WHAT ANODE MATERIAL SHOULD I USE ON MY BOAT?
Aluminum anodes generates 1.1Volts ~ Magnesium anodes generates 1.6 Volts ~ Zinc anodes generates -1.05 Volts
There are three common metals used as galvanic anodes, magnesium, aluminum and zinc. They are all available as blocks, rods, plates or extruded ribbon. Each material has advantages and disadvantages.
Magnesium has the most negative electropotential of the three (see galvanic series) and is more suitable for areas where the electrolyte (soil or water) resistivity is higher. This is usually on-shore pipelines and other buried structures, although it is also used on boats in fresh water and in water heaters. In some cases, the negative potential of magnesium can be a disadvantage: if the potential of the protected metal becomes too negative, hydrogen ions may be evolved on the cathode surface leading to hydrogen embrittlement or to disbonding of the coating. Where this is a possibility, zinc anodes may be used.
Zinc and aluminium are generally used in salt water, where the resistivity is generally lower. Typical uses are for the hulls of ships and boats, offshore pipelines and production platforms, in salt-water-cooled marine engines, on small boat propellers and rudders, and for the internal surface of storage tanks.
Zinc is considered a reliable material, but is not suitable for use at higher temperatures, as it tends to passivate (becomes less negative); if this happens, current may cease to flow and the anode stops working. Zinc has a relatively low driving voltage, which means in higher-resistivity soils or water it may not be able to provide sufficient current. However, in some circumstances — where there is a risk of hydrogen embrittlement, for example — this lower voltage is advantageous, as overprotection is avoided.
Aluminium anodes have several advantages, such as a lighter weight, and much higher capacity than zinc. However, their electrochemical behavior is not considered as reliable as zinc, and greater care must be taken in how they are used. Aluminium anodes will passivate where chloride concentration is below 1,446 parts per million.
One disadvantage of aluminium is that if it strikes a rusty surface, a large thermite spark may be generated, therefore its use is restricted in tanks where there may be explosive atmospheres and there is a risk of the anode falling.
Since the operation of a galvanic anode relies on the difference in electropotential between the anode and the cathode, practically any metal can be used to protect some other, providing there is a sufficient difference in potential. For example, iron anodes can be used to protect copper.
COMMON ANODE TYPES
In brief, corrosion is a chemical reaction occurring by an electrochemical mechanism (a redox reaction). During corrosion there are two reactions, oxidation, where electrons leave the metal (and results in the actual loss of metal) and reduction, where the electrons are used to convert water or oxygen to hydroxides.
In most environments, the hydroxide ions and ferrous ions combine to form ferrous hydroxide, which eventually becomes the familiar brown rust.
As corrosion takes place, oxidation and reduction reactions occur and electrochemical cells are formed on the surface of the metal so that some areas will become anodic (oxidation) and some cathodic (reduction). Electrons flow from the anodic areas into the electrolyte as the metal corrodes. Conversely, as electrons flow from the electrolyte to the cathodic areas the rate of corrosion is reduced. (The flow of electrons is in the opposite direction of the flow of electric current).
As the metal continues to corrode, the local potentials on the surface of the metal will change and the anodic and cathodic areas will change and move. As a result, in ferrous metals, a general covering of rust is formed over the whole surface, which will eventually consume all the metal. This is rather a simplified view of the corrosion process, because it can occur in several different forms.
CP works by introducing another metal (the galvanic anode) with a much more anodic surface, so that all the current will flow from the introduced anode and the metal to be protected becomes cathodic in comparison to the anode. This effectively stops the oxidation reactions on the metal surface by transferring them to the galvanic anode, which will be sacrificed in favour of the structure under protection.
For this to work there must be an electron pathway between the anode and the metal to be protected (e.g., a wire or direct contact) and an ion pathway between both the oxidizing agent (e.g., water or moist soil) and the anode, and the oxidizing agent and the metal to be protected, thus forming a closed circuit; therefore simply bolting a piece of active metal such as zinc to a less active metal, such as mild steel, in air (a poor conductor and therefore no closed circuit) will not furnish any protection.