Technical Presentations at the October 2007 Meeting
2.1
‘Natural Products for Anti-fouling Coatings’,
Lily Chambers, Southampton University & Keith Stokes, DSTL
Biofouling
of marine structures and platforms results in both economical and
environmental penalties. Current approaches to marine antifouling increasingly
adopt strategies to minimise their environmental impact. One approach is
to successfully mimic nature’s methods to control biological growth. A
key biomimetic development for marine antifouling coatings is the
isolation and use of marine natural products. Such chemicals are needed
for secondary metabolic requirements of plants and animals, including
defence chemicals. Recent work has focused on isolation and bioassaying
techniques but few studies have trialed natural product compounds in a
functional coating system.
A
recent project in our laboratories has used a multidisciplinary approach
to develop an antifouling coating system using environmentally acceptable
and naturally occurring products. A red algal natural product extract from
Chondrus crispus has been evaluated as a potential antifoulant. The
ethanol extract was successfully screened with a bioassay which included a
range of biofouling organisms; marine bacteria, microalgae and macroalgae.
The natural product extract was directly incorporated into a proprietary
coating mixture to assess its activity through a realistic delivery
mechanism and to test if its addition affected the coating matrix. The
latter was tested in 3.5 % NaCl solutions using electrochemical impedance
spectroscopy (EIS) and open-circuit potential (OCP) electrochemical
techniques.
The
incorporation of the algal extract into the coating resulted in a slightly
more negative corrosion potential of the coated mild steel by 30 mV (Ag/AgCl
reference), and did not affect the impedance characteristics when compared
to the control coating with no antifoulant. This suggests that the direct
use of the natural product extract in the coating is an effective way to
test antifouling activity for future compounds. The antifouling activity
of the experimental coating was tested in seawater. Biofilm growth
on the coating surfaces was examined using a bacterial viability nucleic
acid stain and an episcopic differential interference contrast (EDIC)
microscope. This proved to be a rapid tool for the examination of growth
patterns and distribution of bacteria in-situ. Field trials were
used in the Solent, England and showed a visual antifouling delay of 6
weeks in comparison to the negative control. The development of a
functional antifouling coating should be possible using an aqueous phase
solution such as a marine natural product. lc701@soton.ac.uk
[L.D.
Chambers, R.J.K. Wood, F.C.
Walsh (Surface Engineering & Tribology & Electrochemical
Engineering Groups, School of Engineering Sciences, University of
Southampton) & K.R. Stokes (Dstl)]
2.2
‘The Replacement of Corrosion-resistant Castings by Fabricated Weld
Overlay Components’, Norman
Cooper, BAE Systems
Copper
alloys have traditionally been used as the major corrosion resistant alloy
for UK submarines, with nickel aluminium bronze being the main material
specified historically. This
alloy suffers from selective phase attack of ΚIII phase,
particularly in stagnant seawater. This
can be mitigated to some degree in welds by heat treatment to convert
ΚIII to the more benign ΚIV phase,
but a better solution, which guarantees a longer product life, is needed.
The CuNiCr alloy NES 824 was thought to offer a solution in the
1980s, but the presence of silicates and oxides of zirconium and titanium
in the cast product made the production of castings which were anything
above a weight of about 50kg, a risky prospect.
For
the Astute submarines a different approach has been adopted for some of
the large components – a weld overlay of 70/30 Cu/Ni on steel (API LX65)
has been specified. This was first tried out 25 years ago, and the components
produced at that time have performed very well in seawater service. A hot wire TIG welding technique is used, and three weld layers are
produced. The first layer is
70/30 Ni/Cu and the other two layers are 70/30 Cu/Ni. The overall thickness used is 8mm.
CT.
4.1
‘Testing
and Experience of Metallic Components in Subsea Hydraulic Control Fluids',
Simon McManus, MacDermid Offshore Solutions
1.
Introduction
MacDermid
are a speciality chemical company supplying the printing, electroplating,
PCB and offshore oil and gas markets. MacDermid has an $800M turnover and 2,500 staff working world-wide.
The offshore oil and gas division, MacDermid Offshore Solutions,
supplies subsea control fluids, blow out preventer (BOP) fluids and motion
compensator fluids for hydraulic operation in the marine environment along
with other corrosion inhibitors and lubricants. The bulk of the chemistry is water based
2.
How does the Fluid Prevent Corrosion?
2.1
Adsorption
The main
corrosion preventative method is by adsorption inhibitors. These are negatively charged water soluble organic materials, they
protect by adsorption on to the metal or metal oxide film exposed to
electrolyte (e.g. seawater or alkaline fluid) by ionic interaction. Bear in mind, metal surfaces are positively charged and these
positive metal ions can be solvated by water or oxidised by oxygen
contained within the water causing corrosion products to form. The adsorbed material prevents detrimental chemicals adhering or
adsorbing themselves on to the metal surface. Examples of organic inhibitors are aliphatic/aromatic amines
(Nitrogen compounds), thiourea (Sulphur compounds) and aldehydes/carboxylic
acids (Oxygen compounds). All these have a charged state. Sulphur compounds bond
strongly to the metal by sharing its electrons with the metal surface.
This blocks solvating water molecules and also stops hydrogen gas
formation. Nitrogen and
Oxygen (cathodic or -ve) containing compounds are less (more weakly)
adsorbed at the metal surface than sulphur type compounds. They tend to select active anodic (+ve) sites on the metal surface,
binding by ionic interaction to the positive sites on the metal surface.
As a general rule, the larger the inhibitor molecule, the greater
the inhibition of corrosion as they displace solvating water molecules
from the active sites on the metal surface. Remember, for rusting to occur, you need both water (the solvating
agent for the metal ions) and oxygen present. Care must be taken in the choice of inhibitor as this can also
effect the lubrication chemicals. In some cases the inhibitor and the
lubricant can be the same chemical, i.e. one product to perform both
lubrication and corrosion prevention. In other cases the lubricant can bind to the ‘tail’ end of the
inhibitor but mostly two chemicals will compete for active +ve sites and a
balance must be preserved.
2.2
pH Passivation
pH can
have a major effect on corrosion rate and corrosion products. Pourbaix provides a large amount of information on metal pH
corrosion products and expected corrosion rates with varying
charge/current drivers. Aluminium
for instance is reasonably protected between pH5 and 8.5, MacDermid have
found that a pH of 9.4 is ideal for iron based alloys. The alkalinity of a fluid and its ability to protect metals can be
greatly enhanced by buffering. This
buffering ability of the fluid quickly neutralises the acid components
manufactured in crevice corrosion preventing further deterioration. All of the fluids supplied by MacDermid are biodegradable when
diluted in seawater. This
does not occur in the concentrated fluid due to the presence of biocides,
however seawater ingress can be an issue and buffering becomes important.
Micro-organism are less likely to grow in an alkaline
environment and if they do the buffering will neutralise the acid
by-products of bacterial growth, again reducing corrosive effects. Mono-Ethylene-Glycol is used in the fluids as a pour point
depressant. At higher
temperatures there is also a risk of glycol degrading to glycolic acid,
once again the alkaline buffering can neutralise the acid and prevent
attack on metals.
2.3
Vapour Phase Corrosion Inhibitors (VPI)
When
equipment is in storage, water-containing fluids will evaporate into air
gaps and then condense on the internals of metallic components. This condensate has been known to cause severe rust, particularly
on the fluid/air interface. The VPI chemicals are weak organic compounds
that become charged in water. They can be added to the fluid at a level
above the solubility equilibrium, when an air gap is present the chemical
will vaporise. When this
vapour comes in to contact with water it will create a solubility
equilibrium in the new solvent. Once
in solution it becomes charges again and will adsorb to the metal surface
as in part 2.1. The protected system must be closed for the VPI to have an
effect.
3.
What is MacDermid Trying to Protect?
The
equipment in the oil and gas sector containing water based fluids range
from valves three meters long to poppet valves containing a poppet ball
less than 1mm in diameter. Obviously
small amounts of corrosion are not going to significantly effect the
former, however small amounts of corrosion could destroy the latter.
If a poppet ball is slightly corroded it will not seat correctly.
The fluid has a high bulk modulus (it is a hard fluid) and the pressures
across the valve are high (200 to 500 Bar). After the small amount of corrosion the main degradation is then
erosion due to high fluid flow through the leak caused by the corroded
area. As oil and gas
production stretches deeper into the Earth’s crust the temperature of
the hydrocarbons being produced also increases. There is now a requirement for the hydraulic fluids to remain
stable and prevent corrosion at temperatures exceeding 200°C. The oil and gas industry also uses a wide range of materials and
coatings including yellow metal alloys, exotic steels and hard or
lubricating coatings. All of these give rise to issues of compatibility over
extended periods. Once a
subsea component is on the seabed it is hoped that it will operate for 25
years without intervention and with long static periods.
4.
Test Methods and Qualification.
4.1
Galvanic Testing
The old
IP329 testing for galvanic corrosion was notoriously difficult to set up
and dismantle, often handling the test coupons would cause more corrosion
than the small weight losses being measured. MacDermid now use an electronic device that can monitor the
corrosion currents in real time. This
is invaluable when formulating as the device can show the effect of the
addition of a chemical instantly. The
device is also very useful as it is not limited to the seven metals and
three couples covered by the IP329. Any
seven materials can be coupled with any of the other inputs in the device
to ascertain which are sacrificial to which. The electronic device can also log corrosion rate against time
giving an indication of film build-up or breakdown. As the test equipment only requires that an electric circuit be
completed, tests can be carried out on actual components retrieved from
service or test materials and coatings proposed by equipment
manufacturers.
4.2
Passivation Measurement
MacDermid
also has a device that logs the current from a material when it is forced
using a direct current. This
indicates the passive layer strength and will show if a component will be
anodically or cathodically protected. Again this device works in real time, so a slight adjustment in pH
or passivation chemistry can be seen straight away.
4.3
Immersion Testing
MacDermid
rely heavily on the simple method of immersion and weight loss
measurement. All of our testing is based on ISO13628-6 part C where
components or coupons are immersed at 60°C and 20°C for 3, 6, and 12
weeks with and without seawater. Some
tests are also carried out at 10°C above the maximum expected operating
temperature of the components. This
testing is conducted with Oxygen regeneration in most cases under a
semi-permeable membrane, however the high temperature testing is conducted
under Nitrogen. After test the components are examined under a microscope to
check if even, inter granular or pitting corrosion has been encountered.
Using the standard 7 metal combinations a total of 324 samples are
required for the ISO testing, however MacDermid have a dedicated team
conducting this testing and have currently examined over 100 alloys with
up to 5 different fluids.
4.4
ICP Testing
An
inductively coupled plasma flame burns as a pure energy at 6 to 10
thousand °K. If a contaminated fluid is Nebulised (aerosol) in to the
flame the frequency and intensity of the spectrum emitted can accurately
give the concentration and identify the atoms/ions present. After exposure to a metal the fluid can be examined by use of
ICP. If the surface area, fluid volume and exposure time are known the
corrosion rate can be calculated very accurately. This is particularly useful on tests with small components or on
samples of fluid retrieved from the field. It also means that corrosion
rates can be gained after only a few days exposure.
4.5
Quick Lab Tes
Other
testing includes the IP287 cast iron drillings check where the metal chips
are places on a filter paper and exposed to the fluid, if corrosion
staining is seen on the filter the fluid has failed. This test is ideal for a quick QC check and to determine the amount
of dilution a fluid can tolerate before corrosion occurs. The VPI test involves suspending carbon steel samples over warm
fluid, a water based fluid with no VPI protection will quickly cause
pitting in the metal. A fluid
with good VPI protection will maintain a clean and brightly polished
surface on the cast iron. The
IP135 bullet test is used to ascertain an inhibitors protection level when
exposed to seawater.
5.
Offshore Experience
MacDermid
have has a large range of incidence offshore which were discussed during
the presentation which include:
•
Aluminium washer corrosion blocking solenoids in Directional Control
Valves (DCV’s).
• Knife edge corrosion on partly immersed cast Nickel Aluminium Bronze, even in the presence of a VPI.
• Tungsten Carbide corrosion of pure cobalt bound WC and corrosion of WC balls where there has been an uneven distribution of Cr through the coating surface. Tungsten carbide corrosion is very small with water based fluids if Chrome is included in the material.
• Removal of Nickel plate, this is not generally an issue as the Ni ions are carried away with the hydraulic fluid in solution. The exposed magnetic ferrous alloy is protected and lubricated by the fluids that are used.
Materials
Technology Ltd. is an engineering materials consultancy firm whose
function is to solve technical problems for virtually all industries.
The paper on ‘Learning from Failures 2’ covered some examples
of the corrosion problems the company has been involved in during the past
25 years, an extract from a longer seminar focussed on ‘Decision
Making’. The paper was a
review of cases, predominantly in the Marine Industry but also from other
industries.
Lessons were drawn relevant to the delegates backgrounds. The cases, which included crevice corrosion, stress corrosion cracking, general corrosion and the synergistic effects of the microstructural changes and residual stresses acting on welds, highlighted the complexity of even simple decisions on materials selection. Some of the cases had resulted in expensive recalls and rework, some in failures of the companies concerned and some in disasters and fatalities. Derek emphasised that the common feature in the majority of failures was poor decision making. [Derek Bates derek@mtechltd.co.uk]