Corrosion affects virtually everything in the marine environment. Steady exposure to salt-laden air and salt water causes many materials to suffer damage and decay. The use of glass-reinforced plastic instead of wood in the construction of hulls and decks, was supposed to eliminate decay processes in those areas. Stainless steel alloys promised to eliminate corrosion problems in standing rigging. Substituting aluminum masts for sitka spruce suggested an end to worries about dry rot at the base of the mast. Aluminum, stainless steel, or rotocast polyethylene tanks were supposed to be leak-proof for the life of the boat. New fabrics, such as the acrylics used for sail covers, hatch dodgers and bimini tops would have very long, if not indefinite lives. Various other "lifetime" products have entered the market, usually demonstrating long, but not quite, lifetime service. In all cases, some form of decay, deterioration, or corrosion has overtaken the new material. Perhaps the worst form of decay in a marine environment is metal corrosion. Since metal parts play such a critical role in the safety and proper operation of vessels, this corrosion deserves careful analysis. By definition, corrosion is a destructive attack on metals. This attack may be either chemical or electrochemical in nature and generally results in the wasting of material and, in many cases, the conversion of one material into a less desirable form. There are other decay processes that can affect a boat, such as: dry rot, mildew, plastic deterioration, gel coat deterioration and the ultraviolet deterioration. Chemical corrosion Chemical corrosion in engines and their cooling systems is most often the result of contamination of the lubricating oil, or the fluid in the closed loop of the cooling system. In addition to producing beneficial energy, engines also manufacture sulfuric acid. This is formed in the crankcase oil of most internal combustion engines when water vapor, a by-product of the combustion process, combines with sulfur, which is a constituent of the fuel, particularly diesel fuels. Over time, a considerable concentration of acid can build up in the lubricating oil. High temperatures increase the incidence and rate of this corrosion. Acid can attack critical bearing surfaces, resulting in greatly increased wear and possibly premature failure. Modern lubricating oils are formulated with acid neutralizers, but, eventually, the neutralizers are depleted. Because of this, frequent oil changes are an important a part of preventative maintenance. Another major factor in electrochemical corrosion is the nature of the electrolyte. Where the electrolyte is sea water, there is no practical way to control its characteristics. Where the electrolyte is the coolant in the freshwater loop of the engine, some control is possible. In general, maintaining the coolant in a basic condition, with a pH above 7.0, will be beneficial. The higher the pH, the greater the excess hydrogen ions, which inhibit electrochemical corrosion. The presence of oxygen, however, works to increase the rate of corrosion, by destroying the protective atomic hydrogen film. A sealed, rather than vented, cooling system, may reduce corrosion by reducing the amount of dissolved oxygen in the fluid. (In industry, specific steps are taken to remove air from cooling water.) Corrosion inhibitors are necessary for the satisfactory life of the components of the cooling system and most anti-freeze solutions contain these inhibitors. Specialized inhibitor packages are available from the manufacturers and distributors of diesel engines. Most of the chemical corrosion problems in engines can be controlled with appropriate lubricants and cooling system additives. Lead-acid storage batteries experience chemical corrosion, both at the exterior terminals and internally. Protection of battery terminals can be achieved with special coatings, in combination with terminal pads that are saturated with neutralizing chemicals. Since chemical corrosion is a fact of life, continuous surveillance and use of preventive techniques are the only practical answers. Electrochemical corrosion Virtually everyone who has operated a vessel in salt water is already familiar with electrochemical corrosion. A common example is the disappearance of the protective zinc anode on the prop shaft, or other underwater metal parts. The ingredients required for this type of corrosion are electrically connected dissimilar metals that are in a conductive solution, or electrolyte. Sea water, it turns out, is an excellent electrolyte. In this type of reaction, the metal that acts as the anode dissolves in the electrolyte. The material that is dissolving does not depart in metallic bits, instead, it leaves in the form of positively-charged ions. This process causes the other metal in the reaction, the cathode, the part we wish to protect from corrosion, to become coated with atomic hydrogen ions. The flowof electrical current from the anode to the cathode neutralizes the build up of positive ions at the cathode, completing the protective process. Although, under some conditions, the entire corrosion process may cease, or greatly diminish, the continued disappearance of a submerged zinc generally proves that the corrosion is continuing. An electrochemical corrosion process can exist in a single piece of metal. The metal need not be immersed in water—a light film of dew, a stray drop, or even high humidity may be sufficient to cause the process. This type of corrosion is possible when a metal surface is heterogeneous and contains minute areas which may be either anodic or cathodic. These areas may be as small as a single grain in the structure of the metal. Surface roughness, mechanical stress, and inclusions or depositions of foreign materials can accelerate the tendency of metals to corrode. When this corrosion process occurs, the anodic areas are eaten away, further roughening the surface, leading to even more corrosion. This corrosion mechanism attacks many fittings on vessels, including those responsible for the security of the rig and highly stressed fasteners on engines and drive systems. Since this corrosion mechanism is aided by surface roughness, it provides justification for the effort and cost invested in achieving highly polished surfaces on metals. Surfaces polished to a mirror finish are the most resistant to this form of corrosion. Since corrosion is accelerated by stress within the metal part, prevention of stress concentration is worthwhile when designing parts. Where possible, sharp internal corners should be avoided, smooth fillets are preferred. Sharp transitions can act like the perforated lines used on paper to facilitate tearing. In extreme cases, sharp corners and transitions can cause sufficient stress concentrations to cause part failure, even without the added deterioration caused by corrosion. When metals are welded, an increased tendency for corrosion is common. For this reason, welded fittings, especially those which must operate at high alternating stress levels and are exposed to salt air conditions, must be inspected frequently. Welded shroud fittings on sailboat masts are a typical example of susceptible fabrications. Where possible, the use of welded fabrication for such components should be avoided. When welding must be used, the weld should be carefully ground and smoothed. Predicting corrosion The tendency of a metal to corrode can be measured and metals can be ranked based on their corrosion potential. This is called the electromotive force, or galvanic series, of a metal. When any pair of metals in the list is placed in contact in water, the one which is higher on the list will become anodic, suffer corrosion, and by so doing, protect the other metal from corrosion. From inspection of the table, a magnesium anode will corrode before any other metal, and in so doing, protect all other metals. The position of gold in the table explains one of the reasons why this metal, even in an extremely thin layer, is so valuable when used as an electrical contact. Everything else, except platinum, will corrode before the gold can corrode. Zinc is a preferred sacrificial anode due to its relatively low cost, its mechanical characteristics and its attractive position in the galvanic series. A number of techniques are available to inhibit corrosion. The familiar sacrificial anode, typically of zinc, works well. An active suppression system, which uses a counter voltage, is also possible. These cathodic protection systems can be made fully automatic, thereby dealing with varying corrosion environments. In some locations, vessels with metal hulls must rely on active suppression systems. Passivation, a method of treating the metal to reduce its solubility in acid solutions can sometimes be used. Barrier coatings, which prevent the metal from coming in contact with the electrolyte solve many problems. Unfortunately, barrier coatings work only when they are total and unbroken, a difficult condition to maintain on voyaging boat. Electrolysis is another cause of corrosion. In this situation, an electrical current, from some outside source, flows between two metals. This current can greatly increase the rate of corrosion. The most common occurrence of this problem is at dockside, where the vessel is connected to a shore power system. Corrosion caused by electrolysis can be greatly reduced by elimination of the current flow, or application of a counter current that neutralizes the stray current. The rate of corrosion of metals can be affected by the manner in which the material was formed. When metal is cold worked, as is common in the attachment of terminals to wire rope for rigging, stresses can build up which increase corrosion rates. If the part is then used in an application where it is subject to cyclical mechanical stress, the combination of effects of the forming process and the applied stress can accelerate corrosion. The number of stress cycles the part can safely withstand can be greatly reduced by the effect of stress corrosion. An interesting example of the effect of stress on the ability of a part to carry a load can be seen in non-ferrous bolts. When such fasteners have been very tightly drawn-up, they will occasionally snap in two, as a result of the stress corrosion effect. Corrosion and various metals Stainless steel is one of the most commonly used maritime metals. There are three main types of stainless steels: austentic, martensitic, and ferritic. The austentic steels contain both nickel and chromium and cannot be hardened, except by cold working. The American Iron and Steel Institute (AISI) type numbers for these steels are in the 201 to 348 group. Two of the most common alloys used in marine applications are types 302 and 316. The nominal composition of type 316 stainless steel includes: carbon, 0.08% maximum; manganese, 2% maximum; silicon, 1% maximum; chromium, 16 to 18%; nickel, 10 to 14%; and molybdenum, 2 to 3%. Austentic steels contain up to 26% chromium. The martensitic steels contain up to 18% chromium, little if any nickel, are hardenable, magnetic and are widely used for springs, cutlery and medical instruments. The balls and seats of stainless steel ball valves used on vessels are usually fabricated of metals of this type. Their AISI type numbers range from 403 through 502. Ferritic alloys can contain up to 27% chromium. They offer good resistance to atmospheric corrosion. These steels, with AISI type numbers from 405 to 446 are the types used to fabricate items such as stainless steel sinks, tanks and the like. The corrosion resistance of stainless steel depends on the presence of relatively large amounts of chromium in the alloy. In the 300 series austentic steels, the amount of chromium ranges from 16 to 26%. Under normal conditions of use, the oxide which forms on the chromium is very stable, thereby protecting the stainless alloy from corrosion. Corrosion resistance can fail when stainless steels are submerged in salt water. Under these conditions, it is possible for the oxide layer on the chromium to become depleted, exposing the material to corrosion. Another cause of corrosion in stainless steels is welding, which can create minute discontinuities in the make-up of the alloy. These discontinuities can become sites for corrosion. Signs of this condition can occasionally be seen in stainless sinks, where parts are welded together. Leaks in tanks fabricated of stainless steel are often caused by corrosion at welds. Crevices in fabricated parts offer similar opportunities for corrosion. When inspecting stainless components for corrosion, it is important to check crevices and sharp interior angles with great care. On occasion, stainless steel parts may show surface rust. This condition is frequently the result of the finishing operation. If finishing included use of ferrous wire wheels, bits of the wire may become embedded in the surface of the stainless steel. These microscopic bits will rust, spoiling the appearance of the stainless. Attempting to remove these surface inclusions by further polishing is often not effective. It is worth remembering that stainless steels are stainless, not stain proof.Monel Monel is an alloy of nickel. The constituents include: 66% nickel, 31.5% copper, 1.33% iron, 0.9% manganese, 0.15% silicon, and 0.12% chromium. This alloy is strong, ductile and has excellent corrosion resistance. Monel is occasionally used for tank fabrication. The safety wire used to insure the security of critical components, such as fuel injection pump locking screws, is often made of monel metal. A related alloy, inconel, is occasionally used for the dry portion of exhaust systems. Aluminum Aluminum is very widely used in marine applications. It owes its corrosion resistance to the rapid formation of a firmly adherent self-healing oxide film. As long as this film is intact, the metal beneath will resist corrosion. Conditions which remove the oxide film, such as the continuous presence of moisture, justify the addition of an additional protective coating, such as paint or anodizing. The anodizing process produces an extremely tough coating of aluminum oxide. Anodizing is normally done electrically, however, similar, but thinner oxide films can be formed by chemical methods. In cases where severe corrosion problems exist, zinc chromate paint may be used, with an aluminum paint applied as the finish coat. Any place where aluminum comes in contact with other metals, particularly in the presence of water, deserves special attention to prevent corrosion. An example is the joint between an aluminum mast and a steel mast step. The inclusion of an insulating barrier is good practice, but remember to provide a good lightning ground. When steel hulls are combined with aluminum superstructure, it is now common to use a specially fabricated joining strip between the two metals. The steel hull is welded to one side of the joining strip, the aluminum to the other. When applying an anti-fouling coating to an aluminum hull, paints that contain copper must be avoided. The copper in the paint can set up an efficient galvanic cell with the aluminum hull material. Since the aluminum is more anodic than copper, the aluminum will corrode. Special, non-copper-based anti-fouling paints are available for use on aluminum hulls. The aluminum oxide that protects the metal is an excellent electrical insulator. It is used for this purpose in many semiconductors. In cases where an electrical connection must be made to an anodized aluminum surface, energetic abrasion of the surface, followed by immediate assembly of the connection is strongly recommended. Titanium Titanium is not extensively used on recreational vessels. Some sailboat mast fittings have been made of the material, which combines great strength with light weight. High-performance (and high price) sailboat winches have been built with titanium winch drums. High-performance blocks are made with this material to save weight. Titanium is quite corrosion resistant. No special coatings are required to insure adequate service life. Zinc Zinc is resistant to atmospheric corrosion, but is susceptible to attack by both acids and alkalis. Although widely used for bright plated components on small craft, zinc's primary use on larger vessels is as sacrificial anodes and as galvanizing, a plating on steel. Galvanizing can be accomplished by a number of methods: Sherardizing, hot process, electrolytic or cold process, and metal spraying. The Sherardizing process provides a highly corrosion resistant coating and is often used to galvanize anchor chain. Sherardizing is accomplished by tumbling the article to be galvanized, at a temperature of 500° to 700° F, in a drum containing zinc dust.Copper, brass and bronze The most common use of copper on board vessels is for electrical wiring and components within electrical devices, such as circuit breakers and the like. Although copper is resistant to corrosion, it is attacked by a moist atmosphere. The resulting green coating, verdigris, is mechanically stable and will generally protect the underlying metal. This layer is an excellent insulator. With copper wire, however, the green oxide layer makes electrical connections difficult. For marine use, stranded copper wire is tin coated to inhibit this corrosion process. Brass is an alloy of copper and zinc. Many brasses have only limited uses in marine applications, due to the possibility of selective corrosion, or dezincification. There have been instances where brass valve bodies have been exposed to salt water and experienced rapid and catastrophic failure. A specific brass alloy, Naval brass, is used for marine shafting. Admiralty brass contains minute amounts of arsenic, antimony or phosphorous, which retards dezincification. An alloy called aluminum brass is preferred for severe sea water service and may be found in engine room plumbing. Bronze is a copper alloy, containing 4.5 to 10 percent tin. Bronzes are very tolerant of the salt water environment. They are easily cast, machined, welded and brazed, making them a good choice for valve bodies, strainer housings and structural elements such as rigging screw bodies. Bronze can become covered with a coating of verdigris. Some bronze parts are chrome plated for appearance reasons. Failure of the plating will not normally effect the serviceability of the part. Iron and steel Iron and steel have been used in ship construction for more than 200 years. During this entire period, someone has had to chip away rust and then reapplying protective coatings, usually a form of paint. However, there is a great deal about steel that can recommend its use. Steel's strength to weight ratio and toughness are very attractive considering the low cost. Steel is also easy to fabricate. And modern paint-like coatings are very effective in protecting exposed steel surfaces. Special slushing oils can be used to coat or fog otherwise inaccessible interior surfaces. Cast iron, such as that used in keels, is relatively resistant to corrosion. In this case, the cast iron will rapidly develop a coating of scale, or rust, which will then inhibit further corrosion. In underwater marine applications it is likely that the iron parts will have to be coated with some type of anti-fouling material. Cast iron, frequently found in equipment such as pumps, generally is thick enough so that a small amount of obvious corrosion, rust and scale will not compromise the safety or function of the equipment. The durability of cast iron is often demonstrated when an old water pipe is unearthed. Some of these pipes have been in continuous use, in quite corrosive soils, for more than 100 years. However, exposing cast iron to heat, salt water and an acidic environment can cause fairly rapid deterioration. The mortality of exhaust risers and manifolds on marine gasoline engines illustrates this well. There is a wide range of corrosion that is possible in the marine environment. Failure to inspect, clean, recoat, or replace important parts prone to corrosion can only cause problems for the mariner. And undoubtedly, these problems will occur at the worst possible time. Contributing editor Chuck Husick is a mariner and aviator based on the west coast of Florida.
