range of ASTM specifications cover all the
forms supplied in titanium and its alloys:
B 265-Strip Sheet and Plate.
B 337-Seamless and Welded Pipe
B 338-Seamless and Welded Tube
B 348-Bars and Billets
B 363-Seamless and Welded Fittings
B 367-Castings
B 381-Forgings
B 861-Seamless Pipe (to replace 337)
B 862-Welded Pipe (not yet approved)
B 863-Wire
(Welding consumables are covered by AWS Specification A5.16.)
Using design codes
The design codes for items of plant and equipment,
pipework, pressure vessels, etc., set maximum stress
levels against metal operating t,emperature. The
ASME Boiler and Pressure Vessel Code is typical of
codes which disfavour titanium with an excessively
high factor of safety. Codes vary as to the fraction of
the tensile strength used in the calculation of maximum
aIIowable stress. ASME B31.1 uses a quarter of the
UTS, ANSI uses a third of UTS.
Even the use of conservative code values to the
associated design formula for metal thickness frequently
produces surprising resuIts. The ANSI B31.1 pipe wall
thickness formula shows how the high strength and
corrosion resistance of titanium combine to provide
low-weight and low-cost pipework and tubulars of
high integrity:
PD
Tm=----+A
2(S
where, Tm = Minimum required wall thickness (mm)
P = Systems Pressure (MPa)
D = Pipe outside diameter (mm)
S = Maximum allowable stress for material
at design temperature (MPa)
Y = Coefficient equal to 0.4 for non-ferrous
metals
A = Additional corrosion allowance (mm)
(zero for titanium)
Commonly used schedules for pipework wall thickness often resuIt in a substantial overspecification
where titanium is concerned (see Table 3). Low-cost,
thin-waII, welded grade 2 titanium piping is an ideal
product to handle low pressure ambient temperature
seawater, for example, in service water, ballast water
or fire main duties [2].
NACE MR-0175 alloys for sour service
AIIoys currently approved for sour service are specified
in NACE MR-0175. The limited range, comprising
commercially pure titanium and four aIIoys, does not
reflect the performance now available from new alloys,
particularly those containing palladium in the ASTM
range [1]. It is anticipated that the forthcoming
changes to the NACE procedures for rating aIIoys for
sour service will soon result in the Iisting of several
more titanium alloys in MR-0175.
* TIMET
and TIMET AL are registered trademarks of
Titanium Metals Corporation, USA and TIMET UK Ltd
Spring 1996
+ PY)
24
Underwater
Technology
Table 3 Pipe wall thickness for grade 2 titanium
Pipe wall thickness
Pipe NB ins
Class 200
90/10 Cu/Ni
2
3
6
12
2.1
2.4
3.4
6.4
Gr2 Ti
Calculated
mm
Gr2 Ti
Sched 55
Gr2Ti
Sched 105
Gr2 Ti
Sub Sched 5s
1.6
2.1
2.8
3.1
1.2
2.8
3.4
4.0
4.6
2.0
3.0
0.5
0.7
1.3
2.6
1.6
corrosion and general fouling resistance of titanium,
and its ability to withstand higher fluid velocities,
permits design for equal and frequently superior
overall heat transfer rates than can be achieved with
metals of higher thermal conductivity. Titanium
alloys, with the exception of grades 7, 11, 12, 16 and
17, generally have much lower conductivity, approximately 30-40% that of the pure metal. In subsea use,
e.g., passive cooling loops, careful design study will be
needed to optimise material and manufacturing costs
(heavy-wall, seamless DS. thin-wall, welded tube and
pipe), with thermal and mechanical performance
(heavy-wall, high-conductivity, pure titanium vs.
thinner-wall, low-conductivity alloys).
The caIculated wall thickness is perhaps the most
conservative (ASTM) code for service at 200 psi
(13.3 bar) at 38 oe. Schedule 5 and 10 wall thicknesses
are per ASME B337. Sub Schedule 5 is suggested for
seawater service at 200 psi at 38 oe.
Impressive savings ofweight are available when this
design strategy is followed. For example a 6"NB
cupro-nickel pipe weighs over 1500 kg/loo m. Titanium
pipe for the same rating weighs less than 650 kg/l00 m,
a weight saving elose to 60%. Higher pressure systems
usually require titanium alloys, but the weight savings
pro rata can be equally impressive.
Physical properties of titanium and its alloys
Density
The physical properties of primary importance in
mechanical design are the metal density, 4.5 kg/l,
which is alm ost half that of other corrosion-resistant
alloys, and the modulus of elasticity. Titanium alloy
density is generally similar to that of commercially
pure titanium, but the highly-alloyed beta formulations
are typically more dense at up to 4.95 kg/l.
Magnetic
susceptihility
Titanium has a very low magnetic susceptibility
(typically in the range 3.0-4.0 x 10-6 S.1. units), and
can be effectively elassified as non-magnetic.
Fatigue, fracture toughness and creep
Fatigue
Fatigue strength of smooth test specimens of titanium
and its alloys is typically 50-60% ofthe tensile strength
values. Notched specimen tests give lower values [5].
Care is required in design and manufacture to avoid
stress concentrating factors where frequent or high
cyelic stress is applied. Poor surface finish, sharp
sectional transitions, unblended radii and corners are
typical conditions to avoid. Corrosion fatigue (the
reduction of fatigue limit due to the presence of a
corrosive medium) is generally not a problem for
titanium and its alloys. Fatigue crack propagation
rates in seawater for commercially pure titanium and
grades 9, 12, 18, 19,20,21 and 22 are similar to those
in air, but rates for alloys Ti-6AI-4V (grades 5, 23,
24, and 25) are marginally higher in seawater and other
corrosive environments compared with those for the
same alloy tested in air. The absolute crack propagation
rate will vary slightly with specific alloy composition,
microstructure, crack orientation and loading, and in
the presence of hydrogen, genera ted galvanically or
from impressed cathodic potentials [6].
Modulus
In applications where weight is critical, the weight
savings achievable with a properly designed titanium
system will be appreciable. Where titanium is used to
replace a heavier metal subsea, care must be taken to
check the existing design for dependence on dead
weight or buoyancy.
Attention to the modulus is essential for the correct
design and application of struts, ties and unsupported
pipe spans. If necessary the section modulus should
be adjusted to offset the lower material value.
However, the relatively low modulus is a factor
contributing to the substantial shock resistance of
titanium and its alloys. Shock resistance is caIculated
from the maximum allowable stress divided by the
square root ofthe product ofthe density and the elastic
modulus. These two properties are both low for
titanium and its alloys, and strengths are relatively
high. The shock resistance of titanium is greater than
that of many steels and copper- and nickel-based
corrosion resistant alloys [3]. The low modulus is also
a major factor in the selection of titanium alloys for
f1exible catenary riser systems [4].
Fracture toughness
Commercially pure titanium and titanium alloys
possess a low ductile to brittle transition temperature
and have useful levels of fracture toughness even at
sub-zero temperatures. All titanium alloys are mechanically safe at temperatures down to -100 oe. (The
ASME Code allows the room temperature stress levels
Thermal conducti"ity
The thermal conductivity of commercially pure
titanium at 18.6 kCal/m2/hr;oC/m
is about 30%
higher than that of many stainless steels. The strength,
25
Volume 21 Number 4
Underwater Technology
welds of many superaustenitic and duplex steels which
have inferior corrosion resistance to the base metal,
effectively placing restrictions on the service temperature of the weldments to between 10 and 35°C,
and creating a dangerous situation whenever an
unexpected increase of temperature occurs.
The oxide film on titanium is maintained, or its
formation is supported, in neutral to oxidising
conditions over a wide range of pH and temperature
values, and in these conditions titanium will survive
erosion, cavitation, pitting, and crevice corrosion.
Enhanced resistance to crevice and under-deposit
corrosion is obtained from alloys containing nickel
and molybdenum, ASTM Grade 12, or palladium,
ASTM grades 7, 11, 16, 17, 18,20,22,24 and 25.
.7'
cn
o
o
.7'
o
o
Fig.l
50
MPavm
Typica/ K1C and K1SCC va/ues Jor Ti-6A/-4V
Ti-6A/-4 V in various conditions
EU
and
Guidelines to cost-effective
design
of grades 1, 2, 3, 7 and 11 to be used in design for
operational service down to - 60°C and alloys may
be similarly treated.)
Several titanium types have fracture toughness
(KId levels in excess \of 80 Mpaß
in air, but
reduced levels of fracture toughness in seawater
(K1scd and other aggressive environments (Fig. 1)
Long-established applications oftitanium, in seawater
and in petrochemical and refinery applications analogous to subsea environments, has produced the
following guidelines for design. These, if applied,
should result in optimum use of material, best design
for manufacturing and greatest cost savings.
[7].
1. Titanium requires no corrosion (pitting) allowance
Impact
Pitting, or the threat of pitting, is a persistent problem
for stainless steeIs of all grades, and there has been
frequent minor adjustment of composition by manufacturers, aimed at improving the pitting resistance
equivalent for conditions in which titanium and many
titanium alloys are totally immune.
The breakdown potential of the titanium oxide film
is high in subsea environments, and pitting, which
occurs when the potential of the metal exceeds the
breakdown potential of its protective oxide, is rarely
encountered.
Pitting potentials for titanium in chloride solutions
are +81 + 10 volts at room temperature, falling with
increasing temperature, but still + 41+ 5 volts at
100 oe. In contrast, pH has very little influence on
pitting potential. Titanium does not require the
'pitting allowance' familiar to many other metals, and
the practical benefit in designing titanium systems to
handle natural or saline waters, is that equipment
may be designed cost-effectively, using the minimum
thickness ofmaterial to satisfy the mechanical requirements of the system.
tests
Titanium alloys are strain rate senSItive. Results
obtainable from Charpy and other high-rate impact
tests point to performance which may be strongly
unrepresentative of-the alloy in practice. Commercially
pure titanium and the low to medium strength lilloys
have historically been considered 'safe', because they
give values above the critical steel value of 27 Joules.
In looking to use high strength titanium alloys, which
will not return such values, it is vitally important not
only to evaluate performance based on testing that
realistically reflects the service conditions, but also to
avoid specifying acceptance criteria based on historic
test data of other metals and alloys.
Creep
Creep is rarely of concern in subsea applications other
than down hole. Metallurgical and surface stability of
titanium and its alloys is excellent within the working
temperature range set by the values of useful strength.
Creep values for commercially pure titanium to 0.1%
plastic strain in 100000 hours are approximately 50%
those oft he tensile strength at appropriate temperatures
up to 300 oe. Alloys offer comparable or superior
performance at higher temperatures up to 600 oe.
Creep values for weIds normally lie in the same range
as those for base metal.
2. Titanium resists erosion and cavitation
All titanium alloys possess excellent resistance to
erosion. Seawater flowing at velocities up to 30 m/s
can be safely handled. The presence of abrasive
particulates lowers the maximum allowable velocity
but any titanium alloy will outperform most other
materials in conditions where its oxide film, if
damaged, will self repair. This permits safe increase of
system flow rates, permitting design with smaller
diameter pipework, and tighter pipework bend radii.
Some reported values [9, 10J for comercially pure
titanium are:
• sea water at 7 m/s shows no erosion
• seawater at 36 m/s erodes at 0.008 mm/yr
Designing a corrosion resistant system
It is the stable, substantially inert, tenacious and
permanent titanium dioxide film which provides the
metal with outstanding resistance to corrosion in a
wide range of aggressive media [8]. The oxide film
forms equally on weIds as on parent metal, and there
is consequently no reduction of weid or HAZ corrosion
resistance. This compares very favourably with the
Spring 1996
26
Uoderwater Technology
microbiologically influenced corrosion (MIC) include
subsea and offshore activities. When water is stagnant,
or only limited or intermittent flow occurs, the
problems are aggravated. The multitude of species of
bacteria and marine life, the wide range of conditions
under which each can survive and propagate, and
uncertainty about the specific circumstances required
to develop conditions that will eause eorrosion failures
in metallic systems, have considerably hindered the
understanding of MIe.
Marine biofouling will occur on titanium in static
or slow moving seawater, but unlike many other
nominally corrosion resistant alloys used offshore, the
integrity ofthe titanium oxide film is maintained under
the deposits. They ean be removed safely, if neeessary,
by mechanical methods or by regular or shoek ehlorination, or prevented in pipework by maintaining water
velocity above 2 m/s. Fouling factors between 0.90 and
0.99 are regularly used for titanium in heat exehangers.
In many other fouling situations, the hard smooth
titanium oxide film will resist deposit adhesion, and
facilitate deaning. Titanium will also resist damage
from oxidising hiocidal agents such as hydrogen
peroxide, sodium hypochlorite, ete., as weil as the
non-oxidising organic treatments used to remove
fouling. Recommended inhibited cleaning solutions
should be specified for use when hard seale removal
is necessary. To date, with the benefit of over
thirty years experience of extensive exposure of
titanium inconditions favoUrable to a wide speetrum
of microbiological activity, hot one single failure of
titanium has occurred which can be ascribed to MIe.
Effectively, titanium and its alloys can be taken as
unaffeeted by mierobiologically influenced corrosion.
• seawater with 40 g/l 60 mesh sand at 2 m/s erodes
at 0.003 mm/yr
• seawater with 40 g/110 mesh emery at 2 m/s erodes
at 0.013 mm/yr
• seawater with 40% 80 mesh emery at 7.2 m/s erodes
at 1.5 mm/yr
Titanium alloys generally perform in relation to their
corrosion resistance comparably with commercially
pure titanium. For example Ti-6AI-4V (grade 5)
• seawater at 36 m/s erodes at 0.01 mm/yr
Beta alloy grade 21, TIMETAL™ 21S, has corrosion
resistance substantially superior to commercially pure
titanium, this being primarily due to the high molybdenum content, and will have erosion and abrasion
resistance equal to or better than the above figures
for commercially pure titanium [11]. Despite the
above recorded performances there are instances where
the combination of particuhltes and their carrying
media have caused ·severe erosion damage, e.g., in
drilling risers. The solution has been to provide the
titanium with a suitable abrasion resistant lining.
3. Titanium
cracking
is highly
resistant
to stress
corrosion
Commercially pure titanium grades 1 and 2 are
essentially immune to stress corrosion cracking (SCC)
in seawater and chlorides. Special, stronger grades of
alloys, in some cases with addition of palladium, are
required in conditions likely to cause SSe. Beta alloy
grade 21 provides an example of extreme resistance.
The normal oxide film stability and resistance is
enhanced by molybdenum, providing immunity over
a wide range of pR and temperature in neutral,
oxidising and mildly reducing aqueous environments.
The basic alloy shows no crevice corrosion in 5%
sodium chloride at 98 oe, pR 0.2, and is resistant to
SCC up to 200°C [12].
Slow strain rate tests have been conducted on
soIution-treated and aged grade 22, (TIMET AUM
21SPd) bar (tensile strength 952 MPa (138 ksi), yield
strength 883 MPa (128 ksi), elongation 25%, reduction
of area 55%). The test environment was: deaerated
20 wt% NaCI, 6.9 MPa (1000 psi) R2S, 3.5 MPa
(500 psi) CO2, 1 g/l sulphur, and 0.5% acetic acid. The
strain rate was 4 x 1O-6/s. This is a most aggressive
medium in which most titanium alloys fail once the
temperature exceeds 190 oe. For the subject evaluations the test temperature was 218°C (425°F).
Two sam pIes were exposed, and time to failure was
almost ten hours in both cases. (Ratio RA(Env)/
RA(inert): 0.99 and 1.05; Ratio TF(Env)/TF(inert):
0.97 and 1.02.)
4. Titanium is immune to microbiologically
corrosion
Subsea application design criteria
Current subsea projects, particularly riser systems,
which call for high levels of strength, fracture toughness and corrosion resistance require attention to the
following:
• Only those alloys which eombine SCC resistanee
and high toughness should be selected for critical,
highly-stressed components.
• Ensure that stressed component surfaces are smooth
and free offlaws and cracks. Fully machined surfaees
are recommended for critical parts with eompressive
surface treatment as an advisable extra.
• Design so that the maximum allowable stress is
below the fatigue limits for the seleeted alloy.
Fatigue crack initiation is in sensitive to the presence
of seawater but crack propagation may be accelerated
by it, particularly if there are periods of stress dweil,
ripple loading, or if the stressing frequeney is less
than 10 Hz.
infiuenced
Practical concerns of working systems
Galvanic corrosion and hydrogen uptake
Where titanium is incorporated into mixed metal plant
or equipment, it will usually be the cathode if a
galvanic couple exists, or is created. Galvanie eorrosion
of less resistant metals may be harmful to titanium as
the cathode if conditions lead to the uptake of
The ability of bacteria to cause or contribute to the
destruction of met als has been acknowledged for many
years. Present in almost all environments, certain
microbiological organisms naturally generate oxidising
or reducing conditions which are highly localised and
corrosive towards metal surfaces with which they are
in contact. Operations affected by, or susceptible to,
27
Volume 21 Number 4
Underwater Technology
and rust stains on tl,e surfaee of titanium pose no
problem.
The solubility of hydrogen in pure titanium and
alpha-beta alloys is limited (150-300 ppm). Onee the
limit is passed titanium hydride is precipitated leading
to local loss of ductility. At ambient temperature
diffusion is slowed, and hydride precipitation will be
limited to the surface layers which is unlikely to be
harmful. The solubility of hydrogen in beta alloys is
typically 4000 ppm, with up to 2000 ppm having no
effect on strength or ductility ofthe alloy and in the
beta annealed condition. The potential for beta alloy
embrittlement is enhanced by ageing to higher levels
of strength [16]. Uptake of hydrogen caused by
corrosion of titanium in reducing acids or other
environments for which it is not suitable is of
secondary concern.
hydrogen. In seawater or sour service, hydrogen
absorption may be caused or aggravated by:
• coupling of titanium to a less corrosion resistant
metal;
• cathodic protection systems producing potentials
>-0.9 V SCE;
• tensile load or residual stress if absorption is
occurnng;
• pH less than 3 or more than 12 which increases the
risk of uptake;
• higher temperatures which cause an increase of
corrosion at the anode and higher hydrogen activity
at the cathode;
• hydrogen sulphide which will accelerate hydrogen
uptake in the presence of a cathodic potential.
Despite these factors, it is only in certain environments,
and under specific conditions, that titanium alloys will
absorb hydrogen. In addition, absorption of hydrogen
does not necessarily cause a problem or limit service
life. Hydrogen uptake can normally be avoided by the
proper design of equipment and control of operating
conditions. In many subsea applications neither
service temperature, nor the potential fromcorrectly
selected sacrificial anodes or properly regulated
impressed potential cathodic protection systems will
be within the range for the absorption or diffusion of
hydrogen into titanium [13].
Design strategies to prevent or limit hydrogen
uptake and protect adjoining, less nüble, parts of the
syste~ include: electrical isolation oftitanium in mixed
metal structures, or coupling to galvanically nearcompatible alloys such as the molybdenum-bearing
austenitic and duplex steels and high· nickel alloys.
These alloys in their passive state are only marginally
less noble than titanium, but if activated, e.g. by pitting,
the risk of their damage by localised attack is greatly
increased.
Several operators have coated exposed titanium
surfaces to reduce the cathode/anode ratio. Under no
circumstances should coatings be applied only to the
less corrosion-resistant material. Any coating defects,
damage or breakdown in localised areas will immediately cause problems unless cathodic or chemical
protection is available, or unless the adjoining
titanium structure is also coated. Impressed potential
cathodic protection of the base metal should deli ver
no more than -0.8 V SCE [14]. Similarly, sacrificial
anodes must be selected to produce negative potentials
of less than - 0.8 V SCE. Aluminium and magnesium
anodes cannot be recommended as their potential is
too high.
Review and potential redesign of the cathodic
protection system is essential when a significant area
of titanium replaees steel subsea [15]. The surface
condition of titanium components is an important
control; the normal oxide film is a barrier to hydrogen
diffusion. Carbonate films formed by cathodic charging
in seawater protect against hydrogen absorption.
Concern that iron embedded or smeared on the
titanium surface may aceelerate hydrogen uptake
is limited to eertain chemical plant at high temperatures, and in subsea environments, iron, rust
Spring 1996
Crevice corrosion
Localised pitting or corrosion, occurring in tight
crevices and under scale or other deposits, is a
controlling factor in the application of unalloyed
titanium. Attack will not normally occur on commercially pure titanium or industrial alloys below
70 oe regardless of solution pH. Attention to design
offlangedjoints using heavy flanges and high clamping
press ure, and the specification of gaskets, choosing
elastic rat her than plastic or hard materials may serve
to prevent crevices developing. Palladium-containing
alloys have enhanced resistance to crevice corrosion
at higher chloride concentrations and lower pR. An
alternative strategy is to incorporate a source of nickel,
copper, molybdenum or palladium into the gasket, as
wire, foil, or oxides impregnated into the gasket
material.
Methanol and hydrofluoric acid
Specific environments and conditions in which titanium
and its alloys should not be used are:
• Methanol. Commercially pure titanium and all
titanium alloys are susceptible to stress-corrosion
cracking in methanol if not inhibited with water.
Water content of 4% will make all alloys fully
paSSIve.
• Hydrojluoric acid and jluorides. All titanium alloys
are rapidly attacked by hydrofluoric acid, even in
very dilute coneentrations and also in fluoridecontaining solutions below pH 7. Titanium should
not be specified if regular HF acidising is anticipated.
Gasket materials which may release active fluorides
must not be used with titanium.
Design for fabrication and installation
In practical terms, although construction in solid
titanium may be possible, restraints on fabrication
capacity and total cost may point to alternative
solutions. Thin-walled vessels and pipes will almost
eertainly be cost effective if made from solid titanium.
Very heavy-walled items will cost less if a stronger
backing base metal is titanium clad. Heavy flanges
will prove to be expensive unless steel is used as a
backing to a titanium flared taft or stub end [17].
28
Underwater Technology
machining and fabrication provides the route
requirements for engineering products.
Long shafts may require extra bearing points if
made from solid titanium---dadding
of a stiffer base
metal may provide a more effective answer, mechanically as weIl as economically. In each ca se consideration of all of the options should provide the optimum
solution to combine requirements
for mechanical
performance, corrosion resistance and fabrication.
Under all circumstances an effectivelydesigned
and
fabricated titanium structure, or plant and equipment
will always weigh substantially less than the same item
in steel, nickel or copper-based alloys. The benefits in
weight-critical
applications
will go far beyond the
normal benefits arising from reductions in down time,
extension of intervals between maintenance and other
factors which historically favour titanium.
to all
Practical guidelines to design cost contro!
Do:
• Check available standard products and specifications
to obtain best availability and lowest cost.
• Use design strategies based on using minimum
ma terial thickness.
• Exploit corrosion resistant characteristics
to the full.
• Consider the use of liners and c1adding in preference
to solid design where heavy sections are unavoidable.
• Use welded fabrication where practicable
in preference to forging and machining.
• Use cold bending or induction bending to limit the
use of bends, flanges and welding for commercially
pure titanium pipework.
• Consult suppliers and fabricators
at the earliest
stage of design.
Do not:
• Simply substitute titanium into existing designs.
• Budget for titanium
project
costs by weight,
especially not by the weight of steel or copper alloys.
• Specify little-used alloys or forms.
Design for welding
Commercially pure titanium presents no difficulties to
welders once the requirements for weld shielding are
understood and used. WeId joint designs for titanium
are similar to those for other metals, and many
excellent welding guides are now available to assist
designers [18].
Gas tungsten are (GT A) is the most commonly used
process and open shop and field welding of large and
sm all fabrications have been routine for many years.
Plasma, laser, electron beam and resistance welding
are all suitable for the weldable titanium alloys.
Friction welding is a particularly successful method
for titanium and is a satisfactory method for alloys
that cannot be fusion welded. The development
of
rotary friction welding appears to offer the opportunity to cut dramatically the field welding times for
titanium tubulars.
Conclusions
Titanium has many existing and potential applications
across a wide range of industries. The metal should
be considered wherever corrosion
is a problem or
weight is a factor. Titanium offers subsea equipment
designers a unique combination
of useful mechanical,
physical and corrosion resistant properties which can
be exploited to improve product and process quaIity
and service life of equipment working
in the most
demanding
environments.
Effective design can be
achieved by attention to the practical differences, the
most relevant ofwhich have been discussed, and which
distinguished titanium from the traditionl;ll engineering
metals.
Stress relieving
Pipes and other fabrications in commercially
pure
titanium will normally not require stress relief. Stress
relieving may be required during machining of bar,
forgings or castings especially where thin sections are
being developed or where metal removal is asymmetrie.
Thirty minutes at 600°C maximum
is normally
adequate for both CP and alloys. Lower temperatures
(400-500 oe) should be effective in many cases and
should be used to limit oxidation of machined faces.
Alloy suppliers should be consulted for a suitable heat
treatment cycle for alloys requiring solution treatment
and ageing.
References
1. Grauman, J. S. 1992. Corrosion
Behaviour
of
TIMETAUM 21S for Non-Aerospace
Applications:
7th W orId Conf. on Titanium, San Diego, USA,
TMS, pp. 2737-2742.
2. Titanium
Information
Group.
1989. Titanium
Pipework Systems.
3. Schutz, R. W. & Scaturro, M. R. 1988. Titanium:
An Improved Material for Shipboard and Offshore
Platform Seawater Systems, I. Corr/NACE
UK,
Corrosion, Brighton, UK, 1988.
4. Berge, S. et al. 1995. Titanium Risers and Flowlines
Feasibility Studies and Research Activities, 14th
Int!. Conf. Offshore Mechanics and Arctic Engineering (OMAE), Copenhagen,
Denmark.
June
1995.
5. Luterjing, G. & Gysler, A. 1984. Fatigue-Critical
Review, Titanium Science & Technology, Proceedings ofthe 5th Intl. Conf. on Titanium, Sept. 1984,
Munieh, Germany, pp. 2065.
Production and range 01 semi-finished products
Titanium
ingots are the starting
point of metal
manufacture and are handled as the other industrial
metals and alloys by forging or rolling to produce
intermediate billet or slab. Billet provides the material
for manufacture of forgings, bar, wire, and extruded
or rolled/drawn seamless tube and pipe. Slab provides
the starting material for plate, sheet, welded tube and
welded pipe. Castings are produced directly from
ingot. Cast weldments
enable large components
exceeding the limits of individual casting weight
(2000 kg) to be supplied. A combination
of forging,
29
Volume 21 Number 4
Underwater Tecbnology
6. Azkarate, I. et al. 1993. Environment Sensitive
Cracking of Titanium Alloys in Offshore Equipmen. 12th Intl. Corrosion Congress, Sept. 1993,
Houston, TX, USA, pp. 2492-2505.
7. Stress Corrosion Cracking-Materials
Performance and Evaluation, ASM International, July
1992 Materials Park Ohio, USA, pp. 265-297.
8. Schutz, R. W. & Thomas, D. E. 1987. Corrosion
of Titanium and Titanium Alloys, Metals Handbook, 9th Edition, Vol. 13, pp. 669-706.
9. Danek Jr, G. J. 1966. The Effect of Seawater
Velocity on the Corrosion Behaviour of Metals.
Naval Engineers Journal, Vol. 78, No. 5, p. 763.
10. IMI (Kynoch) Ud. 1970. Corrosion and Erosion
of Titanium in Seawater. Titanium Information
Bulletin, May 1970.
11. Schutz, R. W. & Grauman, J. S. 1986. Fundamental
Corrosion Characterisation
of High Strength
Titanium Alloys, Applications of Titanium and
Zirconium. STP 917, ASTM, Vol. 4, pp. 130-143.
12. Grauman, J. S. & Mild, E. E. 1992. Corrosion
Resistance of Titanium Alloys in Simulated
Acidising Environments. OMAE Conf. June 1992,
Edmonton, Canada.
13. Liv Lunde et al. 1992. Hydrogen Absorption of
Titanium in Offshore Related Environments
Under Cathodic Charging. 7th World Conf. on
Titanium, San Diego, CA, USA. TMS, pp. 20872094.
14. Schutz, R. 1991. Applicability of Titanium to
Offshore Systems 10th Int!. Conf. OMAE
Stavanger, Norway, 1991.
15. Osvoll, H. & Gart!and, P.O. 1995. Computer
Modelling of Cathodic Protection on Risersl
Tendons. Metal Progress, Feb. 1995, pp. 35-41.
16. Yöung Jr, G. A. & Scully, J. R. 1992. The Influence
of Hydrogen on the Mechanical Properties of
TIMETAUM21S. 7th World Conf. on Titanium,
San Diego, CA, USA, TMS, pp. 2201-2208.
17. Peacock, D. K. 1990. Effective Design Using
Titanium. Int!. Conf. Titanium in Practical Applications, Norwegian Ass'n of Corrosion Engineers,
Trondheim, Norway, Part 6.
18. Titanium Design and Fabrication Handbook for
Industrial Applications, TIMET T/S 2000, Denver,
USA, 1993.
Specialist Consultants in
Underwater and Marine-Related Acoustics
Subacoustech Ud, Burnetts Lane, Horton Heath, Hants 5050 7DG, UK.
Tel: 01703 602428. Fax: 01703 602101.
Services include:
• Research and consultancy into all aspects of
underwater acoustics •
• Advice on noise and diving, including Noise
Audits for diving installations •
• Independent estimates of noise and
environmental effects from underwater seismic
surveys, blasting, etc .•
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Spring 1996
30