This article provides guidance on cargo tank coatings for chemical and product tankers.
Tank coating systems need to be routinely inspected and maintained. Re-coating is generally required at some point during a vessel’s operational life, typically carried out during dry docking. Vessels are also required to keep the tank coating Manufacturer’s Cargo Resistance Guide onboard at all times and refer to it when planning cargo sequencing and tank cleaning.
This bulletin reviews and compares zinc silicate and epoxy coatings, though stainless steel tanks present an alternative to traditional coated tanks.
Stainless steel tanks
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.
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).