Cathodic Protection the Definition and What to do

 

Any Metals of differing electrochemical potential when in contact with one another form Galvanic cells and will corrode. The metal with a lower potential in the galvanic cell structure will be anodic and will corrode. The same effect can occur in areas of different electrochemical potential, such as a steel plate, Aluminium, prop shafts , keels and more worryingly skin fittings on GRP hulls . In a nut shell any metals below the water will corrode and the material with the lowest potential will go first.

Any craft moored and or being used in fresh, salt or estuarine water is at risk from corrosion and the results can be very costly.

The nature of waters in which pleasure craft are kept are becoming more and more varied as enclosed Marinas and Tidal Barriers and locks becoming more common place. Although there appears to be debate about being moored near lots of steel piling I do feel this will also have an effect. I also feel that the antifouling that we use having a copper content will also have a consequence.

 

The result is that many vessels are now moored in waters with a considerably lower salt content than in years gone by. Added to this is the trend that vessels are regularly moving between fresh water estuarine and salt water areas.

Owners need to pay particular attention to the threat of corrosion in these situations.

Many anode manufactures are taking this into account and are able to offer the complete range of Zinc, Aluminium and Magnesium anode systems needed to protect your vessel!

 

Cathodic protection is an electrochemical process which halts the natural reaction (corrosion) of metals in a particular environment by superimposing an electrochemical cell more powerful than the corrosion cell. Sacrificial Anodes are fitted or bonded to the metal to be protected which in turn as it has a greater electrical potential than the anode material becomes cathodic and causes the anode to waste instead of itself.

 

On GRP and also wooden vessels, The areas of  problems are mainly the stern gear i.e. The propellers, shafts, shaft brackets, stern tubes and rudder stocks and skin fittings and valves which are expensive to replace and vital to the vessel integrity and the users safety, the failure of a propeller or skin fitting, rudder could have disastrous consequences. I do feel that many people tend to overlook this as a new shiny GPS is more fun until you have water cascading into your boat because you have not looked after you skin fittings and pipe work.

 

The effects of corrosion can be seen from as little as pitting of propellers and shafts to the decomposition of the alloy within your propeller and or skin fittings. The failure of something as small and as inexpensive as a split pin can result in the loss of your propeller. Now the loss of a propeller may not be at the time, life threatening, but could become very fast a real problem for motor boats with only one engine. You can not get out and push like you can a car when ashore.  

 

Stray current leakage is quite often can be attributed to the cause of corrosion on all types of vessel however more often than not the problem can be traced to a galvanic action. Stray current leakage is the action of electrical current from an external power source such as a battery or the shore power supply which because of some electrical system fault on board the vessel it passes out through the hull or a fitting in the hull and flows through the water causing "Electrolytic" corrosion. Stray current leakage is usually a result of damage or wear to the wiring system or poor installation of wiring or electrical equipment. We all like to add our new toys with little regard to the hidden problems we may be coursing.

 

Corrosion on Steel & Aluminium boats and the potential corrosion is fare grater for obvious reasons  and can be identified as either areas of localised pitting to the hull plate, rudders, bilge keels etc. or less obviously, in the form of general thinning of the hull plating and can be very costly. It will occur below or if you like under the paint coating and can be very hard to detect with all the lovely epoxy paints that are being used these days. Pitting can lead to the complete erosion of the hull below the waterline. General erosion of the steel can be just as critical, weakening the hull and necessitating expensive re-plating.

Corrosion on Aluminium vessels is also generally in the form of localised pitting to the hull plate, rudders, bilge keels and particularly in way of weld seams. Pitting can lead to the complete penetration of the hull below the waterline necessitating expensive re-plating also.

 

The selection of materials is of prime importance in the construction of craft. Boat builders select metals which are as far as possible compatible to each other but there is always a financial demand . So not so expensive new and older repaired boats may suffer. when this is the case metals must be isolated from one another, with an Anode.

 

1/ Use only high grade insulated wiring of suitable capacity. Undersized wires will cause resistance and consequent voltage drop.
2/ Clip or support all wires at suitable intervals to prevent fatigue and eventual fracture.
3/ Use only corrosion resistant terminals and connectors and make sure that all are clean and tight and check them regularly.
4/Attach only the main battery leads to battery terminals so you can isolate it with your battery isolator switch.
5/ make totally shore any bilge pumps plugged into the battery all the time are fitted properly. 6/ Ensure that all battery circuits are correctly fused.
7/ Keep all wiring, connections and junction boxes above the bilge area and other areas likely to become wet. Electrical and Electronic work is best carried out by a qualified marine electrician we will always try and help.

Ongoing maintenance on your boat is essential. Metal work, paint coatings and electrical installations all require regular inspection. We at Lauren Marine can help you with is and we have all the relevant testing instruments.

I hope you have found this informative and if you would like a chat do give us a ring