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.  

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  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 Tests

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.

 4.2  ‘Learning From Failures 2’, Derek Bates, Materials Technology Ltd  

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]

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