1
"Electrochemical imaging of corrosion processes", David
Williams, University College London
Pitting
corrosion of stainless steel involves locally extreme conditions of chloride
concentration and pH, which are maintained as a consequence of the very high
local current density. It is
relatively easy to understand how, once these local conditions are established,
they might be maintained, and hence discuss the conditions for continued stable
propagation of a pit. However, it
is much less clear how, on a surface over which the average passive current
density is on the scale of nA cm-2, such large gradients of local
solution composition could be generated in the first place. As a consequence, there has been a very large number of models
proposed, all of which tend in some way to be equivalent in that they either
propose some special species of low diffusivity (such as clusters or defects),
or highly occluded zones to restrict diffusion (such as rim-dimension cracks in
the passive layer).
The
puzzle now appears to have been solved, using a combination of novel in-situ
microscopy methods. Scanning
electrochemical microscopy with sub-µm resolution shows that the local current
density for dissolution of certain MnS inclusions in stainless steel can be
extremely high (>1A cm-2). The
dissolution is chloride-catalysed and results in the formation of a sulphur-rich
crust extending over the inclusion and the surrounding metal. Photo
electrochemical and optical microscopy indicate that formation of a sulphur-rich
stain around an inclusion is a necessary preliminary to the initiation of a pit
and show attack on the metal underneath the stain. Therefore
it is reasonable to propose that the very high local current density of
inclusion dissolution leads to a significant local concentration of chloride
under the crust, as a consequence of electromigration to support the current,
and may also cause a significant decrease in the local pH as a consequence of
the chemistry of the inclusion dissolution reaction, especially if the inclusion
also contains some Cr. It is then
further reasonable to propose that the conditions generated under the sulphur
crust might be sufficiently extreme to cause the stainless steel to depassivate
and a pit to trigger.
2
"A new anode material to minimise the risk of stress corrosion on
high strength steels in seawater", Valerie Debout, DCN
Zn
or Al-Zn-In sacrificial anodes are commonly used to protect submerged marine
structure from general corrosion and galvanic corrosion. However, such electronegative alloys can also induce stress
corrosion cracking or hydrogen embrittlement on high strength steels. Decreasing
the electronegative potential applied to the structure, in the convenient range
(around -800 mV/SCE) can significantly reduce the amount of hydrogen produced by
the cathodic reaction and thus limit the risk of hydrogen embrittlement.
The
present work has consisted of:
·
The
determination of the criteria for a new cathodic protection system with low
voltage anodes.
·
The
selection of one anode that matches the determined requirements, on the basis of
laboratory electrochemical tests.
Among
the various alloys tested the Al-0,l%Ga anode provided the most promising
results and therefore was selected. Then,
full scale marine tests were performed on an industrial casting of this anode. The
Al-0,l%Ga anode has proven to be effective in the chosen range of potential with
a satisfactory galvanic efficiency in real environments.
3
"Performance of riser splash zone protective cladding",
Paul Badelek, BP
The
protection of riser pipes carrying oil or gas often at elevated temperatures
against external corrosion under aggressive splash zone conditions demands high
integrity cladding systems. The two
most common sheathing/cladding systems are monel and polychloroprene, commonly
called neoprene. As any failure of
these systems by either damage or deterioration will result in very high
corrosion rates on the carbon steel riser pipe, it is essential to undertake
close visual annual inspections in this difficult to access location.
Failure
risk assessments of splash zone protection systems must take into account all
the potential failure modes. One
example of each type of cladding is described which highlights that unexpected
failure modes, often never seen before, can present major difficulties in their
satisfactory resolution.
4
"Carbon films on Cu alloys", Adrian Graham, AEA
Technology
Carbon films may be produced during the heat treatment of copper alloys contaminated with organic lubricants under reducing conditions. The films are porous and cathodic to the copper alloys. In certain natural waters pitting initiates in the copper under these films. This form of attack has been observed in domestic copper tubes and CN107 heat exchangers.
1
"Effect of minor constituents in seawater on corrosion",
Dr Brian Todd, NiDi Consultant
The
corrosivity of seawater is largely determined by its high inorganic salt content
and in particular its high chloride ion content. However,
many other constituents of seawater have an influence and other quite minor
amounts of naturally occurring or added elements can have a pronounced effect. The
purpose of this talk is to discuss the effects of some of the more common minor
constituents including microbial effects, pollution (H2S, S and
ammonia), ferrous sulphate, chlorination, bromides and sand. Often
these minor constituents are present together causing further complications.
2 "Metallic coatings by HVOF
spraying for corrosion protect in marine environments", Dr Andy
Sturgeon, TWI
High
velocity oxyfuel (HVOF) spraying is a relatively new thermal spraying technique
which is increasingly being used for the deposition of protective metallic
coatings, both in original component manufacture and for quick on-site repair. The
HVOF process can give better coating protection than the more traditional
thermal spraying processes such as flame spraying and air plasma spraying. Through-coating
porosity can be removed to give coatings which act as true corrosion barriers,
without the need for a post-coating sealant or fusion stage.
Successful
commercial exploitation of protective coatings prepared by HVOF spraying
necessitates that four key questions be addresses. Firstly, detailed information is needed on what constitutes
the correct coating structure to give protection against corrosion (and wear if
also present) in a given environment. Secondly,
it is necessary to determine the range of spraying conditions which produce this
coating structure. Thirdly, a guide
is needed on the most cost effective routes to preparing coatings which give
acceptable levels of protection on components of complex geometry both in the
work ship and also on-site. Finally
it is necessary to have confidence in the protection given by the coatings
through reliable and reproducible coating procedures and through effective
quality control methods.
3
"The use of advanced electrochemical techniques for plant
corrosion surveillance", Dr Dave Eden, Mentor-1 Group
It
is well known that industrial production facilities can be susceptible to
various forms of internal corrosion. A
variety of reasons lead to this condition; however, it has been repeatedly
demonstrated that corrosion rarely takes place at a constant rate during plant
operation.
Historically,
cumulative corrosion losses (as detected by weight loss coupons, for example)
were in fact often the result of discrete episodes of attack which took place
when transient process conditions drifted outside design limits set for
corrosion-free operation. In the
majority of instances, the plant operators were fond to be unaware when such
excursions took place. For field
applications where conventional monitoring was employed, the methods adopted
were often too insensitive or simply recorded measurements of parameters (e.g.
temperature) which directly inferred the corrosion condition but did not detect
important changes in the phase balance or chemical composition of the process
environment.
An
important factor in the development of plant corrosion surveillance has been the
simultaneous application of a combination of separate, but mutually
complementary, advanced electrochemical techniques. This approach differs from
established practice, which normally relies upon a single corrosion rate
indication. The multi-technique
approach allows several individual signals to be compared and, as such, the
corrosion condition can then be characterised in terms of morphology (e.g.
uniform corrosion, pitting corrosion, etc.) as well as rate of metal loss.
Advanced electrochemical techniques (Electrochemical
Potential and Current Noise, Zero Resistance ammetry, Electrochemical Impedance
Spectroscopy) are highly sensitive and react quickly to changes in the corrosion
condition and, once correlated with the appropriate process parameters, the
resulting information enables plant operators to establish the relative
importance of the specific causes and to prioritise their approach to preventing
or mitigating corrosion.
The
use of advanced electrochemical methods for corrosion surveillance shall be
discussed in this presentation with the aid of actual applications data to
provide a general overview of the information which may be derived and the
likely implications with regard to plant health.
4
"The effects of insulation defects on the corrosion of sub-sea
superduplex stainless steel process pipes", Dr Roger Francis, Weir
Materials
There
is an increasing use of CRA's for subsea flowlines. These pipes carry corrosive fluids at high temperatures and
pressures, and insulation is usually applied to prevent excessive cooling of the
process fluids. The present tests
were undertaken to investigate the effect of insulation defects on the
susceptibility to localised corrosion of a super duplex stainless steel at
different internal temperatures.
Four different commercial coating systems were tested, Neoprene, EPDM, Polyurethane and Polyurethane foam. The results show that pitting occurred at an average temperature of 55°C for neoprene and EPDM, and at lower temperatures for the other two coatings. The reasons for this were discussed, and the implications for service applications addressed.
1
"Control of biofouling without tears", Dr Bob Eden.
CAPCIS
Biofouling
is the accumulation of a film of biological deposit which can decrease equipment
performance and/or lifetime, and in the case of oil wells plugging or souring
can occur. For growth to take place
the organsm and both water and nutrients need to be present in the environment.
In addition, it is necessary for the temperature, pH, redox and salinity to be
within particular ranges. If the
biofilm growth is to survive it must be protected from any inhibitors, such as
biocides.
Five
case studies were covered briefly:
i.
Water injection I
ii.
Water injection II
iii.
Heat exchangers
iv.
Sewers
v.
Combined filtration and seawater treatment
It
is concluded that biofouling may be controlled in a number of ways and that
biocide is not always the optimum solution.
2
"Corrosion fatigue of cathodically protected high strength steels
- the role of sulphate reducing bacteria", Dr Mike Robinson, Cranfield
The
presentation described research at Cranfield University and reviewed the results
of similar work reported in the literature. It has been well established that cathodic overprotection can
cause accelerated rates of corrosion fatigue in high strength steels due to the
absorption of atomic hydrogen by the metal. A permeation technique was used to measure the absorption of
hydrogen by cathodically protected steel, both in seawater and in samples of
marine sediment. Hydrogen uptake
was shown to increase by as much as a factor of ten in the presence of sulphides
generated by active populations of SRB.
Fracture
mechanics testing was carried out to evaluate the extent of hydrogen embrittlement
of high strength steels under static loading. The threshold stress intensities required to cause
embrittlement were determined for a range of environments and a failure/safety
diagram was devised to assist in the selection of appropriate levels of cathodic
protection in environments with high levels of microbial activity.
The
results of corrosion fatigue tests in seawater containing sulphides were
presented. Substantial increases in
crack growth rates have been recorded with SRB present. However, very few studies have been carried out to measure fatigue
thresholds for cathodically protected steels in seawater containing sulphides. More information is needed to allow reliable predictions to be
made of materials performance in these conditions.
3
"Practical applications of coatings for preventing corrosion and
wear", Dr Maxine Watson, AEA Technology
Many
plant components which are subjected to abrasive wear processes within a
corrosive environment experience a rapid loss of material, much greater than
would be produced by the action of wear or corrosion alone. Worse still it is the more corrosion resistant alloys such as
stainless steels which exhibit this synergistic effect. The passive layer responsible for imparting the excellent corrosion
resistance is easily removed by abrasives, the metal quickly repassivates, only
to have the layer removed again.
Coatings
such as hard chrome plate and tungsten carbide cobalt coatings deposited by
D-Gun and HVOF techniques have excellent abrasive wear resistance and corrosion
resistance in marine environments and have the potential to break the
synergistic wear/corrosion cycle. Unfortunately
both of these coatings contain pathways through which corrodants can pass
resulting in corrosion of the underlying substrate material and this can
ultimately lead to detachment of the coating. This
lecture gives two practical examples of the use of these coatings in arduous
abrasive/corrosive environments explaining how the coatings can be sealed to
provide an effective barrier to the underlying 17/4PH substrate material and
significantly extend the life of oil field components.