Sacrificial Anodes, Merchant Shipping

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Transcript of Sacrificial Anodes, Merchant Shipping

Emission estimates for diffuse sources Netherlands Emission Inventory

Sacrificial anodes, merchant shipping and fisheriesVersion dated June 2008

NETHERLANDS NATIONAL WATER BOARD - WATER UNIT in cooperation with DELTARES and TNO

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Anodes, Shipping & Fisheries

Table of Contents........................................................................................

1 2 2.1 2.1.1 2.1.2 2.1.3 2.2 3 3.1 3.2 3.2.1 3.2.2 3.3 3.3.1 3.4 4 4.1 4.2 4.3 4.4 4.5 5 6 6.1 6.2 6.3 6.4 7 7.1 7.2 7.3 8 8.1 9 9.1 10 10.1 10.2 10.3

Introduction and scope Description of emission source Causes Passive protection by means of sacrificial anodes Active protection by means of impressed current Ballast tanks Measures Explanation of calculation method Exterior of vessel Wet surface area Calculation of surface areas based on volume Correction for incomplete draught Corrosion rate Emissions at sail Interior of vessel Activity Rates Assessment using statistical data Interior of vessel Time series, 1990-present Time series, present-2027 Annual data setting Description of emission pathways to water Emission factors Emission factors Application percentages Time series, 1990-present Annual data setting Emissions calculated Emission figures 2004 Emissions 1990-2006 Emissions forecast, 2009-2027 Comments and changes in regard to previous version Difference in figures due to mathematical errors Accuracy and indicated subjects for improvement Most significant areas for improvement Spatial allocation Seagoing vessels and fishing vessels on NCP Seagoing vessels in Dutch territory Fishing vessels in ports

11 21 21 21 23 24 25 31 31 32 34 35 36 38 39 41 41 43 44 44 46 58 61 61 62 62 62 71 71 71 73 85 85 91 91 101 101 103 104

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References

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Anodes, Shipping & Fisheries

1 Introduction and scope............................................................................... The source of the emissions is the anode material placed on the exterior and interior (in the ballast tanks) of seagoing merchant vessels and fishing vessels for the purpose of cathodic protection of metal surfaces. In the National Emission Inventory, this emission is assigned to the governmental target sector Transport. The emissions in question are zinc, aluminium and cadmium. Cadmium is present as a contaminant in the zinc, and is released when zinc anodes decay. This report is based on a previous quantification of emissions of anodes in shipping and fisheries for the Dutch Continental Shelf (NCP) and in ports conducted for the Traffic and Transport Advisory Service (AVV) under the EMS (Emission Inventory and Monitoring for the Shipping Sector). The quantification in this report can be considered to be an update of two EMS protocols: - EMS protocol for Emissions by Shipping and Fisheries: Anodes on ships on the NCP (Kuiper, 2003a) - EMS protocol for Emissions by Shipping and Fisheries: Anodes on ships in ports (Kuiper, 2003b) Here, the quantification of emissions for NCP and ports is integrated into a single report. The method of quantification of the two types of emissions is different, however, and consequently this distinction will be referenced frequently throughout this report.

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Anodes, Shipping & Fisheries

This quantification implements a number of recommendations for improvement of emissions assessment from the protocols listed above, and also incorporates a few new insights. The most significant changes in reference to the protocols are: - Calculation of the Wet Surface Area (WSA) is improved, with a WSA computed for each individual ship in Dutch waters, taking partial loading of the ship into account; - A traffic and transport database based on the Lloyds traffic file has been created for the NCP, which, in combination of the WSA per ship, was used to compute the total average WSA present in Dutch waters; - Emissions from floating tank cooling1 appear to play a much smaller role than described in the protocols. Floating tank cooling is a cooling system used primarily in inland waterway shipping and possibly a few smaller seagoing vessels. Larger seagoing vessels have pipe or plate cooling systems. Consequently, emissions from floating tank cooling are not reported separately in this report; - Along with historical development in emissions, this report provides a forecast of emissions up to the year 2027; - The emissions are spatially allocated by body of water identified in the Water Framework Directive. This data is provided as a separate database.

1

Cooling system for ship engines involving a steel tank welded to the hull, in contact with the

water, and containing a bundle of thin, corrosion-proof pipes. In some versions, the tank is an inverted box on the bottom of the ship with an opening in the bottom. The tank contains a heat exchanger consisting of a package of many thin tubes that come into contact with the outside water on all sides.

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2 Description of emission source...............................................................................

2.1 CausesShips are coated to prevent corrosion. This protective layer, however, is not sufficient to fully protect the ship from corrosion. For this reason, as well as to protect the uncoated sections of a ship (screw, damage, etc.) and ensure that the ship remains protected as the coating deteriorates, cathodic protection is used................................ figure 1 Functioning anode

If two metals are electrically connected in an electrolyte (such as seawater), the electrons of a base metal will flow to another, more noble metal. This is due to the difference in electrical potential. The more noble metal is referred to as the "cathode" and the other as the "anode." As the anode supplies electrons to the cathode, it gradually dissolves into ions, with the result that the cathode becomes negatively polarised and thus protected against corrosion. See figure 1. Cathodic protection can be classified as passive or active. This is explained in more detail in the following subsections.

2.1.1

Passive protection by means of sacrificial anodes

Passive cathodic protection of a ship involves the use of "sacrificial anodes". As already indicated, these sacrificial anodes must be of a metal that is less noble (more base) than the metal to be protected. The two metals used as anodes in shipbuilding are zinc and aluminium. Table 1 shows the various metals in order of nobility. The effectiveness of the anode material in seawater is determined by the composition of the alloy. Because the anodes dissolve in the seawater, the blocks must be replaced at regular intervals. On average, the blocks are replaced every two to two-and-a-half years, when approximately 15% of the original weight remains. For fishing vessels, the ratios are different. A fishing vessel goes into dry-dock every year, and so the blocks are replaced every year, when some 30% of the original weight remains.

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............................... table 1 Metals in order of nobility

Zinc anode The most commonly material used for cathodic protection of seagoing vessels is zinc. The electrical capacity (also indicated by the symbol , see chapter 4.1) of a zinc anode in seawater is 780 Ah/kg (Ampere per hour per kg of zinc anode). This is a function of the amount of valence electrons that can be moved from the zinc to the less noble metal per hour. If the amount of valence electrons the metal to be protected gives off under the influence of seawater is known, the rate at which the zinc anode dissolves can be calculated. The zinc anodes installed in ships are generally designed for a lifetime of between 1 and 3 years. Aluminium anode Aluminium is being used as an anode material more and more frequently. The electrical capacity of an aluminium anode in seawater is 2,600 Ah/kg. Aluminium anodes perform better than the zinc anodes (2,600 valence electrons per hour per kilogram versus 780 for zinc), and as such require fewer to achieve the same effect. Although aluminium is a more expensive material than zinc, the end cost of aluminium anodes is less because they require 3.33 times less material. Another significant environmental advantage is that the aluminium alloys used do not contain cadmium, unlike the zinc alloys used (and prescribed by standardization; see chapter 3.2).

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Nobility of various metals

Nobility of various metals Metal Symbol Potassium K Sodium Na Calcium Ca Magnesium Mg Aluminium Al Zinc Zn Iron Fe Chromium Cr Nickel Ni Tin Sn Lead Pb Copper Cu Mercury Hg Silver Ag Platinum Pt Gold Au

2.1.2

Active protection by means of impressed current

In addition to passive protection, active cathodic protection is an increasingly common method of protection. The impressed current (IC) system moves potentials to protect the metal. An IC system uses a transformer, an adjustable rectifier as a power source and inert, or "non-consumable", anodes. The adjustable rectifier can be set so that the connected anode(s) provide exactly the protective current to provide the desired protection potential. See figure 2. Theoretically, an IC system could be used to protect the entire exterior of a vessel, but in practice this system is often used in combination with passive anodes. The parts of the vessel fitted with passive anodes are the bow thruster tunnel, the screw and the rudder. These parts require a higher level of protection because they use unfinished metal (screw), the coating on these parts has a higher breakdown factor, and the speed of the water that passes along these parts is higher than at other par