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Leisure craft sacrificial anodes as a source of zinc and cadmium 3 to saline waters 4

Sacrificial anodes are attached to the hulls of boats and marine structures to prevent 12 corrosion. Their use inevitably leads to release of zinc as well as impurities in the zinc alloy 13 such as cadmium to the saline environment. Risk assessments and source apportionment 14 exercises require accurate assessments of the potential loads of chemicals into the 15 environment. This research has surveyed a wide variety of zinc anodes for their composition 16 to compare against a reported industry standard as well as using differing methodologies to 17 determine the dissolution rate of zinc and cadmium from anodes. A zinc dissolution rate of 18 477 g/yr/kg of anode is proposed. Although most anodes tested had concentrations of 19 cadmium within the prescribed limits set by the reported standard, calculated leaching rates 20 from laboratory dissolution experiments suggested as much as 400 g per year of cadmium 21 could leach from zinc anodes used on leisure vessels within UK waters.


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Sacrificial anodes attached to the steel on boats (typically on hulls and propeller shafts) and 25 marine structures to prevent corrosion. It is thought they corrode at varying rates due to 26 factors such as salinity variations and stray currents associated with any number of leakages, 27 including for example electrical hook-ups in marinas. This is particularly significant in well as other biota, found within such habitats can be sensitive to elevated levels of metals 31 in the water column and therefore in locations where there are high boat densities (e.g. ports, 32 marinas or channel moorings), dissolution characteristics of anodes needs to be assessed 33 (Nam et al., 2005;Denton et al., 2009;Pearson et al., 2018). This in turn would allow for 34 introduction of better management and mitigation measures that would reduce impact on 35 ecosystem health (Rees et al., 2017). Whilst sheltered estuaries such as the Hamble attract 36 1000's of leisure craft to moor throughout its length (Rees et al., 2017), this issue is 37 exacerbated in marinas where lock gates ensure boats are continuously afloat, thereby 38 restricting tidal flushing leading to further elevated zinc concentrations (Bird et al., 1996;39 Cathery 2014; Harrison 2015; Wood 2014). In addition to zinc, anodes contain a range of 40 impurities which may also present a threat to the aquatic environment. Of particular concern 41 is the highly toxic element cadmium (Cd), a priority hazardous substance under the Water 42 Framework Directive (2000), which regulators are further required to control. 43 Zinc is a specific pollutant under the WFD (2000) and the current Environmental Quality 44 Standard (EQS) in UK estuaries is 7.9 µg/l for dissolved zinc (which includes a background 45 level of 1.1µg/l) (Maycock et al, 2012). This value is significantly lower than the previous 46 value of 40 µg/l. Within estuaries and marinas with high boat density, under certain 47 conditions zinc released from anodes has the potential to contribute to concentrations 48 exceeding the EQS (Bird et al., 1996;Boxall et al., 2000). Previous studies have reported 49 concentrations of up to 19.9 µg/l dissolved zinc in Poole Harbour, for example, which was 50 significantly above the control sites of 2 µg/l (Bird et al., 1996). Similarly, elevated Camargue in the Mediterranean also had raised zinc levels in sediments likely due to marine 55 paints and zinc anodes. The harbour has around 500 moorings and maintenance yards, with 56 zinc concentrations reanging from 17 and 475µg/g within the Harbour sediments, an 57 enrichment factor (compared with control areas) of 9 was observed in areas of boat 58 maintenance (Briant et al., 2013). Studies on anode use and dissolution rates were also 59 carried out in the Plymouth area by Wood (2014), Cathery, (2014) and Harrison (2015), 60 which showed marinas to have higher zinc concentrations in water and sediment samples, 61 compared with nearby control sites. The alternative material to zinc anodes in the marine 62 environment is aluminium (Mao et al, 2011), although they seem to be used less frequently 63 than zinc, mainly due to habits of boat owners related to perceived performance and cost.

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In order to safeguard vulnerable ecosystems it is necessary to manage the sources of 65 contaminants. Therefore, to determine the risk posed by zinc anodes in estuaries, ports and 66 marinas it is necessary to accurately determine their rate of dissolution in order to derive 67 3 predicted environmental concentrations which can then be compared against environment 68 quality standards to assess risk. However, determining dissolution rates is not nescessarily 69 straightforward as environmental factors such as salinity may play a significant role in the 70 dissolution rates. Furthermore, there is also a question as to whether the elemental 71 composition of the anode varies among suppliers and if that could also impact the zinc 72 dissolution rate. Anode impurities may also pose a risk to the aquatic environment, 73 particularly for metals of international concern including cadmium. Concerns regarding the 74 quality and effectiveness of zinc anodes resulted in the current U.S. Military Specification,

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A combination of chemical composition analysis combined with survey data and in situ 97 monitoring was undertaken to determine zinc anode quality, the presence of impurities which 98 could impact on anode performance (e.g. iron) or negatively impact on the environment (e.g. 99 cadmium) and to determine a definitive dissolution rate with respect to salinity.  England. Based on initial size, replacement rate and estimated wear, loss rates for zinc per 121 kg of anode could be calculated along with total loads emitted into the receiving water.   Hanging anodes were also acquired for an in situ anode experiment, these were weighed to 131 two decimal places before the experiment and again after the experiment to determine the 132 5 dissolution rates at each site in the river. The anodes were analysed using X Ray 133 Fluorescence (XRF) to determine their elemental composition (see section 2.3 for details).  Salinity profiling was carried out to determine the salinity variations in the estuary and used 159 along with Environment Agency data to determine salinity regimes. This was carried out at 160 6 each pile with anodes present at high and low tide on spring and neap tides during 2016 on July 161 4 th (spring tide), July 15 th (neap tide), October 12 th (neap tide) and October 21 st (spring tide).

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The salinity was measured using a YSI 556 MPS probe at 1m intervals from the surface to 163 sediment (between 3 and 8m depending on site and whether neap or spring tides). All data were 164 pooled at each site and the mean used to determine the salinity to which the anodes were 165 exposed. The anodes were gently cleaned during salinity profiling, with a toothbrush to remove 166 algae, mud and any iron and/or zinc oxides/hydroxide that may have built up on the anodes.

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Care was taken not to abrade the surface of the anodes. This occurred as the anodes were not 168 moving through the water as would be the case on a vessel, although tidal currents either side 169 of slack water would obviously ensure a certain movement of water across the anode surface.  supplied certified reference materials were analysed to provide analytical quality control.

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Although XRF determination meant only the surface of the material was analysed, it was 183 assumed composition was consistent throughout, and it is noted that any dissolution by saline 184 water is a surface-based process.

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Zinc anodes of different sizes and weights, from a number of suppliers were tested:      Iron impurities are the main concern regarding passivating and poor anode performance and 8 285 out of 10 of the tested anode types exceeded the 0.005% limit set within the US Military 286 specification. The presence of cadmium within anodes is of a concern regarding environmental 287 health. Owing to the environmental toxicity and threat to human health of cadmium, the

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The lack of a significant difference in dissolution rates between the channel moored and marina 349 moored boats suggests that the potential for stray currents from electrical hook-ups in marinas

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The reasons for such variation are likely due to salinity changes (discussed further below), 367 inaccuracies in estimating the loss of zinc from the anodes at replacement, inaccuracy in the 368 reported replacement frequency, variations in the quality of the anodes impacting their 369 performance as noted above.

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The boat owners with more anodes on smaller vessels had generally experienced corrosion 371 issues so consequently used more anodes. This once again suggest a lack of knowledge and 372 awareness as more anodes on the same metal item will not protect it better than one, placed

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Taking all of the survey data together (131 survey results) the loss of zinc to receiving water 379 has a calculated mean of 477 g/yr/kg (SD=287 g/yr/kg) of anode (CI 95% [428 to 526 g/yr/kg]).

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The median loss rate is 500 g/yr/kg of anode which shows the normalisation of dissolution to  The mean and median for the dataset were well within the errors reported for the survey data 396 and therefore provided further evidence for the consistency and accuracy of the datasets.

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Previous estimates for the Hamble have reported 2.4 kg/yr/vessel (based on their own survey 398 data) which equates to 391 and 600 g/yr/kg of anode using mean or median number of anodes 399 per boat and their weight respectively from the survey data (Boxall et al., 2000). This is again 400 within the range reported here. at low water or if boats are inactive for long periods of time (Gavrila et al., 2000).

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The data in Figure 5 suggests corrosion rates do decrease at lower salinities, but the range is

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Based on the varying methodologies presented here, it is recommended that for future risk 519 assessments or source apportionment exercises that a value of 477 g/yr/kg of anode be applied.  Accepting that the laboratory tests may not be a true representation of the actual environment 534 this is still a significant discharge for a priority hazardous substance.

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This data therefore clearly shows that there are significant benefits to limiting the amount of 536 cadmium present in the commercially available anodes, without impacting on their efficiency.

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It may therefore be recommended that the quality of zinc anodes be more consistent and inline 538 with the specification set out by the US Military, with a review of the cadmium content to set 539 it as low as practicable.