METAL CORROSION IN BOATS

Why does my Boat Rot, and what can I do about it?

From the minute your boat enters the water it starts to rot, corrode or rust. This process will continue relentlessly and your Boat Hull, or any metal components under the waterline, will never be in as good a condition again.

All you can do as a responsible Boat owner is monitor the Condition of your Hull and take some remedial measures at the appropriate time to prolong the serviceable life.

Why does Metal corrode in water?

Metals are strong. The metal atoms are arranged in rigid structures and they are surrounded by electrons which are relatively fluid. This allows metal to conduct electricity which is fundamentally the flow of electrons.

Your Hull and any other Metal in contact with the water therefore has a ready supply of electrons which are negatively charged.  Unfortunately the water that your Craft is floating in has a positive charge and is permanently looking for sources of electrons to become more stable. Any metal in water is a perfect target.

Not all water is the same.  If the water is slightly acidic, more Salty or has more Oxygen (fast flowing or white water) it will be more positive and able to steal electrons more readily from your Boat.

Removal of electrons is a process called Oxidation other words for this include corrosion or rotting.

Fortunately, you can measure both the availability of electrons in your Hull and the corrosive potential of the water and take appropriate action!!

Hull potential.

Measuring Hull Potential is very simple and quick.  Using the Boaty meter kit simply connect the Hull to the positive terminal and place the electrode in the water near the hull. The readings for new boats will be as follows e.g.

Aluminium Hulls   -900mv

Steel Hulls     -600mV  

The results are negative as electrons are negatively charged.  The more negative the result is the more electrons you have locked up in the Hull. This is a good thing.

Water Potential (REDOX)

As the water steals electrons and corrosion occurs this result will become less negative.

Otherwise known as REDOX (Reduction-Oxidation) Potential. This is even easier to measure just connect the two electrodes up to the Boaty meter and place them in the water and take a reading.

Normal river water for instance will be around +200mV. Notice that this is positive!!!  The water is therefore looking to take electrons from anywhere it can.  The more positive the result the more corrosive the water is. Slightly acidic water is often +300mv or more. Swimming pool water for example is around +700mV

How to protect the Hull

To protect your Hull, you need to prevent the loss of electrons. Outside of applying coatings this is not usually possible on small boats.  The Hull is highly negative and the water is strongly positive so it is going to happen!!

You can however use Sacrificial Anodes. The principle is simple. You bolt a piece of metal to your Hull under the waterline made of a material that has more available electrons than the Hull metal.

For river Boats this would be Magnesium. Magnesium has a potential of -1800mV. Zinc is also commonly used and has a potential of -1200mV. Basically when the water takes electrons from the Hull it takes the path of least resistance.  The electrons in these sacrificial Anodes are more readily available and are therefore removed thereby saving the electrons in the Hull.

Electrons are removed more readily at Sea so it is not practical to use Magnesium or Zinc in many cases as they would disappear too quickly. It is therefore more common to use Aluminium or even Steel. This will depend on the metal you are protecting.

Monitoring the Hull or Drives.

Once you have installed Sacrificial anodes you should measure the Hull potential.  The reading will be more negative with the right size and number of anodes. e.g., a Steel hulls potential may change from -600mV to say -700mV.

The reading should be taken weekly or monthly. It will become more positive. Before the reading reaches the value it was before anodes were added they should be replaced. At this point the anodes are no longer effective and need replacing before the water starts taking electrons from the hull.

Drives and any other metal under the waterline should be monitored and protected in the same way.

Measuring the Water,

The REDOX potential of water shouldn’t change much but remember you can measure this potential and decide, within reason to moor your boat in the least corrosive location. In a Marina these are usually in more stagnant water. If the reading is say +200mV near the boathouse and say +260mV near the entrance (where the water moves faster) that equates to being 10 times more corrosive.  Every +60mv equates to 10 X the corrosive power!  So if the result is +320mV that means 100 times as corrosive and so on.

 Stray DC and why it increases corrosion

The best reason to measure the REDOX potential of water is to pick up catastrophic stray DC (direct Current) or Oxidizing conditions brought about by faulty ground or earth contacts on nearby boats or in the Marinas grounding set up. (I’ve Used Stray DC generally as a term to cover all of these issues as they are varied and complex))

DC accelerates the movement of electrons and provides clearer paths to the Anodes.  The good news is that is it pretty easy to locate the source of the problem.

All you need do is measure the REDOX potential of the water where you believe there is a problem.

If there is stray DC the REDOX result will be High (typically over +300mV) and may be variable. In some cases, depending on the source, it can be negative this will depend on the proximity of the REDOX probe to an active Anode or Cathode.  Make a note of this reading and then remove a bucket of water taken from the same place the reading was taken. If you take a reading in the bucket it will be the base reading of the Marina water with no Stray DC. The difference in the result shows the extent of the Stray DC.  The bigger the difference the bigger the problem.

Don’t forget every 60mV increase is a 10 times more corrosive power. It is common to see 1000 and 10,000 times more corrosive activity with stray DC.

The first course of action is to isolate the issue by disconnecting the shore power on the suspected source and then re-test the water. 

Summary

Monitoring and logging your hull potential will ensure Sacrificial Anodes are replaced at the right time. Not too soon and not too late. Either way you save time and money and prolong the life of your valuable boat.

Checking the water for optimum moorings and detection of Stray DC or other Oxidation issues will prevent more rapid and catastrophic damage.

Tags: Corrosion REDOX