1. Abstract:
This study investigates the physio-chemical parameters affecting the dissolution rate of zinc sacrificial anodes on pleasure craft and the impacts on the aquatic environment.
The Hamble estuary is densely populated with around 3000 vessels. Anecdotal evidence suggests zinc anodes decay faster within this estuary compared to other UK
estuaries. Estuaries can sometimes exceed the zinc EQS (Environmental quality standards) limits for the UK of 7.9µg/l causing concern regarding potential impacts on marine
life.
A survey of boat owners determined the type of anodes used and possible reasons for the observed dissolution rates within the Hamble. This enables ‘hotspots’ of anode
premature wear to be identified and compared with physical parameters from these areas, which helps determine factors contributing to anode dissolution. These include
stray electrical currents, salinity variation and the number of boats present. Along with the MAMPEC model and water quality data collected from the Hamble the zinc load
from anodes can be determined for individual marinas and the estuary as a whole.
Modelling and determining the varying dissolution rates of sacrificial zinc anodes
on pleasure craft on the Hamble estuary, UK and its environmental implications
Introduction:
Sacrificial anodes are used to prevent corrosion of boat hulls, components
on hulls including propellers and other marine structures from corrosion,
which is known as cathodic protection. Zinc anodes are most commonly
used in seawater and aluminium in brackish water (Young et al., 1979; Bird
et al., 1996; Rousseau et al, 2009). Most pleasure craft have at least one
anode, with some having up to fifteen, the majority use zinc anodes (see
Figure 2).
Anecdotal evidence from boat owners and authorities suggests anodes
decay quicker in the Hamble estuary than elsewhere in the UK. The Hamble
has around 3,000 pleasure craft alone and is situated in Southern England
(see Figure 1 and 3). Reasons suggested for this varied anode decay include
salinity, stray electrical currents and metal debris in the estuary.
UK Environmental Quality Standards (EQS) of 7.9µg/l occur for zinc under
the WFD (Maycock et al., 2012). It is possible estuaries with high boat
numbers and large populations could exceed these standards.
Method:
A survey to boat owners on the Hamble determined anode decay rates and
awareness regarding anode use. The responses showed the types of anodes
used and allowed the zinc load from anodes to be calculated; using the
average number of anodes on vessels (2.86) and a calculated average zinc
release rate of 0.54kg/yr.
The MAMPEC model was also used to calculate zinc release from anodes for
the Hamble on a marina basis, using the release rate of 0.54kg/yr. Using
results from control sites a background concentration of 5.57µg/l was used
(see Figure 4).
Water samples were collected along the Hamble estuary and analysed using
a voltammeter for total dissolved and labile zinc concentrations. Sample
sites focused on mid channel sites and sites within marinas.
References:
Bird P., Comber S.D.W., Gardner M.J. & Ravenscroft J. (1996) Zinc inputs to coastal waters from sacrificial anodes. The Science of the Total Environment, 181: 257-264
Maycock, D., Peters, A., Merrington, G., and Crane, M. (2012) Proposed EQS for Water Framework Directive Annex VIII substances: zinc (For consultation). Water Framework Directive - United Kingdom Technical Advisory Group (WFD-UKTAG). SNIFFER /
Environment Agency
Rousseau C., Baraud F., Leleyter L and Gil O. (2009) Cathodic protection by zinc sacrificial anodes: Impact on marine sediment metallic contamination. Journal of Hazardous Materials, 167: 953-958
Young D. R., Alexander G.V., and McDermott-Ehrlich D., (1979) Vessel-related contamination of Southern California Harbours by copper and other metals, Marine Pollution Bulletin, 10: 50-56
Acknowledgments: The project is funded by The International Zinc Association, Hamble Harbour Board and Solent Protection Society, thank you for your support and advice. Thanks must also go to Southampton Solent and Plymouth University.
Figure 1: Map of the Hamble and location within the UK
Figure 4: Total dissolved zinc levels from the Hamble estuary, samples were collected in marinas and mid
channel, the zinc EQS is exceeded
Aldous Rees1, Anthony Gallagher1, Sean Comber2, Laurie Wright1
1Southampton Solent University, East Park Terrace, Southampton, SO14 0YN
2Plymouth University, Drake Circus, Plymouth, Devon, PL4 8AA
E-mail contact: aldous.rees@solent.ac.uk
Results and Discussion:
Results from the survey indicate a number of hotspots for anode decay occur on
the river Hamble and other estuaries. These hotspots tended to occur in the upper
estuary with lower salinities, around Bursledon on the Hamble (see Figure 1) or
within marinas with power supplies. This suggests salinity and stray electrical
currents are mainly responsible for this decay rate. Metal debris is unlikely to
contribute as not cathodically connected to the vessel (Rousseau et al, 2009). The
survey also indicated a lack of awareness among some boat owners about anode
use, with the wrong metals being used, some not using anodes and others not
replacing enough. Low spec anodes were used in some cases, these tended to
corrode at unsteady rates. A varied anode decay rate will occur in many estuaries
not just the Hamble.
The calculated release rate of 0.54 kg/yr and the average number of anodes per
vessel (2.86) gives a zinc load of 4633 kg/yr for the 3000 vessels on the Hamble
(see Table 1). The zinc levels released from anodes indicate why EQS levels can fail
(Maycock et al., 2012). Hamble zinc levels reached 35µg/l, with most ranging from
4-20µg/l (see Figure 4). The winter zinc levels tended to be higher suggesting
sources such as sewage works also contribute to zinc levels in the Hamble, with
less boats present in winter months. Labile zinc tended to be half the total
dissolved.
Conclusion:
Anodes tend to decay at varying rates depending on a number of factors mainly
marina power supplies and salinity. The release of zinc from these anodes in some
cases is a contributing factor to EQS levels not being met.
Marina/estuary Berths Zinc kg/yr released Dissolved zinc Hamble Average µg/l MAMPEC µg/l
Hamble 3000 4633 - -
Mid channel 600 927 - -
Hamble point marina 230 355 9.2 5.64
Mercury Marina 360 556 6.8 5.69
Universal Marina 250 386 9.4 5.67
Stone Pier Yard 67 103 8.4 6.02
Cabin boatyard 73 113 14.2 5.78
Table 1: Annual zinc load from anodes
The MAMPEC model predicts total zinc released from anodes of 5.64-6.02µg/l,
with the background of 5.57µg/l (see Table 1). This indicates anodes contribute a
very small proportion of zinc to the Hamble. This is due to limitations in the model,
which is designed for enclosed marinas and not open, like on the Hamble. Further
work is needed to calibrate this model. Collected data and calculations suggest
anodes could provide up to half the zinc present.
Figure 2: A new and used anode from the Hamble
Figure 3: Aerial view of the Hamble, showing high
boat numbers
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1 FB 1 FB 2 2a 2 SM 1 3 SM 2 UM 1 4 UM 2 MM 1 5 MM 2 6 PM 1 7 PM 2 8 9 10 Control
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Total Dissolved Zinc June Total Dissolved Zinc October Total Dissolved zinc January Salinity June Salinity October Salinity January
The River
Hamble Harbour
Authority