Corrosion on Yachts - simply enough, if you mix different metals and add salt water, you will get corrosion. That's a fundamental law of physics. It's also a fact of life that boats live in salt water, and often contain many different metals. It's a wonder that some of them still float!
The problems of corrosion fall into these broad categories:
- Corrosion of steel, and to a lesser extent aluminium and alloy hulls.
- Corrosion of external hull fittings, including stern tubes, exposed propeller shafts, propellers, and other exposed metal parts underwater.
- Corrosion of internal metal fittings such as electrical and electronic components.
Metal Parts Underwater
Metal parts underwater are particularly susceptible to corrosion, both galvanic and stray current corrosion. This includes steel hulls which must be protected both by using suitable cathodic protection systems as well as a coating system.
Types of Steel, Stainless Steel and Other Metals and Alloys
There are many different types of steel, including different alloys of stainless steel, and you should always check your alloys.
The first rule to avoid corrosion is to ensure that you have compatible alloys in any situation where both are likely to come into contact with each other in the presence of salt water (e.g. a nut and bolt, or fastenings on the topsides or outside of the hull). Stainless steel is the steel that is most resistive to corrosion in the presence of oxygen, however it can corrode badly when things go wrong. For example:
- Generally, only the 300 series of stainless steel alloys are suitable for marine use. In particular if you see something made from a stainless steel with a number in the 400's, even if it is just one small bolt, washer, or nut, don't use it.
- Stainless steel gains its corrosion resistance in the presence of oxygen because it contains chromium, which bonds to the oxygen and forms a corrosion-resistant layer (in much the way that aluminium does). If you take away the oxygen, e.g. by painting or immersion in water, then the stainless steel loses its corrosion resistance.
- To avoid galvanic corrosion, use identical grades of stainless steel. e.g. do not use 304 grade stainless steel fastenings to affix a 316 grade stainless steel water tank.
- If stainless steel is to be welded, or has been welded, then ensure it is a grade that can safely be welded and still provide corrosion resistance in the marine environment. These are usually either the low carbon grade stainless steels, indicated by an "L" in the grade (e.g. 316L), or "stabilised" stainless steels which are grades 347 and 321.
Be aware that stainless steel is galvanically quite different to mild steel. Therefore in any place where stainless steel and regular steel come into constant contact, e.g. a stainless steel samson post welded to a steel deck, there are likely to develop small areas of galvanic corrosion. Regular steel can be protected from corrosion by using a suitable barrier coating -- epoxy, polyurethane, etc. Putting one of these barrier coatings on stainless steel actually reduces its corrosion resistance. Continued vigilance over any such areas is required.
See steel grades.
Galvanic corrosion is caused by two dissimilar metals, with different galvanic potentials, joined by two separate electrical paths including an electrolytic path (e.g. salt water, which is a liquid electrolyte). Similar to what happens inside a lead-acid battery, although on a much slower scale, an electrical potential (voltage difference) develops between the two metals when an electrolyte can pass ions between them, and all that is needed is an electrical path between the metals outside of the electrolyte solution, and you have an electrical current as well as corrosion happening.
There are many sources of this, including:
- Two dissimilar metals being immersed in salt water, with an electrical path between them. e.g. a propeller shaft and a bronze thru-hull fitting, or a bronze propeller and a steel hull.
- Small drops of water (especially salt spray) making a connection between two electrically connected metals aboard the boat. For example, a small drop of salt spray getting between an exposed copper wire and a steel bolt, or into a small crack between an aluminium mast fitting and the steel base that it is welded to.
In all of these cases it is the least noble metal on the galvanic scale, or in other words the most reactive metal that will be affected by the corrosion. This can work in your favour or against you. Two cases in point:
- A brass hull fitting is more noble than a steel hull, and will cause the hull to be corroded.
- A zinc anode attached to the hull (make sure there is a good electrical connection to the hull) is more reactive than the hull, the propeller, or any other exposed metals. Therefore the zinc will be quickly destroyed by corrosion and the hull will be protected.
Below is a "galvanic series" chart which shows the various alloys and their position in the series. Alloys to the left will cause alloys to the right to corrode, while the alloys to the left will be protected.
Stray Current Corrosion
Stray current corrosion is caused by having a potential (voltage) difference between the hull of the yacht, or a metal part of the yacht touching the water, and the water itself. The source of this voltage difference is usually an earth circuit leakage on board the boat when connected to shore power. Alternatively it can be caused by shore power earth having a potential difference to the water in the marina, in combination with a connection between shore power earth and the hull of the yacht -- this can be a direct connection or caused when power tools connected to shore power are used on board the yacht.
Where there is a voltage difference between shore power earth and the water, the process is reasonably simple to understand:
- AC shore power has 3 connectors, earth, live and neutral. The live and neutral circuits bring current on board the boat. On shore, neutral and earth are connected together. It's normal to find that the on-shore earth and the water in the marina are at the same voltage, however it is frequently the case, due to various causes that they are not.
- AC shore power earth is often connected to AC on board earth.
- AC on board earth is often connected to the hull of the boat (for a steel boat), or a metal earthing plate in the hull of the boat (for other boats). This metal earthing plate will be connected to all of the other metal parts of the boat that are in the water, such as the propeller shaft, the propeller itself, and possibly any chainplates.
- Any voltage difference between the water and the hull, or the water and the metal parts of the boat causes an electrical circuit, which will cause corrosion.
Some of the reasons why shore power earth and the marina AC earth include:
- Faulty (incorrect, or old and corroding) circuitry at the marina.
- Bad wiring on one or more of the boats at the marina, especially steel or ferro-cement boats.
The solution to the issue is to not bring shore power earth aboard the boat to the point where it comes in contact with on-board earth. This can be done by installing an isolation transformer or by limiting the shore power connection to a battery charger and using an AC inverter to produce on board AC (isolating the on board AC from the on shore AC). Note that the idea of bringing only live and neutral aboard the boat, connecting it to on board AC from which on board devices are powered, and leaving the on-shore earth disconnected from the boat is simply unsafe -- this makes on board AC fuses useless, and means that RCDs will not trip if there is earth current leakage (even a large leakage) which can be fatal to anyone on board the boat or even nearby.
Even with an isolation transformer or battery-charger-only type shore power, there can be a possible problem with electrical tools being brought on board the boat, especially on a steel boat. If a power tool is connected to shore power and then left sitting on the deck of the boat, the chassis of the tool (usually connected to AC earth) will form part of a circuit between shore power earth, the hull of the boat and the water. If there is a potential difference between the hull and the water then this will cause galvanic corrosion.
Unfortunately the only solution to this problem with a potential difference between shore power earth and the water is to get all marina owners and all boat owners to clean up their acts to stop any potential difference. This is simply not feasible. When entering a new marina, unless all possible precautions have been taken to prevent any current leakage between the hull and the water, it may be best to simply put a multimeter between the shore power earth and the water to examine any potential difference present. Especially on a steel boat it also pays to have a silver/silver chloride half cell which can be used to test for any galvanic or stray current corrosion issues.
Stray current corrosion can also be caused by a leakage between AC live on board the boat and AC earth on board the boat (AC earth also being connected to ground). Where this leakage is large it will usually blow a fuse, but if there is only a small leakage (perhaps even under a volt) then a Residual Current Device (RCD) is needed to protect the boat from corrosion. A RCD will trip if there is even a small leakage between live and earth, and will shut the power off until the problem is resolved.
Note that this form of stray current corrosion can affect not only your own boat but other nearby boats. The reverse also applies -- AC current leakage on a nearby boat can cause stray current corrosion on your own boat. So beware of berthing next to rusty steel hulks that are connected to shore power!
The methods to avoid corrosion include:
Exclusion of Water from Electrical Contacts
On board the yacht, galvanic corrosion protection is best achieved by keeping circuits dry. Any time there is a stray droplet of water (especially salt water) between the copper wiring of an electrical cable or circuit and, say, a steel nut, bolt, or connector, galvanic corrosion will happen.
Methods to keep circuits dry include:
- Using properly insulated and tinned cable in all places.
- Using appropriate waterproof insulation around all cable connections -- around where the cable connects to the connector itself and around any joints where the connector may come in contact with dissimilar metals (such as a ring connector bolted to a bus bar). Waterproof insulation includes things like liquid electrical tape, heat shrink (the double walled type with an internal hot melt glue), plastic spray (e.g. CRC Plasticote or similar), etc.
- Using drip loops to limit the amount of water running down the inside of cables. Drip loops are best placed where they can be seen -- if water is dripping from the drip loop then it's a sign that something is going wrong somewhere else, e.g. water is leaking in through a deck gland or similar.
Regular inspection of all circuits and connections is also strongly advised.
Cathodic Protection (Zinc)
As can be seen from the galvanic chart above, zinc as a metal is much more reactive than most metals used in shipbuilding. In fact it is the most reactive metal other than magnesium and a few others not shown on the chart, however neither magnesium nor the other metals are suitable for use as a protection method due to the fact that they cause over-protection, which leads to alkali corrosion of the metal you are trying to protect.
(In fact magnesium is suitable for use as a protective anode, however only in fresh water).
So in simple terms, cathodic protection involves turning the metal that you want to protect into a cathode of an electrical cell, while the zinc (also in the water) becomes an anode. In effect you encourage the galvanic corrosion of the zinc and in exchange you will prevent galvanic corrosion of the protected metal.
In order for this to work, the zinc and the metal you are trying to protect must be:
- In the same body of water -- zinc on the outside of your boat will not protect metal fittings in a wet bilge; and
- Electrically connected, outside of that body of water. For example, the zinc should be welded to a steel hull, or in the case of protecting thru-hulls in a fibreglass or wooden boat, connected electrically using a bonding circuit in the inside of the boat to the metal that it is there to protect.
There are many ways of sizing zinc anodes for a particular sized hull but the most common involves a simple formula involving the length (and wetted hull area) of the boat which will give you a size in kg of the zinc anode requirements.
Install Bonding Circuits
"Bonding is the practice of electrically tying together major metal objects on a boat (e.g. rigging and chainplates, engine and propeller shaft, stove, metal fuel and water tanks, fuel deck-fill fittings, metal cases on electrical equipment, etc.) and connecting them to the boat's ground." (Calder p212).
A good diagram of a bonding circuit is in Calder p212, with an electrical schematic on p213. Note that quite heavy wire gauges are used, typically 6AWG or so (for those more used to dealing with metric wire sizes an AWG to European standard wire gauge chart is on Calder p167). The idea being that the bonding circuit is a "husky, low resistance circuit with electrically tight connections" and that any stray currents that may cause galvanic corrosion will choose the path of electrically least resistance to ground, that is the bonding circuit rather than any metal fittings on the surface of the boat.
Water and Oil Exclusion
Especially on a steel boat, it is very important to try to exclude water from the inside of the hull, notably bilges, etc. Anywhere water can pool inside a boat is a potential source of rust and corrosion.
Steel hulled boats offer special challenges when it comes to corrosion. In addition to having suitably sized sacrificial zinc anodes, offering internal and external protection, excluding water and oil from the bilges, a good coating system is needed.
Coating systems come in many different types and grades from all sorts of different manufacturers, but the basic principles are the same. To best protect my steel boat I use the following:
- A single rust-penetrating and sealing barrier coat (sometimes called a pre-prime), and I always find that the two pack systems are the best. Although there are many different single pack formulas around, and I could name a few "miracle rust cures" but I'm sure we've all seen the advertising, they are generally rubbish.
- Two coats of a two-pack epoxy surfacer. This could be a high-build surfacer for uneven areas or a general purpose two-pot primer and undercoat. Two-pack epoxy gives the best general coating and sealing properties, however it is generally affected by UV light so needs a top coat.
- Two coats of a two-pack polyurethane topcoat.
This is by no means the cheapest solution but if you are the owner of a steel boat you will find that rust is your worst enemy and the best system you can manage to keep it at bay will repay the cost many times over.
Single pack systems are available that at least claim to meet most of the criteria for coating systems, however they have the following limitations:
- They are never as effective as a two-pack system. Two-pack epoxy or polyurethane paint sets by the effect of a chemical reaction causing polymerisation of the paint. Single pack systems are dissolved in solvent, which evaporates causing the setting process to occur. The evaporation of the solvent can leave microscopic pores in the paint surface, leading to rust occurring.
- A two-pack system may never be painted over the top of a single pack system. This is because of chemical reactions that can occur between the incompatible surface layers.
Two-pack systems have their own limitations, however, which need to be noted:
- There is a trade-off between usability and effectiveness. This can manifest itself in a number of ways. For example, the barrier coat (often a transparent layer that may dry to a light red or pinkish hue) often has a long drying time -- overnight or longer. There are "quick dry" formulations of barrier coats, however the faster a barrier coat dries the less effective it will be in penetrating and locking off any surface rust.
- Epoxy systems come in various grades, from high-build (where a thick coat is applied) to a thinner formula. The thicker the formula, the more difficult it will be to apply and the more waste is had when spraying. Thicker coats are often best applied by brush.
- Polyurethane topcoats are highly UV resistant, whereas epoxy coats are generally not UV resistant. However the polyurethanes do not generally have as good a barrier resistance effect as the epoxy coats. This is why it is advisable to put a polyurethane coat over the top of an epoxy coat. Make sure that the coats are chemically compatible -- contact the paint manufacturer if in doubt.
- Polyurethane topcoats often come in a thicker variety which is generally best applied by spraying, and a thinner "brushable" version. Of course you won't get as good a texture by brushing as you will by spraying, but it is most important to choose a brushable polyurethane if you are brushing.
- Feel free to ignore any manufacturer's advice about a polyurethane topcoat's ability to mask the colour of an underlying epoxy layer. Generally, it won't, and at worst it will allow the epoxy colour to bleed through. I have had salesmen assure me that a white topcoat will be perfectly OK applied over a red or grey undercoat (generally a story told when they are out of white undercoat) however this is baloney. So these days I apply one layer of a pink, grey, or buff primer, followed by a layer of white two-pack epoxy, and a white two-pack polyurethane over the top of that.
Once you have corrosion, what do you do about it?
The most accurate way of testing whether galvanic corrosion is occurring on your yacht is with a Silver/Silver Chloride half cell and a multimeter. The testing process is reasonably simple.
Silver/Silver Chloride Half Cell
A silver/silver chloride half cell (or reference electrode) is essentially what it sounds like -- half of a very small battery. If you put the half cell into the water and electrically connect it (outside of the water) to something bonded to any components in your hull that are electrically connected to the water (e.g. through hull fittings, propeller shafts, etc) then it will measure the potential (voltage) difference between the cell and whatever it is connected to.
The half cell is provided with a long electrical lead so that it can be lead out of the water and into your yacht. On a steel boat for example the lead only needs to be connected to somewhere on the hull. On a fibreglass boat with a number of hull protrusions it needs to be connected to each one in turn unless they are all bonded together, in which case it just needs to be connected to the bonding circuit.
The multimeter should be placed between the electrical lead of the half cell and the item that is being measured. The multimeter will then measure the potential between the item and the half cell, and because the potential (relative to the water) of the half cell is known, this will indicate the potential of the item relative to the water.
When galvanic anodes have been correctly designed and installed, the electrical potential of the vessel and its individual components should be in the range of -800mV to - 1,100mV in sea water with respect to the half cell. This is the voltage that should be read on the multimeter -- any lower or higher and galvanic corrosion is likely to occur.
Half cells can be purchased either individually or as part of a testing kit. For example:
List links to discussion threads on partnering forums. (see link for requirements):
- Corrosion on yachts (on CruiserLog)
- To bond or not to bond (on CruiserLog).
- Stainless to stainless underwater (on CruiserLog)
- What paints for a steel boat? (on CruiserLog)
References & Publications
- Nigel Calder, Boatowner's Mechanical and Electrical Manual: How To Maintain, Repair, and Improve Your Boat's Essential Systems — 3rd Edition, International Marine/McGraw-Hill, ISBN 0071432388
- Miner Brotherton, Ed Sherman, The 12V Bible for Boats, International Marine, ISBN 0713667036
- Stainless steel on wikipedia
- Steel grades on wikipedia
- Corrosion on wikipedia
- Half Cell on wikipedia
- Corrosion, zincs and bonding - An excellent article (pdf).
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