... Corrosion
When corrosion is found, especially in a marine installation, electrolysis is often blamed as the prime suspect. However, this is not always the case. Corrosion can and does occur without any stray currents being involved at all. Understanding how and where corrosion can occur will help to prevent or at least minimise what can be costly damage over time. Basically there are two types of corrosion: corrosion caused by mechanical friction like sandy water or high speed water flow, and corrosion caused by electro-chemical change, which includes electrolysis. Where friction corrosion is concerned, the ability of quality metals to withstand such impacts, especially where rudder blades and propeller shafts on high speed vessels are concerned is obvious. Understanding the process of electro-chemical corrosion is rather more complicated. Metals contain thermodynamic energy – positive and negative charges – which is freed by the process of corrosion, and are rated in their ability to resist electro-chemical corrosion on the Scale of Nobility and on the Galvanic Series chart which shows the electrical potential of metals in seawater (see Chart on reverse). More noble, less active metals have a neutral or negative electrical potential, so will not generate a flow of positive ions. At the other send of the scale, metals are more active with high positive charges which do generate electrical current flow. Gold, platinum and palladium are among the most noble and are generally safe from being corroded, whereas Zinc, unalloyed aluminium, iron and steel are among the least noble, with graphite and carbon being most highly charged. Bare steels (like 4130) are very active. When exposed just to normal atmospheric moisture and oxygen, a layer of red rust can build up through oxidation – and with each rise in the temperature of 10 degrees the rate of oxidation is doubled.
Of course not all metals are pure. There is a very wide range of alloys. Alloys mix metals and these mixtures can be a recipe for corrosion. For example, all bronzes, brasses and stainless steels, are not the same. So it is important to know how noble a particular alloy is, especially when you are putting it in the marine environment. Moreover, not all metals look like metals. Rubber does not have a metallic appearance, but it contains carbon, and carbon rubber mated to stainless steel can produce quite a reaction!
Electro-chemical corrosion has been given a number of names derived from the circumstances in which it is found. The most common forms you will see are galvanic, crevice, pitting, uniform and stress corrosion, as well as electrolysis. Galvanism occurs when two metals with different electrical potential are in close contact, creating a current flow. In dry conditions, the resultant flow is very small but when the metals are joined by water, the water acts as an electrolyte and there is a significant increase in electron flow from the more active metal to the less active metal, resulting in particles of metal being deposited from the less noble to the more noble metal.
 This is very prevalent at sea due to the excellent conductivity of salt sea water, which, in effect, forms a natural battery. Oxidation occurs at the more active (less noble) metal, known as the anode, creating a flow of electrons to the less active (more noble) metal called the cathode. Salt spray can be enough to set the process in motion if the metals are close together. The further the metals are apart on the Scale of Nobility or Galvanic Series Chart, the greater the potential for electro-chemical corrosion to occur and the greater the extent. Commonly you can find aluminium being transferred to more noble copper as the result of electrol-chemical corrosion. The potential is also greater where the more active metal is small and the less active metal has a large surface area. An aluminium rivet in a stainless steel tube connected by water will corrode very quickly indeed. To avoid problems it is wise not to mix metals below the waterline or at least to use metals that are as close as possible to each other on the galvanic scale. Either the same metal or more noble metal should be used for small fasteners and bolts. If you do have to joint dissimilar metals, you will need to ensure that the less noble metal has the greatest exposed area. and mating surfaces are insulated either by jointing compounds or properly designed washers, inserts and sleeves. Bare in mind that materials like cellulosic reinforced plastics (metals reinforced essentially by wastes from agricultural crops, forest products and paper), carbon or metal loaded resin and asbestos-cement composites, can initiate corrosion of metals they are joined to, even if they are classed as non-metalic.
 Obviously it is important to ensure that insulators have good water resistant properties. Nylon 66 is in common use as it has lower water absorption characteristics than Nylon 6, for example. Corrosion of this nature can also occur when dissimilar metals are not in physical contact but are connected electrically by stray electrical currents transferred via a conductive solution like saltwater. In the latter case the process is known as Electrolysis. All vessels can suffer from electrolysis, even wooden hulls where salt water permeating the joins acts as the conductor. Because the metals are not actually in physical contact, electrolysis is not always easy to track down. Stray currents energise all the underwater metals in the boat. The raised electrical potential seeks a path to ground through the weakest metals starting with the least noble metal, zinc, and moving up the Scale of Nobility. The culprit is mainly DC current as AC current needs to be converted into DC before it can cause metallic corrosion. However, naturally occurring diodes in crystalline forms of metallic oxides that are involved in this process do occur on ships. Aluminium oxide that forms inside aluminium boats is commonly involved. Substandard bilge pump wiring is the most common source of stray current on board ship, as are batteries mounted in wet environments, like decks. When it occurs, electrolysis is hard to track down amongst the mass of wiring. It is wise to ensure from the outset that you have a separate earth for communications equipment. Electrolysis is a vast subject and if you would like to know more, please see our article Electrolysis, the silent enemy.
This in turn increases the electrolytic function and results in the generation of an electrical current, dissolving the metal involved. The moisture deposited on the metal absorbs carbon-dioxide from the atmosphere to form a mild carbonic acid. This process will continue as long as the acidic water is present or exposure to oxygen eliminates the acidity of the water. Poor sealing and protection practices when applying and rinsing off acid etch cleaning solutions before painting invite this type of corrosion. As does permitting salt deposits to build-up in joints. So it is essential to ensure that faying surfaces, for example very close joints that you won’t be able to inspect without dismantling, are sealed and water-tight.
A combination of crevice corrosion cells combined under heavy stress can create Stress Corrosion. This can be found mostly on yacht rigging where water is trapped in swaged cable, between the mast and bolt-on parts or under welded parts, and also on propeller shafts of power boats, where lower grade stainless steel alloys are used. Some metals like aluminium, lead, copper and stainless steel produce a protective oxide film over the surface when exposed to normal air. However, this protection can be short lived when the metals are starved of the oxygen needed to generate the oxide. This passive film can also be broken down by the chlorine from the salt in seawater. When access to oxygen is restored, the film is readily restored.
Cavities can be much deeper than they are wide and can even be concealed by powder-like corrosion products. Once the reaction has commenced it will continue to accelerate.  Creating a corrosion free environment may seem impossible at sea, but there are a number of steps that can be taken to prevent major catastrophes. Uniform Corrosion or weathering occurs more or less uniformly over the entire exposed surface of a metal and proceeds at approximately the same rate over the exposed metal surface. This type of corrosion can be minimised by choosing metals, materials and equipment that are designed to withstand the destructive nature of the environment they are intended to be in. Obviously metals and alloys with high electrical potential need to be avoided, as does mixing metals, particularly when installing new equipment. Highly polished stainless steel is more corrosion resistant than unpolished metal which has machine marks that act as corrosion inviting crevices, but will not make up for inferior grades.
  Uniform Corrosion, before and after Poor quality fittings will probably cost you much more in the end, so for screws, nuts and bolts, and the like, use only all marine grade stainless steel all brass or all chromed stainless steel which provides an additional barrier. Equipment not intended for life at sea, like car batteries, will invite corrosion and electrolysis when exposed to seawater and salt air. Zinc sacrificial anodes are a wise option to protect metal hulls. The high current flow emitted by the zinc has the affect of equalising the electrical potential of all the metals in the system, so that the zinc is eroded and deposited on the more noble metals. This protects other metals higher on the Scale of Nobility that may have been similarly attracted to metals more noble than themselves. Aluminium anodes may be an option depending on the actual metal they are allied with. Obviously any sacrificial element should be on the anodic side and smaller than the metal it is protecting. Efficiency is around 40-50 per cent for magnesium, 50 per cent for aluminium with 5 per cent zinc and 90 per cent for zinc.
If your anodes are not corroding you could have a bigger problem somewhere else! If they have been seriously corroded, the area of zinc should be increased rather than the weight of the zinc. A good electrical connection between the zinc and the hull is essential. Epoxy paint is commonly used for protecting steel and aluminium vessels. Preparation needs to be thorough: cleaning, grit blasting , surface priming and epoxy painting. When looking at specifications, you need to be aware that it is the primer that is the water barrier not the top powder coat. This applies especially to porous metal substrates such as hot dipped galvanised steel and alloy castings. Aluminium vessels only need to be painted in any crevices that might be formed where equipment is mounted and below the waterline if a vessel is left in the water year-round. Copper based bottom paints obviously cannot be used on aluminium hulls. Painting cathodes may also be an advantage, provided that the paint composition is suitable and it doesn’t interfere with what the equipment is designed to do. But, never paint an anode. Earth plates, copper or aluminium, require direct contact with the sea. A coat of paint will prevent this from occurring. To quote Dr Harvey P. Hack, Northrop Grumman Corp, “One of the best 300 series stainless steels is type 316. Even this alloy will, if unprotected, start corroding under soft washers, in o-ring grooves, or any other tight crevice area in as little as one day, and it is not unusual to have penetration of a tenth of an inch (2.5 mm) in a crevice area after only 30 days in seawater.” And just a few final tips:
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