Chemical tanker officers working on coated ships must know the contents of this chapter. This is also important for new build superintendents and delivery masters.
It is a shame if a coated tanker , with millions of dollars in investment is spoilt by an ignorant officer in just one voyage.
Cargo tank coatings can be categorized into two main groups:
a) Inorganic coatings – zinc silicates and ethyl silicate types
b) Organic coatings- epoxy and modified epoxy systems
Inorganic coatings are one-layer coatings, comprising of inorganic silicates pigmented with high percentage of zinc powder. The paint film is porous so the cargo, after the discharge of the ship, can be completely removed from the coating by evaporation but it cannot be happened the same with high-density cargoes like lube oils. Generally the life of those coatings is proportional to the thickness of the coat.
Organic coatings consist of an organic resin system which when is mixed with a hardener it forms a cross-linked array of chemical bonds between the resin molecules. Those types of coatings have the ability to resist in more strong acids or alkalies than inorganic coatings and they tend to absorb significant quantities of cargo and contamination problems can still occur
Today’s state of coatings can be categorized as:
Alkaline zinc silicate
Ethyl zinc silicate
Cyclosilicon epoxy ( siloxirane )
One important factor, which determines the performance of coating, is the curing process and the adhesion of coating on the metal surface. Curing is done in ambient temperature or with high velocity hot air applied into the tanks. Very important factor regarding curing is the time and relative humidity.
If we consider a coating system as an epoxy/amine coating where the epoxy is the first component of the coating and the amine is the curing agent, then the epoxy and the amine will react together. As the curing continues the molecules will become greater and they will continue to grow until a “gel” is formed. When the “gel” is formed, the epoxy molecules cannot be more soluble to the solvent. This means that the molecules do not move free as previously.
When the greatest amount of reaction has taken place and the biggest amount of the solvent has escaped the system then the system can be consider to be cured.
The coating system can be further be cured in order to achieve better properties of the coating by increasing the “environment” temperature. In this way we obtain to assist further the molecule mobility and thus further reaction between epoxy and amine molecules. At the end most of the solvent has escaped the system. This heat treatment is known as the “post-curing” and the final film has better properties than the simple cured film.
A vessel with mild steel tanks costs almost the half that of Stainless Steel and further more applying the latest coating technology, the income could greater than the one with stainless steel cargo tanks.
Corrosion can be defined as “the destruction of a metal by an electrochemical reaction with its surroundings”. Experiments have shown that iron will not rust when it is in dry air, nor in water which is free from dissolved oxygen so both oxygen and water are necessary in the corrosion process. The process of corrosion will be accelerated with the presence of an electrolyte into the solution, especially when it is acid or base.
The chemistry of corrosion is described above.
In simple terms corrosion can be expressed by the chemical reaction
where A is the metal and B the non –metal reactant (reactants) and C, D are the products of the reaction. In other words it is an electrochemical reaction of a metal with its environment
The iron reduces to iron ions at anode, the oxygen is reduced by combining with water and electrons passed from the anode (by iron changing to ions) to hydroxyl ions. The oxygen reacts with the Fe2+ to form ferrous oxides (Fe2O3, FeO), which are a reddish brown loose deposit. In chemical tankers most of the cargoes are organic and inorganic substances.
So it is essential to coat them in order to prevent corrosion of tanks. Corrosion is accelerated more in pH ranges 0 to 3 and from 13 to 14 due to break down of coating system corroded areas and spots may be developed most of the times coating fail because of several reasons like bad application or weather conditions. The coating protection mechanism is composed by three sub-mechanisms:
By the barrier protection, the dried film blocks oxygen, moisture and any corrosive environment from the metal. However, all coatings cannot prevent by 100% their penetration by the corrosive elements and this property is called permeability. Typical barrier coatings are two-part epoxies (e.g. epoxy amines).
Coatings that protect by inhibition contain special pigments to inhibit the corrosion reactions on the steel surface.
Coatings with sacrificing action contain zinc in powder form. Zinc is more active than steel. So if zinc is in contact with steel and this system is present into a corrosive environment, then the zinc will corrode to protect steel. Most of the times zinc is contained in the primer.
Depending the kind of coating we are going to use, we define the number of layers that steel is going to be coated. For example, zinc silicate coatings are applied as one layer while epoxies can be applied as two or three.
So if we examine a three layer coating system, then each layer will have its purpose
The primer is the first coat applied to the surface. It is very important because it ensures the adhesion of coating on the surface. Surface preparation helps the coating stick by removing contaminants that interfere with bonding and by creating a profile or roughened surface. Many primers for steel also contain anti-corrosive pigments that actively assist the control of corrosion.
The intermediate coat or undercoat, is required in many coating systems and may provide one or all of the following functions: improve chemical resistance, serve as an adhesion coat between the primer and topcoat when the primer and topcoat are not compatible, and increase the thickness of the coating system.
The finishing coat is the final coat applied. Topcoats are formulated to improve the chemical and weather resistance of the coating system, and provide characteristics such as: colour, gloss and wear resistance.
Generally the coating is consisted by three components:
Each one has its purpose and their mixture provide the final coating properties.
The pigment is used to distinguish the types and number of coatings. Is a relatively insoluble element of the coating system. It is well known for the colour characteristic, which gives to the coating. Additionally, it assists corrosion resistance, adhesion characteristics and decreases moisture permeability. They can be categorized as inorganic and organic. They are used because they enhance the anti-corrosive coating characteristics and for aesthetic purposes
The resin plays the most important role at the film formation. It holds the pigment particles together and binds the coating to the metal surface. Also it has significant effect on the durability, strength and chemical resistance of the final film. Additionally forms the final membrane upon which depend many of its basic physical and chemical properties. Generally the coating systems are categorized according the
type of resin.
The primary role of solvent is for application. The solvent provides the coating to be taken out of the can and be applied on the surface, dissolves the film-forming ingredients and provides flow out of the coating once it is on the surface and contributes to the drying, adhesion, of the final film. Furthermore the solvent is the main component, which helps the resin and cure agent molecules to react.
Additionally the solvents evaporate, in a their greatest percentage, and they are not taken into account as parts of the dried film coating. Some times the coating system might contain more than one solvent each of which has a certain role like to dissolve the resin and control evaporation
Zinc silicates are generally a two system formulations, consisting of zinc powder which has particles size of 5-9 microns and inorganic or organic binder. The zinc powder may be blended with lead and iron oxides to provide improved spray application properties.
The silicate binder may be water based with potassium (inorganic) silicate blend or alcoholic (organic solvent) solution, in order curing take place.
Post-cured silicates normally have an aqueous base and require application of a chemical curing solution to harden properly. Self- cured silicates may be aqueous or solvent -based and do not require application of curing solutions Most of the times are applied as one coat, which acts as a barrier between steel and corrosives. However, they are not resistant to strong acids and bases. This means that in practice these coatings are suitable only for cargoes, which have PH range of 6 to 9
Zinc silicates are unusual coatings, are one of the few coatings which are designed so that all of the solid pigment particles are not coated with polymer and all of the gaps between particles are not filled with polymer, i.e. they are designed to be porous films.
It is obvious that the best performance in chemical resistance will be achieved with the maximum zinc percentage.
Coatings, which are water based are the Alkaline Zinc Silicates, they may be composed of water-dissolved sodium silicate, potassium silicate or lithium silicate. The curing of coating occurs by the reaction between the zinc powder (pigment) and the binder silica gel (binder). The binder is supposed to react also with the steel substrate, forming a chemical form that results in outstanding adhesion. This chemical bonding to steel surfaces avoids undercutting of coating.
The curing mechanism and coatings formation is supposed to occur in three stages:
1. Initial reaction involves concentration of the components by water evaporation. This brings the zinc and silica into close contact, providing a moist coating on the substrate. During this stage, wetting agents in the paint enhance contact of the coating film with the steel surface.
2. At the second stage insolubilisation of the coating film, caused by the reaction of zinc ions with the silicate, and formation of the initial zinc silicate will occur. After this reaction a solid coating on the metal surface has formed. The mechanical and chemical properties are acceptable but the film has a porous structure.
3. The third stage of reaction is composed by the action of carbonic acid formed by the carbon dioxide and moisture on the coating surface. The carbonic acid, when penetrates the coating film reacts with the free zinc particles completing the formation of a dense zinc silicate matrix.
Curing or hardening of the coating takes place by hydrolysis of the soluble silicate followed by interaction with the zinc to form an insoluble zinc/zinc silicate complex. For self -cure formulations only atmospheric moisture is needed to complete the chemical reaction since atmospheric CO2 creates carbonic acid with moisture.
Ethyl Zinc Silicates are solvent borne coatings consisting of ethyl silicate and zinc powder. The curing procedure is similar to the Alkaline Zinc Silicates but now instead of water, solvent is evaporated.
Alkaline Zinc Silicates, cannot be applied by airless spray equipment due to high content of metallic zinc powder, dehumidification and ventilation during the drying and curing stages 1&2 are critical, they are applied in a single coat with a range of 75~125 microns due to crack formation.
On the other hand, Ethyl Zinc Silicates can be applied by airless spray since organic zinc silicates have lower content of zinc powder, the drying conditions are less critical with relative humidity be greater than 60%. The main problem is the difficulty of respraying low dry film thickness areas since adhesion problems may occur at the first coat.
Although the physical properties (i.e. hardness and abrasion resistance) vary according to the type of silicate used, chemical resistance and cargo compatibility are very similar. These coatings are normally applied as a single coat of 75~125 microns to a blast clean metal surface. They are sensitive to quality of surface preparation and blast cleaning to a white metal finish is necessary
Generally, the above coatings have an extremely high resistance and tolerance to aromatic hydrocarbon solvent such as benzene and toluene, alcohols and ketones. They are not resistant to acids or alkalis, including sea water which has a slow deteriorating effect. Vegetable oils and animal fats are unsuitable but halogenated compounds are suitable provided that tank surfaces are free of moisture. Any moisture will react with the cargo and release acids, which will damage the coating. Also the cargo should not contain any moisture for the same reason.
So it is important that both tanks and cargo will be free of moisture.
Epoxy coatings are generally suitable for the carriage of alkalis, glycols, seawater, animal fats and vegetable oils but, they have limited resistance to aromatics such as benzene and toluene, alcohols such as ethanol and methanol. In other words are blends of polymers of varying molecular weights. They contain curing agents in order to cure fast and they are 75~90 % solids by volume. They have very good chemical resistance and they applied as two or three layers.
These coatings have a tendency to pick up slight traces of the product carried, especially those chemicals which have only a limited suitability. Alcohols, esters, ketones have a tendency to soften the coating and in this condition the coating is more likely to absorb small amounts of cargo. A “fingernail test” can be used to establish the hardness of the coating. If the fingernail is able to penetrate the coating, it is still considered to be soft, in that case the tank is vented thoroughly before water washing is carried out.
Generally these coatings are suitable for the carriage of animal and vegetable oils provided the acid value does not exceed 10 (i.e. free fatty acid content of 5%). However, oils or fats with acid value between 10 and 20 may be acceptable for limited time of carriage.
Example of Sigma paints:
The max acid value determined by ISO 660 ( 1996 ) are related to approx weight % of FFA as below—
Phenguard/ max acid value, no limit/ approx free fatty acid, 100%
Novaguard/ 100/ 30 to 50%
Guard EHB/ 20/ 6 to 10%
Silguard MC/ 5/ 1.5% to 2.5%
Molasses is acceptable in epoxy provided the PH is above 4, although dilute solutions may become acidic and attack the coating. Such situation is remedied by adding an alkali to keep PH in acceptable level.
Epoxy coating can be categorized according to the resin that they will be mixed as follows:
Pure epoxy coatings are based on bisphenol and epichlorhydrin resins reacting, through their terminal epoxide groups, with hardeners having polyfunctional –NH2 groups which are called polyamines.
The properties of cured pure epoxy products depend on:
The type of epoxy
The type and quantity of hardener
The degree of cross-linking
The nature and quantity of additives
Chemical resistance and mechanical properties of epoxy coatings may vary. The factors which, influencing these properties are the molecular weight of resins, the type of hardener (curing agent) and the pigmentation and solvent mixture.
Low molecular weight epoxy resins results in coating films with a higher density of three-dimensional crosslinkings as well as a lower number of hydroxyl croups. Therefore, low molecular weight epoxy resins offer better chemical and water resistance than medium molecular weight epoxy resins, which, on the other hand offer better mechanical resistance and flexibility.
The most valuable property of epoxy resins is their ability to transform from the liquid state to tough, thermoset solids. The conversion is accomplished by the addition of a chemical compound, the curing agent. Depending the type, curation may occur at ambient temperature or may require post heating.
Amine cured agents provide good chemical resistance to epoxy coatings, while polyamide-cured epoxies show more surface tolerance and better mechanical properties. The later are more preferable because they offer superior solvent resistance.
Generally this type of coating is used because of its versatility, resistance range and application properties. A pure epoxy coating can be applied by airless spray at medium to high dry film thickness without sagging, cracking or pinholing. However, the maximum overcoating intervals are relatively short (three to five days),
requiring a tight application schedule
Epoxy Phenolics are multifunctional epoxy resins made by the epoxidation of phenolics resins with Epichlorhydrin. This type of amine-cured resins result’s in polymers with very high crosslink density, offering outstanding chemical resistance. However, most epoxy phenolic coatings require heating to 50 ~70 C for four to five days to reach their full resistance range.
Generally, the chemical resistance of heat-cured epoxy phenolics against strong solvents and fatty acids is better than pure epoxies. From practical point of view, however, heat post-curing poses several problems. To keep the cargo tanks at the required temperature, they must be loaded with an inert cargo (i.e. lube oil) and heated with the heating coils. This procedure is usually insufficient to reach 50-70 C in areas such as deck –heads and bulkheads, requiring the use of auxiliary heaters in the double skin compartments as well as the construction and heating of provisional air casings (void spaces made of staging which trap into blown hot air) on the deck areas above tank ceilings. Today the heating can be easily achieved by blowing hot air into the tanks but it is quite difficult to ensure that all tank areas are kept constantly and uniformly at the required temperature for long periods.
It has been observed that without heat treatment, the chemical resistance of epoxy phenolics improves after a service tome of at least three months if only moderately aggressive cargoes are carried, but it does not acquire the full resistance range of heat –cured coatings.
Properly formulated epoxy phenolics coatings have application properties similar to pure epoxies but usually longer overcoating times, making recoating less critical. On the other hand, they may create more overspray due to their stronger solvents, which are evaporated faster. A coating system with a dry film thickness of more than 700 to 800 microns, which may occur at critical areas such as angular welding seams on bottoms and ceilings may cause cracking through the whole coating film. Usually this phenomenon appears only after a salt-water test and cannot be detected during application
Epoxy Isocyanates: Higher molecular weight epoxy resins can be crosslinked with polyisocyanate with polyisocyanate compounds. This reaction occurs at room temperature and the isocyanate reacts with the hydroxyl groups of epoxy resin. So a densely crosslinked structure with excellent chemical resistance is obtained.
Cured epoxy isocyanates offer a resistance range similar to heat-cured epoxy phenolics, the only exception is that cannot carry alkaline cargoes with high concentrations. Most cargoes can be carried after a curing of ten days. Very aggressive cargoes such as methanol can be carried after a three-month service period.
It has been mentioned the cure occurs at ambient temperature however, epoxy isocyanates are more difficult to apply than pure epoxies or epoxy phenolics and they have more critical application properties. For example, because they need rapidly evaporated solvents, overspray may be a problem and they are sensitive to overthickness. So the dry film thickness of the whole system is small and crack may occur at 150 microns. Most of the times the crack can be observed with naked eye after drying, however some times it is visible only with magnifying lens because the crack does not split the whole coating film. Therefore, it will not result in rusty spots during the salt-water test.
Areas usually affected by cracking are angular welding seems and corroded spots (pitting). Stripe coated areas, if overcoated before they are completely dry, can cause cracking or blistering. To eliminate this problem, each coat must be inspected for cracking and defected areas should be repaired. Paint defects such as sagging and orange peel must be also eliminated because they are associated with cracking.
These application problems as well as health problems are the main reason for reduced usage of epoxy isocyanates. However, if they are used they can offer excellent resistance to aggressive cargoes, especially in the case of newbuildings. In the case of repair they may not be recommended because at heavily corroded steel it will be difficult to avoid overthickness on pitted areas.
Cyclosilicon Epoxies: These coatings are based on a new resin, which is essentially a cyclic silicon structure with five epoxidised phenol groups, that are cured by means of catalyst to give a highly crosslinked polymer. In other words these coatings are a two-component paint based on Siloxirane, a patented polymer with an organic/inorganic matrix.
More precisely Siloxirane consists of SiO- rings as a backbone forming a homopolymerized thermoset (heat cured) coating resin with high chemical resistance and good mechanical properties.
The homopolymerized thermoset resin has an oxygen to carbon linkage with high dense and cross-linked molecular structure. Also the absence of –OH eliminates the failure of building other types of polymers.
Manufactures claim that cyclosilicon coatings can resist up to 98% of the sea- trade cargoes, including cargoes which are unsuitable for stainless steel. Additionally these coatings have very low absorption characteristics. As a result they can offer significant advantages over conventional coatings regarding the cargo range, cargo handling and tank cleaning.
The coating is applied as a two-component paint. It can be applied like a conventional organic coating with partial curing taking place at room temperature, then the curing time will range from four to five days. However, for the full chemical resistance range, heat curing at 80 C for at least eight hours with hot air is necessary. Moreover, the coating system is sensitive to overthickness. Maximum dry film thickness should not exceed 500 microns because of the risk of solvent entrapment or cracking. Also overthickness could be a problem when recoating older tankers with corroded structures since the barrier of 500 microns could be exceeded the coating should have a degree of elasticity to remain on the steel surface without any crack to occur.
Coatings are said to have good slip when they have a low coefficient of friction and poor slip when they have a high coefficient of friction Slip is an important characteristic of coated tanks for it is the property that allows easy removal of cargo during tank cleaning.
The compressed air used for abrasive blasting should be oil free, be cooled after compression and must not have higher temperature and humidity than the air fed into the tanks by the dehumidifiers. By balancing the ventilation of the tanks, oxidation of the blasted surface is effectively prevented.
The abrasive should to be used should be dry, sharp, of good quality with a content of soluble salts which should not exceed a specific limit.
An easy test for detecting oil/grease on a surface is the "water break method", where a drop of water is added on to the prepared surface. The drop will spread out rapidly on the surface if no oil/grease is present, but will remain on the surface in a drop-shaped form in the presence of oil/grease.
After the finish of blasting we use suitable industrial-type of vacuum cleaners in order to remove residual grit and dust from surfaces
Sa 1: Light blast cleaning
When the surface is examined using the naked eye, it has to seen to be free of traces of oil, grease, dirt and lightly attached mill scale, rust, old layers of protective coatings and other bodies.
Sa 2: Thorough blast cleaning
When the surface is examined using the naked eye, it has to be seen to be free of traces of oil, grease and dirt and of the greater part of attached mill scale, rust, old layers of protective coatings and other bodies. Any remaining dirt has to be well attached to the surface.
Sa 2.5: Very thorough blast-cleaning
When the surface is examined using the naked eye, it has to be seen to be free of traces of oil, grease and dirt and of the greater part of attached mill scale, rust, old layers of protective coatings and other bodies. Any attached dirt has to have the form of light stains in the form of narrow strips or points.
Sa 3: Blast-cleaning to visually clean steel
When the surface is examined using the naked eye, it has to be seen to be free of traces of oil, grease and dirt and of the greater part of attached mill scale, rust, old layers of protective coatings and other bodies. The surface should have a metallic shine.
In the shipbuilding industry, we often encounter an initial condition A (good condition) or B (rust condition), whereas the required preparation grades are, according to regulations Sa 2, Sa 2.5 and Sa 3. Preparation grade Sa 3 is desirable but requires expensive cleaning installations whereas at the same time it produces an increase in surface roughness beyond the desired level. Thus, in superstructures a preparation grade of Sa 2 is often sufficient whereas for underwater regions of the hull and the inner structure of compartments a preparation to grade Sa 2.5 is required.
Steel temperature, can be measured by a contact thermometer The steel temperature should always be 3 C above the dew point before painting.
The dew point is the highest temperature at which moisture will condense from the atmosphere. The dew point is essential to determine if the climatic conditions are acceptable for paint -work. The calculation, can be done from tables or by a so-called dew point calculator When you have measured the dry and wet bulb temperature, the dew point and relative humidity can be read from the dew point calculator.
The lining material, which is stored under controlled temperature is mixed and thinned in the correct proportion before use, and after mixing must be used within the specified “pot life” set by the manufacturers. To avoid errors in mixing ratios the components are supplied appropriately sized containers.
The application of coating starts from the bottom of the tank to the ceiling, because during application the evaporated solvents go to the bottom of the tank. So the air in the tank is both renewed and dehumidified to keep clean atmosphere and steady temperature and humidity conditions.
The advantages of air-less spray include the provision of a smooth paint film with less change of air entrapment, greater versatility for the operator, less turbulence in the spray pattern and greatly reduced risk of contamination with moisture and oils from improperly cleaned compression equipment.
After the first coat is sufficiently dry the tank should be inspected and any uncoated spots should be coated.
However, there are some critical areas where due to their structure it is impossible to achieve the appropriate dry film thickness. For that reason stripe coats are applied with roller or brush. Typical stripe coat areas are:
As the stripe coatings have been applied and inspected we apply the second coating, which has different color from the first, using the air-less equipment.
When the coating system has adequately dried then dry film thickness measurements are done At the same time hardness of the coating are taken using a hardness-pen and inspection for any uncoated areas is done for the whole tank.
The tanks may be charged with sea -water and then discharged in order any uncoated areas to be discovered. These areas will be corroded by the sea -water.
At the last stage all heating coils and pumps are fitted on their initial places. Hot air is applied into the tank continuously to enhance curing of coating. This procedure is called post-curing and it can be done even while the ship is in service with these tanks empty.
Temperature determines the maximum amount of moisture which air can hold. When warm air is trapped in the tank, it is cooled by contact with the cold structure of the vessel so the relative humidity level rises until the dew point is reached and water droplets begin to form on the cold surfaces. So in order to avoid the above mentioned phenomenon we try to keep steel temperature few degrees of C above the air tank temperature.
Ventilation is required during each stage of the process: blast cleaning, application of coating, and curing of coating. It can be described in terms of airflow and the exchange of clean incoming air and dirty outgoing air. The balancing of incoming and outgoing air is an important feature of a ventilation system. If a high volume of clean air is blown into the tank while a lower volume of dirty air will be extracted then air turbulence will be created.
Spaces of 60 m3 and less shall have an air change every minute.
Spaces from 60 m3 to 850 m3 shall have an air change every three minutes.
Spaces from 850 m3 to 2,800 m3 shall have an air change every five minutes.
Spaces over 2,800 m3 shall have an air change every ten minutes.
The ventilation rate should sufficient to dilute solvent vapour to 10% or less of the lower explosive limit (LEL) of the specific solvents being sprayed. LEL is the lower limit of flammability or explosiveness of a gas or vapour at ordinary ambient temperature. It is expressed in percent of the vapor in air by volume.
Additionally we force the solvents to leave the coating system so that molecules can move easily and further hardening reaction take place.
Proper ventilation is obtained with equipment for moving air, directing the air, and the efficient set-up of the equipment. The major air movement components of a ventilation system are fans, ducting, and system layout
The reason of setting the suction pipes near the bottom of the tank is because dust particles and vapour solvents are heavier than air they concentrate at the lower tank height. Additionally as both blasting and painting are applied from the tank bottom to the top, we keep the atmosphere for workers clean.
Dehumidification may be required or desired for three reasons. One reason is that a coating specification may require a maximum relative humidity (RH) that is below the ambient RH of the tank For most shipyards in the USA, Europe, and Asia, the ambient RH will normally be higher than 50%, so dehumidification is needed to meet the specification.
A second reason for requiring dehumidification is to prevent condensation on a steel substrate. Condensation will occur when the dew point is at or above the surface temperature. Most coating specifications require the surface temperature to be at least 3 degrees C above the dew point temperature. If these conditions are not met, dehumidification can be used to lower the dew point. These conditions will then allow blasting and painting to proceed. Some shipyards prefer a spread of 6 or 8 degrees C, especially for tanks. In many locations, the surface temperature is less than 3 degrees C above the dew point temperature, so without dehumidification, there would be a substantial risk of condensation.
A third reason for requiring dehumidification is that it can create working conditions that can improve productivity. Dehumidification can raise or lower the ambient temperature while reducing the RH in a tank. Coating work on tanks under dehumidification can continue despite cooler ambient temperatures and high RH. Dehumidification can also lower the ambient air temperature, so it can reduce hot and humid conditions inside a tank that make the workers tired.
Generally the coatings are porous, this means that cargo can physically penetrate the film and be captured into it. The sequent of this behaviour is the possible reaction between the previous and the following cargo, which might lead to cargo contamination. Regarding inorganic coatings (i.e. zinc silicate coatings), very volatile cargoes can be easily removed using evaporation-ventilation techniques from the coating because this type of coating does not absorb large quantities of cargo. However, “heavier” cargoes like lube oil cannot be easily removed from the film. That might cause contamination of the next cargo, especially when the next cargo is a “good” solvent The organic tank coatings, despite they are more resistant to corrosive environments they tend to absorb greater quantities of cargo than zinc silicate.
The main factors influencing absorption-desorption characteristics are:
Also some coating absorption/desorption characteristics are influenced by water. Some coatings have considerably lower rate of absorption when they are saturated with waterThe coating breakdown has a form of blistering, which increases the tendency of the coating to absorb cargo.
It has been observed that the absorption rate of a substance into a coating film is rapid and increases in a linear way and then falls to zero when the film becomes saturated. In other words no more cargo is absorbed by the coating. On the other hand the desorption rate is rapid too, at the first stages, and at the end it falls to a steady value. That means than the absorbed substance has not fully escape the film absorbed cargo quantity becomes maximum during the first three days and until the 13th day the absorption rate relatively does not change. By the 13th day desorption occurs, and lasts four days. As we can see the desorption rate does not change during the last two days (17-19), which means that an amount of absorbed substance will retain into the coating.
Cargoes having small molecules are able to penetrate organic coatings easier than those cargoes with greater molecules. For this reason methanol is one of the most aggressive cargoes.
High cross-linked molecule structure of coating the percentage of adsorbed cargo is reduced• The absorption/desorption characteristics of the paint systems differ significantly. Some paints absorb less amount of cargo than others and desorb the cargo more efficiently. The selection of such coating system reduces the risk of contamination.
Allow coatings to desorb as long as possible. The rate of desorption increases as the tank temperature increases. An important point is that continuous ventilation of tank is not as effective as the increased air temperature in the tank. Avoid the stowage of “sensitive” cargoes such as ethanol, methanol, isopropanol etc., in tanks where incompatible cargoes have been previously stowed.
It is often necessary to clean or ventilate cargo tanks when changing cargoes in order to prevent undesired interactions between cargo residues and the next cargo. Such interactions can form substances that may attack the coating system, enhance danger of steel corrosion and contaminate or discolour the next cargo.
For example, when residues of a cargo, which contains ester groups in its chemical composition, may create acetic acid by hydrolysis. Hydrolysis will take place as soon as the residues of a cargo will come in touch with water molecules. This reaction will cause corrosion and may attack the coating. The same will happen when, cargo residues, contain chlorinated hydrocarbons. They can form hydrochloric acid upon contact with water or water containing cargo.
To avoid such interactions, all esters and chlorinated hydrocarbons must be transported in dry cargo tanks. Methanol cargoes can be especially problematic. Besides having a softening effect on organic coatings, methanol residues in a coating can cause water vapour permeability, causing osmosis between coating and steel substrate. In addition, methanol can extract residual solvent and low molecular weight materials from the coating. This induces stresses in the coating that can lead to cracking.
Only highly crosslinked coatings are resistant to methanol. Most coatings suppliers do not allow transportation of water-containing cargoes after transportation of methanol. Additionally cargo-compatibility tables result of collaboration of chemical companies and organizations are available to ensure the cargo purity and corrosion steel prevention . Guidelines for tank cleaning procedures when changing cargo should be followed carefully to ensure that cargo residues are sufficiently removed before loading a new cargo. Organic tank lining systems can absorb materials from cargoes, and the amounts after different time periods are not well defined.
Variable and unpredictable absorption/desorption characteristics are found not only among different coating types but also within the same generic type of coating from different manufacturers. In addition, different rates of absorption/desorption are found among different cargoes. This can make it difficult to select the correct cargo tank coating system.
According to the coating manufacture specifications, the coating should have a specified d.f.t. of 300 microns and the minimum d.f.t. should not be below the 290 microns. In other words, the final d.f.t. should not have a diminution more than 10% of the specified.
Temperature and humidity sensors can be attached on the tank plates using magnets in order to monitor air temperature and humidity. These sensors can be removed after the completion of curing.
CAPT AJIT VADAKAYIL ( 28 YEARS IN COMMAND )