Technical Presentations at the April 2001 Meeting

2.1 'Corrosion and Biofouling performance of 90-10 and 70-30 Copper nickels in Marine Environments', C.A.Powell (Consultant to the Copper Development Association).

Copper-nickel alloys have been specified over the last 70 years for marine service and have an ongoing technological role as they possess a remarkable combination of good corrosion resistance and biofouling resistance in seawater. They are readily welded and fabricated making them prime materials for seawater pipework, heat exchanger and condenser service for many of the world's navies and merchant ships. They are also used in desalination, power plants and offshore fire water systems, the sheathed protection of oil/gas platform legs and risers and boat hulls.

There are 2 main grades of copper-nickel alloy used in marine service. These contain 10% and 30% nickel and are described as 90-10 and 70-30 copper-nickels respectively. The 70-30 alloy is stronger and can withstand higher seawater velocities but, for many applications, the 90-10 alloy will provide good service and being the more economic tends to be the one that is more widely used. Both alloys contain small amounts of iron to develop the best combination of resistance to flowing seawater and overall corrosion resistance.

Seawater Corrosion Resistance.

Copper-nickel alloys have extremely good corrosion resistance to seawater under a wide variety of conditions. This results from the formation of a thin, protective and predominantly cuprous oxide surface film, which forms naturally on exposure to clean seawater. The film forms fairly quickly over the first couple of days and will take 2-3 months to fully mature.

Corrosion resistance is maintained at higher flow rates than steel and most copper alloys due to the resilience of the surface film. General experience has shown that 90-10 copper-nickel can successfully be used in condensers and heat exchangers with velocities up to 2.5 m/s, and 3 m/s for the 70-30 alloy. For pipeline systems, higher seawater velocities can safely be used in larger diameter pipes as indicated by BS MA18, "Salt water Piping Systems in Ships", which calls for a maximum design velocity of 3.5m/s in pipes of 100 mm and larger for 90-10 copper-nickel and 4 m/s for the 70-30 alloy. Although these values are now considered to be conservative, such guidelines have worked well to avoid impingement attack because they take into account normal velocity raisers within pipework systems such as bends which can cause areas of high local flow rate. Nevertheless, extreme turbulence should be avoided which may occur at tight radius bends, partial blockages and areas downstream of partially throttled valves.

Low flow conditions of less than 0.5m/s in systems are not normally a problem in clean seawater. However, if the seawater contains entrained sand or silt and particularly if the water is polluted e.g. in harbours, with sulphides, higher velocities are preferred.

Copper-nickels have a good resistance to chloride pitting and crevice corrosion in seawater even at higher temperatures and are not susceptible to chloride or sulphide stress corrosion cracking or hydrogen embrittlement. Unlike brasses, they have not been found to suffer cracking due to ammonia in seawater.

Exposure to polluted seawater should be restricted wherever possible and particularly during the first few months of contact with seawater. However, if an established cuprous oxide film is already present, then periodic exposure to polluted water can be tolerated without damage to the film. Normal harbour turn around times which often involve exposure to polluted water have rarely led to significant problems. If formed, the sulphide film can be gradually replaced by the oxide film with subsequent exposure to cleaner waters.

Ferrous additions to seawater in the form of ferrous sulphate dosing or by fitting iron anodes will encourage effective film formation and has been found to enhance corrosion resistance of copper-nickel in both polluted and unpolluted conditions.

Biofouling Resistance.

Copper-nickel has a high inherent resistance to macrofouling as service experience for applications such as piping, boat hulls, water boxes, screens and tubesheets has demonstrated. The biofouling resistance of copper-nickel tubing allows shipboard condensers to maintain good heat transfer capability for several months between mechanical cleanings without the need for onboard chlorine generators required for other tubing materials. Optimum biofouling resistance requires the alloy to be freely exposed such that it is electrically insulated from less noble alloys or free of cathodic protection.

Important Considerations.

• Obtain copper-nickel from a reputable supplier and to international standards
• Use 70-30 copper-nickel consumables for similar welds in 90-10 and 70-30 copper-nickel.
• Heed maximum velocity limits for the alloys.
• During commissioning, avoid polluted water. Add ferrous sulphate to enhance the protective film formation if extra caution is required.
• To get the best biofouling resistance insulate copper-nickels from less noble alloys.

A recent publication "Copper-Nickel Fabrication" describes good fabrication and operation practices and can be obtained from the Copper Development Association or the Nickel Development Institute.

2.2 ‘Calcareous scales formed by cathodic protection- an assessment of their characteristics and kinetics', A P Morizot and A Neville (Corrosion and Surface engineering Research Group, Heriot-Watt University).

An electrochemical technique using the assessment of the rate of oxygen-reduction at a rotating disk electrode (RDE) has been used in conjunction with surface analysis by X-ray photoelectron spectroscopy (XPS) to study the first layer formation of calcareous deposits under cathodic protection in various solutions. The metal electrodes were polarised in the cathodic regime at a potential of –1V SCE and the current as a function of time was measured over 5 minute, 1 hour, 3 hour and 3 day test periods.

The study has shown that the electrochemical technique is an effective, quantitative means of monitoring scale formation and that the existing models which predict scale formation by a basal layer of Mg(OH)2 followed by CaCO3 are over-simplified. The advantages of using an integrated approach of electrochemical analysis and a surface sensitive analysis technique in characterising the initial scale as a function of the solution composition are that the sequences of scale formation and the kinetics of formation can both be assessed. In the presentation it was demonstrated that Mg ions in the solution promote the formation of a very thin basal layer which, when Ca is present, contains no Ca. Also, nucleation and growth of calcite are promoted when Mg is absent but when Mg is present in the solution aragonite.