1
“High strength cupronickels — past, present and future”,
Clive Tuck (Meighs Langley Alloys Division Ltd)
Since
the 1930’s, when it was discovered that the addition of aluminium to a solid
solution of Cu-Ni produces a strengthening effect through Ni3A1
precipitate formation,
a series of high strength engineering alloys based on Cu-Ni-Al have been
developed. The initial commercial alloy of this type, HIDURAX SPECIAL, had poor
grain size control, but this was improved (together with impact toughness) with
the development of HIDURON 191 in the 1960s. At the present time, super high
strength cupronickels typical proof stress 730N/mm2) have become
established for widespread applications in the offshore oil and gas industry due
to their advantages in possessing seawater corrosion resistance and hydrogen
embrittlement resistance, together with mechanical properties complying with
ASTM A193 Grade B7 carbon steel.
Principal
applications of high strength cupronickels have included high integrity
fasteners, high strength connectors and the wide variety of actuator devices,
the main properties utilised being strength, corrosion resistance, hydrogen
embrittlement resistance, anti-galling behaviour and bio-fouling resistance. In
particular, over 250 t of the principal alloy of this type, MARINEL, has been
deployed as an engineering alloy in marine environments and it has demonstrated
a consistent resistance to hydrogen embrittlement and passivation in hydrogen
sulphide media.
However,
evidence has been that if super high strength Cu-Ni-Al alloys are tensioned to
levels significantly above the recommended engineering stress regime (i.e. they
are abused during installation and service), stress corrosion cracking can
result in particular environments if cathodic protection is absent. Slow strain
rate testing has suggested that such debilitating environments include aqueous
solutions which contain ammonia, organic amines or elevated temperature
concentrations of chloride ions. However, recent development of MARINEL has
produced the improved alloy, MARLNEL+, which shows marked resistance to stress
corrosion and therefore possesses a substantial degree of abuse to tolerance.
Development
work on high strength cupronickels is continuing, and computer-aided (neural
network) alloy design methods are being used to further improve the overall
mechanical properties of these alloys with a view to enhancing yet further their
properties.
2
“Latest developments in the conservation of large ships”, Des
Barker (Portsmouth University)
A review
of the conservation programmes being undertaken on the Holland I and Minerva was
given. Holland I was the first submarine built for the British Royal Navy and
was launched in 1901. Holland I sank in 1913 but was recovered in 1981. Both
vessels are undergoing treatment to remove chloride and reduce corrosion rates
to avoid further deterioration of the structure.
3
“Influence of Biomineralisation and Colloids on Marine
Biofouling”,
Mark Varney
(University of Southampton)
The
commercial problems of biofouling are well known and documented. Spores, diatoms
and other marine micro-organisms quickly populate underwater surfaces - in the
order of days to weeks. Some species have surfaces which appear to offer very
effective natural antifouling properties: corals, sponges, macroalgae, for
example. Antifouling agents are very much needed against Enteromorpha
intestinalis (green alga) and Balanus amphrite (barnacle) which are very common.
While a number of very promising molecular agents have been isolated and the
search for better ones continues, there is still no fundamental understanding of
the various mechanisms underlying (or preceding) biofouling.
Topics
discussed included:
1.
Currently effective agents
2.
Electrochemical Measurement of Biofilming
3.
Colloids
4.
Colloid stabilisation
5.
Biomineralisation
6.
Biomineralisation & colloidal formation
Biofouling
(filming) is only encountered in the field, and cannot be reasonably (or
accurately) repeated in the laboratory. Possible control mechanisms to minimise
the effects of biofouling centre on understanding the elements of biofilming (on
timescales of seconds to minutes). Ca2* binding and colloidal formation seem
fundamental, and their involvement in precipitating and stabilising films on
underwater surfaces would appear to be plausible. Future control mechanisms
would probably centre on the control of colloidal conformation and/or steric
effects. There is a clear need for further work in this respect.
4. “Fundamentals of localised corrosion and SCC of high alloy stainless
steels in marine environments”, Roger Newman (UMJST)
Advances in the understanding and control of stress-corrosion cracking (SCC) often relate to the definition of the local environment, either within a localized corrosion cavity or within a thin liquid layer influenced by evaporative concentration. Several examples of such advances will be given, involving crevice corrosion and its role in SCC initiation, magnesium ions in seawater and their role in creating concentrated environments, and the behaviour of H2S in pits and cracks. Aspects of cathodic hydrogen embrittlement of duplex stainless steels will also be considered.
1. “Scanning tunnelling microscopy (STM) and marine corrosion”, Peter Farr (Birmingham University)
The
superduplex stainless steel studied was Zeron 100. This had a 50:50
austenite ferrite structure with a grain size around 30um and composition
(wt%) C 0.24; N 0.21; Cr 24.40; Ni 6.83; Mo 3.70; Mn 0.77; Cu 0.63; Si 0.18; S
0.002; P 0.025; W 0.625. Test surfaces were carefully polished, to minimise the
extent of the damage layer, and were cleaned with solvent before electrochemical
treatments.
The
electrochemical behaviour of the steel in acidified NaC1 solution (equivalent to
activated brines) was established using both potentiodynamic (“dc”) and
impedance (“ac”) techniques. In brief, the anodic polarisation curves
compared well with those for austenitic (18:8) stainless steel. The breakdown
(pitting) potential was far anodic (ca 0.9V SCE). There was evidence of Faradaic
activity within the passive region, also for regions of semi-conductor behaviour
in the passivating oxide.
Under
electrochemically well-defined breakdown conditions scanning probe microscopy (SPM)
showed that 1) even the damage layer on austenite is remarkably stable 2) the
earliest attack is at grain boundaries between austenite and ferrite phases 3)
the ferrite is
much more strongly attacked than austenite, yet does not show pitting
until several hundred monolayers have been removed from this surface 4) pitting
in deeply etched ferrite is strongly anisotropic, that on austenite is
isotropic.
The
results were discussed in the light of current theories of the onset of
corrosion on stainless steel and with concern for risks in the engineering
application of this superduplex stainless alloy.
The
study was a good vehicle for the assessment of available scanning probe
microscopies. These were described briefly (plenty of literature is available
for the enthusiast). The techniques are attractive and not prohibitively
expensive: but possibilities of imaging artefacts are legion, especially in in
situ” electrochemical experiments. Hence most of the images described were
produced by atomic force microscopy (a “nanometric” profilametry), the most
reliable mode of SPM. The observations were supported by SEM work, at lower
magnification.
2
“Incorporating lessons learned into the corrosion engineering of the BP
Bruce Phase II Subsea Development”, Charlie Barraclough (Kvaemer Process)
Lessons
learned from previous problems experienced on other offshore installations were
fed into the detailed design of the BP Bruce Platform, Pipeline and Manifold.
The through life cost of pipeline operation led to the selection of a CRA lined
pipeline and duplex stainless steel manifold. Historical information on riser
and pipeline damage led to the use of carrier pipes for both pipelines and
risers. The heat-exchangers were channel side seawater in order to prevent the
crevice corrosion experienced in some coolers, and the operating temperatures
are kept down to prevent pitting which can occur in CRA clad coolers.
Incidences
of hydrogen cracking of duplex stainless steel required all duplex items to be
subject to finite element analysis which lead to modifications in design to
reduce stresses beyond those normally permitted. In addition no anodes or other
such attachments were made to duplex material, and all duplex material was
coated with rubber or polyurethane. Crevice corrosion of flanged and hub joints
was dealt with by
3 “Cathodic
protection: Hydrogen pick-up and transport”, Alan Turnbull (National Physical
Laboratory)
The
primary cathodic reactions in seawater are reduction of oxygen and reduction of
water, the latter resulting in the generation of hydrogen atoms on the metal
surface. There is some indication that the presence of oxygen can retard
hydrogen atom ingress into the material slightly but this becomes irrelevant
with the formation of calcareous scale as access of oxygen to the surface is
restricted and the oxygen reduction reaction becomes insignificant. The kinetics
of water reduction in seawater and in 3.5%NaC1 at about the same pH are
identical for a C-Mn steel but data for other metals are limited. It is noted
that the kinetics of water reduction decrease significantly with increasing pH
and will hence be low in crevices and cracks (in the absence of dynamic
straining) as a consequence of the alkalinity developed.
The
hydrogen atoms produced from cathodic reduction will be absorbed into the metal
or recombine as molecular hydrogen which will dissolve in the solution. As the
potential is decreased to more negative values the solubility limit of hydrogen
in solution will be exceeded and hydrogen gas will form. Under deep sea
conditions, hydrostatic pressure will suppress the formation of hydrogen gas and
hence the rate of removal of hydrogen. This may encourage increased hydrogen
absorption.
Surface
films can have a very significant effect on hydrogen entry. There is no
unambiguous study of the effect on hydrogen entry of calcareous scale but for
low alloy carbon steels and probably for corrosion resistant alloys (CRAs) it
seems unlikely to be marked. In contrast, the oxide films on CRAs cause
significant retardation of hydrogen entry. Correspondingly, if the film is
ruptured mechanically, hydrogen entry locally can be high relative to adjacent
areas.
In
testing for resistance to hydrogen cracking it is important to obtain
information about the concentration of hydrogen in the metal and the hydrogen
diffusivity. The reliability of such data in the past has not always been ideal
but measurement of these parameters by the electrochemical permeation method is
now on a more sound basis with the publication of a new standard (BS 7886 1997).
Further confirmation of the validity of the technique has been demonstrated by
good agreement between measurements of hydrogen content in a duplex stainless
steel using both permeation and total extraction methods.
Values
of the diffusion coefficient can range from about 1 .7x10-6 cm2s-1
for a 4360 SOD steel to 2.8x10-11 cm2s-1
for a 22Cr duplex stainless steel at
ambient temperatures. This brings up the issue of how long a laboratory test
should be to ensure that service behaviour is adequately represented. Corrosion
fatigue studies on some low alloy carbon steels cathodically protected in
seawater suggest that pre-exposure for a year may be required. There is less
certainty about CRAs because of the importance of local hydrogen entry
(film-ruptured areas) compared to bulk hydrogen entry. The indications are that
test times need not be too long despite the low diffusivity,
provided cracking is initiating at the surface. However, further work for
different alloys is required.
The
importance of film rupture does point to the need to conduct testing of CRAs
under conditions of dynamic strain. Current activity at NPL is focusing on the
impact of dynamic straining on hydrogen cracking of duplex stainless steel using
plain and notched specimens.
4
“Cathodic Protection and Hydrogen Embrittlement”, Robin Jacob (Global Corrosion
Consultants Ltd)
Offshore
cathodic protection (CP) systems utilising conventional sacrificial anodes, and
designed to current recommended practices (DNV, NORSOK), will reach potentials
(relative to the Ag/AgCl/seawater reference electrode) between the required
level for protection of carbon steels (-800mV) and the anode potential
(-lO5OmV). With the formation of calcareous deposits, most will equilibrate at
around —95OmV.
Although
satisfactory for normal offshore steels, such an outcome may not be acceptable
if materials susceptible to hydrogen embrittlement are required. In recent years
cracking, due to a combination of applied CP, inappropriate metallurgical
structure and operational stress levels, has been observed in both high
strength, and duplex stainless steels. Subsequent laboratory investigation has
shown that potentials more negative than ~—-800mV can, under some operating
conditions, lease to hydrogen embrittlement. Clearly, where these materials need
to be used subsea, a novel approach to CP is required.
Such
an approach can be provided by incorporating voltage limiting devices, such as
Schottky barrier rectifiers into the sacrificial anode connections. These ensure
that the potential difference between anodes and structure cannot be less than
—300mV, leading to protection potentials in the range ~-800 to —75OmV. This
does, of course, mean that the generally accepted criterion for protection of
any carbon steel in the structure (-800mV) may not be fully achieved, but in
practice, corrosion rates even at