Comprehensive Guide to Cargo Tank Coatings for Chemical and Product Tankers
Loss Prevention Bulletin
The purpose of cargo tank coatings is to protect the mild steel construction from corrosion and provide an insulation barrier between the steel and the cargo. Maintaining the integrity of tank coatings is essential to ensure cargo purity, thus avoiding contamination and subsequent claims.
(Loss Prevention Officer, West P&I)
This bulletin was created in collaboration with RR&CO.
Tank coating systems require regular inspection and maintenance. Re-coating is typically necessary within the vessel’s trading life and is usually completed during dry dock periods. Vessel’s must carry the tank coating’s Manufacturer’s Cargo Resistance Guide onboard at all times and consult it when planning cargo sequencing and tank cleaning.
Zinc
Zinc silicate is an inorganic paint system. The silicate coating base is either water-based alkali or solvent, typically ethyl silicate. These coatings are very hard, corrosive and temperature resistant with galvanic protection. Zinc coatings have good resistance against neutral pH cargoes, but are not resistant to strong acids and bases, which may cause the resins to dissolve. They exhibit excellent resistance to strong solvents and chemicals.
Cargoes carried in zinc coated tanks get adsorbed or entrapped into the porous structure of the coating, making cleaning between different cargoes essential. Zinc coatings can typically tolerate higher temperatures than epoxy coatings and have a shorter desorption time. If cleaning is skipped, cargo residues can desorb into future cargoes, risking contamination.
RR&CO Remark: Cargo can physically penetrate into the ‘gaps’ formed between the zinc particles and silicate structures.
Image supplied by RR&CO
Importantly, the type of cargo will affect how easy it is for residues of the cargo to be removed from the coating:
(a) Volatile solvent-like cargoes can be rapidly and almost completely removed from the coating by evaporation / ventilation of the tanks following completion of discharge.
(b) Whereas, removal of heavier oil-like (residual) cargoes, which will also become retained in the coating system, will not be removed by evaporation/ventilation and will require more rigorous cleaning.
Such oil-like (residual) cargoes are therefore more problematic for zinc coatings.
Jet fuel requires special considerations as it may suffer from zinc pick up. As a result, most charterers will not load jet fuel into zinc coated tanks. This also applies to edible oils intended for human and animal consumption.
Advantages of Zinc Coatings | Disadvantages of Zinc Coatings |
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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.
Image supplied by RR&CO
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.
Advantages of Epoxy Coatings | Disadvantages of Epoxy Coating |
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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.
RR&CO Remark: 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.”
Image supplied by RR&CO
Image supplied by RR&CO
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.
Coating Type | Solvent resistant | Absorbs solvents | CPP | Absorbs oil residues | Acids & Alkalis | High FFA% | Edible Cargo | Heat Resistant |
Zinc | Yes | Evaporates easily | No | Yes | No | No | V. Restricted | Yes |
Epoxy | No | Yes | Yes | No | Yes | Limited | Limited | Limited |
Image supplied by RR&CO
Coating Type | Capabilities & Limitations | Suitable Cargoes |
1. Inorganic Zinc | Porous surface area. Resistant to Solvents. Oil-like cargo residues can get trapped in coating film. Not resistant to strong Acids and Alkalis, Vegetable Oils and Solvents prone to hydrolysis. Suitable for Methanol carriage. | All cargoes within pH range 6- 9.
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2. Pure Epoxy | Resistant to strong Acids and Alkalis. Dose not commonly absorb significant amounts of oil-like substances. Suitable for carrying CPPs, Aromatics and most Alcohols. Issues with absorption of previous cargoes desorbing into later cargoes causing compatibility issues, cargo sequencing restrictions and cargo contaminations. Restrictions on aggressive solvent-like cargos such as Methanol. | Alkalis, Aromatics, most Alcohols, Oils, Crude Oil and CPP cargoes. |
3. Phenolic Epoxy | Carriage of an extensive range of aggressive cargoes including Solvents and Chemicals, CPP, DPP, Vegetable and Animal Oils. Post-curing required for certain cargoes, Methanol carriage for maximum 90 days, restrictions on cargo carried prior to and after Methanol. | CPP, DPP, almost all chemicals except strong Acids and Alkalis.
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4. New Low Absorption Epoxy:
(Siloxirane/Bimodal) | New polymer resins offer reduced cargo absorption and faster desorption rates enabling the carriage of a wide range of cargoes including Methanol. Siloxirane: Advanced Polymer Coatings- MarineLine 784:
A patented heat cured polymer with advanced cross-link technology. Suitable for the carriage of an extensive range of Product and Chemical cargoes including aggressive cargoes, Solvents, strong Acids, Alkalis, CPPs, DPPs, Vegetable and Animal Oils. Requires post curing. Ventilation required post Methanol carriage. Jotun Tankguard - Flexline: Suitable for the carriage of an extensive range of Product and Chemical cargoes, CPPs, DPPs, aggressive cargoes, Solvents, some Acids, Alkalis, Vegetable and Animal Oils. Requires post curing. Allows stowage of various aggressive cargoes for periods longer than 6 months. Restrictions on cargo carried prior to and after Methanol. Not resistant to strong acids and requires cleaning after carriage of strong alkalis. Bilodal: International - Interline 9001: Suitable for the carriage of an extensive range of Product and Chemical cargoes, CPPs, DPPs, aggressive cargoes, Solvents, some Acids, Alkalis, Vegetable and Animal Oils. Requires post curing. Not resistant to strong acids and requires cleaning after carriage of strong alkalis. Methanol carriage for maximum 45 days, with restrictions on cargo carried prior to and after Methanol. | CPPs, DPPs and very wide range of Chemicals. Some Acid and Alkali cargoes including Vegetable and Animal Oils and aggressive Solvent cargoes such as Methanol (post carriage ventilation required and some restrictions on carriage time). MarineLine 874 is the only coating that can withstand strong and weak acids as cargoes. |
An alternative to coated tanks is to use stainless steel tanks, which are non-corrosive and do absorb or retaining cargo residues, making tank cleaning less hassle some. However, these tanks are expensive and therefore considered uneconomical for certain trades. They may be required by charterers for certain high-specification cargoes, such as MEG, styrene monomer, or strong acids. It is worth noting also that some cargoes cannot be carried in stainless steel, such as hydrochloric acid which requires rubber lined tanks.
Image supplied by RR&CO
RR&CO Remark: Stainless Steel (SUS) cargo tanks do not face the same issues as coated tanks and can carry a much wider range of aggressive acids and alkalis, plus more confidently transport higher sensitivity and/or food grade chemicals. In particular, SUS avoids the need for paint touching up and such strenuous continuous maintenance by the crew, coating blistering, cracking, delamination, rust weeping through from underlying mild steel substrate and/or over application of paint. Plus, SUS does not absorb or retain previous cargo residues within the coating system. Most importantly, the SUS itself is a “easy clean” surface and due to its high chromium content has a passive layer of chromium oxide that forms when dry, clean and smooth and exposed to air (between cargo carriages and from ppm of oxygen inside cargoes), which protects the SUS from cargo attack thereby minimises risk of corrosion and/or pitting – save for the cargoes of sulphuric acid (cleaning after discharge) and wet-process phosphoric acid (chlorides and fluorides levels in the provided cargo).