Aluminium and its alloys
Vyv Cox explains all about the properties of aluminium and cast and wrought aluminium alloys – and outlines their varied uses in a marine capacity
Varied uses in a marine capacity
Sometimes you have to feel sorry for aluminium. As the third most abundant element in the Earth’s crust, after oxygen and silicon, it might be thought that people would use its name correctly, but they don’t.
To most, the word ‘aluminium’ is synonymous with ‘alloy’, widely and incorrectly used to describe car wheels; but this is a misuse of the word. The reality is that almost every metal used by the human race is formed by the combination of two or more metals (and some non-metals) such as cast iron, steel, brass, bronze and hundreds of others. All of these are alloys.
Unusually for such an abundant element, aluminium metal was not produced until the mid-1800s due to its extreme reactivity, particularly with oxygen. This reactivity means that it requires huge energy to separate it, these days carried out electrically on the molten ore at around 950°C. It also means that aluminium has a strong tendency to return to its lowest energy state by reaction with air, water, acids and alkalis. Fortunately for us, as with stainless steel the oxide that forms on its surface is passive, resisting further oxidation and helping the metal to remain bright and decorative. One of its principal properties is its low density, providing light weight with adequate strength for many uses.
Items in aluminium are produced in two principal forms – either as castings, or wrought, ie formed into shape by rolling, extrusion or forging. In common with most pure metals the engineering properties of unalloyed aluminium are limited, requiring the addition of various other metals to give it the desired strength, stability, corrosion resistance or whatever is needed for the application. However, pure (99.9%) aluminium can be hard rolled (work hardened) to provide useful properties in sheet and strip form.
Alloys for marine use
When it comes to selecting aluminium alloys for use on the boat, our choices are fairly limited. Corrosion resistance is by far the most critical factor, with mechanical strength somewhat less important as section sizes can be increased where necessary. The strongest alloys may suffer selective corrosion between the metals in the alloy, which is especially the case with aluminiumcopper in seawater but also problematic in fresh.
AA514 (LM5) is probably the most widely used alloy for casting marine hardware such as cleats, fairleads, steering pedestals, saildrives, mast fittings and similar. All of the 5xx series alloys are nonheat-treatable, with excellent corrosion resistance, machinability and surface appearance, ideal for highly polished fittings.
The aluminium-silicon alloy A413, equivalent to LM6, is the most widely used aluminium casting alloy for general purposes. It is ductile and has lower corrosion resistance than the 5xx series, but it casts
well into intricate shapes and can be used for marine parts.
Wrought aluminium alloys for boatbuilding and marine onshore applications are exclusively from the 5xxx and 6xxx series, as shown in the next table (below). The values preceded by a letter H indicate the extent of work hardening in these non-heat-treatable alloys, whereas those preceded by a T refer to levels of heat treatment (temper).
Incidentally, 6061 is the alloy generally used for hot-forged automotive wheels. Many of us will be aware that this alloy can suffer quite severe pitting corrosion, even in the relatively non-saline conditions of UK roads.
Anodising is the process by which the protective oxide film on the surface of aluminium is artificially thickened. It is thus not a coating but is a conversion of the parent metal. Anodised films protect the metal from corrosion and provide a durable and attractive finish that can be coloured to further improve the appearance of the finished product.
The anodising process is carried out in an electric cell. The object to be treated forms the anode of the cell, with cathodes attached to the cell walls. The electrolyte in the cell is an acid, typically sulphuric acid. A direct electric current is passed between the anode and cathode, causing oxygen to be formed at the anode, which combines with the aluminium to form aluminium oxide. The oxide film formed is porous and needs to be sealed to harden and solidify the surface. In some cases the sealant incorporates a coloured dye. Various methods are used for these final stages. It is possible for the owner to carry out DIY anodising, as detailed in David Berry and Amanda Potts' article ‘DIY anodising’ (PBO March 2012).
The anodised film thickness depends upon the length of time for which the object is exposed to the anodising process. Film thickness is specified to cope with the intended application:
■ 25 microns for marine and heavy-duty architectural duties
■ 15 microns for most general purpose exterior duties
■ 10 microns for interior duties and some decorative exterior duties where frequent cleaning is likely.
Zinc is probably the most widely used anode metal for marine leisure vessels, but this metal develops a white layer of zinc hydroxide in fresh and brackish water, preventing its efficient function. In brackish water, or varying fresh/sea water, it is common to use an aluminium alloy as the anode metal. Aluminium is alloyed with zinc and some minor metals in a variety of ways, a typical composition being shown in the table above.
Aluminium is increasingly being deployed for the construction of yachts, in general using all-welded construction in 5086. This alloy grade provides great strength and resistance to holing in combination with weight savings of 10% over GRP and 35% over timber. Aluminium itself has good resistance to corrosion in both fresh and seawater but it is highly susceptible to galvanic corrosion when in contact with almost every metal in the galvanic series, with the exception of zinc. There is a grain of truth in the story of a copper coin dropped into the bilge of an aluminium boat that subsequently corroded its way right through, although I am not sure I have ever seen the photographs! The two areas that need to be considered very carefully are the attachment of stainless steel or other fittings and electrical paths.
No metal other than stainless steel should be attached to the hull, with the exception of
sacrificial anodes. Fasteners should be plastic or stainless steel, avoiding any copper alloys. Metal objects such as fishing weights, fish hooks, coins and tools should not be left in contact with the hull for any appreciable time. Stainless steel fittings should be bedded on an isolating film such as Duralac, Tefgel or plastic film.
Electrically, the main requirement is that all wiring is made using twin leads, not using the hull for the negative return. Shore power should always be checked for polarity. Electrical devices such as isolation transformers and leak detectors are good precautionary measures and should be fitted.
Conventional copper-based antifouling paints if applied to aluminium hulls would result in galvanic corrosion and should never be used. Most paint manufacturers produce copper-free paints specifically for aluminium hulls. Some fouling organisms are known to create conditions that render aluminium more susceptible to corrosion.
Saildrives are increasingly common on the yachts of today, and the same rules apply. Copper-based antifouling paints should not be used on them. The recommended paint system should be applied regularly to prevent local attack. Volvo saildrives are isolated from the engine and the remainder of the boat by rubber and plastic insulators and these should not be compromised by additional wiring, or even dirty oil on the gasket. Yanmar saildrives are not isolated in the same way. Both rely heavily on zinc anodes attached ahead of the propeller and these must be replaced in good time.
Adding aluminium fittings
When it comes to adding fittings by the owner, there can be problems. Castings such as cleats and fairleads bought in a chandlery are almost certainly made in the appropriate 5xx alloy. However, there is a very good chance that wrought material, sheet or strip, bought at your local DIY store, will be of 1xxx manufacture and therefore susceptible to corrosion. Grades 6063 and 6082 are the most widely extruded alloys, sold by many good-quality suppliers in a wide range of shapes. General extrusions such as angles and hollow sections may not have the anodising thickness recommended for marine use. For above-water use these are generally acceptable, but for anything underwater – especially keel bands and rubbing strips that are likely to lose any anodised finish – only 5xxx alloys will have a long life. I have been unable to source any extrusions in this material using internet searches.
Fittings added by the owner are most commonly attached to aluminium plate, sheet and extrusions using either pop rivets in aluminium, stainless steel or Monel, or stainless steel bolting. Aluminium pop rivets are available in two grades, either ‘soft set’ in 1xxx aluminium or ‘alloy’ in 5xxx grade. The latter is recommended for all marine applications and is available from good quality suppliers. In all cases it is advisable to bed the fastener in a sealant to limit galvanic corrosion. Fittings attached by the boatbuilder are likely to be in 5xxx material.
Sometimes owners are tempted to use recycled aluminium for marine purposes without knowing their composition. This can have serious consequences, as shown in ‘The jury’s out!’ (Making a jury rudder, PBO January 2016). A length of aluminium obtained from a scrapyard was used to make a rudder. Despite having been tested for strength, after three or four years the rudder snapped off at the stock (see photo above). Although most aluminium alloys are ductile, this fracture face appears brittle, with no evidence of ductility. Aluminium-copper alloys of the 2xxx series are highly susceptible to stresscorrosion fractures, particularly in chloride environments. The appearance of this type of failure indicates brittleness, and it is my guess that this alloy was aluminium-copper.
Aluminium is highly susceptible to galvanic corrosion, being at the anodic end of the galvanic series with only magnesium and zinc above it. However, although its position gives a good indication of the likely result of combinations of metals, there is another factor: relative size. A simple analogy explains this, a single rivet in a sheet of another metal. If an aluminium rivet is placed in a steel sheet and immersed it will corrode very rapidly as the reaction is being driven by a large cathode (the steel). In the reverse case of a steel rivet in an aluminium sheet, the large surface area of the anode protects the small cathode and the reaction will be slow. In reality, not all of the aluminium can protect the rivet, and corrosion will take place in a fairly small area around it. ■ Vyv Cox’s book Metals In Boats (Crowood Press) is available now.
A fairlead for the leisure marine industry, cast in a 514 alloy. Fittings for yachts need excellent corrosion resistance and long-term attractive appearance
A zinc anode suspended from the transom of a berthed aluminium boat, providing cathodic protection. This 42ft boat had four such anodes distributed around the hull. Fixed zinc anodes should be renewed regularly to overcome the passivation that occurs due to oxidation
After three or four years, this recycled aluminium rudder snapped off at the stock
Here is another example, this time around a rivet securing the foot of a mast to its base. In this case the rivet appears to be an aluminium one fitted professionally, in which case almost certainly in 5xxx material. It would seem that some surface damage removed the anodising locally, perhaps when the rivet hole was being drilled. In both these cases it would seem that there is no isolating sealant between the rivet and aluminium
Galvanic corrosion is shown in this photo of rivets in a boom. The light corrosion damage is quite localised. These rivets appear to be in a stainless steel material