CTM 9.7 A5 Chemical Tanker Manual
CTM
Aggressive Cargoes in Epoxy Coated Cargo Tanks 		Doc No.: CTM 9.7 A5
Revision: 01
Date: 15 Oct 2024
Issued by: DPA
Approved by: MD

1. GENERAL

The epoxy coating film will be influenced by all types of liquid cargoes, but aggressive cargoes are those with the greatest influence. Some examples of aggressive cargoes include ketones, low molecular weight alcohols, ethers, and esters.


Such commodities must only be carried in fully cured epoxy-coated tanks. The rule for all types of epoxy-coated tanks is that no aggressive types of cargoes should be carried within the first three months following coating application.


Most of the aggressive cargoes are mentioned in the cargo resistance list with a note, but it must be remembered that some aggressive cargoes are not mentioned with a note in the resistance list. In these cases, cleaning/ventilation, etc., must be done as for aggressive cargoes. The question will be in the case of no note: “What is an aggressive cargo?” In this case, we have to rely on experience and common sense. Even if the epoxy coating is fully cured, it must be restored after the carriage of an aggressive type of cargo. Some aggressive cargoes will be absorbed more than others.


These cargoes must be carried in clean, dry tanks. Condensation within the ullage space must be avoided, and the moisture content of the cargo must be below 0.01% (100 PPM). These products must be completely removed from the tank, and the epoxy coating must be RESTORED before any tank cleaning operations involving the use of water or steam.


It is strongly recommended that the remains of such cargoes are removed by ventilation. THE TANK MUST BE VENTILATED UNTIL GAS FREE. The serious risk of damage to the tank lining is encountered in the case of sequences containing aqueous products and water-miscible chemicals.


Most aggressive cargoes will soften the epoxy coating to a certain extent, and if the coating is not restored/re-cured (conditioned) by ventilation, the absorbed cargo will not be removed, and the restoring/recurring of the coating will not take place. This will result in deep blistering, which will expand and break open. It will then be visible as a lot of small half-round rust-striped (hair-like) so-called ‘Horse shoes blistering’.


After the discharge of an aggressive type of cargo, the epoxy coating must be restored. Restoring the epoxy coating in an epoxy-coated tank can only be done by ventilation until “GAS FREE” and not by washing with water or cleaning with detergent or other cleaning fluids. To avoid damage to the epoxy coating, an aggressive cargo should never be followed by another aggressive cargo.


The cargo after an aggressive cargo should be a fully approved non-aggressive cargo without any remark in the cargo resistance list. The time required for restoring an epoxy coating after an aggressive cargo depends on the following:


If the absorption is water or another less volatile cargo, removal of absorption by ventilation must be done with dry air with a relative humidity below 40%.


A too high a temperature does not speed up the restoring process. The epoxy has become soft due to the aggressive cargo, and with too high a temperature, the epoxy film will remain soft even after the absorbed cargo is ventilated out of the coating.


The best way of restoring the epoxy coating is as many air changes as possible with an air temperature of 20-30°C.


Approximate Times Required for Restoring of an Epoxy Lining After an Aggressive Cargo (This is only a guideline, and the exact time depends on the type of cargo, the volatile properties of the cargo, the temperature, and the number of air changes.)


Steel Temperature in Celsius

Rel. Humidity 100°C 150°C 200°C 250°C 300°C
20% or less 17 days 13 days 9 days 6 days 4 days
20-30% 17 days 9 days 6 days 4 days -
30-40% - 17 days 12 days 8 days -

2. THE COATING FILM THICKNESS IN RELATION TO THE ABSORPTION OF AGGRESSIVE CARGO

The thickness of an epoxy coating is usually specified to be between 280 and 300 microns dry film in a 3-coat system. Due to circumstances such as accessibility, etc., the coating thickness will vary, usually between 290 and 500 microns. In certain areas with overlap, small areas may be above 500 microns.


Damage to an epoxy coating by absorption usually occurs in overlap areas and in areas where a stripe coat was applied. During re-coating, extremely high film thickness should not be accepted, and small areas will be repaired by sandpapering and larger areas by re-blasting. Practically, it is not possible to avoid having a film thickness over 500 microns on stripe-coated areas and areas of overlap. High thickness coating is more absorbent and will take much longer to restore to a condition suitable for loading the next cargo.


3. ABSORPTION AND DESORPTION OF EPOXY COATING AFTER AN AGGRESSIVE TYPE OF CARGO

Phenolic epoxy is resistant to solvents, such as ketones, low molecular weight alcohols, ethers, and esters. However, caution must still be used when cleaning the tanks and selecting the next cargo. After containing these types of solvents, the tank should be well ventilated to remove all traces of the chemical prior to cleaning. The next cargo should not be any water ballast or diluted aqueous solutions such as liquid fertilizer solution or latex emulsions.


3.1 Esters, Phthalates, Etc.

Certain products such as esters (acetates, phthalates, etc.) and chlorinated or brominated materials can react with water to form acidic compounds. These products are suitable for storage in PHENOLIC EPOXY 373 coated tanks, but the presence of any moisture will make them aggressive and unacceptable. Such products must therefore be dry, free from acidity, and carried in completely dry tanks. Water contamination must be avoided, and water content of the cargo should not exceed 0.02% (200 PPM).


3.2 Ketones, Alcohols, Nitrites, and Esters

These aggressive cargoes can only be carried when the coating is fully cured, which will be obtained after a service period of 3 months with non-aggressive cargoes (without notes). After transportation of cargoes marked with a note, the coating has to be restored to its original condition. This will be obtained by ventilating the tank thoroughly until the tank is in a gas-free condition, followed by the carriage of non-aggressive cargoes without a note (in the resistance list) for a period of at least 10 days.


3.3 Aggressive Water Miscible Cargoes

Before loading these aggressive, water miscible cargoes, it is essential to remove all traces of water from the tanks. After carriage of these cargoes, no steam, ballast water, wash water, slops, or aqueous cargoes must come into contact with the coating before its original coating condition is restored. The coating will be restored by thorough ventilation until the tank is gas-free, followed by the carriage of non-aggressive cargoes (without a note) for a period of at least 10 days. Ventilation of the tank must be carried out after an aggressive/water miscible cargo has been discharged. This is to ensure the removal of any cargo that may have been absorbed into the coating. This must be done prior to any tank cleaning procedures being carried out.


Any residue left in the tank after cleaning must be removed immediately and drying completed by venting. Cargo residues or absorbed cargoes in the coating are a potential source of contamination of subsequent cargoes. Cargo residue and absorbed cargo in the coating film may cause a chemical reaction, and produce products, which could cause breakdown of the coating system.


There is also a category of aggressive cargoes, which can only be stowed in epoxy-coated tanks for a limited number of days. This number of days must not be exceeded, as the coating will be softened and the absorption of chemicals will increase, which will obviously damage the tank coating. The limitation of days is mentioned in the cargo resistance list for the cargo in question.


4. EXPLANATION OF ABSORPTION

When a liquid cargo is loaded into a tank coated with an organic coating (Phenolic epoxy), part of the liquid will be absorbed by the binder phase of the coating through a diffusion process. Absorption will continue until the binder phase is saturated with the liquid. When the cargo is later discharged from the tank, desorption will take place, again by diffusion process and will ideally continue until all absorbed liquid has been removed from the coating.


5. FACTORS AFFECTING DRYING OF EPOXY COATING

Reiterating that drying is to be understood as the removal of absorbed chemicals, it will first take place after a visible dry state of tank surface has been reached. The dry film thickness is very important, especially during drying. Doubling the film thickness means drying times four (4) times longer in order to obtain the same degree of drying. The drying time in a given tank, (desorption time) is governed by the largest thickness. This could be one of the reasons for the premature damage of coatings on the tank top and the lower part of a tank, as the higher film thickness is usually found here. Also, lower temperature and less ventilation for desorption could be the reason for premature damages to the coating in the lower areas of the tank.


During the coating of new tanks and application during re-coating, extra attention should be given to the control of the coating thickness. Coating of a traditionally constructed cargo tank with specified minimum thickness of 240 microns dry film for three coats, will often result in an average thickness of approximately 350 microns with peaks up to 500 microns, which is 40-50% more than specified. In some overlap zones and on flat bar faces, a film thickness of more than 500-600 microns may be found.


The amount of absorbed cargo into the coating film finally depends on the coating film thickness, and especially areas of extreme thickness as mentioned above, of 500-600 microns, will absorb noticeably more chemicals than the minimum thickness of 240 microns.


Cargoes such as methanol, benzene, and methyl acetate are absorbed into the coating film to a higher degree than other chemicals. Among the various chemical families, glycol ethers, represented by diethylene glycol, mono methyl ether, are absorbed the most, followed by esters, nitriles, aromatic hydrocarbons, and alcohols. The selected families are aggressive to epoxy coating due to their solvency power and water miscibility. Absorption of cargo by the epoxy coating may occur, and subsequent desorption purely depends on ventilation and tank cleaning procedures. Insufficient ventilation may lead to cargo entrapment in the coating, which could damage the coating and contaminate subsequent cargoes.