Technical Presentations at the October 2008 Meeting
1.1
‘Galvanic
Corrosion Performance of High Strength Copper-nickel Alloys in Seawater’,
Julian Wharton, University of Southampton
High
strength copper-nickel alloys (Cu-Ni-Mn-Al-Nb) alloys have been widely used for
marine applications due to their excellent resistance to seawater corrosion,
high inherent resistance to biofouling and good fabricability.
However, in-service experience has shown that these alloys may encounter
variable performance worldwide, i.e.
due to local environmental conditions.
In particular, the corrosion performance of high strength Cu-Ni alloys
may be affected by the seawater environmental factors, which include biofouling,
temperature, salinity, and dissolved oxygen content.
This work reports an investigation into the galvanic corrosion
performance between two wrought high strength copper-nickel alloys, Cu-15Ni and
Cu-19Ni, with regard to the influence of surface oxide films/deposits and
biofouling when immersed in seawater.
Analyses of galvanic currents, coupled and decoupled potentials routinely
monitored for two years are presented.
1
National Centre for Advanced Tribology at Southampton (nCATS), School of
Engineering Sciences, University of Southampton, Highfield, Southampton, SO17
1BJ, UK
2
Platform Sciences Group, Dstl Porton Down, Salisbury, Wiltshire, SP4 0JQ, UK]
Accelerated Low Water
Corrosion (ALWC) is a particularly aggressive form of microbiologically-induced
localised corrosion affecting steel piling and other immersed structures, which
is most commonly found close to the level of the Lowest Astronomical Tide (LAT).
To mitigate this, cathodic
protection (CP) and/or coatings are currently used in combination with surface
sterilisation.
However, it is expensive and
difficult to apply conventional coatings to existing corroded structures under
water to prevent further corrosion.
In some cases, a calcareous
film, a deposit of Calcium Carbonate (CaCO3) and Magnesium Hydroxide (Mg(OH)2)
produced on steel structures has been proved to be an effective and an
inexpensive substitute for a conventional corrosion control coating.
When deposition conditions are
appropriately controlled, it provides corrosion protection for steel in
seawater.
BAC Corrosion Control Ltd (BAC) and partners Mott
MacDonald and The University of Manchester have developed a patented treatment,
LATreatTM, to counter the problem of ALWC by applying a calcareous
deposit in a controlled process. The
process can be used to treat local areas
where ALWC is occurring.
LATreatTM is an electrochemical process which works in a similar
way to CP simply using the seawater and electricity to generate all the
necessary active agents.
The treatment comprises three
optimised stages, Cleaning, Sterilisation and Coating which are typically
carried out sequentially using BAC control equipment located within easy access
of the structure to be treated.
At present the endurance of
the calcareous film is being studied both in the laboratory and the field.
The treatment should be commercially available in mid 2010.
A Patent has been filed for the process.
[barryt.lamb@bacgroup.com, 01952 208506]
Note: This presentation is available in pdf format to staff of MCF member companies. Please contact the Secretariat for details.
4.1 ‘Corrosion in Seawater Cooling Systems’, Clive Tuck, Lloyd’s Register EMEA
A number of typical instances of corrosion in seawater cooling systems on board a ship were considered and explanations were given as to the possible causes of deterioration. The examples shown included corrosion of heat exchangers (copper based shell and tube type and titanium plate type), pumps, pipework and items which were subject to galvanic corrosion.
Attention was drawn to the recently
published DEF STAN 02-781 ‘Protection of
Seawater System Pipework and Heat Exchanger Tubes in HM Surface Ships and
Submarines’ which gives
details of the best methods of conditioning copper alloy pipework for seawater
use. Microbial-influence and
biofouling have been found to be important factors in accelerating corrosion in
seawater cooling systems. Also,
seawater flow needs to be matched to the materials used and free of turbulence,
particularly with respect to the pump speeds employed.
Conclusions from an analysis of the main causes of corrosion were:
- Ensure that adequate
quality control standards are set for equipment and components
- Make sure that
the design takes account of possible galvanic corrosion issues –
dissimilar metals need to be electrically
separated
- Pre-conditioning
(passivation) of CuNiFe has to be effectively carried out
- Set up and
regularly carry out a maintenance programme for heat exchangers
- Ensure that the
anti-fouling system is effective and switched on at all times
The
development of flexible pipelines and risers has been a key driver in allowing
the oil and gas industry to develop fields in deeper and deeper waters,
utilising floating production technologies.
During service, the annulus environment located between the two polymer
layers of the pipe may become corrosive. Permeation
of corrosive species from the produced fluids along with permeation and
subsequent condensation of water in the annulus of the pipe may result in the
development of an acidic aqueous environment.
If this
situation occurs in service the integrity of the carbon steel tensile armour
wires located in the annulus may be at risk.
As such, it is the aim of the current research project to investigate the
effect of simulated corrosive oilfield environments on the fatigue life of the
tensile armour wire component of the pipe.