BREAKING THE CORROSIVE CIRCUIT

An Article by Paul Fay

The yacht had performed really well, they had sailed ten thousand miles in the last year with no problems and now that they were back on their half tide mooring on the West Coast, the owner was catching up on some maintenance. While walking around at low tide having a look at the underwater areas, he suddenly noticed with alarm that the anodes were disappearing very quickly. This needed immediate investigation, as the yacht is 37 feet, built of steel and normally only needed the anodes replacing every four to five years. The way they had suddenly started wasting away showed that something had gone dramatically wrong.

This was not an isolated incident or one that only happens to metal boats. In another case the yard that had hauled a new fiberglass yacht out of the water, found that the alloy sail drive leg was corroded away. The only thing keeping the water out was the internal rubber gaiter. A new sail drive leg that came with a new anode was fitted but three days after going back into the water the anode had corroded so badly that there were holes appearing large enough to insert a finger.

Horror stories like this are all too common and with our increasing reliance on electronics are being reported more and more.

These stories are all made worse by the almost mystical way that many people view galvanic and electrolytic corrosion. The marine industry has also compounded the situation because most marine engineers, both mechanical and electrical, were never taught the simple and easily understood theory behind the subject while they were being trained. When called upon to solve a corrosion problem they start casting around for all sorts of complicated solutions, often making the problem worse and costing the owner a small fortune.

What always seems to be misunderstood is that for there to be a galvanic or electrolytic problem there MUST BE A FLOW OF ELECTRICITY.

For there to be a flow of electricity THERE MUST BE A CIRCUIT.

TO EXPLAIN

Remembering our days at school and what we learnt in the science classes will remind us that if we hang two different metals in an electrolyte liquid and connect them then electricity will flow. This is a simple battery. One piece of metal will be corroded; the other will be virtually unaffected. This is galvanic corrosion.

Another form of corrosion happens when two pieces of metal are in an electrolyte and are then connected to an electrical supply. This can be either a battery or a mains supply. The metals can be different or the same. The corrosion is dependent on the current we are applying and the direction that we apply it. This is electrolytic corrosion.

There are several rules here that must be fulfilled for corrosion to happen.

To produce galvanic corrosion the metals must be different. The further apart in the galvanic series of metals then the higher the current that will flow and the faster the wasting away of the anodic metal.

Second, the metals must be in an electrolyte. At school we would have tried various conducting liquids and found that they each gave different results. The one that we are interested in is salt water, which is a good electrolyte.

Third, THE CIRCUIT MUST BE COMPLETE. This is the same as any circuit, if the metals, or battery plates, or anode and cathode, whatever the term used for them, are not electrically connected then NO CURRENT WILL FLOW.

This electrical connection can be made either by touching or by some other conducting path, such as a wire or the damp interior of a yacht.

This therefore means that if we can isolate the different metals that we use below the waterline then we have broken the circuit. We have stopped the flow of electricity and therefore stopped the corroding or wasting away of the metal.

The point that is to be made here is that when we have electrolytic or galvanic problems we need to LOOK FOR THE CIRCUIT AND BREAK IT.

PRACTICAL APPLICATIONS

In practice, there are certain places where it is virtually impossible to isolate the metals. For instance, we normally use stainless propeller shafts with bronze propellers; also very often mild steel rudders have stainless pins welded on as the bearing surfaces. When this is the case the two metals are chosen to be as close as possible in the galvanic series so that the corrosion is minimal.

The circuit in both of these instances is through the two metals that are touching, through the water and back into the metal.

To reduce the small amount of galvanic damage that will inevitably happen, even though the metals are close in the galvanic scale, a sacrificial anode is fitted. A sacrificial anode is a piece of metal that is low in the galvanic series and is attached to the part that needs protecting. They are usually made from an alloy of zinc and because of it’s low galvanic series properties this will be the first to corrode away, thus protecting the other metal.

Another place where it can be difficult to isolate the metals is in timber boats. Here the circuit is from a skin fitting, through the water to another skin fitting, then through the damp timber of the hull back to the first skin fitting.

It is common practice to connect many or all of the fittings together on wooden boats by joining them internally with copper wire. The copper wire is connected through the hull to a series of anodes that protect the fittings. These anodes need replacing at regular intervals. The only problem with this procedure on some wooden boats is that around the fittings being protected an alkali will be formed. Some timbers will be quickly damaged by this and expert advice should be taken when proposing to fit a new anode system to a wooden boat.

On early GRP hulls, the skin fittings were often joined in the same way as on wooden boats, but it is now more usual to leave the fittings isolated. This uses the insulating properties of the plastic hull to ensure that no circuit is formed, but care is needed to make sure that the fittings are not continually wet by the bilge water, as this would form an internal connection. On many modern GRP hulls, the only anode that needs fitting is to the propeller shaft, as all other fittings are well insulated from each other and so no circuits are formed.

On metal hulls, various methods are employed to minimize galvanic problems. There are now available excellent plastic sea cocks that can be fitted. These will totally cure the problem, as they are electrically inert.

If bronze fittings are used then they must be electrically insulated from the metal of the hull. This is achieved with plastic washers, gaskets and sleeves to form a barrier between the bronze and the hull, so that no circuit is formed.

Welding in the stern tube and then using a resin cased stern tube bearing; a rubber mounted packing gland and a plastic coupling between the gearbox and the shaft, will insulate the propeller and shaft from the hull. (Shaft bearings are available with either bronze or resin outer cases. Only a resin case should be used where ti will be in contact with the metal hull.)

These are just a few of the many ways that under-water circuits leading to corrosion can be avoided. With the boat out of the water, the resistance between fittings can be checked with a cheap multimeter, or even a simple battery, wire and a bulb. This should show that there is no contact between the fittings.

A commonly made mistake is to connect the meter between two fittings when the boat is afloat, to see if current is flowing. What actually happens is that the meter instantly makes a circuit. This produces an alarmingly high current reading, leading the operator to believe that there is a problem when in fact there may be none.

INDUCED ELECTROLYSIS

The alternative method to the galvanic corrosion that has just been discussed is to cause corrosion by introducing current to the fittings. On ships, this is often deliberately carried out by supplying controlled current to special anodes. This is so that the vessel's engineers are in direct control of the electrolysis. This is known as an impressed current anode system.

In the case of small boats and yachts, induced current normally happens by accident. A wire or electronic fitting fails and allows ‘stray’ electric current into the water through a seacock or other underwater fitting, usually causing fast and quickly noticed electrolytic corrosion.

It is fortunate for us boat owners that most corrosion problems are galvanic as when electrolytic corrosion happens it can be difficult to trace the circuit. The reason is because in this case the circuit will often be from the positive of the battery, through an instrument or radio; the instrument is connected to a skin fitting, keel bolt, or the hull to obtain a radio ‘ground’; the current then passes through the water to the engine block that has a heavy cable connecting it back to the battery. Although tortuous, the current will find it’s way round this circuit and even though the chassis of electronic gear is connected to the negative of the battery the electric leak can put different underwater fittings at a differing electric ‘potential’. Although this is only slight it is all that is needed to cause electrolytic corrosion.

It is a fact that the negative on many boats is connected to the water without the owner being aware of it, making it easy for any stray current to form a circuit.

This can be through the engine block, through control cables, and commonly via the outer braid or ground on VHF and SW radios and other radio receivers such as weather faxes.

An often missed, but common way for the circuit to be made is this: - The chassis of radio receivers is connected to the negative of the battery. The outer braid of the co-ax cable from your VHF is also connected to the chassis. The other end of the co-ax is connected to the antenna bracket, which is connected to an alloy mast. Many boat owners will have the mast or rigging connected to a skin fitting or keel bolt for lightening protection. This again connects the negative of the boats supply to the water thus making it possible for stray current to form a circuit.

As you can see locating an electrolytic circuit can be difficult. However, there will always be a circuit and experience has shown that these rarely just ‘happen’. There will normally have been some work on the boat that the problem can be traced to. Perhaps some new instruments have been fitted, or it could simply be that a shelf was put up and a screw has damaged a wire.

So what about the stories quoted at the beginning of the article?

A few weeks before noticing the problem, the owner of the steel yacht had checked the bolts holding a bronze stern tube in place and slightly over tightened them. This had caused the insulation between the bronze and the steel hull to fail, making a circuit. The insulation was repaired and the problem solved.

The GRP yacht with the corroded sail drive leg had a new weather fax fitted. The radio engineer had joined many of the underwater fittings together with wire to obtain a good ‘ground’ for the instrument. Between the alloy sail drive and the bronze seacocks this caused massive galvanic corrosion that cost the owner thousands of pounds to repair.

Most galvanic and electrolytic problems have a fairly simple solution. As boat owners and engineers we don’t need to have a deep understanding of the science that goes into the construction of marine grade metals, this can be left to the experts who make the skin fittings and anodes. All we need is a basic awareness that to avoid or cure these corrosion problems NO CIRCUIT MUST BE PRESENT.