Q.2 — Pitting Corrosion in Cargo Tanks

What is pitting corrosion?

Pitting is a highly localized form of corrosion attack in which material loss is concentrated at discrete points on the steel surface rather than occurring uniformly across it. Instead of a general, even wastage of plate thickness (general/uniform corrosion), pitting produces small-diameter, deep cavities — sometimes only a few millimetres across but penetrating a significant fraction, or even the full thickness, of the plate.

In cargo tanks, pitting is most commonly seen on:

• Bottom plating, particularly where water settles (crude oil carries free water and produced water that separates and sinks to the tank bottom)
• Under sediment/sludge accumulations (wax, scale, asphaltenes) that sit on horizontal surfaces
• Areas where coating breakdown has occurred at isolated spots, leaving small bare-steel “holidays” surrounded by intact coating

Mechanism — why it occurs

1. Anode/cathode area ratio effect (the core electrochemical driver) When a coating is damaged at a single small point, that exposed bare-steel spot becomes an anode, while the surrounding large area of intact, well-coated steel — plus any adjoining bare steel that has developed a protective scale — acts as the cathode. Because the anodic area is tiny relative to the cathodic area, all of the corrosion current generated by the cathodic reaction over that large area is forced to discharge through the very small anodic spot. This produces an extremely high current density at the pit, and hence a very high rate of metal loss confined to that point — this is the fundamental reason pitting proceeds so much faster locally than general corrosion.

2. Autocatalytic pit growth (self-accelerating chemistry) Once a pit initiates, the chemistry inside it becomes self-sustaining and self-worsening:

• Metal dissolution (Fe → Fe²⁺ + 2e⁻) occurs at the pit base.
• Fe²⁺ hydrolyses with water, producing H⁺ ions, so the solution inside the pit becomes progressively more acidic than the surrounding bulk liquid.
• Chloride ions (present in any water phase, especially from ballast/seawater residues or produced water in crude) migrate into the pit to balance the excess positive charge, forming aggressive metal chlorides that further break down any protective film.
• Oxygen is depleted inside the pit (restricted access due to geometry and product buildup), making the interior anodic and strongly differentiated from the oxygenated exterior, which remains cathodic — this oxygen concentration cell reinforces the same directional attack.
• Corrosion products (rust) often form a cap or dome over the pit mouth, further isolating the interior chemistry from the bulk and accelerating the autocatalytic process underneath.

3. Presence of settled water and sediment

• Free water that settles at the tank bottom, particularly where it is stagnant beneath sludge/sediment, provides the electrolyte and traps chlorides and sulphur compounds against the steel, initiating pits at these specific locations rather than uniformly.
• Microbiologically influenced corrosion (MIC) can also occur where sulphate-reducing bacteria thrive in this stagnant, oxygen-depleted water layer, producing highly localized and aggressive pitting.

4. Coating discontinuities

• Pinholes, mechanical damage from tank cleaning tools, gauging/sounding equipment impact, or poor surface preparation before coating application create the initial bare-metal sites where pits are seeded.

Effect on potential for oil pollution

Pitting is disproportionately dangerous relative to its visual/statistical significance because:

• Thickness measurement (gauging) surveys report average or spot thickness readings and can miss or understate pitting, since a UT (ultrasonic thickness) reading taken even a few millimetres from a pit will show apparently healthy plate, while the pit itself may have penetrated far more deeply — this is a key limitation discussed further in Q.7.
• Because pit growth rate is high (due to the anode-cathode area effect above) and often continues undetected between surveys, a pit can progress through full plate thickness leading to perforation well before scheduled thickness measurements or visual inspection would otherwise flag general wastage as a concern.
• A single perforation at the tank bottom or at a bulkhead separating cargo from ballast/sea creates a direct pathway for cargo oil to escape — either into the sea (bottom plating pitting-through) or into an adjacent ballast tank (which itself may then be pumped overboard, causing pollution indirectly), or conversely, seawater ingress into cargo. Even a small-bore pinhole leak, sustained over a voyage, can result in a significant pollution event.
• Pitting at tank bottom-to-shell connections or in way of suction wells and bell-mouths is especially critical since these are lower points where any pollution consequence (direct hull breach) is most severe and where oil accumulates as the last remaining un-pumped residue, keeping those spots wetted and anodic for extended periods.
• Because pitting is localized and unpredictable in location, it undermines confidence in “average condition” assessments of tank structure — a tank can appear to be in good condition (high average thickness, low general wastage percentage) while carrying multiple advanced pits that represent near-term perforation risk. This is why classification societies require close-up surveys and specific pitting assessment (extent and depth measured against original thickness) as part of Condition Assessment Schemes (CAS) for tankers, rather than relying on average diminution figures alone.

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