Corrosion is a direct factor in safety, repair costs and survey risks.
Effective corrosion protection is always a system comprising coating, surface preparation and inspection.
Innovations help primarily in better monitoring and more targeted preservation planning.
Superintendents should treat corrosion as a portfolio topic across the entire fleet.
Corrosion on seagoing vessels follows several mechanisms that act differently depending on the area and loading. The most important are: general surface corrosion (uniform material loss), pitting (localised corrosion with deep holes but small area loss), crevice corrosion (in gaps and beneath deposits), galvanic corrosion (contact corrosion between dissimilar metals) and erosion corrosion (combined effect of flow and corrosion).
For the superintendent, understanding these mechanisms is critical because the correct countermeasure depends on the corrosion type. Surface corrosion can be combated with coatings. Pitting additionally requires cathodic protection. Galvanic corrosion must be prevented through electrical isolation or sacrificial anodes. Choosing the wrong protection wastes money and defers the problem.
Surface preparation is the most critical step in any coating operation. The SA 2.5 standard (near white blast cleaning per ISO 8501-1) is the minimum requirement for ballast tank coatings under the IMO Performance Standard for Protective Coatings (PSPC, MSC.215(82)). In practice this means: the steel surface must be free of rust, old coating residues and contaminants. If this level is not achieved, the new coating will not adhere permanently – regardless of its quality.
Coating systems themselves have evolved. Epoxy-based systems dominate in ballast tanks and underwater areas. For ballast tanks, typical systems comprise a primer plus two topcoats with a total dry film thickness (DFT) of at least 320 micrometres. More modern systems employ glass-flake reinforced epoxies that offer greater chemical and mechanical resistance.
Cathodic protection complements the coating. On seagoing vessels, sacrificial anodes made of zinc or aluminium are predominantly used (galvanic cathodic protection). For the underwater hull and ballast tanks, anode weight, distribution and service life are central planning parameters. A typical docking interval of five years requires adequately dimensioned anodes that remain effective throughout the entire period.
A common misconception is that corrosion protection only happens during docking. In reality, effective corrosion management begins on board – between dockings. Regular ballast tank inspections, documentation of coating conditions and timely repair of small damages prevent a localised coating defect from becoming a large-scale corrosion problem.
Class requirements for ballast tank inspections (Enhanced Survey Programme, ESP) mandate annual close-up surveys of selected structures on older vessels. Those who do not conduct their own inspections between surveys may face substantial repair requirements at the next class survey – which then feeds into docking planning and drives costs upward.
Thickness measurements (ultrasonic thickness measurements, UTM) are an essential tool for monitoring between dockings. They reveal where material loss is occurring and whether residual wall thickness remains within class limits. Systematic thickness measurements over several years enable trend analyses and prediction of when steel repairs will become necessary – an important input for docking budget planning.
For deck and superstructure areas, touch-up painting is an ongoing concern. The crew should be able to repair small coating damages independently – provided the correct materials (compatible paint, primer) and a minimum level of training are available. This is frequently a weak point: on-board paint stocks are not always compatible with the existing coating system.
A typical scenario: a 15-year-old Handymax bulker faces its third special survey. Thickness measurements from previous years show increased material loss in the upper side tanks – an area particularly vulnerable due to temperature cycling (loading/unloading), humidity and poor access. The original coating in these areas is largely degraded after 15 years.
Class will likely require extensive coating renewal and possibly steel repairs (crop and renew). Costs can range between USD 200,000 and 500,000 depending on scope. Had the operator conducted regular ballast tank inspections with photo documentation and UTM over the preceding years, the deterioration would have been detected earlier. Smaller repairs at an earlier stage would have significantly reduced the total cost.
This example is not the exception but the norm for older bulkers and tankers. The IMO PSPC has improved the situation for newbuilds, but for existing fleets, proactive corrosion management remains the only way to control costs.
Prioritisation of corrosion protection measures should follow three axes:
Safety relevance: Structures whose failure jeopardises vessel safety (ballast tanks, cargo holds on bulkers, deck plating) have the highest priority.
Economic impact: Areas whose deterioration leads to high repair costs or off-hire take precedence over cosmetic areas.
Accessibility: Areas only accessible during docking (underwater hull) must be covered in the docking scope. Areas accessible on board (decks, superstructure, ballast tanks) should be maintained continuously.
Fleet managers responsible for several vessels of different ages face a recurring problem: docking budgets are set annually, but corrosion damage accumulates unevenly and does not respect the calendar. A vessel that received only touch-up coating at its last docking may need a full ballast tank recoating at the next one, while a sister ship with a stronger original coating system might need only spot repairs. Treating every vessel in the fleet with the same standard budget line for corrosion work either overspends on the healthy vessels or leaves the deteriorating ones underfunded.
A more reliable approach ranks vessels by measured condition rather than by age alone. Thickness measurement trends, coating condition surveys and previous repair scope give a numerical basis for comparing vessels that class surveys alone do not provide, since a class survey confirms compliance at a point in time rather than predicting where the next problem will appear. Budget allocation based on this ranking directs money to the vessels that need it most in the coming docking cycle, while vessels with a healthy coating record can defer major work by another interval without additional risk.
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