1
"Overview of materials used for seawater handling on offshore
platforms", Alan Warburton. Brown & Root
The
presentation gave an overview of materials used for handling seawater offshore,
starting with an explanation of the various applications where seawater is used
on an offshore platform.
The
early use of carbon steel piping, and lined versions of it, was related to its
use in tankers and onshore refineries.
The
development of the use of corrosion resistant materials was then discussed, including
copper alloys, stainless steels, titanium and composites, together with their
relevant advantages and disadvantages.
A
brief review was then given of materials for valves and items of equipment such
as pumps and heat exchangers, where galvanic corrosion can be a problem in
seawater service.
2
"Galvanic coupling of duplex stainless steels", Aeronwen
Griffiths, NPL
Galvanic
corrosion is always a concern when duplex stainless steels are used with
dissimilar materials. The issue
with coupling to noble alloys and graphite is possible ennoblement of the duplex
stainless steel with the attendant implications for localised corrosion and
stress corrosion cracking. The
galvanic series of metals in seawater at ambient temperatures is well defined
and there are considerable data in the literature on galvanic corrosion of
duplex stainless steels at temperatures between 10°C and 40°C. A research project is being conducted at NPL to provide guidance on the
galvanic compatibility of duplex stainless steels at elevated temperatures.
The test programme has included measurement of open circuit potentials,
cathodic polarisation curves and galvanic currents. Tests have been conducted in aerated ASTM seawater at 22°C, 60°C and
120°C using 22Cr duplex, superduplex, a Ni-based alloys (C276), titanium and
graphite. The results indicated
that coupling duplex stainless steels to titanium, C-276 and other grades of
duplex is unlikely to induce localised corrosion. Coupling to graphite is a potential concern but in practice these
materials will usually be coupled at a crevice. In this context, a model of crevice chemistry was used as a
tool to understand the factors controlling galvanic corrosion in crevices.
The preliminary results demonstrated that the kinetics of the internal
cathodic reaction kinetics have a significant effect on the pH in a crevice.
This must be taken into account in evaluating the effect of galvanic
coupling on crevice corrosion.
3
"Use of GRP pipes in ships and offshore", Mr George
Grim, Shell Trading & Shipping Company
The
introduction and use of Glass Reinforced Plastic (GRP) pipes up to 900mm
diameter in the cargo and ballast piping systems in ships was described. Generally their use in the many ships in which STASCO have been involved
has been an outstanding success, utilising an "Application
Engineering" approach i.e. applying existing and proven engineering
principles to particular system installation. Success has also been assisted by the requirement that GRP pipe
manufacturers provided finished systems i.e. ready for assembly, hence avoiding
any on-site gluing.
The
use of GRP piping in the Draugen GBS involving design principles developed in
shipboard systems was also described. This
arrangement used pipes up to 800mm diameter, a ballast main of 40m diameter and
extensive use of 600mm diameter oil piping. Few problems were experienced in its installation and has continued to
operate trouble-free.
Undoubtedly
the extended use of GRP in marine/offshore applications will continue, giving
(assuming system designs are correct) a service reliability that metallic
materials would often, economically, find impossible to achieve.
4
"Some aspects of the environment sensitive behaviour of marine
fastener alloys", Dr Clive Tuck. Columbia Metals
A
plethora of materials is currently available for possible use as fasteners in
marine applications. The list
covers alloys of varying strengths and corrosion resistance, and encompasses
those based on iron, nickel, titanium, aluminium and copper. All of the materials are known to react with and absorb hydrogen, both
from the gaseous state and via electrochemical charging. The latter is of particular importance when considering bolting
applications on offshore structures where cathodic protection is used, as a
number of studies have shown that the presence of internal hydrogen in most
potential fastener alloys causes the materials to become embrittled. Some
alloys have been demonstrated to be less susceptible to this phenomenon, namely
the nickel alloy 625, some titanium alloys, and, in particular, high strength
(age-hardenable) cupronickels.
It
is not entirely clear how the hydrogen affects the formation of brittle cracks
as no clear link can be found between hydrogen solubility, hydrogen diffusivity
and hydrogen embrittlement susceptibility, although it is known that
dislocations can assist movement and that discontinuities in the structure can
act as internal hydrogen traps. Rice
and Thompson have developed an empirical relationship between the lattice energy
terms for a material both with and without hydrogen which is able to predict
whether the hydrogen can bring about crack blunting, this process resulting in
ductile rather than brittle fracture. The
model has been demonstrated to work in predicting the behaviour of nickel,
aluminium and copper alloys.
As
hydrogen diffusion kinetics play a key role in the availability of hydrogen to
affect the yielding behaviour of materials, care must be made to design any
accelerated testing techniques, such as slow strain rate testing, so as to
represent as close match as possible to natural conditions. In
particular, duplex stainless steels have very low hydrogen diffusivities at room
temperature and this may be too slow to cause embrittlement effects under normal
slow strain rate testing conditions.
High strength cupronickels have repeatedly been shown to be resistant to hydrogen embrittlement. However, their use as subsea fasteners has highlighted their susceptibility to other environment-sensitive behaviour, namely stress corrosion cracking in the presence of ammonia, organic amines and high concentrations of chloride. Recently. Columbia Metals has developed an improved high strength cupronickel, NIBRON SPECIAL, which shows resistance to both hydrogen embrittlement and stress corrosion and this alloy has started to be extensively used offshore.
1.
"Alloy materials for CP systems", Dr Bob Crundwell, Britannia
Alloys and Chemicals
With
the exception of certain magnesium and zinc alloys covered by US Military specifications,
the majority of sacrificial anode alloys are proprietary compositions. Some
of the alloying elements are deliberate additions and others impurities from the
raw materials. This presentation
reviewed the major alloy groups and outlined the importance of the alloy
components and the effects on alloy performance.
2
"Correlation of the microstructure of a 6% molybdenum stainless
steel with performance in a high aggressive test medium", John Grubb,
Allegheny Ludlum
The
6% molybdenum stainless alloys were developed for and are used in many highly
aggressive environments. It is known that these materials will not perform to
their potential if improperly heat treated or inadequately pickled. Second phases which can form from improper heat treatments and
surface layer alloying element depletion can degrade corrosion resistance in
seawater and similar high chloride environments. Data have been developed which correlate the microstructure
resulting from thermal treatments with performance of UNS N08367 alloy in an
aggressive laboratory test solution. Additional
data examines the effects of low chromium surfaces in aggressive solutions.
3 "Pitting corrosion of
duplex stainless steels", Mr L Garfias-Mesias, University of Oxford
The
effect of annealing temperature on pitting resistance of a 25% Cr DSS (UNS
S32550) has been studied to throw light on the significance of phase balance and
phase composition on pitting resistance. Measurements
of critical pitting temperature (CPT) and pitting potential showed best
performance after annealing at the lower temperatures. Tests
on similar DSS with increased Cu content were used to show the influence of Cu
in the pitting resistance of this alloy. In
1M HCl, used to simulate conditions within a pit, the maximum current density in
the active state was substantially reduced for the Cu-containing alloys (or if
copper ions were added to solution) provided that the potential was initially
taken to values where metallic copper could deposit. In
3.5% NaCl solutions, pitting potentials were slightly better for the Cu
containing alloys, whereas the CPT was not substantially improved. Finally, pit stability in these alloys was briefly discussed.
4 "The performance of
concretes in chloride containing conditions", Mr Ray Cox, BRE
In
the middle of the last century, the practice of improving the tensile properties
of concrete with steel reinforcement was introduced. Since then, the practice has developed and now plain carbon
steel reinforced concrete is one of the major structural materials. Whilst
in general reinforced concrete has been a satisfactory material, there have been
instances of premature failure. The
risk of premature failure of reinforced concrete structures is increasing with
the increase in the number of structures being constructed which are subject to
very aggressive conditions such as road bridges exposed to chloride de-icing
salts in the winter months.
When
hydrated, the cement components in concrete produce alkaline compounds which
raise the pH of the concrete matrix to between 12.5 and 13.5. Plain
carbon steel remains passive within this pH range; the limited oxidation
required to maintain passivity has no practical physical effect on the metal or
the surrounding concrete. This
protection remains effective as long as the high pH is maintained and there are
no deleterious materials present.
Alkaline
materials are usually unstable in the atmosphere as they react with the highly
acidic solutions of carbon dioxide in moist conditions. As the result of this
reaction, alkalinity is lost on the surface of the concrete, a process known as
carbonation. The rate at which the
carbonation front spreads into the concrete depends on a number of factors such
as porosity, permeability cement content etc. Perhaps the most important of these are porosity and
permeability of the concrete: the more permeable and porous the concrete the
easier it is for the carbon dioxide to diffuse into the concrete. Once the carbonation front has spread down to the steel, the
steel is at risk to corrosion. The
steel will then corrode provided that there is sufficient moisture and oxygen.
The
presence of deleterious materials such as chlorides in the concrete can disrupt
the passive film in alkaline concrete and lead to corrosion of the steel. Deleterious
materials can enter the concrete as contaminants of the concrete contents, as a
deliberate addition e.g. calcium chloride used as a set accelerator in cold
weather (a practice which is no longer permitted in concrete containing metals),
or may enter the concrete during its service life. The
amount of chloride permitted in concrete as cast is now restricted and is
normally not more than 0.4% chloride ion with respect to the cement content, but
can be lower in some instances e.g. pre-stressed concrete.
There
are several ways of reducing the risk of the steel corroding in concrete, and
for normal conditions it is normally sufficient to ensure that the concrete is
of a suitable quality and there is the appropriate depth of cover. In
more onerous conditions where the reinforcement is at risk from corrosion as the
result of the ingress of chloride, additional steps may be necessary. Chloride diffuses into concrete in the form of a solution.
Therefore,
if moisture can be prevented from entering the concrete, the risk of chloride
ingress is reduced. The ingress of chloride can therefore be prevented by the
application of surface coating or a hydrophobic coating. Similarly,
if the porosity and/or the permeability of the concrete can be reduced by
modifying the concrete's structure either by changing the casting process or by
additions to the concrete, the risk of chloride ingress will be reduced.
Alternatively the durability of reinforced concrete can be improved by increasing the corrosion resistance of the reinforcements. The corrosion resistance of plain carbon steel can be increased by coating with powder epoxy or galvanised steel or stainless steel can be used.