Q.1 — Accelerated Corrosion in the Vapour Space of Cargo Tanks (VLCC)

The vapour space (ullage space) of a crude oil cargo tank — roughly the upper 10–15% of tank height above the highest normal liquid level, extending to the deckhead — is consistently found to suffer the highest corrosion rates in the entire tank structure, often 3–5 times the rate seen in the wetted/immersed zones. The reasons are both chemical and mechanical/operational:

1. Presence of corrosive gases in the atmosphere above the cargo

• Crude oil vapour is not inert. It contains hydrogen sulphide (H₂S), especially from sour crudes, which combines with moisture to form corrosive sulphur compounds.
• Sulphur dioxide (SO₂) and other sulphur oxides form from combustion of sulphur-bearing residues when inert gas is used.
• Inert gas itself (flue gas from IGS) contains SO₂, SO₃, CO₂, and traces of NOx. When these combine with condensed water vapour, they form sulphurous acid (H₂SO₃), sulphuric acid (H₂SO₄), and carbonic acid (H₂CO₃) — all highly aggressive to unprotected mild steel.

2. Condensation cycle (the primary driver)

• The vapour space undergoes continuous temperature cycling — heating during loading/laden passage and cooling during ballast passage, especially when a ship moves from warm to cold climates.
• Steel plating in the upper tank/deckhead is in direct contact with outside air/sea temperature through the deck plate, making it the coldest surface in the tank.
• Warm, moisture and acid-gas laden vapour condenses preferentially on this cooler steel, forming a thin, acidic aqueous film on the surface — an ideal electrolyte for electrochemical corrosion.
• This condensation is repetitive and renews itself with every voyage cycle, unlike the wetted zone where the same cargo remains in contact for the voyage duration.

3. Absence of a protective liquid buffer

• In the wetted/bottom zone, any acidic condensate that does form is diluted and buffered by the bulk cargo or by settled water, and the oil film itself offers some degree of physical protection to the steel.
• The vapour space has no such buffering — the acidic film sits directly and undiluted on bare steel.

4. Structural/geometric factors specific to the vapour space

• The underdeck structure (deckhead, longitudinals, brackets, and the underside of deck transverses) has a complex geometry with stiffener webs, flanges, and connections that trap condensate, preventing it from draining back into the cargo.
• Corrosion products (rust scale) that form on these surfaces are often loosely adherent and non-protective, unlike some passive oxide films, so they do not arrest further attack — this is sometimes described as “top-side” or “deckhead” corrosion and is a well-documented classification society concern (particularly flagged in IACS Unified Requirements and CSR for oil tankers).

5. Coating breakdown accelerates the process

• Even where protective coatings are applied to the upper 1.5 m of tank sides and complete deckhead (a SOLAS/MARPOL requirement since the 2004 amendments – see Q.5), coating failure due to mechanical damage during cargo operations (Butterworth machines, tank cleaning equipment impact, gas-freeing operations) exposes bare steel to this uniquely aggressive acidic-condensation environment, and once breached, corrosion at the holiday/damage spot proceeds rapidly because it is now a localized anodic site surrounded by a large cathodic coated area (an unfavourable anode-to-cathode area ratio).

6. Oxygen availability

• The vapour space, particularly during gas-freeing, tank cleaning, or when the IGS is not maintaining a positive, oxygen-deficient blanket properly, can have higher oxygen ingress than the oil-covered zones, and oxygen is essential to sustain the cathodic reaction in the corrosion cell, further accelerating attack when combined with the acidic film.

Net effect: The combination of (a) a continuously renewed acidic electrolyte film from condensation, (b) sulphur-bearing corrosive gases from crude and inert gas, (c) an unfavourable, undiluted exposure with no cargo buffering, and (d) complex structural geometry that retains moisture, makes the vapour space — especially the deckhead and upper strakes — the most corrosion-prone zone in a VLCC cargo tank. This is precisely why SOLAS Ch. II-1/3-2 mandates protective coating of the entire vapour space (deckhead and 1.5 m of adjacent side shell/transverse structure) on all new oil tankers regardless of size.

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