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Aluminium Alloy Construction (Page 3) Page 1 2 |3| 4 5 6 7 8

From their position in the galvanic series of metals it would be expected that marine alloys would corrode quickly in sea water. However the product of corrosion is an invisible, protective oxide film that is in itself inert and strongly bonded to the parent metal. The film forms instantly upon exposure of the material to an oxidizing environment (the atmosphere or sea water in this case) and is instantly reformed when damaged. It is this property which helps to provide the excellent corrosion resistance of the material

Pitting of the surface of the metal does occur but is usually very mild, leaving the surface with a roughened appearance. Underwater, the pits can be larger but they are normally shallow and few in number. Both types of pitting become dormant quite quickly and are seldom of structural significance.

Electrolytic, or galvanic, corrosion is potentially much more serious. It is caused when dissimilar metals are immersed in an electrolyte (such as sea water). Aluminium and its alloys by themselves in sea water cannot suffer electrolytic corrosion.

The metals are arranged in a galvanic series, part of which is shown below:

Metal Steady Potential Negative to Calomel Cell
Stainless steel 316 (passive) 0.05 These metals will be the cathode and aluminium and the alloys will suffer attack when there is corrosion.
Stainless steel 316 (active) 0.18
Nickel/nickel alloys 0.20 - 0.28
Copper/copper alloys/brass 0.29 - 0.40
Tin 0.47
Lead 0.51
Steel & cast iron 0.61
Aluminium 5083 0.74
Cadmium 0.79 These metals will be the anode and they will be attacked when there is corrosion, protecting the aluminium
Zinc 1.03
Magnesium 1.60

Metals far apart in the series will have a more violent reaction than those close together. When two metals are immersed, the metal lower down the table will suffer galvanic corrosion (it will be the anode). Thus if copper and aluminium are immersed, the aluminium will be corroded and, because the metals are far apart on the table, the corrosion will be serious.

This effect is used to protect aluminium alloys from possible corrosion by fitting zinc 'sacrificial' anodes to the hull, which will be corroded in preference to the aluminium, should another metal be present. Zinc being relatively close to the aluminium alloys will have a fairly gentle reaction and thus the anodes will not be consumed too quickly.

The case of stainless steel is anamalous as it forms a protective film on itself, which tends to inhibit the corrosive effect

The corrosive effect is also affected by the relative surface area of the metals, their proximity and positional relationship, and a number of other variables such as the temperature of the water and the speed at which it is passing the vessel. This makes eletrolytic corrosion a rather complicated subject.

However, proper design, avoiding the use of dissimilar metals wherever possible and the fitting of zinc anodes to the underwater hull will ensure that no electrolytic corrosion of the hull can occur. Often the anode systems fitted to a carefully designed and built craft last very much longer than their expected life, indicating minimal electrolytic effect.

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© George Whisstock.