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I take no credit for any of the information below, but did find if very informative. If any one feels that it contains mis information, please let me know, and I will remove it. Before you start reading you may want to grab a bag of chips and a cold one... It is kind of long.... METAL BOATS Potential corrosion Owners of metal vessels must forestall electrochemical damage by Nigel Calder Ocean Navigator NO. 63 September/October 1994 Years ago, I read of a yacht built of riveted monel plates (an expensive, corrosion resistant, alloy of nickel and copper) which sank at the dock soon after launching. The plates simply began to drop off the bottom of the hull. After an investigation, many of the rivets were found to be made of steel - galvanic corrosion had eaten them up in no time at all.
More recently, I was told of the proud owner of a new aluminum yacht who did a little home wiring project. He grounded a DC appliance to his hull in the same manner in which he was used to grounding automotive wiring to the frame of a car. In a matter of days, stray current corrosion did thousands of dollars of damage to his formerly pristine boat.
Galvanic and stray current corrosion can be a threat to any underwater metals on any boat, so most of what follows is applicable to all boats, but clearly the owners of metal boats have to be especially careful in the ways in which they mix metals and carry out wiring projects. To see why this is so we have to delve into a little electrochemistry.
It is a fact of nature that all metals, when immersed in an electrically conductive fluid (an electrolyte) have a specific electrical potential. This can be measured by placing a very stable reference electrode in the same fluid and connecting a sensitive voltmeter (a millivolt meter) between the metal and the electrode. The voltage that is measured will vary from metal to metal and will also change depending on the nature of the electrolyte; but for a given metal in a given electrolyte, it will always be the same.
One of the reference electrodes used for these measurements is what is known as a silver/silver chloride half cell. Table 1 lists the voltage of common boatbuilding metals, with respect to a silver/silver chloride half cell, when immersed in moving seawater. Static seawater, or freshwater, would give different readings, but the order of the metals in the table would be largely the same.
Voltage is electrical pressure, akin to pressure in a water tank. Where there are voltage differences between different metals, there is the pressure needed for an electrical current to flow, but for this to happen not only must the metals be immersed in the same body of electrolyte, but there must also be a direct electrical connection between them; i.e., a circuit (the equivalent of connecting a pipe between two water tanks at different pressures, at which point, the higher pressure tank will flow into the lower pressure tank until equilibrium is reached).
Without a circuit between different pieces of metal, nothing happens. But any time a circuit is made (through, for example, physical contact or a bonding wire) an electrical current, measurable with a sensitive ammeter, will pass through the circuit from the higher-voltage metal to the lower-voltage metal. This is usually not a problem - electric cables, after all, carry currents for years without suffering damage - except when the circuit is completed by a current flowing through an electrolyte from a low voltage metal to a high-voltage metal. The current flowing in the electrolyte is being generated by an electrochemical reaction that causes the lower-voltage metal to dissolve.
Lost Anodes
The metal from which the current is flowing into the electrolyte, which is known as the base, less noble, or anodic metal, is gradually eaten away, losing all semblance of its original shape and strength - it suffers from galvanic corrosion. The metal to which the current is flowing, which is known as the most noble, or cathodic metal will suffer no damage. (Note that this relationship is relative - a particular metal can be anodic in relation to some metals, and cathodic in relation to others, depending on their positions in the galvanic table).
The rate of current flow, and therefore, the rate at which the anodic metal dissolves, will depend on a number of factors, the most important of which are the following:
1.The voltage difference between the two metals (the greater the difference, the more potential for corrosion). Any time the voltage difference, as show in the accompanying table, exceeds 0.25 volts, corrosion is fairly certain.
2. The relative exposed surface areas of the two metals-the area effect. A large cathodic area connected to a small anodic area, as in the case of the monel plates and steel rivets mentioned above, will soon destroy the anode (the rivets). On the other hand, a large anode in relation to a small cathode will only corrode slowly (ironically, a hull of steel plates fastened with monel rivets would have lasted much longer).
3. A process known as polarization. When two metals form a galvanic cell they are initially quite reactive, but soon some of the by-products of that reaction (various salts and oxides of the metals) insulate the surface of one or other metal, reducing the rate of reaction and thus of corrosion. The effect of polarization varies markedly from one metal to another, and varies according to such factors as whether the water is moving or not (moving water tends to flush the salts and oxides away, keeping the reaction going).
4. The conductivity of the electrolyte. Salt water is relatively conductive, whereas fresh water, unless it contains dissolved salts and minerals is not.
Corrosion in alloys
So far, we have considered two different and dissimilar metal objects. But most metals in marine use are alloys: mixtures of more than one metal. Even "pure" metals contain impurities. Add a little salt water and many can generate internal galvanic corrosion-a notorious example being screws made of brass, an alloy of copper and zinc. In the presence of an electrolyte, these two metals, being widely separated on the galvanic scale, become highly interactive, eating up the zinc in the alloy and leaving a soft, porous, and worthless fastener.
To a certain extent, the prerequisites for the establishment of these internal galvanic cells vary from metal to metal, and alloy to alloy. Steel, for example, requires not only an electrolyte but also oxygen. However since oxygen is readily available in dissolved form in seawater, galvanic corrosion (evidenced by rust) is an ever-present possibility. The waterline area on a steel boat is especially vulnerable, for here the hull is constantly wetted in a highly oxygenated atmosphere.
Untreated steel is particularly susceptible to corrosion. It comes from the foundry with a coating of mill scale. Mill scale has a voltage 0.3 volts above that of regular steel. In the presence of an electrolyte and oxygen, a galvanic cell is established between the underlying plate and the scale, with the plate as the anode pitting the plate until the scale becomes loosened and falls off. Once the mill scale is gone, impurities in the steel itself ensure that the process of galvanic interaction (rusting) continues.
The manner in which aluminum and stainless steel establish internal galvanic cells is somewhat different.
First of all, while there is very little difference in the rate of rusting of one grade of steel as compared to another, there are marked differences in the susceptibility to corrosion of different grades of both aluminum and stainless steel--it is absolutely essential to use marine-grade alloys, fasteners, and hardware in boat construction.
Secondly, in contrast to steel, the presence of oxygen slows the corrosion process, with both aluminum and stainless steel developing an inert oxide film. When oxygen is removed, this oxide skin breaks down. If an electrolyte is added, corrosion is likely to be rampant.
Oxidized stainless steel is called passivated and is one of the more corrosion-resistant metals available for marine use. Without the oxide layer, however, stainless steel becomes active, offering little better corrosion resistance than its principal component, iron. Active areas of stainless steel frequently evidence themselves by rust stains. Since oxygen is readily available in the atmosphere and in seawater, there is little risk of corrosion in most circumstances; but, if stainless steel is used in an area where stagnant water can collect, sooner or later its passivity will break down, and it will become active (normally this occurs in isolated pinholes and crevices). It will begin to “feed” on itself, just as the copper in a brass screw feeds on the zinc. The active area is the anode; the passive area the cathode. As corrosion progresses, the electrolyte becomes more conductive, accelerating the rate of corrosion.
Galvanic protection.
What we have seen so far is that for galvanic corrosion to occur there must be:
At least two dissimilar metals which are
2) both immersed in the same conductive fluid while
being electrically interconnected in some way (through alloying, physical contact, a cable or other conductive path).
If any one of these preconditions is absent, galvanic corrosion cannot occur. That being the case, it is easy to outline a number of prescriptions for avoiding, or at least reducing, problems on metal boats.
First and foremost, so far as is possible, never mix underwater metals. This minimizes differences between the voltages of the various metals in the boat, reducing the potential for corrosion. Whatever grade of steel or aluminum is used for the hull plating should be used in the hull reinforcement, and for deck stringers, pipe stubs, and any other metal fixtures in physical contact with the hull.
Second is to minimize any area effects by ensuring that all metals used for welding and for fasteners are at least as noble as the metals they are fastening. This will create a small cathodic area in relation to a large anodic mass, generating very little corrosion in the anode.
Third is to design and build the hull framing and interior in such a way as to avoid pockets of trapped dirt and moisture. Particular attention has to be paid to the placement of stringers and floors. Ventilation, to eliminate condensation, is especially important with steel; and on all boats there must be the provision of adequate limber holes for drainage. Without moisture, there is no electrolyte, and without an electrolyte, no galvanic corrosion.
In time, of course, salt crystals find their way into cracks and crevices, holding moisture against the hull, while the exterior of the hull will, in any case, be continuously, or intermittently, submerged.
So the fourth prescription is to coat the metal with an impervious skin that keeps the electrolyte from making contact.
This is especially important with steel, but not so much with aluminum, which will develop a relatively inert oxide skin. (A word of caution has to be sounded at this point. Painting a cathodic metal will always help to slow, or stop, galvanic activity; but should an anodic metal be painted, and flaws develop in the paint job - scratches or pin holes - the corrosive effect of any galvanic current will be concentrated on these flaws, resulting in severe localized pitting. It is sometimes better to leave an anodic surface unpainted, allowing it to suffer mild corrosion over its entire surface, rather than to risk this concentrated corrosion.)
So far as paints are concerned, there are numerous modern coatings that create a surface impervious to moisture; but, in all cases, such coatings have to be planned for at the design stage, both to ensure proper surface preparation of the underlying metal, particularly in inaccessible areas on the interior of the hull, and also to eliminate sharp or rough edges that will result in thin coats which are easily breached. Steel can be given a substantial amount of extra protection with a coating of zinc (galvanizing) prior to painting. Since zinc is below steel on the galvanic table, any time the paint layer is breached, allowing moisture access to the metal and so establishing a localized galvanic cell, the zinc will corrode, protecting the steel and, in fact, plating out on the steel to "heal" the scratch.
The final prescription is to electrically isolate differing pieces of metal breaking any potential galvanic circuits between them (but not, alas, within individual pieces of metal). This also can be accomplished in many cases with a properly chosen surface coating. Additional insulating materials can be used to further electrically isolate specific objects. Such insulation may be no more than a gasket of neoprene or PVC, with an insulating sleeve around any bolt and an insulating washer under its nut. Or it may take the form of a rugged non-conductive insert fitted to the hull to which a through-hull is mounted.
Whatever the method, it is an American Boat and Yacht Council (ABYC) requirement that any galvanically-incompatible, below-the-waterline fitting be insulated from a metal hull.
Beyond this, additional cathodic protection can be provided either via a bonding svstem or with an impressed current system.
Bonding.
Bonding is the practice of electrically tying together, and connecting to the boat's ground, all major metal objects on a boat: rigging and champlates, engine and propeller shaft, stove, metal water and fuel tanks, fuel deck-fill fittings, metal cases on electrical equipment, etc. The only major metal objects exempted by the ABYC are electrically-isolated through-hulls.
The purpose of a bonding system is to provide a low-resistance electrical path to ground between otherwise isolated metal objects. Such a circuit provides a fair measure of safety from many potentially dangerous AC leaks, provides a path to ground for the high voltages and currents associated with lightning strikes, minimizes radio frequency interference, and, at the same time, provides a mechanism for protection against galvanic corrosion.
This latter point is a little hard to grasp at first, for when two dissimilar pieces of metal with different voltages, such as a stainless steel propeller shaft and a steel hull, are immersed in seawater, bonding the two will make precisely the circuit needed to promote galvanic corrosion. However, if the bonding system is, in turn, connected to a piece of zinc immersed in seawater, the zinc, being less noble than any boatbuilding metal, will be the object to corrode, providing protection to all the more noble metals on the galvanic table. When the zinc is gone, the next least noble metal on the table that is connected into the bonding circuit will start to corrode (the hull on a metal boat; OUCH!).
This explains the logic behind cathodic protection using sacrificial zinc anodes. The hull (if metal) and all the boat's metal fittings are bonded, and the bonding system is connected to one or more well-placed zincs. The zincs are eaten up, thus protecting the hull and hardware. In technical parlance, the zincs drive the hull and hardware cathodic. As long as the zincs supply enough current, corrosion will be held at bay.
To protect most metals against corrosion, their voltage must be pulled to around 200 millivolts (0.20 volts) below the voltage as given in the accompanying table.
A steel hull, for example, which the table shows as being -0.60 to -0.70 volts relative to the silver/silver chloride half cell, should be pulled down to -0.80 to -0.90 volts (the ABYC recommends - 0.84 volts).
Aluminum is something of a special case. According to the table, an aluminum hull should be pulled down to somewhere between -0.96 and -1.20 volts (depending on the alloy); but, because aluminum is sensitive to overprotection (see below), it should not be pulled below -1.00 volt.
The voltage of a zinc with respect to a silver/ silver chloride half cell is around -1.00 volt. So, if enough zincs are provided, any steel or aluminum hull can be pulled down to this voltage, providing more than enough protection for steel hulls and just about enough for aluminum.
The amount of current required to achieve the 200-millivolt negative shift needed for cathodic protection will depend primarily on the area of metal to be protected and the insulating effect of any paint layer (or oxide film in the case of bare aluminum the greater the exposed area, the higher the current needed for protection. Current requirements also rise in moving water, and at higher temperatures. Since the ability of a zinc to produce the required current is related to its surface area and its proximity to the metal being protected, the higher the current required, the larger the necessary zincs and the closer their spacing. Sizing and placing of zincs is partly a matter of scientific determination, and partly a matter of trial and error; it should be done by a knowledgeable professional.
Impressed currents.
Zinc anodes work by generating a small galvanic current that makes the boat's underwater fixtures cathodic with respect to the zinc.
The same effect can be produced by feeding controlled amounts of current through the water to fittings (and to a metallic hull). This is the basis for impressed-current cathodic protection systems. (CAPAC?)
A reference electrode senses the voltage of the metal to be protected. A control unit then uses the boat's batteries to send an appropriate current to one or more anodes projecting through the hull.
This current flows to the hull and other underwater hardware connected to the system, driving them cathodic (just as with the current generated by a zinc anode) and so prevents corrosion.
The anodes are made of a very noble metal so that they, too, do not corrode.
To provide cathodic protection to a well-painted steel hull, anywhere from 0.1 milliamps to six milliamps of protective current is required for every square foot of submerged hull area. On a 30-foot boat this translates to between 4.5 and 270 amp-hours per week, with an average probably being around 100 amp-hours a week. With a less effective paint job, this figure could easily climb to 250 amphours. Although these figures are at the low voltages needed for cathodic protection, and will therefore translate to far fewer amp-hours at 12-volts, clearly, impressed-current cathodic protection takes a significant amount of battery power and is only suitable for those boats with adequate power sources. (Note that a zinc produces around 335 amp-hours per pound at protection voltages, so the same boat would require 16 lbs. or more of zinc per year. Allowing for the fact that a zinc should be replaced well before it is consumed, and providing a margin for higher than expected usage, 50 to 80 lbs. of zinc should actually be placed on the boat. Due to polarization effects, aluminum boats consume zincs at about half the rate of steel boats).
Some impressed current systems adjust the protective current automatically, but others have to be set manually. Either way, it is important to avoid overprotection. A side effect of any cathodic protection system (using either zincs or impressed current) is that it makes the electrolyte alkaline in the vicinity of the cathode.
Any time the cathode is pulled below -1.00 volt, this alkalinity will reach levels that are corrosive to aluminum, while, at the same time, hydrogen bubbles will begin to evolve at the cathode, and these are liable to lift paint off both aluminum (if painted) and steel hulls.
Since the greatest current density at the cathode will be found in closest proximity to the anode, this is the most likely area for damage. Therefore, anodes on impressed current systems must be mounted in the hull with an insulating shield that extends over the surrounding area of the hull (shield is determined by the maximum current output of the anode).
(Note that alkalinity is also destructive to wood. Zinc anodes mounted on wooden hulls should be insulated from the hull, with the cathodic current held to the lowest level required for protection. To make this possible, a special current controller is often advisable with zinc anode as well as with an impressed current system.)
Stray current corrosion
Galvanic corrosion can set up currents between fittings measured in milliamps and millivolts-a thousandth of an amp or volt, Faulty electrical circuits can establish currents hundreds of times stronger. Such stray currents can originate from within a boat, from shoreside fittings and ship-to-shore cables, or from neighboring boats. In all cases, a leak from a hot wire allows current to find a path to ground through bilge water, damp areas of the boat, and seawater, rather than through proper channels.
Any current that leads to a flow of electrons in and out of fittings in contact with water may corrode the piece of metal feeding the current into the water. Galvanic corrosion is, by its nature, a relatively slow process (the metals themselves have to generate the flow of electrons), but stray-current corrosion, with its potential for greatly accelerated electron flow, can be devastating. In worst-case scenarios, stray currents can wipe out hardware in a matter of hours.
Bonding, zincs, and impressed-current systems provide little protection against stray current corrosion. Given the low-level currents generated by these devices, it doesn't take much of a leakage current to completely overwhelm the protection system. The position of a metal in the galvanic table then becomes irrelevant as to whether or not it will corrode-whatever metal is feeding a current into the water is likely to be destroyed.
DC currents are particularly destructive, whereas there is some debate as to whether AC leaks lead to any corrosion at all. However, in many instances, DC leaks are superimposed on AC circuits, so careful attention should be paid to eliminating stray currents from both sources.
The key factor in eliminating stray current leaks is proper wiring of boats. Since all electrical currents, whether AC or DC, seek the lowest resistance path back to their source, the wiring system has to be so designed that the boat (especially a metal boat), and its underwater fittings, at no time form a path back to a power source.
So far as the DC system is concerned, the power source is the boat's batteries, and the path back to the battery consists of the various ground wires to appliances. These ground wires have to be designed and installed to minimize any resistance in the ground circuit. This comes down to using marine-quality wiring (tinned), properly sized for the job (a maximum 3% voltage drop at full load in most applications), fitted with marine-quality terminals. Beyond this, cables should be run in such a manner as to keep them out of damp areas of the boat and protected at all points where chafe might create a risk of a short to the hull of the boat. Finally, if the hull is properly bonded, the bonding system will provide a low-resistance path back to the battery negative so that in the event of a leak to the hull, the leak will be recovered' and lead directly back to the battery, rather than finding a path to ground through the water. (It is interesting to note that, in the case of the boat mentioned at the beginning of this article, the combination of the use of the hull as a ground path and an inadequately bonded hull, caused the current to be radiated into the water to the propeller and so up the shaft to a grounded engine block. Severe corrosion ensued where the current was fed into the water.)
Insulated sending units
The engine on a metal boat deserves special attention. The sensing units (for water temperature, oil pressure, and so on) on almost all engines are grounded through the engine block. If these are connected to a gauge (as opposed to a warning device or alarm) a small current passes through the sending unit, and, therefore, the engine block, any time the ignition switch is turned on. Similarly, the alternator is almost always grounded to the block, which becomes a full, current-carrying conductor. Should there be a resistive connection between the engine block and the battery negative (not uncommon), this current, or a part of it, may be encouraged to find another path to the battery negative. If this should include the hull, or any underwater fittings, corrosion is likely.
For these reasons, engines on metal boats should have insulated ground sending units; the sending unit should have a separate ground wire and not be grounded through the block. It should also have an insulated ground alternator. The latter, however, are uncommon. But at the very least, the alternator should have a heavy-duty cable (as large as the output cable) wired directly from its case to battery ground to provide a low-resistance ground path that bypasses the engine.
If the engine is also mounted on flexible feet, with a length of flexible hose in all fuel, water and exhaust lines, it will be, to a great extent, electrically isolated from the hull.
Avoiding corrosion with AC wiring is a little more complex than with DC. The potential for corrosion comes from the (green or bare) grounding wire. If installed according to ABYC and other standards, this wire will be tied into the DC grounding and bonding system. This being the case, if two or more boats plug into shorepower, the AC grounding wire makes a direct electrical connection between the bonding systems on both boats, and thus between any underwater metal surfaces on the boats that are wired into the bonding systems. Should one boat be aluminum-hulled, say, and the other boat have substantial areas of underwater bronze that are not protected with zincs, the aluminum hull will become the anode to the cathodic bronze. Or consider another case: an aluminum boat with such an AC circuit plugs into shorepower alongside a steel dock; the boat will become anodic to the entire dock, and corrosion will be rampant.
On metal boats with a shorepower connection, it is absolutely essential to break the galvanic connection between the boat hull and underwater hardware, and the AC grounding wire. For reasons of safety, this cannot be done by simply cutting the grounding wire. The only safe way to break this connection is with a galvanic isolator or an isolation transformer, with the latter being by far the preferable option (see "Problems on the ground," in Issue No.55).
Keeping corrosion at bay
Both our understanding of metal corrosion, and the materials and techniques available to prevent it, or at least to slow it down, have come a long way in recent decades. Today, with proper design, proper surface preparation and coatings, and proper installation of hardware and electrical circuits, there is no reason why a metal boat should not remain virtually corrosion-free for many years, making the benefits of steel and aluminum even more attractive.
However, part of the price of a metal boat is eternal vigilance. The greatest danger of corrosion is likely to come some years down the road when corrosion in electrical circuits begins to cause resistance, fostering stray currents, or when systems begin to fail and be replaced, or the owner decides to make changes to hardware and electrical systems. With any equipment change, some simple installation error, bred of galvanic or stray-current ignorance, can precipitate rapid and expensive damage.
The owner of a metal boat, particularly an older boat, would be well advised to invest a little time in gaining an understanding of how corrosion occurs, and the means of forestalling it, so that the kinds of disasters with which I began this article can be avoided.
Contributing editor Nigel Calder is the author of several books including Boatowner's Mechanical and Electrical Manual, published by International Marine.
Bonding circuit test
The easiest way to see if a bonding circuit is providing galvanic protection is to obtain a millivolt meter and a silver/silver chloride half cell (obtainable from the Professional Mariner company, among others a good investment for those who wish to do some serious corrosion troubleshooting). The shorepower cord should be unplugged and the battery disconnected from the DC circuits. The half cell should be lowered over the side on the end of a long extension cable, one end of which is plugged into a socket on the meter. The other meter probe is touched to the hull and to the various pieces of underwater hardware.
A voltage reading is taken from each piece of metal and noted. The voltages of the hull and all underwater hardware connected into the bonding system should be the same if they are not, there are problems in the wiring or connections. If the bonding system is working, and the zincs are in good condition, the voltage measured at each fitting should be 200 millivolts or more below the voltage for the least-noble metal included in the bonding system (as listed in the accompanying table). If the voltage shift is less than this, the bonding circuit is not protecting this metal properly and it is likely to be corroding.
Next, the battery is reconnected to the DC system and one circuit after another turned on, checking the millivolt reading at each fitting. If, at any time, the voltage reading changes, there is a leak into the bonding circuit from the DC circuit that has just been turned on. This may simply be the result of poor connections or wiring, or may be the result of a partial short somewhere. (To check the wiring, run a jumper wire from the battery to the piece of equipment on the circuit, and check the millivolt reading; repeat this procedure from the ground side of the equipment to the battery. If either test corrects the millivolt reading, the problem lies in the side of the circuit being jumpered.) Finally, the shorepower cord is plugged in and the AC circuits turned on one at a time, checking once again for any change in the voltage readings as each circuit is turned on. Any leaks-either AC or DC-need to be tracked down and cleaned up.
(Many "corrosion meters" are configured to show all millivolt readings as positive values. In this case, an unprotected metal hull will read around +0.65 volts, while a protected hull will show a 200-millivolt positive voltage shift to +0.85 volts. The values listed in the text remain the same, just reverse the sign.)"McKenna Renee"
1972 '32 FB Express. Twin 318 / Edelbrock 1409s
Chesapeake City Md (Upper Bay / C&D Canal)
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