The Galvanic Series MATERIAL TITANIUM AND ALLOYS
300 SERIES STAINLESS STEELS MONEL METAL COPPER
COPPER NICKEL LEAD
SILICON BRONZE MANGANESE BRONZE
CAST BRASSES AND BRONZES NAVAL BRASS
ALUMINIUM BRONZE HIGH STRENGTH STEELS
ELECTRICAL POTENTIAL RANGE (IN VOLTS)
+0.06 to –0. 05 0.00 to –0.15 -0.04 to –0.14 -0.14
-0.13 to –0.22 -0.19 to –0.25 -0.24 to –0.27 -0.25 to –0.33 -0.24 to –0.40 -0.30 to –0.40 -0.30 to –0.40 -0.60 to –0.63
LOW STRENGTH ALLOY STEELS -0.57 to –0.63 PLAIN CARBON STEELS CAST IRONS
ALUMINIUM ALLOYS ZINC
ALUMINIUM MAGNESIUM
-0.60 to –0.70 -0.60 to –0.72 -0.70 to –0.90 -0.98 to –1.03 -1.25 to –1.50 -1.60 to –1.63
In both aluminium is the anode and stainless steel is the cathode. Corrosion of the aluminium rivet in the first arrangement would be severe. However, corrosion of the large aluminium plate in the second arrangement would be much less even though the potential difference is the same in each case.
The cathodic member of the couple is one of those alloys identified as being influenced by slime films and, if there is a reason not to paint, periodic cleaning of the bare metal surface can minimize the effect of the slime and reduce the corrosion rate of the anode. Periodic cleaning is not recommended for reducing galvanic corrosion of other alloys because removal of corrosion product films from those metals usually increases corrosion rates.
Whenever the vessel is slipped, the existing anodes should be wire brushed back to bright metal and then coated with soft soap before painting the hull. Any paint accidentally applied to the anodes will then wash off with the soft soap when the vessel is placed in the water.
If it is necessary
Corrosion of the anodic metal is both more rapid and more damaging as the voltage difference increases and as the cathode area increases relative to the anode area. The approximate voltage difference for any two metals can be taken directly from the table.
It is worth
noting that marine slime films composed primarily of microscopic bacteria and diatoms can change the potentials of many of the alloys near the noble end of the galvanic series as indicated.
The potentials of these alloys become more positive in the presence of slime films thus increasing the voltage difference when these metals are placed in contact with more anodic alloys. This has been found to increase the corrosion rate of copper, steel and aluminium anodes by a factor of 2 to 5 but to have no effect on the
38 | The Report • March 2017 • Issue 79 corrosion rate of zinc anodes. It
may also change which metal in the couple becomes the anode.
When the two metals in a galvanic couple are close together on the series, such as manganese bronze and silicon bronze, their voltage ranges overlap and either one can be the anode, depending on the exact exposure conditions.
case, more detailed information would be needed to predict the behaviour of the couple. The effect of the second factor above, the cathode to anode area ratio (RC/A
of a rivet in a plate.
at some time for the vessel to change from a salt to a fresh water environment or vice versa then it should be borne in mind that this will have a deleterious effect on the anodes fitted.
Magnesium or aluminium anodes (which are suitable for fresh water) have a much higher driving potential than anodes manufactured from zinc.
If a In this
vessel fitted with magnesium or aluminium anodes passes into salt water for anything longer than about seven days, the anodes will waste away very quickly.
) is illustrated by the example In the first
situation the rivet is of aluminium and is comparatively small and the plate of stainless steel and the RC/A ratio is large.
In the second the
situation is reversed: the stainless steel rivet is small and the RC/A
is also small. ratio
Vessels which are fitted with magnesium or aluminium anodes moving into a salt water environment for longer than a week, should therefore be fitted with a replacement zinc anodic system. Conversely vessels fitted with zinc anodes (suitable for use in salt water) will find over a period
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