exceeding about seven days that the metal will be coated with an impervious off-white crust of zinc salt which will very effectively prevent it working even when returned to salt water.
After any trip into a fresh water environment a vessel fitted with zinc anodes should have these thoroughly scaled clean back to bright metal.
In no case
defect can be often seen near where sacrificial anodes are attached to steel or fibreglass hulls. The process often results in local blistering of the paint coat near the anodes and is often referred to as cathodic disbondment.
It is sometimes mistaken as osmotic blistering. If proceeding into
fresh water for longer than about seven days a vessel should be fitted with replacement magnesium or aluminium anodes.
should magnesium anodes be fitted to the hull of wooden boats.
The selection of sacrificial anodes for a vessel is dependent on the surface areas and type of metals to be protected taking into account their electronegative potentials. The marine surveyor should note that the ABYC inter alia states that a sacrificial anode should induce and maintain a minimum negative shift of 200 mV relative to the least noble (or most negative) metal.
The issue of under protection is, of course, well understood. Put simply, too little protection results in corrosion. The various Formulae and methods given above were designed with that requirement in mind and are backed up by the author’s experience.
Over Protection
The marine surveyor should also be aware that it is possible to put too many anodes on a boat, a danger which is called over-protection. That occurs when the sacrificial anodes generate a current higher than necessary to protect the more noble metallic fittings. Common issues on metal, frp, ferro-cement and wooden hulls given over protection include: -
1. Paint stripping – This is the hydrogen blistering of paint and occurs because of the destruction of the adhesion between the coating and underlying metal to which it has been applied. The
2. The repulsion of anti-fouling and propeller coatings – In the same manner as hull paint stripping, over protection can lead to the non-retention of these coatings due to the incompatibility of the application with sacrificial anodes. It can render the application ineffective and often leads to increased barnacle and other marine growth attachment.
3. Marine Growth – Excessive cathodic protection accelerates the formation of calcareous deposits on a vessel’s hull, rudders and propellers including a calcium carbonate coral like structure.
4. Alkali Attack – This can also arise as a result of excessive cathodic protection systems and can lead to the formation of alkaline conditions on metallic hulls when there is insufficient flow of water to return the water to its natural pH, resulting in accelerated corrosion. This can be accentuated as the paint blisters to create an even more alkaline condition and further lead to rapid deterioration of the submerged metal. Also seen on wood – see below.
5. Embrittlement and Metal Blistering - Studies have also shown that the metal-hydrogen interaction which is effected by the entry of hydrogen into the interstices within the metal’s crystal structure by the over protection potentials may significantly influence the strength behaviour of mild steel by the influence of the hydrogen on the rate of increase of lattice dislocation multiplication i.e. the metal tends to become brittle. Ductility may be retained if the potential does not exceed
The Report • March 2017 • Issue 79 | 39
-900 mV.
In wrought iron, the
hydrogen tends to collect inside the metal causing metal blistering near the anodes.
Defects due to over protection can also be found on wooden hulls. Cellulose is the natural polymer which gives wood its remarkable strength and is a major component of the material where lignin holds the cellulose together. Over protection resulting from the galvanic current flowing between the anode and cathode destroys the lignin and that results in the effects of localised wood decomposition and the defect called angel’s hair.
The images below show the result of installing too many anodes
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