PREVENTING CORROSION

You’ll find various references to corrosion throughout this book, but I feel the subject is sufficiently important to warrant a chapter of its own. In seawater, corrosion is electrochemical in nature, and it’s important that every boatbuilder who works with metal is familiar with the causes and effects of the more common types of corrosion. You should know how to avoid corrosion problems.

  Corrosion is not confined to metal boats, nor to modern boats. Corrosion can damage every type of vessel, including those built of timber, fiberglass, and Ferro-cement. It is because of corrosion that keels and rudders fall off, that stainless steel tangs break, and that rigging fails. Corrosion is often the cause of fastening disease, an age-old problem with wooden boats. The results of corrosion can be severe, to the point of failure for rudderstocks, through-hull fittings, propellers, and sea-cocks. There have been instances where the sea-cocks have been caught in the final stages of disintegration just before they crumbled away and let in the outside water.

   When a metal is immersed in seawater, it will achieve a certain electrochemical potential. Different metals have different potentials. Different potentials can also occur locally—from area to area in a single metal surface, for example, or near a weld area, or between areas exposed to different levels of oxygen. It’s the potential difference between metals in contact with each other, or areas on the same metal surface, that acts as the driving force for corrosion under certain circumstances.

Galvanic Corrosion

   When two different metals are immersed in such a good electrolyte as seawater and connected through a metal path, an electric current will flow, causing corrosion of the metal with the lower potential. The metal that corrodes is called the anode and the metal that has the higher potential (the nobler metal) is called the cathode. When this type of corrosion occurs it is termed galvanic or bimetallic corrosion.

   Although the less noble metal in the galvanic couple will corrode at a higher rate than it might otherwise have done, the more noble metal will corrode at a lower rate. You can use this to your own advantage; in fact, it’s the basis for cathodic protection. The accompanying table shows the galvanic series, and will help you predict which alloy in a metallic couple is more likely to corrode.

   The metals and alloys lower in the galvanic series have lower potentials and will be corroded by those higher in the list. The degree of corrosion that occurs depends not only on how far apart they are in the galvanic series (and thus the size of the potential difference), but also on the relative surface areas of the cathode and anode. Alloys close together in the series, such as copper and bronze, will be less prone to galvanic corrosion than those further apart, like copper and steel. Corrosion can be expected to be greater if the exposed surface area of the more noble metal is large compared to that of the less noble alloy. An example of this is that steel bolts in a Monel structure will corrode very quickly, whereas Monel bolts would corrode insignificantly in a steel structure, unit for unit.

   There are various ways of controlling galvanic corrosion. Choosing metals close together in the galvanic series can be a good way of reducing galvanic problems. If possible, you should use only similar metals throughout the vessel. In this way, no galvanic current will flow.

   It’s not always possible or desirable, of course, to use one metal throughout the hull, deck, and superstructure. Luckily, galvanic current can be avoided by electrically insulating the two metals from each other. Insulating washers and sleeves can be used on bolts; nonconductive gaskets can be used on flanges.

   Paint coatings can also be used as protection against galvanic corrosion. In this case, the temptation is just to coat the alloy that is likely to corrode in the metal couple. But coatings may have imperfections or “holidays” in them, or can be damaged, so the current can pass through in localized areas. The large area of the uncoated cathode produces high rates of corrosion in the small areas exposed through the coating. Always apply coatings to the more noble metal, or to both metals, rather than to the anode alone.

Non-metal fittings are another possibility; however, some classification authorities are reluctant to accept this solution. They suggest that fire and degradation from sunlight could be a problem. We have seen examples of the latter, where plastic (presumably nylon) skin fittings were wiped off when the vessel rubbed against a piling. This vessel had spent a considerable time in a sunny climate and the sun had affected the fittings to such an extent that they had very little strength. There are some plastic sea-cocks, skin fittings, and the like, that are said to be unaffected by the sun and ultraviolet rays. You should check these out for yourself before purchasing them and fitting them to your boat.

   Again, the best overall protection is to stay with one metal, especially in the hull, where you have a good chance of maintaining all-steel or an all-aluminum structure. In practice, the solution is to use a combination of the above methods to minimize chances of galvanic corrosion.

  In the interior of your metal boat, you can choose the closest compatible metal to attach the interior joinery to the metal hull. There is no point in using regular steel screws, as they would soon rust in the marine environment. You can use stainless steel screws or Monel screws and bolts (expensive), or you can plan your interior to avoid as much contact of dissimilar metals as is possible.

   Galvanic corrosion can also occur in the same metal. For example, type 303 free-machining grades of stainless steel suffer extraordinarily severe corrosion in salt water. These metals contain high densities of manganese sulphide, or selenium inclusions, which create many, built-in metal-to-inclusion galvanic cells. These grades should never be used in salt water.

TABLE 12-1. GALVANIC SERIES OF METALS IN SEAWATER

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The position of the metals on the scale may vary slightly depending on the exact composition of the particular metal.

Cathodic or most noble

Platinum

Gold

Graphite

Silver

Titanium

Hastelloy C

Stainless steel (304 and 316 passive)

Nickel

Monel (400, K-500)

Silicon bronze

Copper

Red brass

Aluminum bronze

Admiralty brass

Yellow brass

Nickel (active)

Naval brass

Manganese bronze

Muntz metal

Tin

Lead

Stainless steel (types 304 and 316 active)

50/50 lead tin solder

Cast iron

Wrought iron

Mild steel

Cadmium

Aluminum alloys

Galvanized steel

Zinc

Magnesium

Anodic or least noble

SELECTIVE CORROSION

Selective corrosion can occur in certain alloys when one component of the alloy corrodes away more quickly than another. We’ve seen this with brass sea cocks, which contain copper and zinc. The zinc dissolves in seawater and leaves behind a weak and spongy mass of copper. This is called dezincification. Bronze sea cocks prevent this.

Cast iron also can exhibit a form of selective corrosion called graphitization. The matrix of cast iron contains flakes or spheroids of graphite. The iron can corrode, leaving a weak, brittle network shell of graphite. The external appearance remains unchanged, which can make the condition difficult to detect. A further consequence is that the graphite shell is galvanically very noble and can then cause galvanic corrosion of adjacent parts.

   Stainless steels can undergo selective attack in heat-affected weld zones. You can avoid this completely if you weld with carbon-L grades, or titanium- or niobium-stabilized grades, of stainless steel.

CREVICE CORROSION

In seawater, stainless steels have very low general corrosion rates. If they corrode, it’s normally in localized areas under hard fouling or tight, man-made crevices such as gaskets. When the oxygen in the crevice is used up, an oxygen-concentration cell forms with the oxygenated metal outside the crevice. This can lead to corrosion reactions within the crevice. The 316 alloy has better resistance to this than the 302 or 304 alloys. Cathodic protection by anodes or galvanic contact with other less noble alloys can also help.

STRAY-CURRENT CORROSION

This is another type of corrosion that can be prevented. Unlike galvanic corrosion, which is caused by two different metals in water, stray-current corrosion results from an outside electrical source, such as direct current from the ship itself or alternating current from a shore electrical hook-up. You will find additional information on the cause and remedy for this situation elsewhere, including Del Kahan’s essay in Chapter 14.

In most cases, the villain is one of these sources: a current leak in the wiring from frayed or broken wires; improper or crossed grounds; electrical leaks from loose, broken, or poorly insulated terminal connections; or bad marina shore-power equipment.

   When you’re connected to shore power, you should always use heavy-duty extension cords. The length should be sufficient only to transfer the current from the power source to your boat. Power tools often generate stray currents and onboard radios can also cause problems in this area.

   Unfortunately, while galvanic corrosion occurs relatively slowly and over a period of time, electrolytic corrosion can occur rapidly, depending on the strength of the stray current. The stray current can be a mere trickle of direct current from an area that is damp, to a blue-sparking short circuit on board from a marina’s 220/110-volt system. Stray current can also give shocks to the crew and can cause fire or explosion.

Warning signs can range from blue sparks and a crackling noise from a shorted-out power cable to heavy static on radio speakers because of voltage drop. When electrical equipment does not function up to expectations, suspect a stray-current flow that ends up somewhere other than where you want it.  Voltage can be traced with a multi-meter. Metal damage in the grounded area, resulting from electrical leakage, shows up as massive rusting and scaling on steel parts, abnormal brightness on bronze, and the total disintegration of aluminum parts.

   Stray currents, like galvanic corrosion, can be eliminated when electronic devices are installed on the vessel. A custom builder should take the approach that both types of corrosion are predictable problems that not only can be built in, but can equally be “built out” of the boat and totally eliminated.

   You should use high-quality wiring, fittings, and switches designed for the marine environment by a reputable marine manufacturer to protect yourself against most of the potential problems described in this chapter and elsewhere. Shore power needs particular attention since it has the greatest potential for danger. You’ll want to use heavy-duty cord that is moisture resistant and specifically made for marine use. Use watertight marine connectors at both ends and, if possible, use a “moulded-cord set,” which is the last word in connecting shore power to your vessel.

Install a ground-fault circuit interrupter in the vessel’s panel or fuse box. It will help prevent electrical accidents, especially those that result from current flowing from a hot wire to a ground. It will also reduce your chances of getting a shock.

Cathodic Protection

Many people are under the false impression that boats that cruise exclusively in fresh water do not require any special form of cathodic protection. The most commonly used method of protection is to install anodes to various underwater locations on the outside of the hull. M. G. Duff, the British experts on this subject, have produced two excellent pamphlets. One is for boats operating mostly in salt water and the other covers the freshwater environment. These publications explain the special requirements needed to protect your stern gear, rudder, and associated underwater equipment from the ravages of mysterious gremlins that can damage them and even the hull itself.

Most metals are extracted from ores by various processes, and they are prone to return to their natural state under the action of oxygen and water. We have all seen unprotected metals react in this way. Marine aluminum is one exception, it can be left unpainted and still not deteriorate even in a saltwater environment. The French build aluminum sailboats and leave them totally unpainted above the waterline; the Canadians do the same with fishing boats built in British Columbia. Pity about the appearance of the unpainted boats!

Cathodic protection is a means of transferring the corrosion electrochemically to another less noble metal. The concept is not new. For instance, Samuel Pepys, back in 1681, noted in one of his diaries that the removal of lead sheathing on ships of the line reduced the corrosion on the iron rudderposts. Over 100 years ago, the Italian physicist Luigi Galvini conducted experiments in this field and proved that when two metals were electrically connected and immersed in water, the resulting corrosion of one of the metals was speeded up, while the other received some level of protection. Once we understand this concept, the method of controlling hull corrosion and protecting immersed fittings becomes relatively simple. Sacrificial anodes of reactive metals can be applied to a metal to protect it.

A word of caution: cathodic protection of a copper-nickel hull is unnecessary because the alloy already possesses good resistance to corrosion by seawater. The use of cathodic protection will also reduce the effectiveness of the antifouling of the material. Hull attachments below the waterline should, if possible, be copper-nickel, or if not, the fittings should be made of a slightly more noble metal.

Sacrificial Anodes

Sacrificial anodes are usually made of magnesium, aluminum, or zinc. For metal boats, anodes are either zinc or magnesium, and come in various shapes and sizes. These protective devices are relatively inexpensive and a complete spare set should be carried at all times. An unexpected haul out could reveal the necessity to replace the anodes, so a set should always be on hand. For fresh water, use magnesium anodes and for salt water or heavily polluted water, use zinc anodes.

If you’re building a new boat, the designer will be able to recommend the type, number, and placement of the anodes, and they can be either welded or bolted to the hull. We recommend the bolt-on method, as the replacement of this type will not cause the paintwork or inside foam to be damaged by additional welding. Bolting on replacement anodes is a much simpler process than removing and replacing old spent ones that have been welded in place.

When you’re setting up your anodes for the first time, simply use the anode attachment straps and bolt holes to mark the position of the threaded studs to be welded to the hull skin. The fact that you’re going to reuse the same locations and bolting arrangements, is another good reason to have more than one spare set on hand. On one occasion, we had to re-drill several anode straps to match existing studs when we were unable to buy the same brand with matching holes. The studs and surrounding area should be painted after the studs are installed. Under no circumstances paint the anodes: this stops them from working.

The position and placement of the anodes depends on the size and displacement of your boat. The anode manufacturers have special charts showing the relationship between the size of boat, the number of anodes required, and where they should be located. It’s common practice to have one anode on each side of metal boats up to 25 feet (7.62 m) long; they’re placed below the waterline about 25 percent forward of the stern. Boats up to 35 feet (10.67 m) long require four anodes, two per side, one 25 percent and one 50 percent forward of the transom. Boats up to 44 feet (13.41 m) in length can use the same numbers and positions, but larger anodes. For larger boats, it’s normal to have three anodes per side.

   In addition to anodes already mentioned, every boat should have a small anode placed around the propeller shaft, another on the rudder, and a third in the area of the bow thruster, if fitted. When placing the anodes, either make sure that they’re adjacent to the sea-cocks (if they’re made of dissimilar metal) or fit additional anodes as required. For a foil-shaped rudder, the anode can be fitted with threaded studs about 25 percent below the waterline. On a powerboat’s single-plate rudder, the anode can be through-bolted in position, using a bolt of the same metal as the rudder.

   If you fit a bronze seacock on a steel or aluminum hull, make sure you insert a heavier metal section in the hull skin in the area where the metal standpipe is located. We recommended that the standpipe be carried inboard until the bronze seacock can be fitted clear of the waterline. The seacock will be isolated from the standpipe by liberal bedding compound installed between these two items.

   Please note that the installation of anodes for a metal hull differs from that of a wooden or fiberglass boat in that the metal hull and metal fittings inside conduct galvanic current to the anodes. You don’t have to run wires from the engine, engine shaft, or other similar items to the anodes. The reason is that the engine and other fittings are already grounded to the metal hull and carry the galvanic current to the anodes.

   If you keep the hull of your steel or aluminum boat well painted, especially the area below the waterline, this alone will contribute to your maintenance of the boat and reduce the demand on the anodes. Anodes will need replacing before they are totally used up. Deterioration of the anodes shows that they are working. As mentioned elsewhere, the underwater sections of a copper-nickel hull will not need any painting or antifouling protection.