Boating Advice: Understanding corrosion (Part 2)

Whether you are a boat owner or potential boat buyer, one thing to be aware of is corrosion. It is alive and well on the water. But the lack of in-depth information is amazing and it borders on a cover up or a case of denial. 

Just replace the anodes or more amazingly add more anodes and she’ll be right. No. she won’t be right. She may well be worse. More isn’t better.

In my last article, an introduction to corrosion (Corrosion Part 1), I didn’t really discuss corrosion caused by dissimilar metals. This event is illustrated in the "Galvanic Series of Metals".

GALVANIC ACTION
The scale below lists the softer metals to harder metals commonly found in the marine industry:
>> Titanium
>> Stainless Steel
>> Bronze
>> Naval Brass
>> Copper
>> Mild Steel
>> Aluminium
>> Zinc (common anode)
>> Magnesium
The list is far more complex, but let’s keep it relative to the topic specific in our marine industry.  

Each metal in the chain creates its own galvanic circuit, the harder metal being the positive and the softer being the negative charge. In boating, let’s show some examples and common faults.

YACHTING ISSUES
For the yachting followers and owners, go and have a look at your mast or boom extrusion. Notice how all the fittings attached are secured by stainless-steel fastenings. Now look at the list and see the gap; it’s almost from one end of the spectrum to the next, so we will have the galvanic process in play.  

Now you have looked at the fittings and you have noticed that there is white powdery effect taking place. If the section is painted you will often find the paint is blistering around this area.

If you have ever had to remove a fitting from a mast that has been on there for years and years, you will now the frustration that this form of corrosion can cause. A simple “five minute job, mate” becomes a day’s labour.

'Cocky Poo' Tip: A professional installer at the time would of used a barrier like Lanolin or Duralac (cocky poo) as we refer to it for it bright yellow nature. These compounds are designed to break the contact between the two dissimilar metals. 

Like everything on boats, these fastenings should be pulled off and the compounds reapplied and the fastenings refitted over time. Prevention is better than cure.

SKIN FITTINGS A MAJOR ISSUE
Skin fittings and seacocks are fast becoming a major issue within the marine industry, even more so in the last decade with availability of cheap good-quality stainless steel seacocks or valves. 

More and more often I am seeing vessels that have had valves replaced, the owner accepting this is needed, but the uneducated repairer ends up installing a stainless valve on the original bronze skin fitting.

Although these metals are very close in the Galvanic Series, remember these are submersed in water 100 per cent of the time, so the circuit is amplified.

I personally found over the years that the best answer was to upgrade the skin fitting and valve all in the replacement process and ensure the bonding system is up to scratch. 

(I am covering the bonding system later on in the series in depth, so I’ll leave that one at that for now.)

BILGE PUMPS
Bilge pumps, more specifically float switches, are a bigger contributor to corrosion from stray currents than any other component on the vessel.  The float switch is our insurance policy (so we keep telling ourselves) that the boat won’t ever sink!  

The float switch always has power to it, otherwise it doesn’t work (not rocket science) and in most cases the connection is made under the floorboards and is often wet or even submersed.  

The fact is, a boat that has water in the bilges and a live float-switch connection is now the best conductor and transferrer of current on the vessel. Every metal component is now under charge.

On a yacht, this means the electrolysis process is into the keel bolts and keel; for timber vessels all fastenings at the bilge water level will be active; for a steel vessel, this is extremely harmful and would recommended that only a manual bilge system was installed.

The only prevention here is to ensure your connection to the float switch is waterproofed and well terminated.  The wires on these units are often of low quality, so a regular update on these is advisable, especially for owners that are relying on them too much.

CORROSION AND LEGS
Engines, legs and running gear are huge contributors to the electrolysis process. The engine on your vessel is negatively earthed and on many older vessels this is constant.

In later model production vessels, many have a negative isolation switch that turns the negative source to the engine off.  The installation of an isolation switch on the negative terminal to your engine is one of the best prevention methods of electrolysis and leakages!

THE NEXT INSTALMENT
But I have only scratched the surface here by listing some common faults and cures. Next instalment, I will look at some early warning signs. Some I have encountered all too frequently. I will also introduce expert boat lover and corrosion expert Brian Gatt from Logix Group.  

Between us, we have come across some quite mind-blowing cases of corrosion.

The next article in this Series, Part 4, will deal specifically with four case studies of corrosion on boats ranging from a 1994-model sportscruiser to a 2005-model 42ft production cruiser, from an early-90s production 42ft yacht to a 44ft production yacht built in 2014 with corrosion issues!

*Gavin Clark is a contributor at boatsales.com.au and BoatPoint.com.au who writes under the pseudonym 'The Surveyor'. He is a shipwright and marine surveyor with a business called Slipstream Marine based in Sydney. He has 27 years experience in the boating industry, is an avid boater and owner of a 33ft Sharpie (trawler). Gavin can be contacted at http://boatsurveying.com.au