Marine Chemistry in the Coastal Environment

33 Marine Anti-corrosive and Anti-fouling Coatings MONROE M. WILLEY

Downloaded by UNIV OF PITTSBURGH on February 29, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch033

Ε . I. D u Pont Nemours & Co., Marshall R & D Laboratory, Philadelphia, Penn. 19146

One of the oldest and most important resources we obtain from the world's ocean is the low cost, low energy consuming transportation of heavy bulk goods. It's been determined that a ton of coal can often be carried across an ocean at lower cost than across a state. The English term "ton" by which we measure great quantities of nearly everything,originally referred to a maritime container used for the bulk shipment of wine. The ton, or "tunne" was a wooden container of 252 wine gallons. It occupied about 40 cubic feet of hold space, which became the "cargo ton", and weighed about 2240 pounds, or our current "long ton". This, in turn, is the weight of about 35 cubic feet of seawater, or the "displacement ton". The field of marine coatings is probably only a little younger than the art of navigation, having undoubtedly been born of necessity, when the ancient mariners found that the lives of expensive wooden ships were relatively short in the foreign environment of the sea. The Old Testament tells us that Noah was instructed to coat his ark within and without with pitch. One of the purposes of a bituminous coating would probably have been to help protect the wood exposed to air from rot, and the underwater hull from the infestation and subsequent destruction by the teredo or ship worm. The effect of the ship worm can often be seen on driftwood in the form of holes or tunnels. While the teredo is a shellfish rather than an insect, it might be viewed as a sea-going termite, that for centuries has been the major "corrosive" agent on wooden ship bottoms in warm waters. A number of preventive measures have been employed over a long period of time. Outside wooden sheathing, metal sheathing, charring the wood to some depth, and a large variety of coating concoctions have all been employed. The ancients in the Mediterranian area who were skilled in working with various metals met success against the ship worm with the use of lead sheathing nailed to that part of the hull below the water line. This left

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In Marine Chemistry in the Coastal Environment; Church, Thomas M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.


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only the problem of the attachment to the surface of the hull of such organisms as barnacles, algae, and the like, which, while relatively non-destructive, impart considerable "drag", or resistance as the vessel moves through the water. This accumulated growth is referred to as "fouling". An example of fouling show on 6" χ 12" panel that was immersed in Florida waters for several months. The roughness of a accumulation significantly reduce the ship speed, and had to be removed by scraping every few months. The use of lead sheathing was successfully carried on for centuries until 1758, when the British Navy apparently ran short of lead and decided to evaluate copper sheathing on the 32 gun frigate, HMS Alarm. After some 20 months at sea, largely in Carribean waters, the Alarm was dry docked, and found not only free of ship worms, but also free of fouling. Why copper sheathing wasn't tried 2000 years earlier is appar ently a mystery, but a practical solution to the fouling problem was finally discovered. The use of copper sheathing continued until the advent of steel or iron hull in the mid l&OO's. At this time, it was discovered (the hard way) that copper attached to ferrous metals and then immersed in seawater, caused the iron to dissolve in the ensuing electro lysis. H . M . S . Jackal actually foundered as a result of copper sheath ing on the iron hull. It was found that copper could be used over iron if the two were insulated from each other with a material such as an inter vening wooden sheathing. This rather expensive practice was carried out to some extent (1). The advent of iron hul Is brought with it the problem of severe corrosion of these metals in the marine environment. Standard metal protective paint systems such as red lead pigmented linseed oil primers followed by suitably pigmented oil based topcoats were used to retard corrosion, but were less effective than on land based structural steel. Some improvement was later obtained with the use of synthetic resins such as oil modified phenolic varnishes, and alkyd resins, along with pigment ations that were a little more effective in seawater. Even then, such systems offered limited protection, required constant maintenance by the ships crew, and frequent dry docking for replacement of corroded plates and repainting. Those in the audience who may have served in the World War II Navy or Merchant Marine will no doubt remember "chipping and painting" details with little affection. The corrosion problem varies in degree and mechanism in three general parts of the ship. These are the "topsides" or the part totally above the water line which is exposed in the atmosphere with occasion al wettings in seawater. The "boottop" or splash zone, is that section that remains constantly wet with seawater while simultaneously receiv ing the high oxygen levels of the atmosphere , and the bottom, or that part which is constantly immersed in the sea but exposed to relatively



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low oxygen levels. Steel used in ship construction is received from the mill covered with α blue oxide surface layer called "mill scale" that forms during the hot rolling process. Until recent years, the protective coatings were applied over the mill scale and tightly adhering rust after construction. In the marine environment such systems would rapidly begin to fail by "under-rusting", or the formation of rust under the mill scale and continuing formation in the areas where tight rust existed originally. This situation was held under control by constant chipping away of the under-rusted areas and repainting on voyage. Bottoms, of course, had to be frequently repaired and repainted in dry dock at great expense. With the development of the modern ships of much greater size, but crews of about the same size as on the smaller vessels, it became impractical or impossible to continue on-voyage maintenance to the extent previously carried out. More efficient coatings systems offering longer term protection and lower maintenance therefore became a necessity. In the modern day construction mill scale is removed down to bright steel usually prior to fabrication, by the impingment of abrasive grit or steel shot propelled by compressed air, or from centrifugal wheels. This process, known as shot or grit blasting, presents a steel surface free of oxides or other harmful impurities, and imparts a "profile" to the surface that greatly improves the adhesion of the first layer of protective coating. This first coating layer is applied immediately after blasting plates or pre-fabricated sections to prevent the formation of rust, which otherwise would rapidly develop. It is of sufficient thickness and quality to allow the steel to be stored outdoors for several months while awaiting fabrication. It must be compatible with the coating system that later goes on over it, and must not detract from the in-service performance of that system. Pre-construction primers may be of various types, depending on the intended subsequent coating system and area of use; however, the most commom type is probably "inorganic zinc". The latter is a zinc-rich coating consisting of zinc dust particles bound to each other and to the steel substrate by a polysilica glass. These coatings, being anodic to the steel, provide a high degree of corrosion resistance and are extremely resistant to abrasion from rough handling. They are capable of receiving over them additional coats of inorganic zinc, as well as most other types of coatings that are not easily subject to saponification. The most commonly used inorganic zinc is applied by mixing zinc dust into a vehicle of a partially hydrolized alkyl silicate solution, and spraying the resulting mixture onto the steel · The coating then cures by reaction with atmospheric moisture to split off the alkyl group as alcohol, leaving the silica binder.



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Other vehicles such as sodium silicate and lithium silicate have also been used where an aqueous system is desired. The plates and shapes, or pre-fabricated sections coated with a suitable pre-construction primer are given their complete coating system at a further stage of fabrication, or after completion of construction. The area above the water line and the splash zone usually receives a further coating of inorganic zinc over the thin coat of pre-construction primer. Additional protective and decorative organic coatings are then applied over the zinc to provide a long lasting system of good appearance. The organic system over the zinc usually consists of a "tie coat" applied directly to the zinc to provide adhesion over a long period of time, and to build up sufficient thickness to protect against the elements and provide a smooth surface to receive the final topcoats. The tie-coat must be resistant to saponification in the presence of the inorganic zinc, be easily applied in a minimum number of coats, and have the toughness and general resistance properties required by the intended service. Topcoats are applied over the tie coat to provide the desired color and appearance, as well as resistance to sunlight, rain, and seawater. Special "boottop coatings are sometimes used at the waterline, or in the case of variable water line vessels, between the light and deep load line. These coatings are formulated for maximum resistance to seawater, and may sacrifice some resistance to "chalking", or erosion caused by normal atmosphere weathering. The types of organic coatings used on the topside are variable in generic type, and are selected with regard to the type of service, production schedules, climatic conditions in the shipyard, and the shipowner's desires and needs. Included are vinyls (polyvinyl chloride), chlorinated rubber, catalyzed epoxies, epoxy esters, alkyds, and their various modifications, and, to an as yet limited extend, urethanes. The reasons for selecting a particular coating are too numerous to go into at this time. It will therefore suffice to say that all have their own particular advantages, and provide good service when properly applied and used. Moving below the waterline, we encounter another world where corrosion problems are somewhat different, and the function of topcoats is entirely different from those used in the atmosphere. Zinc rich primers other than pre-construction do not seem to be frequently used below the waterline, since it appears to be a common opinion that organic coatings provide at least equally good protection in these surroundings. The anti-corrosive bottom coatings include the same generic types as those used on topsides, but also include bituminous coatings such as coal tar, and tar reinforced with polymeric materials such as epoxy-po lyamide or e poxy-ami ne resins. Metallic aluminum 11



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flake finds considerable use as a pigment in bottom coatings to further improve corrosion resistance. Aluminum is anodic to steel, and the flakes may be made to orient themselves in an overlaping fashion to help reduce water permeability. Polyester coatings filled with glass flakes and applied at very high film thicknesses have also made an appearance as bottom coatings. Organic coatings in general that are applied directly to steel are very often pigmented in such a way to further add to the corrosion resistance of the resin itself. Zinc chromate, zinc oxide, and red lead are examples. Various so called inert pigments, or fillers, will also have an effect on corrosion resistance as well as on film properties. Talc, mica, clay and calcium carbonate are typical extender, or filler, pigments. In addition to providing ordinary corrosion protection, anti-corrosive coatings below the waterline also function as a separator or insulator between the steel hull and copper containing anti-fouling coatings. Being cathodic to steel, the latter coatings have the potential to accelerate corrosion should they come into direct contact with the hull. The use of adequate thicknesses of anti-corrosive coatings on bottoms is therefore doubly necessary. The protection offered by underwater coatings is often supplemented by a system of "cathodic protection", wherein an electrical potential of less than one volt is imparted to the water immediately surrounding the hull, placing the latter in a cathodic state. This is accomplished either by the attachment of sacrificial anodes of a suitable metal, such as high purity zinc, or by directly impressing a current generated by equipment inside the ship. In the latter case the current is introduced to the water by anodes passing through the hull to the sea. Coatings used in conjunction with cathodic protection systems must be resistant to the alkalinity generaged at the steel surface in order to avoid being lost. Most bottom coatings used today are therefore formulated with cathodic protection being a consideration. As previously indicated, anti-fouling coatings are applied over the anti-corrosive coatings to prevent the attachment of various marine organisms that reduce speed, increase fuel consumption, and in some cases, contribute to corrosion of the steel plating. The anti-fouling coatings currently in use work by the same principle as copper sheathing; that is, they slowly leach into the water a material toxic to the attaching organisms. Copper sheathing does this simply by slowly corroding and dissolving into the seawater. Anti-fouling paints have the toxin or toxin producing material dispersed through the film from which it is slowly released. Anti-fouling paints may be classified in two general categories. In the first type, the binder, or matrix, is slightly soluble in seawater. It is formulated in such a way that as the toxin at the surface of the film is depleted, the surrounding matrix



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dissolves at a similar rate, and exposes a fresh toxic surface. In an ideal situation, this process continues until the paint film is completely gone. This type of coating is known as a "soft" or soluble matrix antifouling coating, and probably finds it greater use on small pleasure craft. The other general category is the "hard" or insoluble matrix type where the binder is insoluble in seawater, but the toxin is present in sufficient concentration to allow it to be slowly dissolved out of the film leaving a porous, sponge-Iike "skeleton" (1). There are also intermediate levels where the soluble portion oflhe film consists of both toxin and a soluble resin. The most common resin used as the soluble binder is rosin and certain of its derivitives. Other materials such as shellac may be used, but rosin works as well, and is low in cost. Other non-soluble resins may be blended with rosin to help control its rate dissolution, or to form a "hard" coating. Polyvinyl chloride and chlorinated rubber are examples of insoluble resins that may be used. The most common toxin is probably cuprous oxide. Cuprous oxide, like metallic copper, is slowly dissolved by seawater to provide a surface sufficiently toxic to repel most attaching organisms. Other toxins may also be employed either alone or in conjunction with cuprous oxide. Examples are tributyl tin oxide, tributyl tin fluoride and cupric hydroxide. Organic compounds have also found some use as marine anti-foulants. Combination of biocides are often employed in order to be effective against a wider range of organisms. Toxic anti-fouling coatings in their present state have certain obvious disadvantages. They have the potential to accumulate in confined waters, such as enclosed harbor areas having little or no current flow, thereby having a possible ill effect on various marine flora and fauna in that location. Secondly, they have by their very nature a limited life span, since their effectiveness depends upon the actuaI removaI of the critical ingredient. Traditionally, we have attempted to obtain maximum effective life of the coating by formulating the matrix in such a way that the toxin is released no faster than is necessary to prevent fouling. In actual practice, a well formulated coating is effective for perhaps 18 months. In recent years some interesting efforts have been made to extend the life of anti-fouling coatings. One of these is a thin hydrophylic acrylic coating that is applied over the conventional anti-fouling coating that allows the toxin to leach out sufficiently while the ship is at anchor or tied to a dock, but is claimed to prevent the rapid and wasteful extraction that normally occurs while the ship is underway. Since fouling attaches itself only when the vessel is standing still, or nearly so, relative to the surrounding water, such a coating would help save the toxin for the time when it's needed (2).



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With many conventional A . F . coatings, the leaching rate of the toxin in the early months of exposure is well above the critical level necessary to prevent fouling. This is done in order to keep the toxin at or above the critical level needed to be effective during the later months. As a result excessive biocide is introduced into the water during the early period, which is both wasteful, and may generate additional pollution to enclosed waters. The U.S. Naval Ship Research and Development Center has announced the development of a series of organometallic polymers consisting of a suitable backbone polymer, such as vinyl, acrylic, etc. chemically combined with an organotin moiety of the following general structure:

Sn R, Ί

R 2

K

rt

Rg

Where the R's may be propyl, butyl, phenyl, or a combination thereof. This in effect makes the polymer toxic in itself, as opposed to the conventional system where the biocide is dispersed through a matrix. It is claimed that these polymers can be formulated to release organotin toxin through hydrolysis at a uniform rate during exposure at or just above the critical level needed to prevent fouling. This, of course, would decrease pollution by avoiding the release of excess toxin, and lengthen the effective life of the coating (3). A development not in the coatings field, but worthy of mention, is the use of hulls made of copper-nickel alloys that in themselves resist accumulation of fouling. Four shrimp trawlers were recently built with copper-nickel hulls for use in the Indian Ocean. The boats are expect ed to provide economic advantages in this part of the world, where fouling is heavy, and dry docking facilities are limited (4). The ideal coating would be one that is not toxic at a l l , but prevents the attachment of organisms because of its "non-stick", or release properties. Du Pont's Teflon® flourocarbon resins were one of the first candidates to be evaluated in this area. Unfortunately, barnacles and the various other marine organisms found the Teflon® surface to be a firm anchorage. More recently it has been reported by Bate I le Memorial. Institute that silicone rubber resists the attachment of fouling purely by virtue of its release properties, without the presence of any toxic materials whatever. While anti-fouling release coatings are not yet commercial, such results would indicate that a totally non toxic, long lasting anti-fouling coating is possible (5).


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Literature Cited (1) "Marine Fouling and its Prevention", Woods Hole Oceanographic Institution, Woods Hole Mass., Published by U.S. Naval Institute, Annapolis, Md. (2) Van Londen, A.M., "A Hydrophilic Bottom System to Improve a Ship's Performance", Presented at the 14th Annual Marine Coatings Conference, Williamsburg, Va., (March 1974). (3) Dykman, E.J. and Monlemarano, J.A., "Performance of Organometallic Polymers as Anti-Fouling Materials", Journal of Paint Technology, (January 1975). (4) Marine Engineering/Log, "Inland and Offshore", (February 1975). (5) Mueller, W.J. And Nowacki, L.J., U. S. Patent No. 3,702,778, "Ship's Hull Coated with Anti-Fouling Silicone Rubber".


Marine Chemistry in the Coastal Environment

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