Cargo tank coatings serve to shield the mild steel structure from corrosion and act as an insulating barrier between the steel and the cargo. Preserving the condition of these coatings is crucial to maintain cargo purity, preventing contamination and potential claims. The bulletin provides a comparison of zinc silicate and epoxy coatings.
Epoxy
Epoxy tank coatings are organic and fall under a group of coatings each with distinct properties, capabilities and limitations, such as pure epoxy, phenolic epoxy (Novolac), Marineline and Bimodal or Siloxirane. These organic resin systems are mixed with a hardener to form a coating film, creating a three-dimensional cross-linked structure of chemical bonds between the resin molecules.
Epoxy coatings are resistant to strong acids and alkalis. They do not typically absorb significant amounts of oil-like substances, making them suitable to carry CPPs (clean petroleum products), aromatics and most alcohols.
Conversely, epoxy coating systems absorb significant amounts of solvent like, highly volatile cargoes into the paint film. The amount of absorption dependents on the cargo, the contact time between the cargo and the coating (e.g. length of carriage time) and the carriage temperature.
This phenomenon of absorption and desorption with epoxy coated tanks is well documented, necessitating careful consideration of cargo sequencing and tank cleaning.
After carrying a solvent like cargo, the coating must be conditioned for as long as possible to promote the desorption of the absorbed cargo species. The rate of desorption is significantly increased by raising the tank temperature, which is more effective than continuous ventilation.
Water washing must not commence until the ventilation process is complete, as premature water introduction can lead to blistering and subsequent damage of the coating. Incomplete forced ventilation following the carriage of highly volatile cargo increases the risk of desorbing cargo residues into subsequent cargoes.
Notwithstanding the above, relatively low concentrations of absorbed cargo species can be retained in the coatings despite following industry cleaning recommendations and best efforts to remove these species using non-standard cleaning methods. In some cases, these low concentrations can be sufficient to significantly contaminate (to single digit ppm levels) not just the immediate next cargo to be carried, but also subsequently carried cargoes.
Fundamentally, it is a matter of evaluating the amount of time since the last problematic cargo was carried and absorbed by the coating system, such as BTX aromatics – i.e. benzene, toluene and xylenes – and when the next sensitive cargo is loaded, such as Ethanol, Methanol or MEG). See an example desorption graph below evidencing that it can take multiple weeks for the desorption levels to become insignificant (the bottom axis counts the number of days).
West Club highlights that sensitive cargoes such as foodstuffs, methanol, ethanol, and mono-ethylene glycol (MEG) should be avoided as the first, second, third and sometimes even fourth subsequent cargoes in tanks previously loaded with incompatible cargoes, due to high risk of contamination. The wall wash test (WWT) and first foot samples tests will not detect the cargo residues absorbed into the tank coating.
If unfavourable stowage is unavoidable, the charterers should be fully informed of the contamination potential. If they request the vessel to load the cargo, a Letter of Indemnity along with a Bank Guarantee must be obtained prior to commencing loading.
This absorption and desorption process also stresses the coating, shortening its lifespan. The coating can hold certain cargo residues for a considerable time, and when a second cargo is absorbed, the two cargoes may react, creating a harmful environment within or behind the coating. This typically results in blistering, coating, and eventual detachment of the coating.
RR&CO remark: The compatibility of successively and previously carried (and potentially absorbed) cargo residues should therefore be considered when planning grade rotations / cycling.
Credit: West P&I Club
During the tank cleaning verification process, a tank may fail a WWT yet still carry the next assigned cargo without contamination, as the dilution effect has not yet been applied. Utilising wash water analysis (WWA) as a verification method can significantly reduce the risk, as it uses of UV-Vis spectrophotometry to analysis the wash water for previous cargo residues, covering the entire cargo tank and line system.
An example of an UV-Vis spectroscan machine is the “Caretech Spectroscan UV-Vis SmartLog”, photo and results graph below. Its marketing materials note: “For support of wash water analysis, a database with 100ppm graphs of the most common cargoes is pre-installed. These graphs can be used as template to create overlays with the actual scans of the wash water, making interpretation for crew easy and fast.”
Additionally, a tank may pass a WWT or WWA and still have oil like cargo residues within the zinc paint film, or solvent like cargo residues within the epoxy paint film. Such instances can best be avoided by applying proper tanker seamanship practices during the planning stage of cargo scheduling and cargo-coating compatibility.