Paper on Hull Protection of a Ship using non-polluting Paints
Hull protection of a ship is one of the major challenges in maritime sector. Sticking of marine species (Bio-fouling) on the hull reduces the speed and also lead to Bio-corrosion. Anti-fouling paints containing toxic chemicals are used to kill the fouling species. But it affect other marine species.This paper gives some modern methods used to prevent bio-fouling
Hull Protection using non-polluting coatings
Abstract:
Corrosion control in marine engineering is basically not just question of choosing corrosion resistant materials on very durable coatings; it is a matter of fitting together all different elements together to reduce corrosion to an acceptable and economic level. Moreover the selection of material and coating has to meet the requirement of both performance and cost.
Protection of the hull of a ship is one the most challenging factor. Ships' hull spends a large proportion of the time submerged in water and as a result they become prone to colonization by marine micro-organisms. Boot topping is subjected to both immersion and weathering and it is the area subjected to severe corrosion. Coating system is the most extensively used process for the prevention of corrosion and bio fouling. A marine coating is a coating that goes beyond an ordinary paint in adhesion, toughness, chemical resistance and resistance to weather, humidity and water. The composition changes according to their application. Coating system prevents current flow by excluding the electrolyte and oxygen and providing no holes or gap in the coating, the corrosion process is suppressed.
But most of the coatings used are toxic to the marine environment. IMO has already banned the use of TBT (Tributylene tin) based coatings to protect endangered marine species. So in the present scenario it is important to protect the hull using non polluting, nontoxic, zero discharge coating that protects ship hulls from marine fouling organisms and corrosion. This paper will discuss the recent developments in this field and a comparative study of their performance in marine environment which are eco- friendly and cost effectiveIntroduction
Ship's hull spends a large proportion of the time submerged in water and exposed to harsh marine environment while it has the combination of highly aggressive chloride environment and high humidity.
Corrosion is a term used to describe the chemical process in which a metal reacts with environment. A metal which corrodes rapidly is of little practical value as a structural material unless measures are taken to reduce the rate of corrosion. This is true for mild steel in a marine environment, but fortunately effective prevention measures due available. Marine related corrosion can be divided into three broad categories;
1) Corrosion in sea water
2) Corrosion in high humidity high sea coastal environments and
3) Microbiologically induced corrosion.
Mild steel is used for the construction of the hull of a ship. These mild steels are not noted for their resistance to marine corrosion, but they are outstandingly important to an economic and strength grounds, availability and fabricability. The hull of a ship is mainly exposed to electrochemical corrosion and bio corrosion.
The methods used for the prevention of electrochemical corrosion are eco friendly and non polluting to marine environment. But bio corrosion arises due to the accumulation of bio-fouling organisms on the hull of the ship. Anti-fouling paints are used to control their accumulation, but they contain chemical substances which are toxic to those species which accumulate on the hull. These chemical substances remain toxic in sea water for a long time and kills non targeted species.Electrochemical nature of corrosion
A typical corrosion cell is illustrated in the figure. This shows the essential features of two electrodes (cathode and anode) which corrodes when current when currents is drawn from the cell in an electrolytic solution, a liquid which conducts electricity by movement of ions.
When corrosion occurs in structures exposed to marine environment, it is usually associated with three main sources.
1) Bimetallic contact
2) Break down of surface films
3) Crevice corrosion
When two different metals come in contact, the metal the more noble metal will act as cathode and the other as anode and the corrosion of anode will takes place.
Attack in the form of pitting may occur on protected surfaces where local breakdown of painting or coating has occurred. When the film is damaged a corrosion cell is established between the exposed cell metal, as a anode and the scale, which readily conducts electrons, form the cathode. It is a highly accelerated localised corrosion.
Crevice corrosion results from two areas of the same metal being exposed to an electrolyte under conditions where different oxygen concentration prevails. The area which has access to a reduced level of oxygen becomes anode and may result in severe corrosion in that position.
Prevention of electrochemical corrosion
Electro chemical corrosion can be prevented by proper selection of material, proper design, appropriate coating and by cathodic protection. Most of the methods employed for the prevention are eco-friendly.Painting
Painting is the most extensively used process for the prevention of corrosion. Paint compositions cover a multitude of proprietary products but the basic mechanisms of corrosion protection are as follows:
1) A barrier coating which prevents ingress of water and oxygen. To be effective the painting must be of adequate thickness. To provide on impervious layer above the prepared surface, free from flaws and firmly adherent throughout its life.
2) An inhibitor carrier in which appropriate inhibitor carriers such as zinc chromate are incorporated in the paint.
3) If cathodic protection provided by incorporating the high concentration of metal powders such as zinc in the paint composition. This type of coating can be effective in preventing corrosion of tops and scratches in the paint film. Hence many paint systems provide protection by a combination of these mechanisms. For painting to be cost effective good surface preparation is essential.Zinc based paints
The durability of a zinc coating is in broad terms, more or less proportional to its thickness, regardless of the method of the method of application, the specific process selected is however frequently dictated by the size of the structure, its configuration and the degree of corrosion and abrasion to be encountered during the service.
Zinc seals the underlying metal from the contact with corrosive environment and is self repainting. The anodic action of the zinc continues until the film is converted into a dense, impervious barrier, resistant to weather, water and fume attack. However if coating is damaged, fresh zinc is readily available to provide further anodic action.
Bio fouling and Bio corrosion
Bio-corrosion, Microbial corrosion or micro biologically influenced corrosion may be defined as an electrochemical process where the participation of the micro organism is able to initiate, facilitate or accelerate corrosion reaction without changing its electrochemical nature. Apart from metal and solution, micro-organisms are required for bio corrosion. So bio fouling on the hull of the ship is the main reason for bio corrosion.
Most cases bio fouling and bio corrosion is due to the discrete deposits of fouling organisms like bacteria, macro-algae, mussels, barnacles, seaweeds and invertebrates on the metal surface. This facilitates the the initiation of corrosion by altering the oxygen concentration at the metal/solution interface.
Localised corrosion is caused by the deposition of fouling organisms. The area with the lowest oxygen availability (under the deposit) is forced to become the anode in the reaction, while the area outside acts as the cathode. The reaction depends on the electrolytic continuity between the anode and the cathode. If there is no electrolytic under the deposition then a crevice effect may be caused, resulting crevice ring of corrosion around the edge of the deposit.
Other effects of bio fouling
As ship's hull become fouled with biological matter, the resulting surface friction also causes a significant increase in the power required to maintain a desired speed the additional power output results in fuel consumption, which adds cost. As power output increases, the air pollution emissions from a shipboard propulsion system also increase. Components of this air pollution include nitrogen oxides, sulphur oxides, particulate matter, and green house gases. Therefore, using anti-fouling coating systems can:-
Increase fuel efficiency
Prevent corrosion
Decrease air pollution emission
Increase operating speeds
Minimize the spread of aquatic invasive species
Prevention of bio corrosion
The golden rule to apply for preventing and controlling bio corrosion and bio fouling in industrial system is to keep the system clean. To control bio corrosion, one has to prevent bio-fouling, that means to prevent the settlement of bio fouling organisms on the hull of the ship. This is normally done by using anti-fouling paints, which are the final coating coated on ships' bottom.
Antifouling paints
Antifouling paints release chemicals into the sea which are poisonous to bio-fouling organisms and prevent their attachment to the surface. The main components of these paints are biocides and an acidic binder. In this type the particles of poison are distributed throughout the film of binder. When this binder comes in contact with sea water which is alkaline in nature, hydrolysis reaction takes place and they release the biocide. Another type used is by leaching process. In this type the poison content is made high enough to ensure that particles of poison are in contact throughout the film. Leaching of poison from particles in the surface of the film is then followed by leaching from the particles deeper in the film as channels are formed from particle to particle. This type of binder need not to be soluble in seawater because the particles of poison can dissolve leaving a honeycomb of insoluble binder-Copper based AF Paints
The best known prior art antifouling paints were the old stand-by cuprous oxide paints which operate on the principle of leaching out, at a controlled rate, a toxic solution to kill or discourage sea life from attaching to the ships' bottom. The cuprous oxide paints leached at a high rate in order to perform their function, and therefore had to be mechanically removed and renewed at frequent intervals. For example, one could expect an effective life of only six to 18 months, thus, one had to accept the low efficiency of a "dirty bottom" or the "down time" of dry docking.-Tin based AF Paints
To overcome these difficulties, and to achieve the desired objectives discussed above, the old cuprous oxide AF paints are being rapidly replaced by new, improved AF paints and coatings containing organo-metallic compounds called TBT-tributyl tin. The advantage of these new organo-metallic AF paints over the previous cuprous oxide type AF paints is that they are far more toxic to sea life and can be designed with very low leach rates to perform their AF function. Their antifouling life may thus be prolonged to a projected five year period. However, ships' hulls bearing these organo-metallic AF coatings do eventually require abrasive blasting to facilitate repainting. Since these organo-metallic compounds, particularly the commonly used organo-tins, are not biodegradable, remain toxic in the sea water for long periods (are approximately 20 times more toxic than cuprous oxide) and they kills non targeted species.Negative environmental effects of TBT
In the 1980s it became apparent that the use of TBT was causing severe damage to non-target species in the wider marine environment, such as deformities in shellfish and mollusca communities, reduced growth of algae and toxic effects in young fish. The effects of TBT were particularly noticeable on dog whelk populations near harbours and marinas where female dog whelks developed into males
The majority of anti-fouling coatings also contain solvents which are harmful by inhalation and by skin or eye contact. They can have a narcotic effect resulting in the following symptoms: headache, dizziness, irritability and mental confusion. These reasons brought out the question of using TBT based AF paints further,
However, in November 1998 the IMO made the decision to introduce a world-wide ban in the use of TBT in antifouling paints for most ships from January 2003. In 1999 the International Maritime Organization (IMO), the United Nations Agency concerned with the prevention of marine pollution, agreed a resolution which calls for:
(1) A global prohibition on the application of organo-tin compounds acting as biocides, in anti-fouling coatings on ships by 1 January 2003; and
(2) A complete prohibition on the presence of such compounds, in anti-fouling coatings on ships by 1 January 2008.
The ban of TBT based AF paints by IMO made the ship industry to think of cost effective non polluting paints to fight against bio-fouling and bio-corrosion.Non polluting paints
At present, the principal substitutes for TBT are copper based system which was used earlier. Copper is far from a perfect solution because it is also associated with negative environment effects, though not believed to be as serious as those of TBT.
Although there are some less toxic alternative biocides under consideration, some of the most promising alternatives may be those that approach the problem by inhibiting adherence of the species to the hull rather than killing the species directly.
A limited range of anti-fouling methods are available which function by physical means rather than by using biocides. However, such methods tend to be more expensive than traditional, biocide anti-fouling coatings and their practical application is more limited. Some of these products are also at an early stage of development. Nonetheless, if such products are chosen carefully they can prove to be useful alternatives.Alternatives used post IMO antifouling treaty
1.CDP- controlled depletion layer
The main ingredient is natural gum rosin. This one is less toxic to marine environment comparing with TBT. They are extremely sensitive to water and release the biocides via diffusion process. It had a exponential leaching rate in which large amount of biocides are released in the early stage making less release afterwards which makes this method less effective. Impurities were contained in the raw material which build up on the surface and a thick leached layer is formed which makes difficult for biocides to leach. They have a life of about 36 months and their cost is 50% above TBT.
2.SPC- Self Polishing Copolymer
In this method the release of biocides takes place by hydrolysis. Co-resin (part) is basically hydrophobic and not hydrophilic. Therefore the reaction takes place only near the surface. This made it to have a controlled biocide release. They have an extended service life of 60 months, but their cost is 200% more than TBT.
3.Hybrid Technology
This method as name suggests is the combination SPC technology and CDP technology. CDP feature of surface tolerance along with the SPC features of controlled biocide release and reduced leached layer is used in this method. They have an expected life time of 60 months.
4.Foul Release System
This system has a non reactive low energy surface system (silicon, fluro polymer) which does not allow the adhesion of the organism on their surface, they have a slippery effect. They are strongly hydrophobic, which means there is no water absorption and no active zone. They are self polishing when the vessel is moving at a specific speed. Their predictable life is about 10 years, but their cost is 400-600% above TBT.ALTERNATIVE METHODS
Using silicon epoxy in antifouling paints
This anti-fouling marine coatings and boat bottom paints are tough and durable. The unique silicone-epoxy chemistry endows the product with extremely good adhesion and tensile strength and adheres readily to most barrier and primer coats. Its elastic modulus enables it to deform and resist impact damage. Inherent lubricant improves fuel economy, reduces engine strain and increases speed.
The unique release properties result in low bio-fouling adhesion from Zebra mussels, barnacles, tube worms and all bacterial and algal bio films. Ecological Coating's Marine Coatings create self cleaning surfaces if the craft is used regularly at reasonable speed.
Manufactured in formulations for fresh and salt water and specific applications including friction reduction (speed) and foul-release
Using Natural fungus in antifouling Paints
Now a new type of paint has been developed which uses an extract from the microscopic fungus Streptomycin avermitilis to poison barnacles. The fungus lives in the ocean and is extremely poisonous to acorn barnacles and other crustaceans, a feature based on the environmentally friendly defence of the fungus against being eaten.
New study from Goteborg University in Sweden has found that when this fungus is added to paint for ship hulls, the surface remains entirely free from barnacles. As little as a 0.1-percent mixture of pure fungal extract in paint is sufficient to affect the nervous system of barnacles and prevent any growth and the fungal extract is toxic only as long as the paint is on a painted surface. The main advantage of these paints is that when the paint is dissolved in sea water, the activation of the poison appears not to take place, making the paint apparently harmless to organisms in the open sea. The fungal extract is probably both cheaper and, above all, more environmentally friendly than the paints based on copper compounds available on the market today.
Sealcoat system
Sealcoat system uses biocide free epoxy resin as a bonding agent that seals the surface, and the finishing layer consists of synthetic micro-fibres that protrude slightly from the epoxy layers, thus resembling the velvet-like skin of the seal. SealCoat works mechanically due to the movement of the fibres in the water, even when the boat remains idle. The system can provide five years of continuous and effective fouling protection, and can be applied to all kinds of surfaces, such as steel, aluminium, plastic, wood, fibreglass etc. Sealcoat is suitable for all types of boats (sailing or cruisers), and especially for those that retain their full displacement when cruising. While Sealcoat is an environmentally friendly antifouling method, but it is expensive and must be applied by a trained specialist. The main disadvantage is that it is easily damaged and cannot be repainted
Electromagnetic impulses
Alternative forms of control that are available include a device which creates and transmits timed, electromagnetic impulses through a metal band fixed to a ship's hull. The impulses radiate out from the band along the hull, preventing the micro-organisms from attaching themselves. Sonic anti-fouling systems have also been developed which utilize low frequency sonic waves to create a micro-thin layer of rapidly moving water over the hull's surface which makes it very difficult for fouling organisms to attach themselves.
'Wrinkled' coating resists barnacles
North Carolina State University engineers have created a non-toxic "wrinkled" coating for use on ship hulls that resisted build up of troublesome barnacles during 18 months of seawater tests. Researchers created the coatings by stretching a rubber sheet, applying an ultra-violet ozone treatment to it, and then relieving the tension, causing five generations of "wrinkles" to form concurrently. The coatings were further covered with an ultra-thin layer of semi fluorinated material. During ocean tests performed, the wrinkled materials remained free of barnacles after 18 months of seawater exposure, while flat coatings with the same chemical composition showed barnacle build up after just one month in seawater.
Sponges-natural antifouling compound
At The Hong Kong University of Science and Technology, the investigators have explored the natural antifouling substances produced by marine organisms such as sponges and marine bacteria.
Altogether, 10 non-toxic compounds produced by marine microbes as natural fouling inhibitors have been identified. Among discoveries in the research was that highly potent antifouling and antibiotic compounds are produced by the bacteria and fungi living on sponges and seaweeds. The products prevent larval settlement of fouling organisms through a non-toxic mode so do not kill organisms but inhibit their settlement. So they are environmentally friendly
Though it sounds unreasonable of this point of time, but a solution can be taken into by interposing fine micro bubbles of air between the water and hull. This will prevent the barnacles to find a substrate for adhering. Solution can be sort with respect to ensuring that air bubbles are constantly attached to hull to and in case of being destroyed by external abrasion, system should replenish them instantly thus can additionally benefit in reducing the resistance of vessel.Conclusion:
Protection of hull of a ship is one of the most challenging factors. Even a minute corrosive element can cause a tremendous loss to the ship. As mentioned, the paints used to prevent the bio-fouling are toxic to marine environment.
If the anti-fouling paints are not used then it will lead to bio-corrosion, as well as increased hull resistance and in turn increased fuel consumption. So antifouling coatings are important as long as life of ship and sea trade is concerned. Anti-fouling hull coating system can provide substantial environmental benefits and a way to save money. But the methods employed may cause severe damage to the environment.
At present there are regional, national, and international regimes in place to control the detrimental effects of anti-fouling coating systems. This is because there is a compelling need to minimize the environmental harm caused by the biocides employed.
Although there is some less toxic alternative biocide under consideration, some of the most promising alternatives may be those that approach the problem by inhibiting adherence of the species to the hull rather than killing the species directly.Acknowledgement:
We are grateful to Dr B K Saxena, Ex-Chief engineer and Principal, Tolani maritime institute for providing the support for presenting the paper. We are also grateful to Dr. S.G. Dixit, Capt. S G Deshpande, Dr. S Kanungo, Ex-Chief engineer and Program chair of Practical Skills Tolani Maritime Institute for providing the information in latest developments in this field. We are also thankful to our library in charge Mr. Praveen Vaidya.bibliography:
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